A n-Channel MOSFET starts conducting when the voltage UGS (voltage from Gate to source) gets larger (usually around 3 volts). A p-Channel MOSFET starts conducting when the voltage UGS gets smaller (usually around -3 volts). It of course all depends on what sort you are using.
Another note to make is that when using them in digital circuits, they draw more current the higher the switching frequency is. This is because the Gate has a capacitance (usually around 500 pF). The reactance
Xc can be calculated with this formula :
1/(2*pi*f*C), where
f is the frequency (Hz), and
C is the capacitance (F). The resistance doesn't have to be regarded (it has a small influence when using high frequencies), so current
I can be calculated with Ohm's law :
I=UGS/Z, where
Z=Xc. If you do choose to calculate the resistance of the wires,
Z=sqrt(Xc^2+R^2).
Another thing you should know is the "ringing effect" at the Gate. I'm not entirely sure what causes it, but whenever a FET switches from low to high (in a digital circuit here), you will get spikes that shoot up to almost twice the voltage. I presume it has something to do with the Gate's inductive properties... To prevent it, it helps to put a small resistor in series with the Gate (I usually use 10 ohms, but you'll need to calculate it).
Quote: "Think of mosfets as digital transistors."
The difference between your normal bipolar transistors and FETs are that bipolar transistors amplify the current flowing into the Base, where FETs are like a variable resistor controlled proportionally by the input voltage (really high Gate resistance). They can both be analog.
TheComet
