Introduction

A p-n junction diode is a two-electrode simple semiconductor device that allows current flow in only one direction while blocking current flow in the opposite or reverse direction. If the diode is forward biassed, electric current is allowed to flow freely through the circuit, while on the other hand, when the p-n junction diode is reverse biassed, it blocks the flow of electric current. In the case of n-type semiconductors, the majority charge carriers are free electrons, whereas holes are the majority charge carriers in p-type semiconductors. A p-n junction forms when an n-type semiconductor gets connected to a p-type semiconductor. Let us study the I-V characteristics of p-n junction diodes through an experiment.

Aim of the Experiment

To draw the forward bias and reverse bias I-V characteristics of a p-n junction diode.

Equipment required 

P-N Junction semiconductor diode, 50V battery, 3V battery, high resistance rheostat, 0-3V voltmeter, 0-50V voltmeter, 0-100mA ammeter, 0-100 μA ammeter, one-way key, connecting wires and pieces of sandpaper.

Theory 

When the n-section is connected to the battery’s negative terminal, and the p-section of a diode is connected to the battery’s positive terminal. The junction is called forward-biassed. The forward current gradually increases rapidly as the bias voltage increases. The current suddenly increases at 0.7 V for Si diodes and 0.2 V for Ge diodes. The value at which forward current rapidly increases is referred to as the threshold or cut-in voltage. 

The junction is reverse-biassed when the p -section of the diode is attached to the negative terminal of a high voltage battery. The n-section of the diode is fed to the positive terminal of the same battery. When the bias voltage increases, a minimal reverse current flow for a start remains almost constant with bias. However, when the reverse bias voltage is more than enough, we suddenly see a jump in reverse current to a bigger value.

Procedure

For a forward bias:

1. Ensure all the connections are connected tightly and note the least count of all the metres.
2. Insert the key K inside the potential divider (rheostat). The voltmeter of unit V and milli-amperemeter of unit mA will both read zero.
3. Increase the forward-bias voltage of the Ge diode to 0.3 V. Current is still zero as the potential barrier of the junction is 0.3V 
4. Boost VF to 0.4 V at this point, the milli-ammeter measures a tiny current. Increase VF in 0.2 V increments and record the corresponding current. You will notice that current increases gradually at first, then rapidly, until VF reaches 0.7 V.
5. Make VF equal to 0.72 V. You will notice that the current suddenly increases. This stage is often termed the “forward breakdown” stage. The forward current doesn’t show significant changes when the VF exceeds the “forward breakdown” stage. 
6. Remove out the key all at once and record your data.

For a reverse bias:

1. Ensure all the connections are connected tightly and note the least count of all the metres.
2. Insert the key K inside the potential divider (rheostat). The voltmeter of unit V and milli-amperemeter of unit mA will both read zero.
3. When you move the contact closer to the negative end to provide a 0.5 V reverse-bias voltage (VR), a weakly reverse current begins to flow.
4. VR should be increased in 0.2 V increments. The current climbs gradually, then quickly, until VR reaches 20 V. 
5. Make VR equal to 25 volts when you notice a sudden jump in the current this stage is known as “reverse break-down”. Take note of the readings for current and immediately remove the key.

Observation:

For a forward bias:

For reverse bias:

Calculation

P-N junction diode under forward bias:

*Plot a graph VF vs IF and mark point A (2.0,20) and B (2.4,30)

For change in VF from point A to point B

△VF= 2.4–2.0 = 0.4V

△IF= 30-20= 10 mA

Junction resistance=r= △VF△IF= 0.4V10mA = 40 ohms 

P-N junction diode under reverse bias: 

*Plot a graph VR vs IR and mark point A (5,1) and B (7,2)

For change in VR from point A to point B 

△VR= 7.0-5.0 = 2V

△IR= 2-1= 1 μA

Junction resistance=r = △VR△IR= 2V1 μA= 2X106 ohms 

Conclusion 

Through this experiment we understood how the I-V characteristic of P-N junction differs when P-N Junction Diode is Under Reverse Bias and when P-N Junction Diode is in forward Bias. Through the graph, we can understand the relationship between both factors. A negative voltage is fed to the N-type material and positive voltage is fed to the P-type material when a diode is coupled in a Forward Bias condition. A positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material when a diode is coupled in a Reverse Bias situation.