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The Two Types of Transistor
There are two basic types of semiconductor transistor: MOSFETs and BJTs. BJT stands for Bipolar Junction Transistor. MOSFETs and BJTs have slightly different electrical properties. One crucial difference is that MOSFETs have higher input impedance than BJTs. Input impedance is the resistance to current flowing into the transistor. High input resistance is a desirable characteristic in transistors used for amplification. However BJTs are able to handle much higher currents than FETs of comparable size. This means when designing electronics for high current applications there is a trade-off between input impedance, maximum current and the size of the transistors used. The IGBT was designed to combine the best attributes of MOSFETs and BJTs.
How Semiconductor Technology Works
Semiconductors are materials that have a level of electrical conductivity between that of a metal and an insulator. Semiconductors are doped with chemicals so that they contain an excess of either negative charge carriers or positive charge carriers. These result in N-type or P-type semiconductors respectively. When P-type and N-type regions are next to each other, the positive and negative charge carriers are attracted to each other. They combine and form a layer called the "depletion region," which contains no charge carriers and is completely non-conducting. The operation of both MOSFETs and BJTs involves controlling the size of this non-conducting depletion region and hence the conductivity of the transistor.
What IGBT and MOSFET Have In Common
Both IGBTs and MOSFETs use semiconductor materials. MOSFETs consist of either two P-type regions separated by an N-type region or two N-type regions separated by a P-type region. Two of the contacts of the MOSFET are attached to each of the two P-type (or N-type) regions. A third contact is attached to the intervening N-type (or P-type) region but separated from it by an insulating layer. The voltage applied by this third contact effects the conductivity between the two P-type (or N-type regions). This is the basic internal structure of both MOSFETs and IGBTs.
Structural Differences
The key structural difference between an IGBT and a MOSFET is the extra layer of P-type semiconductor beneath the standard arrangement. This has the effect of giving the IGBT transistor the characteristics of a MOSFET in combination with a pair of BJTs. This is what makes IGBTs so useful in power applications.