SEMICONDUCTOR DEVICES PART 3
Different types of junction diode:-
- Zener diode:- A specially designed diode in which P and N region are heavily doped due to which the depletion layer junction width is small and the junction field i.e. potential barrier is high and it can operate continuously, without being damaged in the region of reverse break down voltage, is called zener diode.
After the break down a large change in current can be produced by almost insufficient change in reverse bias voltage i.e. for widely different currents, the voltage across the zener diode remains constant so a zener diode can be used as a d.c. voltage regulator.
To use a zener diode as a d.c. voltage regulator, it is joined in reverse bias to unregulated dc input voltage through a resistance R (called dropping resistance) and the regulated output voltage is obtained across load resistance Rl.
If I is current from the supply, Iz the current through zener diode and Il, the current through the load resistance, then
I = Iz + Il
If Rz is the resistance of the zener diode, then
Vo = Vz = IzRz = IlRl
Applying Kirchhoff’s law to the mesh containing R, we get
RI+Vz = VI
Vz = VI-RI
- When Vi < Vz, almost no current flows through the zener diode i.e. Iz=0 and Vo = VI.
- When Vi = Vz, junction diodes operates in the breakdown region and output voltage Vo = Vz = (VI-RI) becomes constant.
- When the input voltage increases beyond a certain limit (i.e. Vi > Vz, then the voltage across the zener diode remains constant (= Vz) but the current rises abruptly as the resistance of the zener diode reduces to almost zero. Due to which the is an increase in voltage drop across R. Since RL is connected in parallel so the voltage across RL remains the same as that of zener break down voltage. Hence the output voltage remains constant.
- Photo diode:- A junction diode made from light sensitive semiconductor is called a photo diode.
Photo diode is always reverse bias. When no light falls on it, a small reverse current flows through the junction. This current is due to thermally generated electron-hole pair and is called as dark current. When the photo diode is illuminated with light photons of energy hυ>Eg then it rejects the valence electron due to which the current increases and becomes maximum. This current is called as saturation current. On increasing the light intensity, the saturation current increases.
A photo diode turns its ON or OFF in nanoseconds. So it can be used as a fastest photo detector.
- In detection of optical signals.
- In demodulation of optical signals.
- In light operated switches.
- In speed reading of computer punched cards.
- In electronic counters.
- Light emitting diode: A light emitting diode is simply a forward biased p-n junction made of gallium arsenide or indium phosphide and emits spontaneous light then the LED is made forward bias then the energy is released due to the recombination of electrons and holes, falls in visible region or infrared region of EM spectrum.
Advantages over conventional incandescent lamps:
- Low operational voltage and less power consumption.
- Fast action and no warm up time required.
- Long life and ruggedness.
- Light emitted is nearly monochromatic.
- Infrared LED’s are used in burglar alarm systems.
- In optical communication system.
- LED’s are used in numeric displays (in watches and calculator).
- In optical mouse for the computers.
- In remote controls.
- Solar cell:- It is a junction diode which convert solar energy into electrical energy and is based on photo voltaic effect ( generation of voltage due to bombardment of photons).
It consists of a p-n junction made of Si or GaAs. A very thin layer of n-type semiconductor is grown over a p-type semiconductor by using diffusion method. (so that the energy falling on the diode not greatly absorb before reaching the junction).
Working: When light falls on the top of a solar cell, it gets absorbed. The absorb energy knocks out electrons, which flows across the junction to create the current. The current flows through the p-layer. A load resistance is connected across the metal contact of cell to store the electricity. After flowing through the load resistance, the current flows back into the n-layer. Thus, a current is generated without mechanical input energy.