An insulated-gate bipolar transistor (IGBT) is a solid-state switch that allows or stops current flow by applying voltage to a semiconductor base. It has advantages over conventional transistors, such as no moving parts, low resistance, and fast turn-on speed. IGBTs are smaller and cheaper to manufacture than MOSFETs and require lower power. Shutdown occurs in two stages, making it slightly slower than MOSFETs.
At its simplest level, an insulated-gate bipolar transistor (IGBT) is a switch used to allow current to flow when it is on and to stop current flow when it is off. An IGBT is a solid state device, which means it has no moving parts. Instead of opening and closing a physical connection, it is operated by applying voltage to a semiconductor component, called a base, which changes its properties to create or block an electrical path.
The most obvious advantage of this technology is that there are no moving parts to wear out. However, solid-state technology isn’t perfect. There are still issues with electrical resistance, power requirements, and even the time it takes for the switch to operate.
An insulated-gate bipolar transistor is an improved type of transistor designed to minimize some of the disadvantages of a conventional solid-state transistor. It offers the low resistance and fast turn-on speed found in a metal-oxide-semiconductor power field-effect transistor (MOSFET), although it is slightly slower to turn off. It also doesn’t require a constant voltage source like other types of transistors do.
When an IGBT is turned on, voltage is applied to the gate. This forms the channel for the electric current. The base current is then supplied and flows through the channel. This is essentially identical to how a MOSFET works. The exception to this is that the construction of the insulated gate bipolar transistor affects how the circuit turns off.
An insulated-gate bipolar transistor has a different substrate, or base material, than a MOSFET. The substrate provides the path to the electrical ground. A MOSFET has an N+ substrate, while an IGBT’s substrate is P+ with an N+ buffer on top.
This design affects how the switch fails in an IGBT, allowing it to occur in two stages. First, the current decreases very rapidly. Second, an effect called recombination occurs, during which the N+ buffer on top of the substrate eliminates the stored electrical charge. With shutdown occurring in two steps, it takes a little longer than with a MOSFET.
Their properties allow IGBTs to be manufactured to be smaller than conventional MOSFETs. A standard bipolar transistor requires slightly more semiconductor surface area than the IGBT; a MOSFET takes more than twice that. This significantly reduces the cost of manufacturing IGBTs and allows for more of them to be integrated into a single chip. The power requirement for operating an insulated gate bipolar transistor is also lower than in other applications.
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