Hot electron phenomena

Abstract

Abstract Much interesting physics, and most device applications, are associated with carriers that have been excited in some manner to have an energy well in excess of their energy when in thermal equilibrium. In some cases the carriers behave in a manner that can be associated with an elevated temperature, giving rise to the name hot carrier. The name is used more widely to refer to any highly excited carrier. We discussed the velocity-field curves for bulk Si and GaAs in Chapter 1, Section 1.7. We described the transition from Ohm’s law behaviour at low electric fields, where the carriers are little perturbed from equilibrium, through to the regime of saturated drift velocity at high electric fields. Here the carriers are in a steady state; they are highly excited but are giving up their excess energy to the lattice (via phonon emission) at the same rate that they acquire energy from the electric field. We also introduced the idea of avalanching in very high electric fields where very hot electrons excite further electron-hole pairs, and we related the uncontrolled form of this phenomenon to dielectric breakdown. In Chapter 5, Section 5.6, we described the reduction of mobility seen in a 2DEG as a function of increased applied field. In Chapter 6, Section 6.6, we briefly discussed the current-voltage characteristics of a QlDEG in the presence of a strong electric field where eventually there is less current for a greater applied bias. In this chapter we gather together the main hot carrier effects in semiconductor multilayers, associated with the non-linear response to strong electric or optical fields. Using hetero junctions, one can inject carriers from wider-gap into narrowergap semiconductors. The injected carriers are initially very hot, having an initial velocity as much as 10 times that of the saturated drift velocity, and we can study the detailed mechanisms by which they lose their excess energy and momenta and return to thermal equilibrium temperatures. The modification to the density of states in less than three dimensions slows down the rate of relaxation of hot carriers.