why does a MOSFET have higher conduction loss compared to a transistor? While designing switching circuits , driver circuits controlled by a microcontroller, how should i select a switching device(i.e; where should i use a BJT/MOSFET)?

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    \$\begingroup\$ Eerrrm, MOSFET = Metal Oxide Semiconductor Field Effect Transistor. A MOSFET is a transistor... You need to clarify your question. \$\endgroup\$ – Tom Carpenter Feb 12 '16 at 3:23
  • \$\begingroup\$ Do you mean compared to a BJT (Bipolar Junction Transistor), or maybe to a JFET (Junction Field Effect Transkstor)? \$\endgroup\$ – Robherc KV5ROB Feb 12 '16 at 3:32
  • \$\begingroup\$ In older texts, "transistor" often means BJT. It's archaic terminology, but I'm not sure I'd call it wrong. That being said, some context for the question would be nice. \$\endgroup\$ – Adam Haun Feb 12 '16 at 3:33
  • \$\begingroup\$ sorry ..! by transistor ,i mean a BJT! it is said that an IGBT combines the best qualities of a BJT and MOSFET. so ,is lower conduction loss of BJT utilised in a IGBT? \$\endgroup\$ – prasanna Feb 12 '16 at 3:39

MOSFETs do not necessarily have lower conduction loss than BJTs- in fact in many cases (especially at low voltage) a MOSFET can do much better. For example a 5m ohm MOSFET switching 100mA (grossly overrated, but I digress) will drop only 500uV. A BJT might drop tens of mV at best.

IGBTs can have competitive conduction losses with MOSFETs if they are both high voltage rated but they tend to be worse if the voltage rating required is low for both types being compared. Often they drop between 1V and 2V at operating current.

BJTs require a lot of base drive current, even just sitting there 'on', whereas IGBTs and MOSFETs require current mostly when they are switching, and only leakage when they are on or off. The base current represents some loss in the transistor and some in the base driving circuitry. MOSFETs and IGBTs can require significant drive current when being driven at high frequency.

IGBTs don't combine the best of both in all ways, or they would be used more often. They're more of a niche product for high voltage (hundreds of volts and up) application. They tend to be a bit sluggish and may require negative bias. They do combine the low static base drive current with the small die size (meaning reasonable cost) for high voltage devices of a given current rating.

If you want a 1000V 10A device to switch at 25kHz the IGBT will look pretty good. If you want a 50V 30A device to switch at 200kHz, a MOSFET will be attractive. BJTs are very useful at lower currents and in some high voltage switching applications.

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  • \$\begingroup\$ thank you..! so the potential drop across the device when "on" is the conduction loss, is that right? if i am wrong please explain what is conduction loss ? \$\endgroup\$ – prasanna Feb 12 '16 at 5:28
  • \$\begingroup\$ The power loss (ignoring drive) is just the voltage across the device times the current it is conducting, so yes, it's directly proportional to the voltage drop. MOSFETs and BJTs (and IGBTs) behave differently as current increases - MOSFETs are almost like a resistor for low drops, the other guys behave in a more complex manner. \$\endgroup\$ – Spehro Pefhany Feb 12 '16 at 5:33

In basic terms:

  • BJT transistors are relatively low-impedance, current-driven devices. They amplify Ibe or Ibc to a higher value Ice or Iec.

  • FET (MOSFET, or JFET) transistors are relatively very high-impedance, potential(voltage)-driven devices. An extremely low motion of electrons (more capacitive that resistive) 'charges' carriers in a channel, allowing (or blocking) current flow through the source & drain of the device.

For my uses, I select a BJT when I want to work with a lower-impedance signal, and select a FET when I need to buffer a high-impedance signal.

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