p n junction diode characteristics experiment

VI Characteristics of a Diode            

Aim of the experiment.

At the end of the experiment, the student should be able to

• Explain the structure of a P-N junction diode
• Explain the function of a P-N junction diode
• Explain forward and reverse biased characteristics of a Silicon diode
• Explain forward and reverse biased characteristics of a Germanium diode

PN Junction Diode Characteristics – Explained in Detail with Graphs

In this article, we learn about PN junction diode characteristics in detail – like how to bias a PN junction (Forward & Reverse bias methods), behavior of PN junction during forward & reverse bias setups, how to plot the VI characteristics, what is reverse breakdown and many other essential concepts regarding a PN junction diode. So let’s begin.

In chapter 1 – Understanding the PN junction , we have seen how a PN junction is formed from a p-type and n-type semiconductor. We have also learned about diffusion current, depletion region, drift current and barrier potential. If you find these terms foreign, just read the chapter about “ understanding the pn junction ” once more. Lets just make some questions. What is the use of a PN junction? Why have scientists created a pn junction device? What kind of problem it solves ? Learning anything is really fun when we question it. So these are our questions. Why there exists a pn junction in this world! ?;)

To get an answer to all these questions, lets first try to understand the characteristics of a PN junction. We know a pn junction has a “barrier potential”. Only if we overcome this “barrier potential” by applying an external voltage to the pn junction, we would be able to make it conducting. This simply means, current will pass through the pn junction only if we apply an external voltage higher than the “barrier potential” of pn junction. In chapter 1, we have seen that  net current inside a pn junction is zero. Inorder to understand the behavior of a pn junction we need to make it conducting by applying an external voltage over a range (say from 0 volts 5 or 10 volts ), and then we study how the current passed through the pn junction varies with increasing voltage levels. To apply an external voltage, we usually connect 2 metallic contacts at the two ends of the pn junction ( known as terminals ); one on the p-side and other on the n-side. A PN junction with two metallic contacts is known as a pn junction diode or a semiconductor diode. 

Note:- I have written an interesting article which tells the story behind invention & discovery of PN Junction diode. If you like to read the story, follow here:- Story behind Invention & Discovery of PN Junction

PN junction diode is symbolically represented as shown in picture. The direction of arrow is the direction of conventional current flow (under forward bias). Now lets try applying an external voltage to the pn junction diode. The process of applying an external voltage is called as “biasing” . There are two ways in which we can bias a pn junction diode.

1) Forward bias and 2) Reverse bias  

The basic difference between a forward bias and reverse bias is in the direction of applying external voltage. The direction of external voltage applied in reverse bias is opposite to that of  external voltage applied in forward bias.

Forward biasing a PN Junction diode

Forward biasing a pn junction diode is very simple. You just need to take a battery whose values can be varied from (o to V volts), connect its positive terminal to the p-side of pn junction diode and then connect the negative terminal of battery to the n-side of the pn junction diode. If you have done upto this, the forward bias circuit of pn junction diode is complete. Now all we need to do is understand how the pn junction diode behaves when we increase the voltage levels from 0 to say 10 volts or 100 volts. We have learned that if we apply an external voltage higher than the barrier potential of pn junction diode, it will start conducting, which means it will start passing current through it. So how we are going to study the behavior of pn junction diode under forward biased condition? Lets get a voltmeter and ammeter and connect it to the forward biased circuit of pn junction diode.A simple circuit diagram is shown below, which has a pn junction diode, a battery (in picture it is not shown as variable. keep in mind we are talking about a variable power source), an ammeter (in milli ampere range) and a voltmeter.

Note:- Assume that the pn junction diode is made from Silicon. The reason is difference in barrier potential for a diode made from Germanium and Silicon. (For a silicon diode – barrier potential is 0.7 volts where as for a Germanium diode barrier potential is low ~ 0.3 volts)

How to plot the characteristics of a pn junction ?

What we are going to do is, vary the voltage across diode by adjusting the battery. We start from o volts, then slowly move 0.1 volts, 0.2 volts and so on till 10 volts. Lets just note the readings  of voltmeter and ammeter each time we adjust the battery (in steps of 0.1 volts). Finally after taking the readings, just plot a graph with voltmeter readings on X-axis and corresponding Ammeter readings on Y axis. Join all the dots in graph paper and you will see a graphical representation as shown below. Now this is what we call “characteristics of a pn junction diode” or the “behavior of diode under forward bias” 

How to analyse the characteristics of a pn junction diode ?

Its from the “characteristics graph ” we have just drawn, we are going to make conclusions about the behavior of pn junction diode. The first thing that we shall be interested in is about “barrier potential” . We talked a lot about barrier potential but did we ever mention its value ? From the graph, we observe that the diode does not conduct at all in the initial stages. From 0 volts to 0.7 volts, we are seeing the ammeter reading as zero! This means the diode has not started conducting current through it. From 0.7 volts and up, the diode start conducting and the current through diode increases linearly with increase in voltage of battery. From this data what you can infer ? The barrier potential of silicon diode is 0.7 volts 😉  What else ? The diode starts conducting at 0.7 volts and current through the diode increases linearly with increase in voltage. So that’s the forward bias characteristics of a pn junction diode. It conducts current linearly with increase in voltage applied across the 2 terminals (provided the applied voltage crosses barrier potential).

What happens inside the pn junction diode when we apply forward bias ?

We have seen the characteristics of pn junction diode through its graph. What really happens inside the diode during the forward bias ? We know a diode has a depletion region with a fixed barrier potential. This depletion region has a predefined width, say W . This width will vary for a Silicon diode and a Germanium diode. The width highly depends on the type of semiconductor used to make pn junction, the level of doping etc. When we apply voltage to the terminals of diode, the width of depletion region slowly starts decreasing. The reason for this is, in forward bias we apply voltage in a direction opposite to that of barrier potential. We know the p-side of diode is connected to positive terminal and n-side of diode is connected to negative terminal of battery. So the electrons in n-side gets pushed towards the junction (by force of repulsion) and the holes in p-side gets pushed towards the junction. As the applied voltage increases from 0 volts to 0.7 volts, the depletion region width reduces from ‘ W’ to zero. This means depletion region vanishes at 0.7 volts of applied voltage.  This results in increased diffusion of electrons from n-side to p-side region and the increased diffusion of holes from p-side to n-side region. In other words, “ minority carrier ” injection happens on both p-side (in a normal diode (without bias) electrons are a minority on p-side) and n-side (holes are a minority on n-side) of the diode.

How current flow takes place in a pn junction diode ?

This is another interesting factor, to explain. As the voltage level increases, the electrons from n-side gets pushed towards the p-side junction. Similarly holes from p-side gets pushed towards the n-side junction. Now there arises a concentration gradient between the number of electrons at the p-side junction region and the number of electrons at the region towards the p-side terminal. A similar concentration gradient develops between the number of holes at the n-side junction region and the number of holes at region near the n-side terminal. This results in movement of charge carriers (electrons and holes) from region of higher concentration to region of lower concentration. This movement of charge carriers inside pn junction gives rise to current through the circuit.

Reverse biasing a PN junction diode

Why should we reverse bias a pn diode ? The reason is, we want to learn its characteristics under different circumstances. By reverse biasing, we mean, applying an external voltage which is opposite in direction to forward bias. So here we connect positive terminal of battery to n-side of the diode and negative terminal of the battery to p-side of the diode. This completes the reverse bias circuit for pn junction diode. Now to study its characteristics (change in current with applied voltage), we need to repeat all those steps again. Connect voltmeter, ammeter, vary the battery voltage, note the readings etc etc. Finally we will get a graph as shown.

Analysing the revere bias characteristics

Here the interesting thing to note is that, diode does not conduct with change in applied voltage. The current remains constant at a negligibly small value (in the range of micro amps) for a long range of change in applied voltage. When the voltage is raised above a particular point, say 80 volts, the current suddenly shoots (increases suddenly). This is called as “ reverse current ” and this particular value of applied voltage, where reverse current through diode increases suddenly is known as “ break down voltage “.

What happens inside the diode ?

We connected p-side of diode to negative terminal of battery and n-side of diode to positive terminal of battery. So one thing is clear, we are applying external voltage in the same direction of barrier potential. If applied external voltage is V and barrier potential is Vx , then total voltage across the pn junction will be V+Vx . The electrons at n-side will get pulled from junction region to the terminal region of n-side and similarly the holes at p-side junction will get pulled towards the terminal region of p-side.  This results in increasing the depletion region width from its initial length, say ‘W’ to some ‘W+x’. As width of depletion region increases, it results in increasing the electric field strength.

How reverse saturation current occurs and why it exists ?

The reverse saturation current is the negligibly small current (in the range of micro amperes) shown in graph, from 0 volts to break down voltage. It remains almost constant (negligible increase do exist) in the range of 0 volts to reverse breakdown voltage. How it occurs ? We know, as electrons and holes are pulled away from junction, they dont get diffused each other across the junction. So the net “ diffusion current ” is zero! What remains is the drift due to electric field. This reverse saturation current is the result of drifting of charge carriers from the junction region to terminal region. This drift is caused by the electric field generated by depletion region.

What happens at reverse breakdown ?

At breakdown voltage, the current through diode shoots rapidly. Even for a small change in applied voltage, there is a high increase in net current through the diode. For each pn junction diode, there will be a maximum net current that it can withstand. If the reverse current exceeds this maximum rating, the diode will get damaged.

Conclusion about PN junction characteristics

To conclude about pn junction characteristics, we need to get an answer to the first question we have raised – What is the use of pn junction? From the analysis of both forward bias and reverse bias, we can arrive at one fact – a pn junction diode conducts current only in one direction – i.e during forward bias. During forward bias, the diode conducts current with increase in voltage. During reverse bias, the diode does not conduct with increase in voltage (break down usually results in damage of diode). Where can we put this characteristics of diode into use ? Hope you got the answer! Its in conversion of alternating current to direct current (AC to DC). So the practical application of pn junction diode is rectification!

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Hey! That’s so helpful

Thickness of diplition layer depend on which factor?

DESC: Diode forward biased 24VDC QTY: 20pcs

DESC: Diode Reverse biased 24VDC QTY: 20pcs

Faith N. Dolorito Procurement Specialist MANILA OVERSEAS INC. TEL:6328004227 FAX:6328004172

thank you so very much…. I am clearly understood to read it……. ……..

As width of depletion region increases, it results in increasing the electric field strength.Why?

What is zener effect and avalanche effect.?

Utmost/extremly thanks ….. For this crystal clear explanation….. I really got something from it…. But sir what is Zener effect.and avalenche effect.?

Why internal electric field generate after diffusion process in pn junction

i hve a question. why the arrow in pn junction thicker????

explain the working of PN junction diode in forward and reverse biasing configuration please ?

why the battery in reverse bias is greater than in forward bias

I think I missed something. You say that the PN junction only starts to conduct current after the voltage aplied on the diode (Vd) reaches 0.7V, the barrier potential as you call it, but all the graphics and equations shows us that there is current through the diode for values of Vd smaller than 0,7V. I mean, even considering the current for Vd near zero negligible, with Vd~0.60V there is current.

As I see it, we just consider 0.7V as a practival value for a conducting diode, where any variation of the current will cause a small variation on Vd, keeping it around the same 0.7V. It would me consistent with the diode current equation Id=Is(exp(Vd/nVt)-1), cause in 0.7V for a regular diode, de slope in the curve is too large to see any change in Vd as the current varies.

I don’t know if I made myself clear, but thats a point that is not really clear in many books about semiconductors physics and it’s annoying me. If you could clarify that for me I would be glad.

Why the forward voltage values are almost constant for source voltage from 5V to 1V during forward-biased?

what is the difference between the connections of forwardbias and reverse bias in pn junction…?

in forward biasis -VE terminal of battery is connected to pentavelent group N and +ve is connected to trivalent group P but in reverse biasis the connection is opposite …

can I get a pdf of this chapter??

very clear presantation if you were around i would offer you a cup of tea or coffee good work

why is the voltmeter connected across the ammeter and reverse biased diode..?

Can a diode work on ac voltage or not

@Anuj – A diode is basically a PN Junction. It is used to convert AC to DC.

diode worked on ac voltage but it will give output is DC why because ac has two half cycles in that case,it will conduct only positive half cycle….do not allow -ve cycles…

it’s working on ac voltage

The junction information is clearly understand so nice of it thanx

for eachelectron hole combination that take place near the junction a covalent bond breaks in the p section near the +ve pole of the battery how it is formed?

it is so helpful and it clears all the confusion…….plz answer meone question thatis why in CB mode the emitter current increases with increase of V(CB)

this is a exellent article……….sir plz letme know about base width modulation

It is very short notes It is very useful i am very happy after read that notes thank u very much

thanks 4 the good explanation. will you please show the one connected image source circuit of both forward and reverse biased a pn-junction

Please see Fig.10

wow it is very much helpful to me. Thanks the author

yes, its very great answer that i want. Thanks.

I really appreciate. Got a clearer explanation that i did in class… Kudos. Thanks Admin

a great work with full clearification. thanx !

Really interesting and clear clarification of every aspect of a junction diode characteristics.Very nice

Brilliant! Very helpful article. It’s clearly explaind and easy to understand. Bravo for the person who has put so much work to make it!!

Thanq So Much 🙂 this helped me a lot 🙂 Is there explanation for Transistor as a Switch and Amplifier?

explanation is little bit invalid

thaks very much for the good explanation.can you describe the current voltage characteristics of a photodiode when light is incident on it?

veryyyy goood explanation, i got it perfectly, please tell me about bridge wave rectifier, we connect 4 diodes in bridge but when the d1 and d2 are forward biased then haw the d3 and d4 are reversr biased

@Nayan – Read this article:- https://www.circuitstoday.com/full-wave-bridge-rectifier

It will help you understand bridge rectifier perfectly.

when we talk about reverse bias ,thn the width of depletion layer increases thn after more reverse voltage(greater than reverse breakdown voltage) how current flow through dide?

At break down, what happens really is that the diode gets damaged. It loses its junction & characteristics associated with the junction. The “diode” almost behaves like a shorted wire & hence current flows through it easily. Theoretically, internal resistance of a diode at breakdown is zero. But in practice, there exists a small internal resistance and hence the current increases with a deviation factor (and not a perpendicular graph).

Hope this helps!

Really helpfull , Thanks sir..

good explanation with neat a diagrams

its very simple to understand ……i like to read a lot in webpage…thank u to author who wrote this.

well explained. really enyoyed.

sir please add the curve charcterstic found when we use ge semiconductor as pn junction diode due to the this experiment

it was very useful and was written in a readble mannar

I like this and I enjoy

its a rely nuc explanation abt pn junctoin m a net qualified scientist

Thank you Pintu 🙂 It was very nice words 🙂

the difference between depletion barrier’s height and width . i mean why they are different and what they indicate?

If depletion region’s width indicates the area covered by defused electrons/holes then read further.

In forward bias condition external electric field ( produced by battery) will be opposite to the internal electric field ( produced depletion barrier ). in this case the external electric field will cancel the internal electric field and more electron will flow from n type to p type material(assumed external voltage is greater than depletion barrier) which increases the depletion region but in real, in forward bias condition the depletion region’s width decreases. And in reverse bias condition the depletion region increases instead of decreasing. (I am familiar with the increase/decrease of potential of depletion barrier and agree with the books)

I am very confused with this question. so please help me. Thank you

What really matters is the “barrier potential” of a diode. In a Silicon diode, the “barrier width” is higher than a Germanium diode. So “barrier potential” of a Silicon diode is higher than Germanium diode. I hope you understood.

cool great approach. hoping that 2 give more information about electronics

Please help me out.. In forward bias if battery voltage is 2v , drop across si diode cant be more than 1v i.e. Vd<1v… So now my qusetion is where this remaining 1v of battery is if no resistor is in series with diode?

In that case, 1 volt will be dropped across the wires with the help of a very large current.

Awesome explanation.thank you

Crystal Clear approach, awesome!!

it’s very useful thank you

Really amazing! I have never seen a website this successful in explanation! You can’t imagine how much this helped me! Thanks

Keep adding more and more info….

owsam… PERFECT …!!

Thanks so much. That was a comprehensive expose. Keep keeping

oh thank u..i am very confused to read my text book but now every thing is clear….thank you very much ..

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Characteristics and Working of PN Junction Diode

• March 27, 2024
• By Ravi Teja

In this tutorial, we will learn about PN Junction Diodes and in particular the characteristics and working of PN Junction Diode. This understanding will lay a better foundation for exploring further into different aspects of semiconductor electronics.

Introduction

PN junction is a significant building block and it is one among the indispensable structures offered by the semiconductor technology in electronics. Electronic components such as bipolar junction transistors, junction FETs and MOSFETs, or diodes such as light-emitting diodes (LEDs), and analog or digital integrated circuits (ICs) are all supported in semiconductor technology.

The exciting property of semiconductor diode is facilitating the electrons to flow exclusively in one direction across it; as a result it acts as a rectifier of Alternating Current. The indispensable operation in semiconductor diode is the basis for understanding of all the semiconductor diodes.

The diode can be observed as a straightforward bipolar semiconductor device. The characteristics of diode look to be a graph of current that a diode produces when the voltage applied to it. A perfect diode can be absolutely distinguished by its current and voltage curve.

It permits the current to flow solely in forward direction and effectively blocks the current in the reverse direction. It is vital to recognize that the semiconductor is entirely a single-crystal material, made from two separate blocks of semiconductor opposite kind.

One block is doped with trivalent impurity atoms to create the P region that acts as acceptors with holes as majority charge carriers and the adjacent block is doped with pentavalent impurity atoms to create the N region that acts as donors with electrons as majority charge carriers.

The boundary splitting the n and p region is referred to as the metaphysical junction. The concentration of doping is same all over in every block and there will be an abrupt modification in doping at the junction. When the two blocks are placed nearer to each other, the electrons and holes diffuse towards the region of lower concentration from the region of higher concentration.

In the process of diffusion, electrons from N region diffuse towards the P region whereas holes from P region diffuse towards the N region. Once holes enter the N region, they will recombine with donor atoms. At the same time, donor atoms admit additional holes and become positively charged stationary donor atoms.

The electrons spreading from N region to P region recombine with the acceptor atoms in P region. At the same time, acceptor atoms admit additional electrons and become negatively charged immobile acceptor atoms.

As a result, a large number of positively charged ions are produced at the junction on the N side and a large number of negatively charged ions are produced at the junction on P side.

The net positively and negatively charged ions within the N and P regions induce an electric field in the space near to the metaphysical junction. Merging these two regions wherever the electric field is small and wherever the free carrier density is equivalent to the net doping density can be named as the space charge region.

It can also be referred as a quasi neutral region. Fundamentally, all electrons and holes are swept out of the free space charge region by the electric field. The tapered region in which depletion of free mobile charge carriers takes place is called as Depletion Region.

It is assumed that the depletion region around the metallurgical junction has well-defined edges. It conjointly assumes that the transition between the depletion region and the free space charge region is abrupt.

Depletion region contains preset positive ions on the N-side and preset negative ions on the P-side. The width of the depletion layer is inversely proportional to the concentration of dopants present in each region.

The electric field within the depletion region creates an opposing force that opposes the electrons and holes from diffusing attributable to the impact of charged ions within the depletion region. This opposing force can be often cited as potential barrier voltage. The typical value of potential barrier for silicon is 0.72V and for germanium is 0.3V.

When the electric field and barrier potential are balanced with one another, then the state of equilibrium is reached that result in potential difference Vo connecting the two sides of the depletion layer. The net contact potential difference depends on the type of material and it is high for n-type than the p-type.

In the state of thermal equilibrium, barrier potential provides low potential energy for the electrons on N-side than P-side. Energy bands bend in the free space charge region, since conduction and valence band positions with respect to the Fermi energy levels changes between P and N regions.

No conduction of current takes place in this equilibrium state and the current due to diffusion and drift current cancel for both the electrons and holes. The built-in barrier potential maintains balance between majority charge carriers in the N region and minority charge carriers in the P region as well as between majority charge carriers in the P region and minority charge carriers in the N region.

The built-in potential barrier can also be estimated as the distinction between the intrinsic Fermi energy levels in P and N regions.

PN junction diode is a diode which can be used as a rectifier, logic gate, voltage stabiliser, switching device, voltage dependent capacitor and in optoelectronics as a photodiode, light-emitting diode (LED), laser diode, photo detector, or solar cell in electronics.

Working of PN Junction Diode

If an external potential is applied to the terminals of PN junction, it will alter the potential between the P and N-regions. This potential difference can alter the flow of majority carriers, so that the PN junction can be used as an opportunity for the diffusion of electrons and holes.

If the voltage applied decreases the width of the depletion layer, then the diode is assumed to be in forward bias and if the applied voltage increases the depletion layer width then the diode is assumed to be in reverse bias. If the width of depletion layer do not alters then it is in the zero bias state.

• Forward Bias: External voltage decreases the built-in potential barrier.
• Reverse Bias: External voltage increases the built-in potential barrier.
• Zero Bias: No external voltage is applied.

PN Junction Diode When No External Voltage is Applied

In zero bias or thermal equilibrium state junction potential provides higher potential energy to the holes on the P-side than the N-side. If the terminals of junction diode are shorted, few majority charge carriers (holes) in the P side with sufficient energy to surmount the potential barrier travel across the depletion region.

Therefore, with the help of holes, current starts to flow in the diode and it is referred to as forward current. In the similar manner, holes in the N side move across the depletion region in reverse direction and the current generated in this fashion is referred to as reverse current.

Potential barrier opposes the migration of electrons and holes across the junction and allow the minority charge carriers to drift across the PN junction. As a result of it, a state of equilibrium is established when the majority charge carriers are equal in concentration on either side of the junction and when minority charge carriers are moving in opposite directions.

A net zero current flows in the circuit and the junction is said to be in dynamic equilibrium. By increasing the temperature of semiconductors, minority charge carriers have been continuously generated and thereby leakage current starts to rise. In general no conduction of electric current takes place because no external source is connected to the PN junction.

Forward Biased Pn Junction Diode

With the externally applied voltage, a potential difference is altered between the P and N regions.When positive terminal of the source is connected to the P side and the negative terminal is connected to N side then the junction diode is said to be connected in forward bias condition. Forward bias lowers the potential across the PN junction.

The majority charge carriers in N and P regions are attracted towards the PN junction and the width of the depletion layer decreases with diffusion of the majority charge carriers. The external biasing causes a departure from the state of equilibrium and a misalignment of Fermi levels in the P and N regions, and also in the depletion layer.

So an electric field is induced in a direction converse to that of the incorporated field. The presence of two different Fermi levels in the depletion layer represents a state of quasi-equilibrium. The amount of charge Q stored in the diode is proportional to the current I flowing in the diode.

With the increase in forward bias greater than the built in potential, at a particular value the depletion region becomes very much thinner so that a large number of majority charge carriers can cross the PN junction and conducts an electric current. The current flowing up to built in potential is called as ZERO current or KNEE current.

Forward Biased Diode Characteristics

With the increase in applied external forward bias, the width of the depletion layer becomes thin and forward current in a PN junction diode starts to increase abruptly after the KNEE point of forward I-V characteristic curve.

Firstly, a small amount of current called as reverse saturation current exists due to the presence of the contact potential and the related electric field. While the electrons and holes are freely crossing the junction and causes diffusion current that flows in the opposite direction to the reverse saturation current.

The net result of applying forward bias is to reduce the height of the potential barrier by an amount of eV. The majority carrier current in the PN junction diode increases by an exponential factor of eV/kT. As result the total amount of current becomes I = I s *  exp(eV/kT), where I s  is constant.

The excess free majority charge carrier holes and electrons that enter the N and P regions respectively, acts as a minority carriers and recombine with the local majority carriers in N and P regions. This concentration consequently decreases with the distance from the PN junction and this process is named as minority carrier injection.

The forward characteristic of a PN junction diode is non linear, i.e., not a straight line. This type of forward characteristic shows that resistance is not constant during the operation of the PN junction. The slope of the forward characteristic of a PN junction diode will become very steep quickly.

This shows that resistance is very low in forward bias of the junction diode. The value of forward current is directly proportional to the external power supply and inversely proportional to the internal resistance of the junction diode.

Applying forward bias to the PN junction diode causes a low impedance path for the junction diode, allows for conducting a large amount of current known as infinite current. This large amount current starts to flow above the KNEE point in the forward characteristic with the application of a small amount of external potential.

The potential difference across the junction or at the two N and P regions is maintained constant by the action of depletion layer. The maximum amount of current to be conducted is kept limited by the load resistor, because when the diode conducts more current than the usual specifications of the diode, the excess current results in the dissipation of heat and also leads to severe damage of the device.

Reverse Biased PN Junction Diode

When positive terminal of the source is connected to the N side and the negative terminal is connected to P side, then the junction diode is said to be connected in reverse bias condition. In this type of connection majority charge carriers are attracted away from the depletion layer by their respective battery terminals connected to PN junction.

The Fermi level on N side is lower than the Fermi level on P side. Positive terminal attracts the electrons away from the junction in N side and negative terminal attracts the holes away from the junction in P side. As a result of it, the width of the potential barrier increases that impedes the flow of majority carriers in N side and P side.

The width of the free space charge layer increases, thereby electric field at the PN junction increases and the PN junction diode acts as a resistor. But the time of diode acting as a resistor is very low. There will be no recombination of majority carriers taken place at the PN junction; thus, no conduction of electric current.

The current that flows in a PN junction diode is the small leakage current, due to minority carriers generated at the depletion layer or minority carriers which drift across the PN junction. Finally, the result is that the growth in the width of the depletion layer presents a high impedance path which acts as an insulator.

In reverse bias condition, no current flows through the PN junction diode with increase in the amount of applied external voltage. However, leakage current due to minority charge carriers flows in the PN junction diode that can be measured in micro amperes.

As the reverse bias potential to the PN junction diode increases ultimately leads to PN junction reverse voltage breakdown and the diode current is controlled by external circuit. Reverse breakdown depends on the doping levels of the P and N regions.

With the increase in reverse bias further, PN junction diode become short circuited due to overheat in the circuit and maximum circuit current flows in the PN junction diode.

Reverse Biased Diode Characteristics

V-I Characteristics of PN Junction Diode

In the current–voltage characteristics of junction diode, from the first quadrant in the figure current in the forward bias is incredibly low if the input voltage applied to the diode is lower than the threshold voltage (Vr). The threshold voltage is additionally referred to as cut-in voltage.

Once the forward bias input voltage surpasses the cut-in voltage (0.3 V for germanium diode, 0.6-0.7 V for silicon diode), the current spectacularly increases, as a result the diode functions as short-circuit.

The reverse bias characteristic curve of diode is shown in the fourth quadrant of the figure above. The current in the reverse bias is low till breakdown is reached and therefore the diode looks like as open circuit. When the reverse bias input voltage has reached the breakdown voltage, reverse current increases spectacularly.

PN Diode Ideal and Real Characteristics

For ideal characteristics, the total current in the PN junction diode is constant throughout the entire junction diode. The individual electron and hole currents are continuous functions and are constant throughout the junction diode.

The real characteristics of PN Junction diode varies with the applied external potential to the junction that changes the properties of junction diode. The junction diode acts as short circuit in forward bias and acts as open circuit in reverse bias.

• Semiconductors contain the properties in middle of conductors and insulators.
• Commonly used material for semiconductor is silicon.
• Semiconductors contain electrons and holes as charge carriers.
• The charge carriers in semiconductors are free to move throughout the device, so they are called as mobile charge carriers.
• Holes are positively charged particles and electrons are negatively charged particles.
• Charge carriers are responsible for conducting electric current.
• Semiconductors are of two types namely intrinsic and extrinsic semiconductors.
• Intrinsic semiconductors are purest semiconductors as they don’t have any impurities in it.
• Extrinsic semiconductors contain impurities called as dopants that change the electrical properties of semiconductors.
• Extrinsic semiconductors are classified into two types. They are N-type and P-type.
• N-type impurities are called as donors because they contain electrons as majority chare carriers.
• P-type impurities are called as acceptors because they contain holes as majority charge carriers.
• PN junction is formed in a single crystal by joining two N-type and P-type semiconductors.
• PN junction diode is a two terminal device, the characteristics of diode depends on the polarity of the external potential applied to the PN junction diode.
• The junction of N and P semiconductors is free of charge carriers; hence the region is called as depletion region.
• The width of depletion region alters with the external applied potential.
• When no external potential is applied to PN junction, the condition is called as zero bias. The junction potential for silicon diodes is 0.6V – 0.7V and for germanium diodes is 0.3V.
• When the junction is biased in the forward direction, the majority carriers are attracted towards the junction and get replenished at the junction. In this condition, width of the depletion region decreases and with the increase in external potential diode acts as short circuit that allows the maximum amount of current to flow through it.
• When the junction diode is biased in the reverse direction, the majority charge carriers are attracted by the respective terminals away from the PN junction, thus avoiding the diffusion of electrons and holes at the junction. There will be a small amount of current called as leakage current due to minority charge carriers at the junction. This small current is called as drift current. When the reverse bias potential is increased further the diode acts as open circuit, thereby blocking the current to flow through it.

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• PNPN Diode or Shockley Diode
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• CBSE Class 12
• CBSE Class 12 Physics Practical
• To Draw The I V Characteristic Curve For P N Junction In Forward And Reverse Bias

To Draw The I-V Characteristic Curve of a P-N Junction In Forward Bias and Reverse Bias

In a standard diode, forward bias and reverse bias takes place. Let us know how to draw the I-V characteristic curve of a P-N junction in forward bias and reverse bias.

To draw the I-V characteristic curve of a P-N junction in forward bias and reverse bias.

Materials Required

• A P-N junction diode
• A 3-volt battery
• A 50-volt battery
• A high resistance rheostat
• One 0-3 volt voltmeter
• One 0-50 volt voltmeter
• One 0-100 mA ammeter
• One 0-100 μA ammeter
• One way key
• Connecting wires
• Piece of sandpaper

Forward bias characteristics

The junction is said to be forward-biased when the p-section of the diode is connected to the positive terminal of the battery and the n-section of the diode is connected to the negative terminal of the battery. With an increase in the voltage, the current also increases. For Si diode, at 0.7 V the current increases suddenly.

Reverse bias characteristics

The junction is said to be reverse-biased when the p-section of the diode is connected to the negative terminal of the battery and the n-section of the diode is connected to the positive terminal of the battery. With an increase in the voltage, there is a small change in the current but the reverse current increases to a higher value with an increase in the voltage.

For forward bias

• The circuit connections should be as shown in the diagram.
• All the connections should be neat, clean and tight.
• For voltmeter (V) and milli-ammeter (mA), the least count and zero error should be noted.
• To get the zero reading from the voltmeter and milli-ammeter, rheostat should be brought near the negative end by inserting the key K.
• To apply the forward bias voltage (V F ) of 0.1V, the contact should be moved towards the positive end. The current remains zero.
• Keeping current zero, increase the forward bias voltage up to 0.3 V for Ge diode.
• To record a small current using milli-ammeter, increase the V F to 0.4 V.
• Increase the V F by 0.2 V and record the corresponding current. When the V F becomes 0.7 V, the current will increase rapidly.
• When V F = 0.72 V, the current increases suddenly and this is known as forward breakdown stage.
• Take out the key if the forward current won’t change as V F increased beyond forward breakdown.
• Record the observations.

For reverse bias

• Note the least count and zero error of voltmeter (V) and micro-ammeter (μA).
• To get zero reading from the voltmeter V and micro-ammeter μA, insert the key K and bring the rheostat near the positive end.
• To apply reverse bias voltage (V R ) of 0.5 V, move the rheostat to the negative end so as to flow the reverse current.
• Increase V R by 0.2 V and record the corresponding current. When V R becomes 20 V, the current will increase rapidly.
• When V R = 25 V, the current increases suddenly and this is known as reverse breakdown stage. Record the current reading and take off the key.

Observations

Range of voltmeter = …….V

Least count of the voltmeter = …….V

Zero error of voltmeter = ……..V

Range of milli-ammeter = …….mA

Least count of milli-ammeter = …….mA

Zero error of milli-ammeter = ……..mA

Table for forward bias voltage and forward current

Range of micro-ammeter = …….μA

Least count of micro-ammeter = …….μA

Zero error of micro-ammeter = ……..μA

Table for reverse bias voltage and reverse current

Plotting of Graphs

Plot a graph between V F and I F taking V F on the x-axis and I F on the y-axis. The graph obtained is known as forward bias characteristic curve.

Plot a graph between V R and I R taking V R on the negative x-axis and negative I R on the y-axis. The graph obtained is known as reverse bias characteristic curve.

Junction resistance for forward bias = …… ohms

Junction resistance for reverse bias = ……… ohms.

Precautions

• The connections should be neat, clean and tight.
• Key should be used when the circuit is being used.
• Beyond breakdown, forward bias voltage should not be applied.
• Beyond breakdown, reverse bias voltage should not be applied.

Sources Of Error

Faulty junction diode might be supplied.

Q1. Define the energy level in an atom.

Ans: Energy level in an atom is defined as the energy value of an electron in the subshell of an atom.

Q2. What are the different types of energy bands?

Ans: Following are the different types of energy bands:

• Conduction band (C)
• Valence band (V)
• Forbidden band (F)

Q3. What are the different types of substances?

Ans: Following are the different types of substances:

• Semiconductors

Q4. What is the SI unit of conductance?

Ans: SI unit of conductance is siemens (S).

Q5. Name the different types of biasing.

Ans: Following are the different types of biasing:

• Forward biasing
• Reverse biasing

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September 2, 2024

This article has been reviewed according to Science X's editorial process and policies . Editors have highlighted the following attributes while ensuring the content's credibility:

fact-checked

peer-reviewed publication

Low-energy ion implantation enables 2D lateral p-n junction construction

by Wuhan University

The feature size of silicon-based transistors is approaching the theoretical limit, which puts forward higher requirements for the atomic level manufacturing of semiconductors. The basic idea of atomic level manufacturing is to process and manipulate matters with atomic level precision, which will greatly reduce the power consumption of the chip and achieve a huge increase in the chip's arithmetic power.

2D materials are expected to address the challenges faced by traditional silicon-based semiconductor devices. The p-n junction is the basic unit of optoelectronic devices in the information age.

Previous studies have shown that 2D vertical p-n junctions can be prepared simply regardless of lattice mismatch. However, due to the van der Waals gap in interfaces and the impurities introduced in the stacking process, 2D vertical p-n junctions will reduce the carrier mobility.

The 2D lateral p-n junction can effectively solve these problems. Therefore, how to realize the construction of high-quality 2D lateral p-n junction is crucial for the practical application of 2D semiconductors.

Ion implantation technique is a mature doping method for constructing p-n junctions in the traditional semiconductor industry, which has the merits of controllable doping concentration and depth, abundant doping elements, uniform doping area and non-polluting doping process.

However, due to the high energy of incident ions (tens of keV), the traditional ion implantation technique will cause damage or even penetrate the atomically thin 2D materials during the implantation process, resulting in device failure. Therefore, it is difficult to directly modulate the electrical and optical properties of 2D materials using conventional ion implantation.

In a paper published in Light Science & Applications , a team of scientists, led by Professor Xiangheng Xiao from School of Physics and Technology, Key Lab of Artificial Micro- and Nano-Structures of Ministry of Education, Wuhan University, Wuhan, China, has developed a low-energy ion implantation system for constructing 2D lateral p-n homojunction.

Low-energy ion implantation technique inherits the advantages of the traditional ion implantation technique. It has a lower ion energy and shallower implantation depth, which is expected to address the problem that traditional ion implantation techniques cannot be directly applied to modulate the performance of 2D materials.

Although a few groups have conducted research on low-energy ion implantation, they have mainly focused on the microscopic characterization and defect modulation. To date, there is a lack of research on using low-energy ion implantation to achieve patterned p-type doping on 2D materials to completely reverse their conductivity types and construct lateral p-n homojunctions.

By precisely modulating the implantation dose, the conductivity type of the WS 2 flake is successfully modulated, which could be converted from n-type to bipolar or even p-type. The universality of this method is also demonstrated by extending it to other 2D semiconductors. In addition, the photodetector based on WS 2 lateral p-n homojunction exhibits satisfactory self-powered photodetection capability.

This work provides an effective method for controllable doping of 2D materials and promotes the practical application of 2D materials.

The authors used the low-energy ion implantation technique to directly implant nitrogen ions into the few-layer WS 2 , and realized precise modulation of WS 2 conductive type by controlling the implantation dose of low-energy nitrogen ions.

"By increasing the implantation dose, the conductive type of WS 2 can be changed from n-type to bipolar- or even p-type. At the ion implantation dose of 1×10 14 ions cm -2 , the current on/off ratio of N-WS 2 FET can reach 3.9×10 6 . The performance of N-WS 2 FET does not deteriorate significantly after three months, indicating the stability of the doping method," said the researchers.

"Low-energy nitrogen ion implantation has been extended to other typical n-type two-dimensional metal chalcogenides materials, such as WSe 2 , MoS 2 and SnS 2 . Their conductivity types were successfully transformed from n-type to p-type, demonstrating the universality of the method," they added.

By combining low-energy ion implantation technique with lithography technique, the authors realized patterned doping of 2D materials. The WS 2 lateral p-n homojunction was successfully fabricated.

"Kelvin probe force microscopy characterizes that there is an obvious surface potential difference in the junction region, and demonstrates the feasibility of constructing lateral p-n homojunction with patterned doping by this method. The p-n junction exhibits significant photovoltaic effect under illumination, and shows satisfactory self-powered photodetection ability under different wavelength lasers."

"Under 532-nm laser illumination at 1.7 mW cm -2 , the self-powered photodetector based on this p-n junction can achieve an open-circuit voltage of 0.39 V, responsivity and detectivity of approximately 35 mA W -1 and 9.8×10 10 Jones."

The researchers say, "This doping method compatible with integrated circuits shows a huge application potential on modulating the performance of 2D semiconductor devices, and provides a reliable strategy for promoting the practical application of 2D materials."

Journal information: Light: Science & Applications

Provided by Wuhan University

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