How it works
The IGBT combines a MOSFET input stage with a BJT output stage. The gate and channel structure is identical to a Power MOSFET — a voltage-controlled, insulated gate drives an N-channel inversion layer — but the drain region is replaced with a P+ substrate, creating a PNP BJT between collector and emitter. When the gate is driven high, MOSFET channel current drives the BJT base, and minority carrier injection from the P+ collector lowers on-state resistance dramatically compared to a pure MOSFET. Turn-on is fast; turn-off has a "tail current" as stored minority carriers recombine, lasting 0.5–2 µs, which limits maximum switching frequency to around 50 kHz for standard IGBTs.
Key points to remember
IGBTs are voltage-controlled (like MOSFETs) but have BJT-like low on-state voltage drop at high currents — typically 1.5–3 V versus 5–10 V for a comparable BJT. Gate drive voltage is typically +15 V / −5 V or −15 V; gate threshold voltage V_GE(th) is around 4–6 V. Latch-up — a failure mode where the parasitic thyristor structure inside the IGBT turns on and cannot be turned off — is suppressed in modern devices by heavily doping the P-body region. Punch-through IGBTs (PT-IGBT) switch faster with more tail current; non-punch-through (NPT) types switch slower but handle short-circuit conditions more robustly. Maximum junction temperature is typically 150°C.
Exam tip
The examiner always asks you to compare IGBT with MOSFET and BJT in a table — know that IGBT wins on on-state drop at high current, MOSFET wins on switching speed, and BJT loses on gate drive simplicity and switching speed simultaneously.