Diodes: Types, Symbol, Characteristics and Applications

Ans: A photodiode is a p-n junction semiconductor device that transforms light into electricity. When photons are absorbed in the photodiode, electricity is formed. Optical filters, built-in lenses, and large or tiny surface areas can all be found in photodiodes. The common, traditional solar cell used to generate electric solar power is a large area photodiode.

Ans: Silicon and germanium are commonly used for the manufacturing of diodes. 

Ans: (i)  

(ii) 

The Zener diode is operated in the reverse breakdown region and the voltage across it remains constant due to this they are used as voltage regulators. 

Ans: Circuit diagram of Photodiode-

Photodiodes are used in the demodulation of optical signals and detection of the optical signals. Zener diodes are used as DC voltage regulators.

Ans: The device D is a Zener Diode. 

The Symbol of Zener diode is 

Ans: With the formation of p-n junction, the holes from p-region diffuse into the n-region and electrons from n-region diffuse into p-region and electron-hole pairs combine and get annihilated.

This input produces a potential barrier, VB across the junction which opposes the further diffusion through the junction. Thus, a small region is formed in the vicinity of the junction, depleting free charge carriers. It has only immotile ions called the depletion region.

Ans: Circuit diagram of forward biased and reverse biased p-n junction diode is illustrated below: 

First diagram illustrates the circuit diagram of forward biased p-n junction diodes and the second diagram illustrates the circuit diagram of reverse biased p-n junction diodes. 

Ans: Zener diode is commonly used as voltage regulator.

Principle: Zener diode is operated in the reverse breakdown region. The voltage across it remains constant. The voltage is equal to the breakdown voltage for a high charge in reverse current.

Ans: When the p-n junction is formed, the holes from p-region diffuse into the n-region and electrons from n-region diffuse into p-region and electron-hole pairs combine and get annihilated.

This input produces a potential barrier, VB across the junction opposes the further diffusion through the junction. This small region formed in the vicinity of the junction which is depleted of free charge carriers and has only immotile ions is called the depletion region.

Circuit diagram of forward biased and reverse biased p-n junction diode is illustrated below: 

(i) The width of the depletion layer decreases in forward bias.

(ii) The width of the depletion layer increases in reverse bias.

Ans: (i) Working of LED: LED is a forward biased p-n junction diode. It converts electrical energy into optical energy of infrared and visible light.

It is always in forward bias. Due to this the thin depletion layer and low potential barrier facilitate the diffusion of electrons and hole through the junction when high-energy electrons of the conduction band combine with the low-energy holes in the valence band. The energy is released in the form of photons and can be seen in the form of light.

(ii) Semiconductors of suitable band gap (close to 1.5 eV) are preferred to make LED size GaAs, CdTe, etc. The other reasons to select these materials are high optical absorption, availability of raw material, and low cost

(iii) Uses of LEDs:

(a) LED can work at very low voltage and consume very less power in comparison to incandescent lamps

(b) They take very less time to operate and have a long life.

Ans: The circuit diagram of full wave rectifier is shown below: 

The input waveform and output waveform is given in the diagram below: 

The working of the waveform is based on the principle that junction diodes offer very low resistance in forward bias direction and high resistance in reverse bias direction. 

Ans: (i) Zener diode works only in reverse breakdown region due to this it is considered as a special purpose semiconductor.

Reverse current is due to the flow of electrons from n-side to p-side and holes from p-side to n-side. The reverse biased voltage increases the electric field across the junction increases significantly and then reverse biased voltage V – V_z. After that the electric field strength is high enough to pull the electrons from p-side and accelerate it to n-side.

These electrons are responsible for the high current at the breakdown.

Voltage regulator converts an unregulated DC output of a rectifier into a constant regulated DC voltage, using Zener diodes. Thus, the voltage drop across RL increases without any change in the voltage drop across the Zener diode. This is because of the breakdown region, Zener voltage remains constant even though the current through the Zener diode changes.

When the input voltage decreases, the current through Rs and Zener diodes decreases. The voltage drop across Rs is reduced without any change in the voltage across the Zener diode.

Now, any change in input voltage results in the change in voltage drop across Rs. This change takes place without any change in voltage across the Zener diode. Thus, Zener diode acts as a voltage regulator.

(ii) Circuit diagram of p-n junction diode as a half-wave rectifier is illustrated below

Diode conducts corresponding to the positive half cycle and does not conduct during the negative half cycle. Hence, AC is converted by diode into unidirectional pulsating DC. This is also known as half-wave rectification.

Ans: (a) The formation of p-n junction takes place by the diffusion of charge. When the p-type semiconductor is joined with n-type semiconductor the diffusion of free charges across the junction starts.

Due to the formation of the p-n junction, the holes from p-region diffuse into the n-region, and electrons from n-region diffuse into p-region. The electron-hole pair combines and gets eliminated.

This input produces a potential barrier, VB across the junction which opposes the further diffusion through the junction. Thus, a small region that forms in the junction which is depleted of free charge carriers and has only immotile ions is called the depletion region.

Potential barrier: The potential distribution near the p-n junction is known as the potential barrier.

The circuit diagram of full wave rectifier is shown below: 

The input waveform and output waveform is given in the diagram below: 

The working of the waveform is based on the principle that junction diodes offer very low resistance in forward bias direction and high resistance in reverse bias direction. 

Ans: Zener diode works in the reverse breakdown region which is why it is considered as a special purpose semiconductor.

Reverse current is due to the flow of electrons from n-side to p-side and holes from p-side to n-side. The reverse biased voltage increases the electric field across the junction increases significantly and then reverse biased voltage V – V_z. After that the electric field strength is high enough to pull the electrons from p-side and accelerate it to n-side.

Voltage regulator converts an unregulated DC output of a rectifier into a constant regulated DC voltage, using Zener diodes. Thus, the voltage drop across RL increases without any change in the voltage drop across the Zener diode. This is because of the breakdown region, Zener voltage remains constant even though the current through the Zener diode changes.

When the input voltage decreases, the current through R_s and Zener diodes decreases. The voltage drop across R_s is reduced without any change in the voltage across the Zener diode.

Now, any change in input voltage results in the change in voltage drop across R_s. This change takes place without any change in voltage across the Zener diode. Thus, the Zener diode acts as a voltage regulator.