Electronic – When is a MOSFET more appropriate as a switch than a BJT

Many power MOSFETs require a high gate voltage for high-current loads, to ensure that they are fully turned on. There are some with logic-level inputs, though. The data sheets can be misleading, they often give the gate voltage for 250 mA current on the front page, and you find that they need 12V for 5A, say.

It's a good idea to put a resistor to ground on the gate if a MOSFET is driven by an MCU output. MCU pins are usually inputs on reset, and this could cause the gate to float momentarily, perhaps turning the device on, until the program starts running. You won't damage the MCU output by connecting it directly to a MOSFET gate.

The BS170 and 2N7000 are roughly equivalent to the BJTs you mentioned. The Zetex ZVN4206ASTZ has a maximum drain current of 600 mA. I don't think that you will find a small MOSFET that can be driven from 3.3V, though.

The main division is between BJTs and FETs, with the big difference being the former are controlled with current and the latter with voltage.

If you're building small quantities of something and aren't very familiar with the various choices and how you can use the characteristics to advantage, it's probably simpler to stick mosly with MOSFETs. They tend to be more expensive than equivalent BJTs, but are conceptually easier to work with for beginners. If you get "logic level" MOSFETS, then it becomes particularly simple to drive them. You can drive a N channel low side switch directly from a microcontroller pin. IRLML2502 is a great little FET for this as long as you aren't exceeding 20V.

Once you get familiar with simple FETs, it's worth it to get used to how bipolars work too. Being different, they have the own advantages and disadvantages. Having to drive them with current may seem like a hassle, but can be a advantage too. They basically look like a diode accross the B-E junction, so this never goes very high in voltage. That means you can switch 100s of Volts or more from low voltage logic circuits. Since the B-E voltage is fixed at first approximation, it allows for topologies like emitter followers. You can use a FET in source follower configuration, but generally the characteristics aren't as good.

Another important difference is in full on switching behaviour. BJTs look like a fixed voltage source, usually 200mV or so at full saturation to as high as a Volt in high current cases. MOSFETs look more like a low resistance. This allows lower voltage accross the switch in most cases, which is one reason you see FETs in power switching applications so much. However, at high currents the fixed voltage of a BJT is lower than the current times the Rdson of the FET. This is especially true when the transistor has to be able to handle high voltages. BJT have generally better characteristics at high voltages, hence the existance of IGBTs. A IGBT is really a FET used to turn on a BJT, which then does the heavy lifting.

There are many many more things that could be said. I've listed only a few to get things started. The real answer would be a whole book, which I don't have time for.