7.5 Power MOSFETs
Compared to BJTs, which are current controlled devices, ﬁ eld effect transistors are
voltage controlled devices. There are two basic ﬁ eld-effect transistors (FETs): the
junction FET (JFET) and the metal-oxide semiconductor FET (MOSFET). Both have
played important roles in modern electronics. The JFET has found wide application in
such cases as high-impedance transducers (scope probes, smoke detectors, etc.) and the
MOSFET in an ever-expanding role in integrated circuits, where CMOS (complementary
MOS) is perhaps the most well-known.
Power MOSFETs differ from bipolar transistors in operating principles, speciﬁ cations,
and performance. In fact, the performance characteristics of MOSFETs are generally
superior to those of bipolar transistors: signiﬁ cantly faster switching time, simpler
drive circuitry, the absence of a second breakdown failure mechanism, the ability to
be paralleled, and stable gain and response time over a wide temperature range. The
MOSFET was developed out of the need for a power device that could work beyond
the 20-kHz frequency spectrum, anywhere from 100 kHz to above 1 MHz, without
experiencing the limitations of the bipolar power transistor.
7.5.2 General Characteristics
Bipolar transistors are described as minority-carrier devices in which injected minority
carriers recombine with majority carriers. A drawback of recombination is that it limits
the device ’ s operating speed. Current-driven base-emitter input of a bipolar transistor
presents a low-impedance load to its driving circuit. In most power circuits, this low-
impedance input requires somewhat complex drive circuitry.
By contrast, a power MOSFET is a voltage-driven device whose gate terminal is
electrically isolated from its silicon body by a thin layer of silicon dioxide (SiO
a majority-carrier semiconductor, the MOSFET operates at much higher speed than its
bipolar counterpart because there is no charge-storage mechanism. A positive voltage
applied to the gate of an n-type MOSFET creates an electric ﬁ eld in the channel region
beneath the gate; that is, the electric charge on the gate causes the p-region beneath the
gate to convert to an n-type region, as shown in Figure 7.25(a) .
This conversion, called the surface-inversion phenomenon, allows current to ﬂ ow
between the drain and source through an n-type material. In effect, the MOSFET ceases
to be an n-p-n device when in this state. The region between the drain and source can be
represented as a resistor, although it does not behave linearly, as a conventional resistor
would. Because of this surface-inversion phenomenon, then, the operation of a MOSFET
is entirely different from that of a bipolar transistor.