Educational issues for power semiconductor devices

Abstract

The teaching of power semiconductor devices often requires a more detailed presentation of the basic physical and electrical parameters than used in semiconductor device physics courses; this is true not only for the standard power devices, like BGTs, GTOs and power MOSs, but also for the more recent ones that merge the MOS effect with bipolar operation, like IGBTs, MCTs, ESTs, etc. In fact, usually in these basic courses many important items for the power devices such as the limiting effects of breakdown, the temperature effects on transport properties, the constraints posed by the large area and low doping, the effects of high injections and the limits imposed by high current effects, like secondary breakdown, are not considered. In this paper some of the points will be briefly recalled that could be considered in courses dealing with power semiconductor devices, with the aim of suggesting a possible and useful link between the basic properties studied in solid state physics courses and the performance of modern power devices. In particular, the dependence of mobility and lifetime on doping, injection level and temperature should be stressed because they play a major role in the electrical operation of the devices. Secondly, the basic effects that limit the ratings of the power devices both in steady state conditions and in the turn-on and turn-off transients must be discussed and used to understand their role in the rating of different types of devices, either 2D effects that act in determining some important unipolar, bipolar, or mixed MOS-bipolar ones. Finally, the phenomena in power devices should be presented both with simplified analysis and with CAD tools; these latter need to be introduced to make the student aware of their use and possibilities.