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Why I Prefer Power MOSFETs

Power MOSFETs began their use in commercial audio amplifiers in the
late Seventies when so-called lateral power MOSFETs became available
primarily by Hitachi. The most well-known of these early power MOSFET
amplifiers were the Hafler designs. Then around 1980 International
Rectifier came out with the legendary HEXFET vertical power MOSFETs. These
were initially aimed primarily at switching power supply and other power
control applications, and so some audiophiles mistakenly overlooked them
as good candidates for audio power amplifier output stages. The HEXFETs
were distinguished by the application of well-developed and sophisticated
fabrication techniques borrowed from the MOS integrated circuit industry.
Think of a Power MOSFET as about 1 million very fast little MOS
transistors connected in parallel with an extremely efficient
metallization technology that keeps resistance down and helps to get out
the heat.

In the early Eighties, I designed a MOSFET power amplifier using the
HEXFETs which dealt with and overcame the drawbacks in certain areas that
MOSFETs had with respect to bipolar transistors, while capitalizing on the
many advantages of the MOSFETs. Among other improvements, this amplifier
was the first to apply the Hawksford error correction scheme to MOSFET
power amplifiers, solving the so-called transconductance droop problem.
The design was published as a paper in the Journal of the Audio
Engineering Society and is available elsewhere on this site.

The most significant advantages of power MOSFETs, particularly vertical
ones are:

They are extremely fast, with equivalent ft's on the order of 300
MHz

They do not suffer from secondary breakdown

They need only the simplest form of short circuit protection

They are capable of extremely high transient currents
(~20-50A/device)

They do not suffer from Beta droop at high currents

They are easier to drive than bipolars, => much smaller driver
transistors

There is little or no danger of driver transistor secondary
breakdown

They are easy to turn off, reducing secondary crossover
distortion

Chapter 11 in Designing Audio Power
Amplifiers is dedicated to the design of MOSFET power amplifiers.
MOSFETs are not without their own shortcomings and design challenges, and
those are discussed as well.