Power Semiconductors: The BJT, MOSFET, and IGBT (3)


A. Physical Characteristics
The MOSFET was introduced in the 1970s and, unlike the BJT, is a voltage controlled device. It also has three terminals, though they differ from the BJT: the source, the gate, and the drain. The source and drain diffusions are separated by the gate.

The MOSFET has a p or n channel and can operate in depletion or enhancement mode. In enhancement, no current flows when the gate voltage is zero. In depletion mode, however, a narrow n channel is formed under the gate such that current will still flow when the gate voltage is zero.

B. Operational Advantages and Disadvantages
In comparison to the BJT, the MOSFET is far superior. It has a high input impedance, reducing complexity and cost, and a low input current drive. At low currents, it also has a higher gain than the BJT. To handle higher currents, it is sufficient to simply put several MOSFETs in parallel, and because there is only one breakdown region, the safe operating region is larger. Additionally, because they are free from minority carrier storage times, MOSFETs are faster at switching than the BJTs.
Despite its advantages, the MOSFET has low gain at high currents. Moreover, it was slow to catch on and had an overall greater cost than a BJT of the same power rating. However, in recent years, the prices have come down and MOSFETs have gained in popularity [2].


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