g:gate； S: source source; D: drain. N-channel power supply is generally connected to D, output S, and P-channel power supply is generally connected to S and output D, and the enhanced depletion connection is basically the same.

This is a MOS tube pyroelectric infrared sensor. The rectangular frame is the sensing window, the G pin is the grounding terminal, the D pin is the drain of the internal MOS tube, and the S pin is the source of the internal MOS tube. In the circuit, G is grounded, D is connected to the power supply, infrared signal is input from the window, and electrical signal is output from S.

On the MOS tube

The transistor has an N-channel, so it is called an N-channel MOS transistor, or NMOS. There are also P-channel MOS(PMOS) transistors, which are lightly doped N-type back gates and P-type sources and drains.

Judging the source s, the drain d

Set the multimeter at R× 1k, and measure the resistance between the three pins respectively. Measure the resistance twice by exchanging pens and meters, and the one with lower resistance (generally thousands of ohms to more than 10,000 ohms) is the forward resistance. At this time, the black pen is the S pole, and the red pen is connected to the D pole. Due to different test premises, the measured RDS(on) value is higher than the typical value given in the manual.