- One assumption implied was that Ohm's law is valid in the carrier drift processes.
- That is, it was assumed that the drift current is proportional to the electric field and that the proportionality constant (σ) is not a function of field ξ.
- This assumption is valid over a wide range of ξ.
- However, large electric fields (> 103V/cm) can cause the drift velocity and therefore the current J = -qnvd to exhibit a sublinear dependence on the electric field.
- This dependence of a upon ξ is an example of a hot carrier effect, which implies that the carrier drift velocity vd is comparable to the thermal velocity vth.
Saturation of electron drift velocity at high electric fields:
- In many cases an upper limit is reached for the carrier drift velocity in a high field (Fig. above).
- This limit occurs near the mean thermal velocity (= 107 cm/s) and represents the point at which added energy imparted by the field is transferred to the lattice rather than increasing the carrier velocity.
- The result of this scattering limited velocity is a fairly constant current at high field.
- This behavior is typical of Si, Ge, and some other semiconductors.
- However, there are other important effects in some materials; for example, a decrease in electron velocity at high fields for GaAs and certain other materials, which results in negative conductivity and current instabilities in the sample.
- Another important high-field effect is avalanche multiplication.