IGBT

IGBT is a new development in the area of power MOSFET technology. This device combines into it the advantages of both MOSFET and BJT. So an IGBT has high input impedance like a MOSFET and low-on-state power loss as in a BJT. Further, IGBT is free from second breakdown problem present in BJT. IGBT is also known as metal-oxide insulated gate transistor (MOSIGT), conductively-modulated f‌ield effect transistor (COMFET) or gain-modulated FET (GEMFET). It was also initially called insulated gate transistor (IGT).

Basic Structure and Working of IGBT

Below figure shows the basic structure of an IGBT. It is constructed virtually in the same manner as a power MOSFET. There is, however ; a major difference in the substrate. The $n^+$ layer substrate at the drain in a power MOSFET is now substituted in the IGBT by a $p^+$ layer substrate called collector. Like a power MOSFET, an IGBT has also thousands of basic structure cells connected appropriately on a single chip of silicon.

When gate is positive with respect to emitter and with gate-emitter voltage more than threshold voltage of IGBT, an n-channel is formed in the p-regions as in a power MOSFET, This n-channel short circuits the $n^-$region with $n^+$ emitter regions. An electron movement in the n-channel, in turn, causes substantial hole injection from pf substrate layer into the epitaxial $n^-$ layer. Eventually, a forward current is established as shown in above fig.

The three layers $p^+$, $n^-$ and p constitute a pnp transistor with $p^+$ as, emitter,$n^-$ as base and p as collector. Also $n^-$, p and $n^+$ layers constitute npn transistor as shown in below fig.

Here $n^-$ serves as base for pnp transistor and also as collector for npn transistor. Further p serves as collector for pnp device and also as base for npn transistor. The two pnp and npn transistors can, therefore, be connected as shown in fig to give the equivalent circuit of an IGBT.