Optimisation of SuperJunction Bipolar Transistor for ultra-fast switching applications

In this work we present a thorough optimisation of the SuperJunction Bipolar Transistor based on a new modelling approach. The Superjunction Bipolar Transistor was first proposed in [1,21 but its potential against the most advanced IGBT structures such as Field Stop (ITS), Light Punch-Through or Trench IGBTs, has only been briefly discussed. For the first time we report here on the impact of varying the net doping of the n and p drift layer pillars to deliver the best trade-off between the on-state and switching performance. This is carried out both at room temperature and high temperatures. In essence the doping charge in the n & p pillars changes the ratio between bipolar and unipolar conduction in the drift region and therefore alters very significantly the speed and on-state performance of the device. This is a unique effect, un-characteristic to any other power devices known in the field. We demonstrate here through extensive numerical simulations that an optimised superjunction IGBT can lower the turn-off losses by a factor of 2 (or more) when compared to a state-of-the-art Field Stop (Soft Punch Through) IGBT while maintaining a similarly low on-state voltage drop. We also show here that the SJ concept can deliver significantly improved overall performance when compared to a FS IGBT at lower drift dopings, where the use of charge balance results in lower drift lengths and at high doping levels, where the effect of unipolar conduction at the cathode side of the drift region leads to better on-state/switching trade-off. However there is a middle range of doping levels where the use of the SJ drift region is in fact detrimental to the plasma distribution in the device leading to worse performance than a FS IGBT.