N-Channel MOSFET Physics
Components of a MOSFET
In order to understand how MOSFETs work, we need to look at their construction. Consider the following simplified cross-section of one of the most commonly used MOSFETs; an N-Channel, depletion mode MOSFET:
In the illustration above, there are two channels of N-type (negatively charged) silicon laid down in a body of P-type (positively charged) silicon. It has three leads; source and drain connect directly to the N-type silicon channels, and the gate almost touches the P-type body, but has an insulator between its metal electrode and the body of the MOSFET.
Depletion Region
Recall from the last chapter that the P-type and N-type junction creates a depletion region because the nearby holes and electrons combine. Without any free charge carriers, the area between the P and N-type silicon becomes an insulator:
Also recall from the previous chapter, that in the P-type silicon, there are still a small number of of electrons available, known as a minority charge carriers.
Making Current Flow
As the name Field-Effect Transistor implies, MOSFETs work by creating an electric field. If we apply a positive voltage to the gate, it creates an electric field that attracts the minority charge carriers from the surrounding material to the area between the channels and the gate electrode:
Now, with free electrons in that area, current can flow between the gate and source leads.
Voltage Device
One of the things that makes MOSFETs so power efficient is that because there is an insulator between the gate electrode and the body of the MOSFET, current doesn't flow in or out of the gate, instead, it creates an electromagnetic field via the voltage force alone. If more voltage is appplied, the field gets stronger, attracting more electrons to the gap, and allowing more current to flow:
Saturation
However, once a certain threshold of charge carriers have been attracted to the area, it becomes saturated and adding more voltage won't enable any more current to flow.