Stepwise Guide: Drawing IV Characteristic Curves for PN Junctions
A PN junction diode is formed by joining the p and the n-type semiconductor with a barrier potential between these two semiconductors.
A diode is said to be forward-biased if the current flows in the forward direction because of this, we often call this current the forward current or I.
The value of the forward current is dependent on the value of the forward voltage with a direct relationship between these two. This direct relationship is called the IV Characteristics of PN Junction.
IV Characteristics of PN Junction
For a PN Junction diode, there are two operating regions and three biasing conditions for the standard Junction Diode. The IV characteristics of PN Junction diode are as follows:
Zero Bias Condition – A conduction when no external voltage potential is applied to the PN junction diode.
Reverse Bias – The voltage potential is connected to the negative terminal i.e., to the n-type material and positive terminal of the battery to the N-type material across the diode which has the effect of Increasing the PN junction diode’s width or the depletion layers between these two semiconductor materials.
Forward Bias – The voltage potential is connected positive, (+ve terminal), i.e., to the P-type material and negative (- ve terminal) to the N-type material across the diode which has the effect of decreasing the PN junction diodes width, or the depletion layer between the two semiconductors.
Now, let’s discuss the IV characteristics of a PN Junction diode in detail:
Forward Bias and Reverse Bias
Forward Bias
From the above context, we understood that if the current flows in the forward direction, but which direction are we talking about? Well! A forward bias characteristic is in the following manner:
A positive terminal of the battery is connected to the p-type conductor whereas a negative terminal is to the n-type. We all know that on applying the potential to the circuit, the current starts from a negative terminal to the positive, i.e., from n-type to the p-type (a built-in electric field is in the direction opposite to that of the applied electric field).
Forward Bias Diagram
Below is the forward bias circuit diagram:
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From the above forward bias diagram, we notice that on applying the electric field to the forward bias circuit diagram, the holes from the p-type semiconductor combine with electrons in an n-type semiconductor.
A forward bias characteristic is that it reduces the potential barrier, which creates an easy current flow through the forward bias circuit diagram. Also, on increasing the voltage, the magnitude of a current also rises sharply, the same can be seen in the following graph:
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Reverse Bias
The connection in a reverse biasing is opposite to that of the forward biasing. In this type of connection, a positive terminal is connected to the n-type semiconductor whereas a negative terminal is connected to the p-type semiconductor.
One of the reverse bias characteristics is, there is a small change in the value of current with the voltage rise; however, the reverse current increases with the voltage rise. When there is an increase in the reverse current, the depletion layer or the potential barrier between the two semiconductors also rises.
Reverse Bias Diagram
The reverse bias diagram can be better understood in the following manner:
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The below graph represents how the value of the reverse current rises on increasing the voltage or simply the reverse-bias current-voltage characteristics:
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PN Junction Diode Experiment
The flow of electrons from the n-type semiconductor towards the p-type semiconductor of the PN junction takes place when there is an increase in the voltage.
Similarly, the flow of holes from the p-type semiconductor towards the n-type of the junction takes place along with the increase in the voltage.
The PN Junction Diode Experiment results in the concentration gradient between both sides of the terminals. Because of the formation of the concentration gradient, there will be a flow of charge carriers from higher concentration regions to lower concentration ones. This motion of charge carriers inside the PN junction diode is the reason behind the flow of current in the circuit.
PN Junction Diode Characteristics
Semiconductors contain two types of mobile charge carriers, viz: Holes and Electrons, where holes are positively charged and electrons are negatively charged.
We can dope a semiconductor with donor impurities like Antimony (N-type doping), so that it contains mobile charges which are basically electrons.
A semiconductor can be doped with acceptor impurities viz: Boron (P-type doping) so that this semiconductor contains mobile charges which are primarily holes.
The junction region that has zero charge carriers is known as the depletion region.
The junction ( or the depletion region) region has a physical thickness that varies with the applied voltage. The depletion region reduces in a forward bias condition whereas it rises in a reverse-biased condition.
FAQs on IV Characteristic Curve for PN Junction in Forward and Reverse Bias
1. What is an I-V characteristic curve for a PN junction diode?
An I-V (Current-Voltage) characteristic curve is a graph that illustrates the relationship between the voltage applied across a PN junction diode and the current flowing through it. This curve is fundamental to understanding the diode's behaviour, as it graphically shows how the diode conducts current differently under forward bias (allowing current to flow easily) and reverse bias (blocking current flow), which is its primary function.
2. How does the I-V characteristic curve look for a PN junction in forward bias?
In forward bias, the P-side of the diode is connected to the positive terminal of the power supply and the N-side to the negative terminal. The I-V curve shows that:
- Initially, for low forward voltages, the current is very small.
- As the voltage approaches a specific value called the knee voltage (or threshold voltage, ~0.7V for silicon), the depletion layer narrows significantly.
- Beyond the knee voltage, the current increases exponentially with a small further increase in voltage. This indicates very low resistance to current flow.
3. What are the key features of the I-V characteristic curve in reverse bias?
In reverse bias, the P-side is connected to the negative terminal and the N-side to the positive terminal. The I-V curve shows:
- A very small, almost constant current called the reverse saturation current (in the order of microamperes or nanoamperes) flows through the diode. This is due to the movement of minority charge carriers.
- This low current remains constant even as the reverse voltage increases significantly, indicating very high resistance.
- If the reverse voltage reaches the breakdown voltage (Vbr), the current increases sharply, which can permanently damage the diode.
4. Why does a PN junction diode offer very low resistance in forward bias and very high resistance in reverse bias?
This behaviour is due to the depletion region at the junction.
- In forward bias, the applied voltage opposes the built-in potential barrier, making the depletion region very narrow. This allows majority charge carriers (holes from P-side and electrons from N-side) to cross the junction easily, resulting in a large current and hence, low resistance.
- In reverse bias, the applied voltage aids the potential barrier, widening the depletion region. This prevents the flow of majority carriers. Only a tiny current from minority carriers can flow, leading to an extremely high resistance.
5. What is the significance of the 'knee voltage' in the forward bias I-V characteristics?
The knee voltage, also known as the threshold voltage or cut-in voltage, is the minimum forward voltage required for the diode to start conducting current significantly. Below this voltage, the potential barrier is not sufficiently overcome, and the current is negligible. Above the knee voltage, the diode's resistance drops dramatically, and it behaves almost like a closed switch. For silicon diodes, this is typically around 0.7V, and for germanium diodes, it is about 0.3V.
6. What happens at the 'breakdown voltage' in the reverse bias I-V characteristics?
The breakdown voltage is the critical reverse voltage at which the high resistance of the PN junction breaks down, leading to a sudden and sharp increase in reverse current. This can happen due to two main phenomena:
- Zener Breakdown: Occurs in heavily doped diodes at low voltages due to the rupturing of covalent bonds by the strong electric field.
- Avalanche Breakdown: Occurs in lightly doped diodes at higher voltages, where minority carriers gain enough kinetic energy to knock out more carriers, creating an 'avalanche' of charge.
7. How do temperature changes affect the I-V characteristic curve of a PN junction diode?
Temperature has a significant effect on the I-V curve. For every degree Celsius rise in temperature:
- The reverse saturation current approximately doubles. This is because higher temperatures generate more electron-hole pairs, increasing the number of minority carriers.
- The forward barrier potential (and thus the knee voltage) decreases by about 2.5 mV. This means the diode will start conducting at a slightly lower voltage as temperature increases.
8. What are some common applications of a PN junction diode based on its I-V characteristics?
The unique one-way conduction property shown by the I-V curve allows PN junction diodes to be used in many applications:
- Rectifiers: To convert AC (alternating current) into DC (direct current) in power supplies.
- LEDs (Light Emitting Diodes): In forward bias, some diodes emit light, used for displays and lighting.
- Photodiodes: In reverse bias, they can detect light by converting light energy into electrical current.
- Zener Diodes: Special diodes designed to operate in the reverse breakdown region for voltage regulation.
- Switching Circuits: Used as electronic switches in digital logic circuits due to their high and low resistance states.
