ECE

Carrier Transport Phenomena

The net flow of electrons and holes in a semiconductor will generate currents.
The process by which these charged particles move is called transport.
There are the two basic transport mechanisms in a semiconductor crystal: 
Drift the movement of charge due to electric fields, and 
Diffusion the flow of charge due to density gradients.
The carrier transport phenomena are the foundation for finally determining the current-voltage characteristics of semiconductor devices.
We will implicitly assume that, though there will be a net flow of electrons and holes due to the transport processes, thermal equilibrium will not be substantially disturbed.



Carrier Drift

An electric field applied to a semiconductor will produce a force on electrons and holes so that they will experience a net acceleration and net movement, provided there are available energy states in the conduction and valence bands.
This net movement of charge due to an electric field is called drift.
The net drift of charge give, rise to a drift current.

However, charged particles in a semiconductor are involved in collisions with ionized impurity atoms and with thermally vibrating lattice atoms.
These collisions, or scattering events, alter the velocity characteristics of the particle.
As the hole accelerates in a crystal due to the electric fi eld, the velocity increases.
When the charged particle collides with an atom in the crystal, for example, the particle loses most or all of its energy.
The particle will again begin to accelerate and gain energy until it is again involved in a scattering process.
This continues over and over again.
Throughout this process the particle will gain an average drift velocity which, for low electric fields, is directly proportional to the electric field.

Carrier Diffusion

There is a second mechanism that can induce a current in a semiconductor.
We may consider a classic physics example in which a container, as shown in Figure , is divided into two parts by a membrane.





The left side contains gas molecules at a particular temperature and the right side is initially empty.
The gas molecules are in continual random thermal motion so that, when the membrane is broken, the gas molecules flow into the right side of the container.
Diffusion is the process whereby particles flow from a region of high concentration toward a region of low concentration.
If the gas molecules were electrically charged, the net flow of carriers would result in a diffusion current. 

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