My professor's textbook says:

(1) The BJT is usually considered to be the ’heavy–lifter’ of the transistor kingdom, oriented toward control of larger levels of current.

(2) The FET is usually associated with the light, fast action, particularly in the design of VLSI circuits.

Is this true? I thought FETs were typically used for large amounts of current (I have used MOSFETs when I built an LED cube; they were rated for nearly an amp).

  • 7
    \$\begingroup\$ Your professor's book is a couple decades out of date, me thinks. \$\endgroup\$
    – Matt Young
    Commented Feb 17, 2014 at 17:31
  • \$\begingroup\$ Used to be true. In the early 1980s, FETS capable of handling high currents were a real novelty. How old is that book? \$\endgroup\$
    – user16324
    Commented Feb 17, 2014 at 18:30
  • \$\begingroup\$ I must be older than the early 1980s. I was working for the HP division that made power supplies, and FETs were already being used then as the switching elements. \$\endgroup\$ Commented Feb 17, 2014 at 18:36
  • 6
    \$\begingroup\$ The important question is what truth will be tested during exams? The professor's-textbook-truth or today's-pratical-truth. \$\endgroup\$
    – jippie
    Commented Feb 17, 2014 at 18:38
  • 1
    \$\begingroup\$ To help everyone - please advise the date of the book. For high voltages - say hundreds of volts and high power (say hundreds of watts up) the IGBT (which Olin mentioned) tends to be preferred. An IGBT is a bipolar transistor which is controlled by an FET input structure. BUT this is almost certainly what the book was referring to. \$\endgroup\$
    – Russell McMahon
    Commented Feb 17, 2014 at 20:29

2 Answers 2


I disagree with your professor's book. Both BJTs and FETs can be made for small signal use or high current use. They are different technologies, so it should be no surprise they have different tradeoffs. There are certainly plenty of examples where each is used in each role. The statement you quote is basically silly, or at the least very outdated.

For example, FETs are overwhelmingly used in power supplies in consumer electronics. Their ability to switch on and off fast and present a low maximum on resistance make them attractive in that role. Up to a few 10s of Amps, a FET with a low Rdson will drop less voltage, and thereby dissipate less heat, than a bipolar. FETs can also be paralleled more easily since their Rdson goes up with temperature, as apposed to BJTs where the saturation voltage goes down with temperature.

There are other cases, particularly over a few 100 Volts, where BJTs usually have better characteristics. Then there is the IGBT which is a FET used to turn on a BJT.

  • \$\begingroup\$ My favorite part of your answer: "The statement you quote is basically silly." Accepted, thanks. \$\endgroup\$ Commented Feb 19, 2014 at 2:05

Serious high power applications are mostly IGBT and Thyristor. If you need to control a diesel-electric locomotive or an electric catapult for your 5th generation fighter aircraft, you're probably not using a MOSFET.

Last I looked (may be out of date) most PC power supplies used BJTs in the power output stage.

A lot of cheap flyback converters may still have a single high voltage BJT, for cost reasons. This is what you might find in a switching AC adapter of a few watts. CFL circuits generally use small high-voltage BJTs, and I would not be surprised to find LED lamps using a similar circuit. Those represent huge volumes of parts.

Almost all the low voltage stuff, and a great deal of the off-line switchmode stuff has migrated to (regicidal) MOSFETs in recent years.

So, it's kind of a dated observation, with, at best, only elements of truth in 2014. Le roi est mort, vive le roi !


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