What is the difference between MOSFETs and BJTs

Question

After reading this page :

http://learn.adafruit.com/rgb-led-strips/usage

I was wondering what is the difference between N-channel MOFSET and a TIP120 transistor. More specifically, why does he add 100-220 ohm resistors at the base when using the TIP120?

Answer 1

http://www.digikey.com/product-search/en?lang=en&site=US&keywords=tip120

TIP120 is a BJT, which is a different family of transistors from FETs. Below is a broad, oversimplified, cartoon version of how they both work. The below assumes NPN and NMOS, as specified in the question. PNP and PMOS would invert some of this.

A BJT has very low base impedance; essentially, there's a diode between base and emitter. This means that if the transistor is "on", the base of the transistor will be ~.7V above the emitter. If you try to drive the base higher than that (say to 3.3V or 5V with a microcontroller I/O pin) an undesirably large amount of current will flow, and bad things will happen. You have to have something between the I/O pin and the transistor base to limit that current. Thus the resistor. The processor side of the resistor goes to 5V (or whatever your microcontroller logic rail is), and the transistor side goes to ~.7V. This voltage differential, divided by the resistance, gives you the current being injected into the base. That, plus the transistor characteristics, tells you how much current can now flow through the BJT collector-emitter.

A FET has very high gate impedance, so no current flows into the gate when it's turned on. You apply voltage between gate and source, and the "switch" closes. The gate can typically go up to 20V above the source, so driving a FET with a microcontroller isn't typically a problem. Instead, you have the opposite concern: some FETs need more gate voltage than some processors can supply!

Now, there are all sorts of additional details. Sometimes you put a resistor in series with the gate of a FET, for filtering purposes. There is actually current flow into the gate of a FET, particularly at turn-on and turn-off, which can matter for some applications. And BJTs and FETs can be driven in an analog mode, where they're neither fully on or off, but somewhere in between. Sometimes that's good, sometimes it's bad.

When I'm wearing my microcontroller hat, I tend to use FETs wherever possible. In general, they're easier to work with and their losses are lower. BJTs are sometimes cheaper, and they're more likely to be the choice for analog control applications.

Answer 2

A brief comparison of mosfets and BJE transistors would include the following:

voltage drop

• Transistors have a considerable voltage drop across the emitter and collector, it is in the range of 1-2v (and even higher for Darlington).
• Mosfets on the other hand don't have a specified voltage drop but rather a drain-source ON resistance (Rds-on) which is in the milliOhm range and results in a low voltage drop.

base/gate drive

• Transistors normally need a resistor to the base (a resistor in the emitter can have a similar effect) to limit the base-emitter current to safe levels. A general rule of thumb is that a transistor needs about 1/10 to 1/20 of the collector current driven to the base in order to be saturated and have a low Vce voltage drop (which is wanted for a transistor used as a switch). For a Darlington that current is considerably lower because of the high gain.
• Mosfets practically need no current to the gate for static operation (it's just a small amount of current while turning on/off) but need to drive the gate at a voltage level that is high enough (depends on the model) for the mosfet to turn fully on and have a low Rds-on.
  When the mosfet is used for fast PWM switching (unlike static operation) then a proper driver (with sufficient current capacity) is usually required and the reason is that the mosfet gate has a capacitance that has to be charged in order for the mosfet to be turned on and discharged for the mosfet to turn off, the faster you can charge and discharge the gate capacitance the faster the mosfet is going to switch between the on/off states spending as little time as possible in the state that is between these two where the dissipated power will be high (this result to overheat.)