Power amplifiers and switch-mode power supplies are two instances where the constituent transistors are required to deliver higher currents, and to withstand higher voltages, than are encountered in the digital, high-frequency and memory transistors discussed previously. In this chapter we briefly describe the structural details of, and discuss the principal properties of, several types of high-power transistor: the GaAs HBT and GaN HJFET for power amplification, and the Si MOSFET and hybrid transistor for power supplies.
High currents mean high carrier densities, which can lead to a modification of the space-charge region at the output junction (collector/base or drain/body), with consequences for the frequency response and/or the breakdown voltage. We begin this chapter with a description of the breakdown process (avalanche breakdown), and of the high-current, space-charge-modifying effect (the Kirk Effect).
Operating transistors at high VCE or high VDS can lead to electrical breakdown of the collector/base or drain/body junction, respectively. Breakdown is characterized by the sudden onset of a large current which, if it is not interrupted, can lead to thermal destruction of the transistor. The breakdown process in a reverse-biased pn-junction is illustrated in Fig. 16.1.
Electrons entering the high field of the junction rapidly gain kinetic energy. If this energy is allowed to exceed the bandgap energy Eg, then, when the electron finally collides with a lattice atom, an energy E ≥ Eg can be transferred to another electron, thereby exciting it into the conduction band.