MOSFETs working in the variable resistance region can be used to regulate current and voltage in circuits.

The variable resistance region of a MOSFET refers to the state where the resistance of the channel changes with the gate voltage when the MOSFET is conducting. This characteristic allows the MOSFET to be used as a resistor, achieving variable resistance functionality.

For example, when the gate voltage increases, the channel resistance decreases, allowing the MOSFET's conduction current to increase. Conversely, when the gate voltage decreases, the channel resistance increases, resulting in a decrease in the MOSFET's conduction current.

Now, let's take a look at the conduction state of the MOSFET.

The conduction state of a MOSFET is determined by both the gate voltage and the drain-source voltage, not just the gate voltage alone.

So, what is the use of MOSFETs working in the variable resistance region?

When a MOSFET operates in the variable resistance region, its resistance value changes with the control voltage, allowing for the adjustment of current and voltage in the circuit.

However, it cannot fully open to pass a large current.

This is because when a MOSFET is operating, it needs to meet certain conduction conditions, which simply means that the control voltage must be greater than or equal to the gate threshold voltage, otherwise the MOSFET will not conduct.

Even if the control voltage is greater than the gate threshold voltage, it cannot allow the MOSFET to pass too large a current, as excessive current can easily cause the MOSFET to overheat and fail.

Furthermore, the drain current will increase slowly with the increase of the drain-source voltage, as the resistance between the drain and source of the MOSFET is very high in the variable resistance region. Therefore, the conduction state of the MOSFET cannot be fully opened, and only a very small current can pass through.

Therefore, in practical applications, when using MOSFETs, it is necessary to choose the appropriate MOSFET according to the specific usage scenario and design the circuit properly to ensure the stable operation of the MOSFET.