Professor Kazuo Tsutsui and his research team at the Tokyo Institute of Technology say that, while the efficiency of IGBTs is good, reducing the on resistance or the voltage from collector to emitter required for saturation (Vce(sat)) could increase their energy efficiency.
Previous investigations have found that increasing the injection enhancement (IE) effect – which creates more charge carriers – can lead to a reduction in Vce(sat). While this can be achieved by reducing the device’s mesa width, it increases the mesa resistance. One solution is to reduce the mesa height, but this impedes the IE effect.
The researchers reduced the mesa width, gate length and the oxide thickness in the MOSFET by a factor of 1/k and compared devices with a value for k of 1 and 3. Because the fabrication of narrow mesas can cause problems, the trench depth was also reduced by 1/k. While this had a slightly negative effect on the IE effect, it made fabrication easier and lower cost, while only having a small effect on the dependence of Vce(sat) on trench depth. Gate voltage was also decreased by a factor of 1/k, while the cell pitch was maintained at 16μm.
The team concluded that ‘it was confirmed experimentally for the first time that significant Vce(sat) reduction can be achieved by scaling the IGBT in the lateral and vertical dimensions, with a decrease in the gate voltage’.
