Here are some notes about MOSFETs:
Structure
A MOSFET is a four-terminal device with terminals called the source, drain, gate, and body. The body is usually connected to the source terminal, making it a three-terminal device in appearance. 
Working
A MOSFET works by varying the width of a channel where charge carriers flow. The width of the channel is controlled by the voltage applied to the gate, which is located between the source and the drain. 
Uses
MOSFETs are used in both analog and digital circuits. They are the most common type of transistor because they use less power, have higher switching speeds, and can be miniaturized easily. 
Small-signal analysis
In small-signal analysis, a small AC signal is applied on top of the DC bias voltage. The resulting output AC voltage is amplified based on the slope of the transfer characteristic at the bias point. 
Drain punch-through
Drain punch-through can lead to leakage or breakdown between the source and the drain. This can cause loss of gate control.