Unit – II

### Circuit Diagram:

Voltage across the diode can be varied with the rheostat. The milliammeter or the micro-ammeter measures the current.

Forward Bias:

a) In forward bias, the emf of the battery and the p.d. across the depletion region oppose each other which reduces the depletion barrier p.d. and decrease in its width. The electrons flow from N-type to P-type in the diode and from positive pole to negative pole in the battery.
b) Above graph shows the V-I characteristics for the forward bias condition. The current increases with the increase in the applied voltage as shown in the graph. The initial increase in current is very less, but beyond a voltage known as ‘cut in voltage’, current increases rapidly. Here, current does not vary linearly as per Ohm’s law.

Reverse bias:

a) In reverse bias emf of the battery and the p.d. across the depletion region are in series and assist each other. The electrons find it difficult to move from N to P-type and holes from P to N type.
b) Above graph shows the V-I characteristics for the reverse bias condition. The current is negligible (of the order of mA) for smaller values of the voltage due to minority charge carriers. The electric current is constant and is known as reverse saturation current. There is a sudden rise in the current on increasing
the voltage beyond a certain point known as breakdown voltage. Normally, P-N junction diode is never used beyond the reverse saturation current in the reverse bias mode.

## Zener Diode:

1. A zener diode is a special case of pn junction in which both the p and n region are heavily doped and are reverse biased.
2. A high current is obtained at the certain voltage Vzand it is almost independent of reverse bias voltage. Hence such semiconductor diodes are called zener diodes and the critical voltage at which the current is maximum is called breakdown voltage or zener voltage.

Working:

1. In zener diode, p and n region are heavily doped (shown by p+ and n+). Due to which the width of depletion layer is small and junction field will be high.
2. When it is reverse biased, due to thin depletion layer, the junction field becomes very high and the conduction band of n+ region comes to very close to the valence bond of the p+ region. At critical voltage Vz, the junction field strips some electrons from the valence band and tunnel them through the thin depletion layer. The breakdown of the depletion layer due to tunneling is called zener breakdown.
3. Once tunnelling starts, flow of electrons from p+ region to n+ region starts at high rate. At
the same time the moving holes get accelerated and knock off some electrons from neutral
atoms. Thus a huge flow (avalanche) of electrons starts and reverse current increases
sharply.

Use of Zener Diode as Voltage Regulator:
1. To get constant voltage power supplies (Battery Eliminator) are used instead of batteries. This power supply consists of a rectifier and electrical filter. These power supplies have following limitations.

D.C. voltage changes due to
a) change in line voltage
b) current drawn by the load
Such supplies are called unregulated power supply.

2.Principle:
The property of zener diode is that at the breakdown voltage Vz, the current becomes independent of the voltage across the diode.
i.e. the voltage across the zener diode remains the same.

3.Circuit Diagram:

4. Unregulated dc voltage supplies reverse voltage greater than the breakdown voltage. Zener diode starts conducting and develops voltage Vz across the zener diode. This
voltage Vz is the regulated voltage.
5.  Effect of change in line voltage:
When line voltage (generally 230V) changes the voltage supplied by unregulated power supply changes. Thus the current flowing through the zener diode changes. But due to zener breakdown, the output voltage remains the same at Vz.
6.  Effect of change in load resistance:
The load is connected in parallel to the zener diode. If the load resistance changes (say decreases) the load current increases. Hence the current through zener diode decreases. But again due to the property of zener diode output voltage remains the same at Vz.

## Photodiode:

1. A Photodiode is a special purpose p-n junction diode fabricated with a transparent window to allow light to fall on the diode.
2. It is operated under reverse bias. When the photodiode is illuminated with light (photons) with energy (h
) greater than the energy gap (Eg) of the semiconductor, then electron-hole pairs are generated due to the absorption of photons.
3. The diode is fabricated such that the generation of e-h pairs takes place in or near the depletion region of the diode. Due to the electric field of the junction, electrons, and holes are separated before they recombine.
4. The direction of the electric field is such that electrons reach n-side and holes reach p-side. Electrons are collected on n-side and holes are collected on p-side giving rise to an emf.
5. When an external load is connected, current flows. The magnitude of the photocurrent depends on the intensity of incident light (photocurrent is proportional to incident light intensity).

## Solar Cell:

1. The solar cell is a device which converts solar energy into electrical energy using semiconductor diode.
2.
Principle:
It works on the principle of the electron excitation from the valence band into conduction band by
photons at the junction of the p-n diode.
3.
Construction:
The substrate is a base or foundation made up of p-type semiconductor. On this substrate, a thin layer of n-type is created with a process of diffusion. Materials used are Si, CdS, CdTe, CdSe etc. The junction is very close to the top surface. A thin layer of metal usually silver is formed at the bottom below p-type layer for contact. On the top of n layer metal lines are deposited called metal fingers. These fingers provide a contact for n – layer.
4.
Diagram:

5. When a photon of energy hn is incident on the p-n junction such that the energy of a photon is greater than the forbidden energy gap Eg, it lifts the electron from the valence band to conduction band. Thus at the junction, a pair of an electron and a hole formed. Such charged carriers are called photogenerated charge carrier.
6:
Uses:
a) A set of solar cells can be used to charge storage battery in the daytime. These storage batteries can be used to give electrical power during the night.
b) Solar cells are used in calculators and watches and lux meters.
c) Solar cells are used in artificial satellites.
d) Solar cells are used in remote areas for supplying electricity to climatic condition measuring devices.

3.V-I Characteristics of Solar Cell

In V-I characteristics ISC is short-circuited (Load resistance zero) current and it is zero. VOC is open circuit voltage when the circuit is open (Load resistance infinity).

## Light Emitting Diode (LED):

LED is pn diode which emits light when is forward biased. the emission may be visible or infrared.
1. Principle:
At the p-n junction, the electrons in n-region are in conduction band while the hole in p region is in the valence band. When the electron falls from n region to p-region i.e. from the conduction band to the valence band to recombine, the difference in energy of the two bands is converted into heat and light energy.
2.
Construction:
The substrate is a base or foundation made up of an n-type semiconductor. On this substrate, a layer of p-type is created with a process of diffusion. p-layer is kept up because electron-hole recombination takes place in this layer.
Semiconductor materials used depends on the wavelength of light required. Junction is very close to the top surface. A thin layer of metal usually silver is formed at the bottom below n-type layer for contact. On the top of p-layer, meta anode is fixed with the open central area to emit light.

3. Diagram:
a) It requires very small current to operate than the torch bulb.
b) Power consumed by LED is very low.
c) Its action is very fast.
d) It is very small in size and has less weight.
It is available in different shapes.
e) It emits light of different colors.
f) It is sturdy and generally unbreakable.
g) It has a long life.
h) They can be arranged in a matrix to form advertising and information displays.
i) They can emit the whole spectrum of light.

5. Uses of LED
a) They can be arranged in a matrix to form advertising and information displays.
b) Infrared LEDs are used in the burglar alarm.
c) In optical mouse and digitisers, LEDs are used.
d) Infrared LEDs are used in remote controls.
e) They can be integrated into seven segment display to show digital output.
f) LEDs are used to read bar code
g) They are used in graphic equalizers and audo apparatus to indicate intensity of sound