What is the causal relationship between transistors, MOSFETs, and IGBTs?

IGBT is an insulated gate bipolar transistor that can withstand high voltage and high current, with fast switching speed. Its principle is similar to MOSFET, but it has stronger current carrying capacity. IGBT combines the advantages of MOSFET and bipolar transistor to achieve fast switching and high flow.

The following text will introduce the causal relationship between IGBT, MOSFET, and transistor in sequence. Firstly, it is necessary to understand the definitions.

The full name of IGBT is Insulated Gate Bipolar Transistor, which is a super electronic switch. With a thickness of only 120um and less than 2 square centimeters, an IGBT chip can withstand a voltage of 1200 volts, and the current flowing through and turning off can reach up to 600A. It can also switch thousands to tens of thousands of times per second.

IGBT has three terminals, namely the collector for incoming current, the emitter for outgoing current, and the gate for controlling on/off.

By applying a voltage of 15V to the gate, the IGBT is turned on and the current path is established, just like opening a valve on a water pipe with our hands. If a positive voltage is applied to the collector at this point, it is like filling a reservoir connected to a water pipe with water, and the current will continuously flow from the collector to the emitter. If you want to turn off this current path, it's also very simple. Lowering the gate voltage of the IGBT to zero is equivalent to closing the valve of the water pipe. The upstream water no longer flows down, and when the water flow in the pipe is clean, the current will be turned off.

The same principle applies to MOSFETs, where the switching of the transistor can be controlled by applying voltage to the gate. What is the difference between MOSFETs and IGBTs?

That's right, IGBT does have MOSFET genes, which can be considered as the biological son of MOSFET and transistor marriage. Let's take a look at the equivalent circuit diagram of IGBT. IGBT can be seen as a combination of an N-channel MOSFET and a PNP transistor. The emitter and collector of IGBT come from PNP transistors; The gate of IGBT comes from MOSFET; The drain of the MOSFET is connected to the base of the PNP transistor, so if a 15V voltage is applied to the MOSFET gate and the MOSFET is turned on, the base of the PNP transistor is pulled low, the PNP transistor is turned on, and the collector current will continuously flow to the emitter.

Why is IGBT composed of a MOSFET connected in series with a transistor?

This is because MOSFET is like a small water pipe with a flexible valve, which can open and close the threshold gate with very little force, and it is not a problem to open and close tens of thousands of times per second, but the water pipe is too thin and the flow rate is small.

So we use MOSFET as a fast threshold gate to control the bipolar transistor as a coarse water pipe, which has fast switching speed and large water flow, making it a strong and flexible IGBT.