There are four noise source classifications in semiconductors: thermal, shot, generation-recombination, and modulation noise. The first three are well understood while the origin of the fourth with regard to semiconductors is less well so.
Thermal noise is a white noise source whose origin is based on fundamental thermodynamic physical laws. For a semiconductor of resistance R the spectral current and voltage noise densities are
Shot noise is due to the discreteness of the charge carriers and is related to the statistical nature of their injection into the semiconductor over a Schottky barrier. The origin of the spectral density may be found by considering Carson's Theorem [1]. This states that for a diode with reverse current I which equals e<n>, where n is the spontaneously fluctuating number of electrons that cross the barrier per second, the spectral density is given by
It can be seen that the shot noise is linearly dependent on the frequency of the
electrons crossing the barrier and
quadratically dependent on the charge of the pulse that the electron creates in
an external circuit.
The spectrum is white because of the very short transit time of the carriers
across the barrier, where the barrier includes
the space charge region.
For the barrier to show full shot noise at all frequencies two conditions must
be met.
The first is that when charge carriers are
injected into the
material, space-charge neutrality is re-established in a very short time, given
by the material dielectric relaxation
time 
[3], where 
is the resistivity of the material.
For SIU-GaAs this is of the order of 10-5s.
The second is that each current pulse should be able to displace a charge
equivalent to that of one electron in the external
circuit, that is to
say a low trap density is required at the metal-semiconductor interface.
Generation-recombination noise can be understood by considering a semiconductor
with a number of traps.
The continuous trapping and de-trapping of the charge carriers causes a
fluctuation
in the number of carriers in the conduction and valence bands. The transitions
are described by equation (2) and thus the
noise spectral density is Lorentzian in nature, with a corner frequency given by
, the lifetime of the electrons in the
conduction band. The current noise density was calculated by van Vliet
[4] to be
where 
.
It should be noted that in observed
spectra the corner frequency of this noise is dispersed, implying a wide
distribution of lifetimes. If the current that flows
through a device is due to generation then the expression for the noise present
will be the generation-recombination one rather
than the expression for the shot noise defined in equation (9).
However in most Schottky junction devices the applied
voltage will increase the injection rate and the noise will show a very close
resemblance to shot noise.
At low frequencies an excess noise spectral density with a 1/f amplitude dependence is observed in semiconductors, which is known as modulation noise. As stated earlier the exact cause of this noise is still not understood.
