When you search for "field-effect transistor" on a certain online marketplace, what you mostly see are insulated gate field-effect transistors, while junction field-effect transistors are few and far between. Both JFETs (junction field-effect transistors) and MOSFETs (metal-oxide-semiconductor field-effect transistors) belong to the family of field-effect transistors. But why is there such little interest in JFETs? Does anyone know the reason? Feel free to leave your thoughts in the comments section!

Today's protagonist is the MOSFET, and here's its full name:

Metal-Oxide-Semiconductor Field-Effect Transistor

Due to its gate being insulated, it's also called the insulated gate field-effect transistor, or simply the MOS transistor.

It has three terminals: gate (G), drain (D), and source (S).

The substrate of a MOS transistor is usually connected to the source. There exists a body diode between DS, and regardless of whether it's an N-channel or P-channel MOSFET, the direction of the body diode's arrow is consistent.

Based on the conducting channel, MOS transistors are classified into two types: N-channel and P-channel, and further divided into enhancement-mode and depletion-mode.

The difference lies in that the gate voltage applied to an enhancement-mode MOS (E-MOS) is always positive and has a threshold voltage. It will fully conduct when the gate-source voltage exceeds this threshold voltage.

The gate voltage of a depletion-mode MOS (D-MOS) can be positive or negative, and it will never fully conduct.

Enhancement-mode MOS transistors are generally used more often.

So how do we typically use MOS transistors as switches? For example, to drive an LED?

In fact, MOS transistor conduction does not require current.

The main difference between NMOS and PMOS is that an NMOS switch will remain open until a gate voltage is provided. It will conduct when UGS exceeds a certain value, namely the threshold voltage UGS(th), usually between 2~4V. When the gate pin receives voltage, the switch (D-S) will close.

On the other hand, a PMOS switch will remain closed until a gate voltage is provided. It will conduct when UGS is less than a certain value, typically between -2~-4V.

If UGS does not reach the corresponding voltage value, the MOS transistor will not conduct.

Therefore, it is often referred to as a voltage-controlled component.

Next time, we will explain the working principle of the MOSFET.