Electronic and Instrumentation

Zener diode is having utmost importance in power applications. In zener diode the doping level of P and N type semiconductors are very high as compared to the simple p-n junction diode. When we connect it into any circuit in parallel or series with the equipments in reverse biased, then crossing the certain limit it break down the circuit and the equipment retain safe that is more costlier than zener diode. The whole-sole purpose of zener diode is to provide immediate protection to the equipments which are very sensitive to the varying current and voltage.

In Zener diode we are using two types of mechanism:

(A)   Avalanche Breakdown

(B)   Zener breakdown

 

Avalanche Breakdown:  In highly doped p-n junction diodes it is occurs, when the accelerated free electrons acquire  sufficient kinetic energy to break the bonding among the atoms on bombarding with other atoms and results in regeneration of high number of free electrons moving in the force of electric field. This process causes rapid increase in current in reverse region and known as break down current.

 

Zener Breakdown:  In highly doped p-n junction diodes it is occurs, when the applied electric field increases continuously and reaches to a level, which provides sufficient amount of force on the outer most shell electron. This force is so higher in magnitude, causes breaking in covalent bond among atom and its electrons. This process produces large number of free charge carriers and hence rapidly increases the reverse current.

 

Application of Zener Diode:

1. As a voltage regulator in circuits.
2. As a fixed reference voltage in transistor biasing circuits.
3. In peak limiters and wave shaping circuits.
4. For protection of electrical and electronics equipment.

 

Tunnel Diode:

It was first observed by Dr. Leo Esaki in 1985, when the impurity level of a normal p-n junction diode is greatly increased i.e. 1000 times more than normal diode its then its characteristics are completely changed. This diode is also known as Esaki diode.

In tunnel diode, the impurity level or concentration is very much higher from any normal p-n junction diode. This amount of concentration makes the depletion layer thin i.e. 10 nanometer of width which is so narrower than normal diode depletion layer i.e. 1 micron of width. In normal p-n junction diode the depletion layer is not crossed by the electron and hole due to lack of sufficient amount of energy. To cross this barrier one may need to increase the voltage around the diode this may lead to burning of diode.

Operating a highly doped p-n junction diode i.e. about one part in 10³ atoms reduces the width of depletion layer i.e. 10^-9 meter. Even on applying small amount of voltage the charge carriers can easily penetrate the depletion layer this phenomenon produces a large amount of forward current. The phenomenon of penetrating the charge carriers, directly through the potential barrier, instead of crossing it, is as tunneling and the diode is known as Tunnel Diode.

Application of a tunnel diode:

1. As a ultra-high speed switching device. The switching time is of the order of 10^-9 sec.
2. As a logic memory storage device.
3. As a microwave oscillator at higher frequencies i.e. 10 GHz.

 

Varactor Diode:

When a p-n junction diode using in reverse bias then along depletion layer charge carrier are accumulates massively at the junctions i.e. P and N type regions. In that case, depletion layer behaves as a dielectric to the junction boundaries which are behaving as capacitor plates and holds charge in the form of charge carries. This capacitance is varying on varying the reverse voltage across the diode and the diode is known Variable Capacitor, VariCap, VoltCap or Tunning Diode and Varactor Diode.

The depletion layer decreases when voltage decreases and increases when reverse voltage increases respectively. This leads to change in width of depletion layer and thereby produces variation in dielectric capacity of the capacitor. This phenomenon is similar to simple capacitor in which dielectric layer decreases hence capacitance increases and vice-versa.

In Varactor diode the capacitance parameter is controlled by the method of doping in the depletion layer. Doing on both side of depletion layer is uniform. The range of capacitance varies (tuning range) from 4 to 1 or we can say in the ratio of 4:1 for example  the maximum limit of capacitance is 100 pF and minimum limit is 25 pF.

 

Application of varactor diode:

1. Mainly in tuning circuit like radio and transistor.
2. In resonance circuit.

 

The Schottky  Diode:

Schottky diode is formed by joining the highly doped semiconductor region (specifically N-type)to metal such as Gold, Silver, and Platinum. It is different from any other diode because in the junction is formed between metal and semiconductor instead of two semiconductor regions.

The reason behind to join a metal piece not a semiconductor is that when the junction voltage gets crossed the limit then a number of large carrier is coming from the metal side to semiconductor region without much increasing the voltage level from the limit. Due to this Schottky diode can be useful in high frequency circuits.

The junction capacitance in Schottky diode is of very low magnitude so that the the operating frequency ranges upto 20 GHz or more than this.

 

Application of Schottky Diode:

1. To rectify very high frequency signals i.e. above 300 MHz.
2. For fast switching in digital circuits and also in digital computers application.
3. In Transistor-transistor logic circuits.

 

PIN Diode:

P-I-N diode is made from the joining the three types of semiconductor materials i.e. two heavily doped and one

intrinsic type semiconductor (undoped semiconductor called intrinsic semiconductors). Thus the PIN diode is formed by joining PN type semiconductor separating by intrinsic semiconductor.

The intrinsic semiconductor offers a high resistance to the current flowing through it. The advantage of PIN diode on a normal PN junction diode is s following:

1. The capacitance between the PN junctions is decreases because of the increase separation between the P and N type regions. This makes PIN diode a very fast and provide suitable applications in very high frequencies circuits.
2. The electron-hole pair generation is much higher than PN junction diode, thus increases the electric field around the P and N regions. This helps  PIN diode to process even weak signals.

 

In PIN diode, the width of depletion layer is controlled by the electron diffusion process across the intrinsic semiconductor region. When forwarded biased is applied across the terminals then depletion layer decreases. Thereby, produces more charge carriers to injected in I region. If the sable bias is applied then it behaves as low resistance device and produces maximum current. PIN diode is working as a variable resistance device.

In reverse biased condition, the depletion layer becomes thick and on increasing the reverse voltage the depletion layer getting thicker till the I region becomes free of mobile charge carriers. The reverse voltage at which this is happen is called as swept out voltage. At this voltage the PIN diode is working as a constant capacitive device.

Applications of PIN diode are as following:

1.  In attenuating circuits to control current.

2.  In DC controlled microwave operated switch.