Page 46 - Power Electronics Handbook

TheIGBT 39

-Silicon
dioxide

The p+ of the substrate causes the IGBT to be a minority carrier device,
resulting in superior conduction characteristics compared to MOSFETs. The
equivalent circuit shows that the device primarily consists of a p-n-p
transistor driven by an n-channel MOSFET. The voltage drop across the
IGBT is the sum of the p-n junction and that across the MOSFET.
Commercial devices minimise this voltage drop by increasing the size of the
MOSFET, so as to reduce its on-resistance, and by increasing the gain of the
p-n-p device.
The turn off time of the IGBT is relatively high, due to the lifetime of the
minority carriers in the n- epi, which is the base of the p-n-p transistor. This
causes the IGBT to have switching characteristics which are not as good as
an equivalent MOSFET. Lifetime killers can be introduced to improve
switching speed, or an n+ buffer added, as shown in Figure 1.22(a). Both
these techniques, however, increase the volt drop across the device, so
compromises are often needed.
As seen from its construction the IGBT contains four alternate layers,
p-n-p-n, which can result in a latching operation, similar to a thyristor, as
explained in Section 1.9. This effect can be reduced by the use of a wide epi