P-N Junction Diode

P-N Junction Diode

Diode: Di+iode = diode is formation of of two electrodes i.e. Anode and cathode.

P-N Junction Diode:

1. It is popular semi conductor device formed by P & N junctions.
2. It has two terminals called electrodes one from p-region and and one from n-region.
3. Using ohmic contacts i.e.metal we can connect P-N Junction Diode in external circuits.

image1: P-N Junction Diode  

Biasing of P-N junction diode:

Biasing:  Applying external DC voltage to any electronic device is called biasing.there is no current in un biased device.

Depending on the polarity of the DC voltage to the diode is classified as  1.Forward biasing and 2.Reverse biasing. 

Forward biasing of P-N junction diode:

An external DC voltage(or battery)  is connected to diode in such way that p-region is connected to positive terminal of the battery and n-region is connected to negative terminal of the battery is called forward biasing.

image2: forward biasing of P-N junction diode

Operation of the P-N junction Diode:

P-N junction diode is unidirectional device, it will conduct when diode is connected in forward bias.this conduction will happends when applied DC voltage is greater then barrier potention of diode.

If we are using Si-P-N junction diode its barrier potential is 0.7V,if the applied voltage is greater or equal to this voltage Si-diode will conducts.    (Si-Silicon)

If we are using Ge-P-N junction diode its barrier potential is 0.3V,if the applied voltage is greater or equal to this voltage Ge-diode will conducts.    (Ge-Germanium)

The minimum voltage required to turn on an electronic device called Cut-in voltage.It is represented by V. Which is 0.5V volts for Si-diode and 0.2V for Ge-diode.

When the diode is connected in forward bias the positive terminal of the battery is connected to p-region and negative terminal of the battery is connected to n-region.At this condition when the applied voltage is increasing from 0V, the majority carriers in p-region are holes repelled from +ve terminal of the battery and the majority carriers in n-region are electrons repelled from -ve terminal of the battery, they came towards the depletion region.If the applied forward voltage (V_f)  increases more than the barrier potential the majority carriers in two regions break the bridge(barrier) and these carries move towards opposite terminals of the battery.i.e there is conduction  of charge carriers exist is called current.This current is forward current which is represented I_f.

The total current is I_f = I_n + I_p        :I_n  is electron current, I_p  is hole current

The forward voltage V_f=V+I_f  r_f      :r_f is forward resistance of the diode

There is minority carriers current( is called conventional current) in junction which is opposite direction to direction of flow of electrons.

image 3: unbiased P-N junction diode	image 4: forward current in a P-N junction diode

From the images 3 & 4 we can observe the depletion region width reduces from unbiased to forward biased diode.

Forward resistance of the diode: The  resistance offered by diode in forward bias is called forward resistance of the diode.

which is defined in two ways  

1. Static forward resistance   R_f  =V_f/I_f     which measured at particular point of V_f to I_f
2.  Dynamic forward resistance r_f =(V_f2-V_f1)/(I_f2-I_f1)   which is measured change in forward voltage to change in forward current.

Reverse biasing of P-N junction diode:

An external DC voltage(or battery)  is connected to diode in such way that p-region is connected to negative terminal of the battery and n-region is connected to positive terminal of the battery is called reverse biasing.

image5: reverse biasing of P-N junction diode

When P-N junction diode is connected in reverse bias p-region is connected to negative terminal of the battery and the positive terminal of the battery is connected to n-region then all the majority carriers are attracted towards the battery,then depletion region width widens.

image6:depletion region width in reverse bias

The reverse current flow through the junction is reverse current which is due to minority carriers which is very small. which is order of micro amperes for Ge-diodes and nanoamperes for Si-diodes.

In reverse bias  reverse current not basically depends on the reverse applied voltage up to certain voltage reverse current is constant but reverse voltage is increases beyond a particular point large reverse current flow,which damage the diode.this is called reverse breakdown of a diode,that particular reverse voltage ia reverse break down voltage.

Reverse resistance of the diode: The p-n junction offers large resistance in the reverse biased condition called reverse resistance.This is also defined in two ways.

1. Static reverse resistance   R_r  =V_r/I_fr    which measured at particular point of V_r to I_r
2.  Dynamic reverse resistance r_r =(V_r2-V_r1)/(I_r2-I_r1)   which is measured change in reverse voltage to change in reverse                                                                           current.

V-I characteristics of P-N  junction diode:

The response of p-n junction diode can be easily indicated with the help of characteristics called V-I characteristics of p-n junction.It is the graph of applied voltage across the p-n junction to the current passing through the junction.

image 6: V-I characteristics of P-N junction diode