A reverse-conducting IGBT (RC-IGBT) integrates an IGBT and a freewheeling diode (FWD) on a single chip. In many IGBT applications, there is a mode in which freewheeling current flows from the emitter to the collector. For this freewheeling operation, the freewheeling diode is connected anti-parallel to the IGBT. Figure (b) shows an example of the internal structure of an RC-IGBT. Part of the P region in the collector electrode is replaced by an N region to form a PIN diode(*1) (P-N--P) with the P region in the emitter electrode. This PIN diode is connected anti-parallel to the IGBT like the FWD shown in Figure (a) and acts as a freewheeling diode. Nowadays, the applications of RC-IGBTs are expanding to the hard-switching fields in addition to voltage resonance.
(*1) A PIN diode is a diode with a high-resistance intrinsic (I) semiconductor region between P and N regions. Because of low dopant concentration, the intrinsic region has high resistance close to that of an intrinsic semiconductor. The FWD in an IGBT requiring high withstand voltage has this structure.