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.