Question

(a) Draw the circuit diagrams of a p-n junction diode in (i) forward bias, (ii) reverse bias. How are these circuits used to study the V-I characteristics of a silicon diode? Draw the typical V-I characteristics? 

(b) What is a light emitting diode (LED)? Mention two important advantages of LEDs over conventional lamps.

Answer 1

(a) There are three possible biasing conditions for a p-n junction diode and these are:

1) Zero bias: In this biasing condition, no external voltage potential is applied to the diode.

2) Forward bias: In this biasing condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material, Due to this type of biasing, the width of the p-n junction diode decreases.

3) Reverse bias: In this biasing condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material, Due to this type of biasing, the width of the p-n junction diode increases.

The circuit arrangement for studying V-I characteristics of a p-n junction diode in forward bias is given below.

The circuit arrangement for studying V-I characteristics of a p-n junction diode in reverse bias is given below.

Typical V-I characteristic of a silicon diode is given below.

(b) A light-emitting diode (LED) is a semiconductor device that emits light when an electric current flows through it. When current passes through an LED, the electrons recombine with holes emitting light in the process. LEDs allow the current to flow in the forward direction and blocks the current in the reverse direction.

Some advantages of LEDs over Incandescent Power Lamps are:

• LEDs consume less power, and they require low operational voltage.
• No warm-up time is needed for LEDs.
• The emitted light is monochromatic.
• They exhibit long life and ruggedness.

Answer 2

(a) (i) Forward Bias: 

(ii) Reverse Bias:

The battery is connected to the diode through a potentiometer (or rheostat) so that the applied voltage to the diode can be changed. For different values of voltages, the value of the current is noted. A graph between V and I is obtained as in figure. Note that in forward bias measurement, we use a milliammeter since the expected current is large while a micrometer is used in reverse bias to measure current.

(b) Light Emitting Diode (LED):

A light emitting diode is simply a forward biased p-n junction which emits spontaneous light radiation. When forward  bias is applied, the electron and holes at the junction recombine and energy released is emitted in the form of light. For visible radiation phosphorus doped GaAs is commonly used. 

The advantages of LEDs are: 

(i) Low operational voltage and less power. 

(ii) Fast action with no warm up time. 

(iii) Emitted light is nearly monochromatic radiation. 

(iv) They have long life.