How to determine if the transistor in your hand is an NMOS or PMOS?

Today, I'll show you the quickest and most efficient way to distinguish between NMOS and PMOS.

Firstly, let's talk about enhancement-mode MOSFETs, and here are their circuit symbols:

You can see that the directions of the two arrows are different.

This arrow represents the substrate, because the substrate inside the MOSFET is connected to the source.

The biggest difference between the circuit symbols of NMOS and PMOS lies in their substrate arrows. In NMOS, the arrow points towards the gate, while in PMOS, it points away from the gate.

This arrow direction is related to the direction of the PN junction between the MOSFET's internal substrate and the channel's inversion layer.

Secondly, the body diodes of NMOS and PMOS transistors are opposite.

In NMOS, the anode (positive terminal) of the body diode is connected to the source, while in PMOS, the anode of the body diode is connected to the drain, and the cathode (negative terminal) of the body diode is connected to the source.

So, how do you differentiate the three pins of a MOSFET?

You can use a multimeter for testing, and here's a simple explanation.

Take a TO220-packaged MOSFET, which generally has a heat sink. The heat sink will be conductive with the drain. Use a multimeter to test which pin can conduct with the heat sink, and that corresponds to the drain.

Because of the diode connection, there are diode characteristics between the drain and source.

Therefore, when using a multimeter to test which pin is the source, both forward and reverse will conduct, indicating it's the source.

The remaining pin is the gate.

In summary:

1. The positive terminal of the body diode of NMOS is generally connected to the source.

2. The heat sink is connected to the drain.