4.5.3.3 Switching Characteristics

The switching performance of conventional IGBTs is limited by the excess carrier recombination process producing a tail current. Additionally, hole injection is not stopped instantly leading to another extension of switching time. There are two distinct phases in the turn-off waveform of the conventional LIGBT. The first phase is characterized by a rapid fall in anode current, corresponding to the electron current flowing through the MOSFET, which is reduced to zero when the gate voltage drops below its threshold voltage. The second phase is dominated by the slowly decaying tail of the anode current. During the second phase, minority carriers (holes) stored in the drift region are removed by recombination processes and the decay of this stored charge is mainly determined by the minority carrier lifetime in the drift region. The reduction of the carrier lifetime of the drift region increases the on-resistance of the devices.

Figure 4.49: Turn-off characteristics of the proposed SOI SA-LIGBT and a conventional SOI-LIGBT at the anode current density 190A$/$$cm^2$, $V_\textrm {A}$ $=$ 50 V, and $V_\textrm {G}$ $=$ 12 V. The tail current dose not exist for the SOI SA-LIGBT.

The $n^+$-anode short of the SA-LIGBT provides an electron extraction path during turn-off. It helps the fast decaying of excess carriers (holes) in the $n$-drift region, and provides a faster switching speed compared to the conventional LIGBT. Figure 4.49 shows the turn-off characteristics of the proposed SOI SA-LIGBT and a conventional SOI LIGBT. Turn-off simulations were performed at an anode current density of 190A/$cm^2$, $V_\mathrm{A}$ $=$ 50V, and $V_\mathrm{G}$ $=$ 12V. The devices were turned off by ramping the gate voltage down from 12V to 0V in 10ns. A carrier lifetime of 10$\mu$s for both electrons and holes was used in the simulation. As can be seen in the figure the turn-off tail of the proposed SA-LIGBT is extremely small in comparison to the conventional SOI-LIGBT. SA-LIGBT gives similar characteristics to that of SOI-LDMOSFETs, thus considerably reducing the turn-off time and the transient power losses [164].