Engineering Physics Lab Experiments

To study the characteristics of PIN Diode in forward bias and reverse bias

Aim

To study the characteristics of PIN DIODE in forward bias and reverse bias.

Apparatus Required:

Pin diode (or nip diode) characteristic study kit, milli ammeter, micro ammeter, voltameter, variable power supply

Description:

Photodiodes are diodes in which charge carriers are generated in responsive to light incident on photodiode. It is a semiconductor which converts photon energy of light into electric signal by releasing and accelerating current conducting carriers with in the semiconductor. P-i-n diodes and avalanche photo diodes (APD) are the most common devices used to convert light at the output of fiber in to electric current. With the rapid development of light wave communications, low cost, high performance optical photodiodes are required for a variety of applications. Photodiodes are used in optical communication networks to convert optical signals to electrical signals. The photo diodes can also be used as light receiving element of a mouse, photo coupler, a remote controller or a wireless transmission device.

Basic Mechanism of p-i-n and avalanche diode is

Input energy is optical energy (or light energy) → output energy is electrical energy

The mechanism of the photodiode is like that of a (miniaturized) solar cell. As light detectors, they reverse biased and the reverse current is linearly proportional to illuminance striking the diode. They are not as sensitive as phototransistor but their linearity can make them useful in simple light meter. The basic structure of p-i-n diode and avalanche diodes are given below.

What is a p-i-n diode?

A p-i-n diode consists of an intrinsic region of semiconductor material sandwiched between end regions of heavily doped p+ and n+ type materials. The device was originally proposed as a low frequency, high power rectifier because it could support a high reverse voltage. The resistance of a reverse biased p-i-n diode is very high. However, under large forward bias, a large number of electrons and holes are injected into the central intrinsic region and modulate the conductivity of this region. As a result the diode resistance becomes very low and can be controlled by the forward bias. Because of this property, the diode can be used as a switch or a variable attenuator at microwave frequencies.

What is an avalanche diode?

The avalanche photodiode has a number of different characteristics when compared to the normal p-n, p-i-n photo diodes, making them more suitable for use in some other applications. The main advantages of avalanche diode is greater level of sensitivity. The structure is more complicated than that of the p-i- n device. There are n+, p, un-doped and p+ regions. Light absorption takes place in un-doped region. The avalanche region occurs between the n+ and p-regions. Light enters the un-doped region of the avalanche photodiode and caused the generation of electron-hole pairs. Under the influence of the electric field the electron migrate towards the avalanche region. The electric field causes their velocity to increase to the extent that collisions with the crystal lattice create further electron-hole pairs. In turn these electrons may collide with the crystal lattice to create even more number of electron-hole pairs. In this way a single electron created by light in un-doped region is responsible for the creation of more number of electrons.

What are the differences between p-i-n diode and avalanche diode?

The avalanche photodiode has a number of differences with when compared to the p-i-n diode. The avalanche process means that a single electron produced by light in the un-doped region in multiplied several times by the avalanche process. As a result the avalanche diode far more sensitive when compared to p-i-n diode. In avalanche process the signal is more noisier compared to the p-i-n diode. N- type guard type ring is also required around the p-n junction to minimize the electric field around the edge of the junction. In avalanche diode the current gain is dependent not only on the bias applied, but also thermal fluctuations.

The Photo diode are characterized by quantum efficiency, responsivety, gain, dark current etc, they are briefly described below

Quantum efficiency: It is defined as the fraction on incident photons having sufficient energy to liberate electrons. It depends both on the wavelength of the incident light and the photo diode material.

Responsivity:It is a practical measure of output current for a given optical power input. It is defined as average output current divided by average incident optical power. Its units are A/W.

Dark current: In the absence of light a small dark current flows in a photo diode which is caused due to leakage in the reverse bias. It is very small and it can be ignored. Photo detectors are used in various different applications, such as radiation detection, smoke detection, and flame detection. They are also used in switching the street lights “on” or “off”, depending on the surrounding light.

Procedure:

To study the forward biased characteristics of Pin Diode:

1. Connect the circuit as shown in fig.1
2. Using variable power supply set LED current (I _L) to about 5mA
3. Using the variable power supply of pin diode, slowly increase the forward. Voltage (V _F) and note the forward current (I _F) for various values of (V _F ) Note the readings in Table – 1.
4. Repeat the above steps by increasing LED. Current in steps of 5mA.
5. Plot graph for V _F V _s I _F as shown in fig.

To study the reverse biased characteristics of Pin diode:

To study the “LED forward current” Vs “Pin diode reverse current” characteristics:

1. Connect the circuit as shown in fig.3.
2. Using the variable power supply of pin diode, set the reverse voltage (VR) constant at 5V.
3. Using variable power supply of LED, increase LED current (I _L) to about (5mA) and not the Pin diode reverse current (I _R) for various values of I _L.

Note the readings in Table – 2

4. Repeat the above steps by increasing reverse voltage (V_R 0 in steps of 1V.)
5. Plot a graph for I _L V _S I _Ras shown in fig.4

To study the “Pin diode reverse voltage” Vs “Pin diode reverse current” characteristics:

1. Connect the circuit as shown in fig.
2. Using variable power supply set LED current (IR) to a value of 5mA.
3. Using variable power supply of Pin diode, slowly increase the reverse voltage (VR) and note the reverse current (IR) for various values of (VR). Note the readings in Table 3.
4. Repeat the above steps by increasing reverse voltage (VR) in steps of 1V.
5. Plot a graph for a VR Vs IR as shown in fig. 5.

Observations:-

Table -1: Forward biased characteristics of Pin diode. (VF Vs IF)

LED current I L=4mA/Forward Pin voltage.V F(V)/Forward Pin current I R(µA)	LED current IL=5mA/Forward Pin voltage.V F(V)/Forward Pin current I R(µA)	LED current IL=6mA/Forward Pin voltage.V F(V)/Forward Pin current I R(µA)
1
2
3
4
5
6

Table-2: Reverse biased characteristics of pin diode (V_RV _SI_R)

LED current I L=4mA/Reverse Pin voltage.V F(V)/Reverse Pin current I R(µA)	LED current IL=5mA/Reverse Pin voltage.V F(V)/Reverse Pin current I R(µA)	LED current IL=6mA/Reverse Pin voltage.V F(V)/Reverse Pin current I R(µA)
1
2
3
4
5
6

Table-3: Reverse biased pin diode characteristics. LED current Vs PIN reverse current (I _L V _S I _R)

Reverse Voltage V R = I V	Reverse voltage VR=2V	Reverse voltage VR 3V
1
2
3
4
5
6

Precautions:

1. Biasing should be done properly.
2. Connections should be made carefully.
3. No external light should be incident on photo diode.

Result:

Hence, by the above graphs the forward and reverse biased characteristics are verified

Reference Books:--

1. ”Vibrations And Waves”,A.P.French,Arnold-Heinemann,New Delhi,1972
2. ”Basic Electronic Instrument” Hand Book ,C.F.Coombs,McGraw-Hill,1972
3. ”Laboratory Manual In Applied Physics”-Second edition- H.Sathyaseelam-New age International

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