Depletion Region Formation

Introduction to PN Junction Diode:

Diode is a device which have two terminals mainly a cathode and an anode. PN signifies the combination of p type and n type material to form it.

When a p type material is joint with an n type semiconductor using a special fabrication technique, PN junction is formed.

P type material has holes as majority charge carriers. When a hole spontaneously leaves its place, a negative ion is created. So hole is represented with a compensatory –ve ion.

N type material has electrons as majority charge carriers. When an electron spontaneously leaves its place, a positive ion is created. So electron is represented with a compensatory +ve ion.

Diffusion Process:  When P type material is joined with N type material, then we can see a concentration gradient (concentration difference) of charge carriers through out whole PN junction. This can understood as follow. Through out complete PN junction diode, hole are large in P side but small in N side as well as electrons are large in N side but small in P side.

So we can see the difference in concentration of holes as well as electrons. Due to this concentration gradient, electrons from N side tend to move towards P side. Similarly holes from P side tend to move towards N side.

The process of moving of charge carriers due to concentration gradient from higher concentration towards lower concentration spontaneously is known as Diffusion Process.

Depletion Region:  

After diffusion process, negative ions are created in P side while positive ions are created in N side. Therefore an electric field is set across the junction of PN diode directed from positive ions towards negative ions. Once electric field is set up across the junction, movement of majority charge carries stop across the junction. This electric field restricts the flow of majority electrons and holes.

The region of positive and negative ions is known as depletion region. Since this region is made of space charges, this is also known as space charge region.

Potential Barrier and Barrier Potential:  

Due to electric field set across the junction, A potential is also set across the junction which behaves as a barrier because it do not allow majority electrons and holes until minimum potential equal to this amount is applied to the diode. Therefore this potential is known as barrier potential and the region is known as potential barrier.

As our whole body activities is performed when knee is energized, in similar ways, potential barrier is the minimum potential requirement for performing diode in ON condition. Therefore this is called Knee voltage. It is represented by Vk.

Value of Barrier Potential for Different Diodes:   

We will study about the diodes made by either Si (silicon) or Ge (germanium). Both have different diode parameters. 

Knee Voltage for Si diode    =     0.7 Volt

Knee voltage for Ge diode   =     0.3 Volt.

When diode is applied a potential, Silicon starts working above 0.7 volt while Ge requires only 0.3 volt for being ON. When diode becomes ON, this amount of potential is dropped across the diode connected in the circuit.

Some questions for Exercise

1. Diffusion is a process which occurs due to

(a) Large Potential Application   (b) Small Potential Application

(c) Concentration Gradient        (d) All of these

2. In a semiconductor, P atom is added. When an electron leaves from it due to diffusion process, what happens

(a) Positive ion is formed            (b) Negative ion is formed

(c) A hole is formed                    (d) None of these

3. Which term does not resemble to the Barrier Potential

(a) Knee Voltage                          (b) Peak Inverse Voltage

(c) Cut-in Voltage                         (d) Threshold Voltage

Answer:  1 - c          2 - a         3 - b

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