June 2015
I’ve read that bipolar transistors are current devices and MOSFET transistors — like old-fashioned vacuum tubes — are voltage-operated devices.
Although I understand the distinction conceptually, what does that mean from a practical perspective? For example, does this mean that bipolar are best for high power applications and MOSFETs are best for low voltage applications?
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Mosfets come in many flavors: there are N-type and P-type of course but there are enhancement mode and depletion mode. Enhancement mode are off at zero gate voltage; you have to apply a positive gate voltage for N-type or a negative gate voltage for P-type to turn it on. N-type mosfets are available with drain voltage ratings from 30 V to 800 V or more. Most mosfets are designed for switching; an 800 V, 10 amp device would quickly burn up unless it could turn on and saturate even quicker.
Mosfets are characterized by their saturation resistance which can be very low (like .01 ohms). Bipolar transistors on the other hand, are characterized by their saturation voltage which can’t get as low power as mosfets. The threshold voltage (the point where the transistor just turns on) is not well controlled so you can’t really know what the drain current will be at a particular voltage. That makes it difficult to design a linear circuit. I avoid that problem by using bipolar transistors in linear circuits, or using pulse width modulation in a switching circuit which can be filtered to produce an analog signal.
Depletion mode mosfets are on at zero gate voltage and you have to apply a negative gate voltage to N-type to turn it off. Junction fets are also depletion mode devices and the zero gate voltage drain current is not well controlled so they are usually binned and labeled so you can have some idea of what you are designing with.
