An overview on PN junction diode
PN junction:
Fig: PN junction open circuit
Fig: PN junction characteristics
Open circuit PN-junction:
Fig: open circuit PN junction
Diffusion current component:
As we have holes as majority in P side and electrons in N side there is concentration gradient on both sides which leads to movement of mobile carriers from both sides towards the other leaving immobile charges at the edges of the P and N side respectively.
This movement of charges causes the current known diffusion current as the charges are being diffused.
Depletion region/junction/barrier potential:
As we see due to diffusion the mobile charges leave the immobile charges which means charges are being depleted off from their respective position forming a depletion region which is also known as space charge region.
Drift current:
The minority carriers of P and N side are swept towards the edge of depletion region with the help of electric fields from N type and P type.
The holes from N type are swept towards the P type and electrons from P type are swept towards the N type. This drifting of charges causes drift current which occurs due to the electric field.
Equilibrium:
In open circuit we do not apply any external supply the diffusion and drift currents are equal as they are opposite to each other thus maintaining the PN junction at equilibrium state.
Idrift = Idiffusion
Contact potential:
As the definition of potential states that the amount of work done to move a charge is called potential.
Due to concentration gradient and electric field we encounter diffusion and drift currents respectively creating potential at the space charge region known as contact potential.
Electric field:
The donor atoms from N side are depleted off creating positive charge and acceptor atoms from P side are depleted off creating negative charge.
As the electric field occurs from positive field to negative field there exists electric field at junction.
Energy band diagram:
Fig: Energy band diagram of open circuit PN junction
Forward Bias PN junction:
Fig: Forward bias PN junction
Diffusion current component:
We apply negative terminal to N type and positive terminal to P type as like charges repel the electrons from N type start diffusing towards P side and the holes from P side start diffusing towards N side.
This movement of charges causes the current known diffusion current as the charges are being diffused.
Depletion region/junction/barrier potential:
As the charges towards the terminal are being depleted off and the space charge region is being filled with the mobile charges there is almost 0 barrier potential i.e. no depletion region or space charge region exists between N and P type in forward bias PN junction. In this region the electrons and holes start to recombine known as electron hole pair generation.
Drift current:
The minority carriers of P and N side are swept towards the edge of depletion region with the help of electric fields from N type and P type.
The holes from N type are swept towards the P type and electrons from P type are swept towards the N type. This drifting of charges causes drift current which occurs due to the electric field.
Equilibrium:
In forward bias PN junction the decrease in depletion region causes increase in diffusion current with respect to the drift current.
E = Idiffusion – Idrift
Contact potential:
Due to electron hole pair generation there exists voltage at the depletion region but the barrier voltage reduces with reduction in the depletion region.
Contact potential = Vo(built in voltage) – Vf(forward bias voltage)
Electric field:
The donor atoms from N side are depleted off creating positive charge and acceptor atoms from P side are depleted off creating negative charge.
As the electric field occurs from positive field to negative field there exists electric field at junction.
But the magnitude of drift current due to reduction in depletion region.
Energy band diagram :
Fig: Energy band diagram of forward bias PN junction
Reverse bias PN junction:
Fig: Reverse bias PN junction
Diffusion current component:
We apply positive terminal to N type and negative terminal to P type due to unlike charge attraction principle the electrons from N type start diffusing towards positive terminal and the holes from P side start diffusing towards negative terminal.
This movement of charges causes diffusion current which is very minute i.e. negligible amount.
Depletion region/junction/barrier potential:
As the electrons from N side and holes from P side move towards the P type and N type respectively due to external supply this leads to depletion of charges at the edge of P type and N type creating depletion region.
Drift current:
As the minority carriers form at the depletion region and majority carriers move towards the terminal the drifting of electrons and holes are equal which could be constant drift current.
Equilibrium:
In forward bias PN junction the increase in depletion region causes almost zero diffusion current and constant drift current.
E = Idrift
Contact potential:
Due to electron hole pair generation there exists voltage at the depletion region but the barrier voltage increases with increase in depletion region.
Contact potential = Vo(built in voltage) – Vbr(barrier voltage)
Electric field:
AS depletion region increases the diffusion of charges decreases which increases electric field.
Energy band diagram :
Fig: Energy band diagram of reverse bias PN junction
Reverse bias breakdown:
The breakdown in reverse biased P-N junction diode is due to the strong electric field in the depletion region when the doping is high.
Fig: PN junction diode transfer characteristics
Zener break down:
When a heavily doped diode is reverse biased then the energy bands become crossed at relatively low voltages.
Zener breakdown occurs when the electric field in the depletion region increases to the point of breaking covalent bonds thereby generating electron-hole pairs. This tunnels the electrons from the P side valence band to the N side conduction band and the holes from N side conduction band to P side valence band. This tunnelling is known as the zener effect.
After the start zener effect a large number of carriers can be generated with negligible increase in junction voltage.
This makes reverse current in the breakdown region large.
Avalanche breakdown:
This occurs when minority carriers cross the depletion region under the influence of the electric field that attracts the kinetic energy to break covalent bonds. This causes ionization collisions as carriers are free and collide.
Capacitance of PN junction
Junction capacitance/transition capacitance in reverse bias:
This is due to dipole in transition region
Cdep or CJ or CT = £A/w
Diffusion capacitance in forward bias:
Rate of change in injected minority carriers in the junction with variation of VF is depletion capacitance.
CD = τI/ηVT
NOTE: W = 2£VJ(NA +ND)/(qNAND)
Open circuit VJ = Vo
Forward bias VJ = Vo - VF
Reverse bias VJ = Vo +VR
VT =T/11600
τ =life time of charge
I = forward current
It was a brief description that would help for a quick recall without missing the key areas
ReplyDeleteThank you Sushma
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