Injection-Enhanced Insulated-Gate BipolarTransistor (IEGT)
The IEGT device was developed to lower the on-state voltage drop (Kitagawa et al., 1993). Of the development, the device rated at 2500 V has a voltage drop of around 4 V at room temperature. The device structure is shown in Fig. 5.38. It has a deep trench with oxide sidewall to inject the electron carriers via the accumulation layer formed by the electric field along the extended vertical gate. The higher amount of electron injection, as shown in Fig. 5.38, raises the
Fig. 5.37. Dual-gate base-resistance-controlled thyristor.
Fig. 5.38. The injection-enhanced IGBT and the excess carrier concentration in the bulk drift region.
electron concentration in the drift region near the p-body then in term it helps to lower the on-state conduction voltage.
MOS-Controlled Thyristor (MCT)
The MCT was developed (Temple, 1984; Bauer et al., 1991) for the similar objectives to have a lower on-state conduction voltage than IGBT devices. The basic device structure is shown in Fig. 5.39 which has both n-channel and p-channel to turn on and to turn off the device. Published data show that a 600 V rated MCT device can be able to carry about 10 times higher current than that of the IGBT device, and about 100 times higher than the MOSFET device (CSIHPDPIC, 1996).
When the Gate 2 is applied a positive voltage, the device functions like an IGBT and due to the prolonged n-emitter, it is very susceptible to latch-up,
Fig. 5.39. MOS-controlled thyristor.
as an intended characteristic. To turn off the device, a negative gate voltage is needed to be applied to both Gates 1 and 2 to short the upper n+-emitter/ p-base junction, so as to reduce the current gain of n-p-n transistor in order to break the regenerative latch-up. Similar to BRT device, the magnitude of the negative gate voltage should be proportional to the amount of thyristor current to be turned off. CSIHPDPIC (1996) gives the relationship of turn-off current densities and gate voltages for single-cell and array-group of MCT devices. The negative gate voltage varies according to different cell pitches and, for a fixed cell pitch it is almost linearly proportional to the thyristor current density.