A diode is a two-terminal device that allows current flow only in one direction.
We look in detail at rectifier diodes and Zener diodes. Rectifier circuits convert an a.c. voltage into a ‘pulsating d.c. voltage’. Zener diodes are used to provide a stable voltage reference from a varying supply voltage.
Semiconductor Diodes
Semiconductor Diodes
Semiconductor Diodes
The symbols for these two types of diode are (after Smith and Dorf):
Rectifier
Zener diode
Semiconductor Diodes
The device is a piece of semiconductor material (typically silicon) part of which is doped to be n-type and the other part p-type (One would typically start with an n-type slab and diffuse p-type impurities into part of it at higher concentration to make that region overall p-type. Metal pads are added so metal wires can be attached.)
Semiconductor Diodes
The essential electrical characteristic of a p-n junction is that it constitutes a rectifier

i.e easy current flow in one direction; restrained current flow in the other.

How is this achieved?
Semiconductor Diodes
There is a high concentration of holes on one side of the junction and a high concentration of electrons on the other side.
This causes the two carriers to diffuse to the other side of the junction. (Holes from the p-type region towards the n-type one, electrons the other way)

Semiconductor Diodes

Semiconductor Diodes
The electrons, which move from the n-type semiconductor to the p-semiconductor, recombine with holes there.
This process leaves fixed negatively charged ions unmatched near the junction on the p-side.
Semiconductor Diodes
Similarly an excess positive charge builds up near the junction in the n-semiconductor.
The 'charging up' exerts a repulsive force on further charges crossing the junction. i.e. a p.d. called the barrier potential difference is produced which opposes the diffusion of charge across the junction. This p.d. will be called Vo.
For a silicon p-n junction Vo ? 0.6-0.7 Volts

Semiconductor Diodes
The barrier p.d. has a strong electric field associated with it.
This region near the junction is called the depletion region since, to a close approximation, it is free of electron and hole carriers. (Any thermally generated carriers in this transition region would be swept away by the field).
It is noted that a junction capacitance is associated with the depletion layer (It effectively acts like an insulating region between two (semi-)conducting plates)
Semiconductor Diodes


Semiconductor Diodes
Reverse bias condition
Here an external potential of V volts is applied across the p-n junction with the positive connected to the n region and negative to the p.
 The net result is a widening of the depletion region, which establishes too great a potential barrier for majority carriers to overcome.
Majority carrier flow is reduced to zero.


Semiconductor Diodes
Reverse bias condition (continued)
A small reverse saturation current can exist due to thermally generated minority carriers that diffuse towards the junction. (When they get there the junction voltage assists their passage over.)
The reverse saturation current is given the symbol IO
IO is typically 10-9 A (1 nA) for a silicon junction.
For very large reverse bias the junction can break down causing an abrupt increase in reverse current

Semiconductor Diodes
Forward bias
Here an external voltage is applied with the positive to the p-type material and the negative to the n-type material.
The barrier height is reduced from Vo to (Vo - V) (This is because bulk n- and p- regions have lower resistance so most of the applied potential is dropped across the depletion layer).

Semiconductor Diodes
Forward bias (Continued)
A large number of majority carriers now have sufficient energy to cross potential barrier.
These majority carriers constitute the dominant component of the forward diode current.
The balance between the tendencies to diffuse and drift is destroyed and ‘diffusion of majority carriers rules’.
Semiconductor Diodes, Summary
In a forward biased diode majority carrier flow dominates.
Hole and electron flow contribute a current in the same direction:
          i.e. Total current = hole current + electron current.
The relative contribution of electron and holes to the diode current depends on the relative doping of the p- and n- regions. (Important for later discussion of bipolar junction transistors.)
The barrier height cannot become zero because bulk resistance effects in the semiconductor regions limit the current. The bulk p- and n- region resistance is around 1?

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