POWER MOSFET FEATURES

Main features of power MOSFETs are:

a) MOS type FETs are basically majority carrier devices; consequently, they differ greatly from bipolar transistors which are minority carrier devices.

b) MOS type FETs are not current controlled devices as are bipolar transistors. They are voltage controlled devices and are controlled by the voltage applied between the gate and source.

c) Since they are majority carrier devices, high-frequency switching operation is possible because there is no storage time-lag due to the carrier storage effect.

d) With bipolar transistors, current concentration occurs in the high-voltage area and junction failure occurs due to secondary breakdown. Therefore, bipolar transistors require considerable derating. Power MOSFETs, on the other hand, have a negative temperature coefficient which makes it difficult for secondary breakdown to occur, making these devices highly resistant to failure and thus enabling use right up to the maximum rating.

e) When power MOSFETs are used for switching operations, their switching times, that is, the rise and fall times, of power MOSFETs are one order of magnitude faster than those of bipolar transistors; therefore, turn-ON and turn-OFF loss are much smaller than those of bipolar transistors.

Incidentally, the ON resistance of power MOSFETs has a positive temperature coefficient; therefore, heat sink and thermal expansion design is necessary, taking into consideration the RDS (ON) at high temperatures.

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