Diodes & Basic Applications
Intrinsic silicon doped so that one half is n-type and other half p-type forming pn junction between the two regions
n-region : free electrons (majority) and holes (minority) carriers
p-region : holes (majority) and free electrons (minority) carriers
The pn junction forms a basic diode and is fundamental for all solid-state devices operation
A diode is a device that allows current in only one direction
For every \(e\) that diffuses across the junction and combines with a hole, a +ve charge is left in the n region and a -ve charge is created in the p region, forming a barrier potential, \(V_B\)
This action continues until \(V_B\) repels further diffusion.
Conduction \(e\) in the n region must overcome both the attraction of the +ve ions and the repulsion of the -ve ions in order to migrate into the p region.
After the ion layers build up, the area on both sides of the junction becomes essentially depleted of any conduction electrons or holes and is known as the depletion region
A single pn junction forms a semiconductor diode.
There is no current across a pn junction at equilibrium.
The primary usefulness of the semiconductor diode is its ability to allow current in only one direction as determined by the bias.
The term bias in electronics refers to a fi xed dc voltage that sets the operating conditions for a semiconductor device.
There are two bias conditions for a pn junction
forward biasing
reverse biasing
Either of these conditions is created by connecting an external dc voltage in the proper direction across the pn junction.
Source -ve terminal to n-type material, and +ve terminal to p-type material.
Battery pushes holes and free electrons toward the junction.
If \(V_s < V_B ~\Rightarrow\), the free \(e\) do not pass depletion layer.
On entering depletion layer ions push them back into n region hence no current through the diode.
If \(V_s>V_B\) battery pushes holes and free \(e\) toward the junction.
Free \(e\) pass through the depletion layer and recombine with the holes.
Holes in p region moving to the right and free \(e\) moving to the left and somewhere in junction vicinity opposite charges recombine.
Free \(e\) continuously moves right end of the diode and holes to left end, there is a continuous current through the diode.
Battery -ve terminal connected to p side and +ve to the n side.
-ve terminal attracts holes, and +ve attracts free electrons causing them to flow away from the junction widening the depletion layer
The newly formed ions increase PD across the depletion layer as the holes and electrons move away from the junction.
The greater PD, the wider the depletion layer.
When the depletion layer’s potential difference equals the applied reverse voltage, it stops growing.
Reverse bias prevents current across the pn junction
The reverse current caused by the thermally produced minority carriers is called the saturation current
Saturation means that we cannot get more minority-carrier current than is produced by the thermal energy
A small current flows on the surface of the crystal. known as the surface-leakage current, caused by surface impurities and imperfections in the crystal structure.
When a diode is reverse-biased, it must be able to withstand the maximum value of reverse voltage that is applied or it will break down.
The maximum rated voltage for a diode is designated as peak inverse voltage (PIV).
The required PIV depends on the application; for most cases with ordinary diodes, the PIV rating should be higher than the reverse voltage.
If the external reverse-bias voltage is increased to a large enough value, avalanche breakdown occurs.
Phenomena occurring:
Assume that one minority conduction-band electron acquires enough energy from the external source to accelerate it toward the positive end of the diode.
During its travel, it collides with an atom and imparts enough energy to knock a valence electron into the conduction band.
There are now two conduction-band electrons.
Each will collide with an atom, knocking two more valence electrons into the conduction band.
There are now four conduction-band electrons which, in turn, knock four more into the conduction band.
This rapid multiplication of conduction-band electrons, known as an avalanche effect, results in a rapid build-up of reverse current.