Built in potential for PN junction diodes

[saad]

built in potential

The built in potential ( or barrier potential ) for pn junctions is given by following relation,
Vbi = Vt ln(Na*Nd/ni²) which is normally of the order of 0.6V to 0.8 V.

It can be seen that Vbi is purely function of Temperature and Doping and independent of the external bias

Incase of forward biased Pn junction, the barrier is reduced to

barrier = Vbi-Vf --------- Eq (1)

where Vf is the applied forward bias potential.

What happens if Vf > Vbi?
From Equation (1) => Vbi-Vf <0

How can we justify a negative barrier across the pn junction?

 

[saruman1983]

built-in potential

The barrier is not negative.It's just that the junction is forward biased. The barrier IS the built-in potential (I think). That's why at about 0.7V (0.6-0.8V as you said) the barrier is about to be overcome (or has been overcome) and the junction can support huge quantities of current while maintaining an almost constant voltage drop across it.

 

[subharpe]

built in potential equation

A forward voltage more than the contact potential makes the barrier completely disappear. This will mean the junction does not restrict the diffusng charge carriers across the junction and therefore the junction forwardly breaks down. The device behaves as a resistive bar with very low resistivity, therefore large forward current flows and it damages the diode. The negative contact potential simulates the condition of an ohmic contact where the jnction does not deplete the region of physical contact between the metal and the semiconductor and therefore the resistivity of the semiconductor material gives rise to ohmic drop.

 

[saad]

contact potential in pn junction

The negative contact potential simulates the condition of an ohmic contact where the jnction does not deplete the region of physical contact between the metal and the semiconductor and therefore the resistivity of the semiconductor material gives rise to ohmic drop.

1. Can you elaborate above statement?
2. What about validity of the equation that evaluates the potential appearing across the pn junction (i.e. potental across pn-junction = Vbi-Vf) which theoretically leads to negative potential across the junction incase of Vf>Vbi (which isn't offcourse possible because built-in field will always be directed from n to p region of diode)

Regards

 

[INS-ANI]

diode built in potential

The barrier potential(bp) is due to presence of positively doped and negatively doped region side by side. bp can be considered as the resistance offered by the junction to the flow of electrons as intended by Vf. and as long as the bp hasnt been broken, the electrons feel a greater force in the direction of bp.
as soon as the Vf exceeds bp(as in ur case), a large current flows.

also, in electronics, there is nothing any thing like negative current or voltage. its just the opposite direction wrt to the reference direction.

 

[subharpe]

built in potential pn junction

When Vf exceeds Vc the contact potential, it means all the space charges that were present at the depletion region to resist the diffusion of the majority carriers are swept away and there is no depletion region for the majority carriers. The cathode and/or the anode are usually heavily doped so that at this condition the resistivity offered by the device is very small leading to excessive current which heats up the device and ultimately burns it. The forward breakdown voltage can be increased by making one of the electrode less doped.

In case of ohmic contacts, if you remember, there are two types of basic metal semiconductor contact diodes, which gives rise to no depletion region depending on the work function of the semiconductor and the metal and the doping type of the semiconductor. In these cases the junction region is rushed with the space charges which actually helps diffusion by adding drift to the majority carriers unlike the case of p-n junction diode where drift occurs for the minority carriers. Therefore there is no unilateral characteristics of the metal- semiconductor junction and depending on the resistivity of the semiconductor current flows in both direction as a linear function of voltage.