What do the three pins G, S, and D of the packaged MOSFET mean?

MOSFET Structure and Pin Configuration

This packaged MOSFET pyroelectric infrared sensor features a distinctive rectangular frame serving as the sensing window. The device incorporates three essential pins:

• G pin (Ground terminal)
• D pin (Internal MOSFET drain)
• S pin (Internal MOSFET source)

In practical circuit applications, G connects to ground, D links to the positive power supply, while infrared signals enter through the window and electrical signals output via S.

MOS Driver and Gate Characteristics

The MOS driver plays a crucial role in waveform shaping and driving enhancement. A suboptimal G signal waveform lacking sufficient steepness can result in substantial power loss during switching stages. This inefficiency manifests in:

• Reduced circuit conversion efficiency
• Excessive heat generation in the MOSFET
• Potential thermal damage

The presence of capacitance between MOSFET GS necessitates adequate G signal driving capability to prevent compromised waveform jump time.

Testing Procedures

Gate (G) Identification:

Short-circuit the G-S pole and use a multimeter set to R×1 level. Connect the black test lead to S and red to D. Expected resistance: few Ω to >10 Ω. The G pole is confirmed when infinite resistance is measured between it and other pins, regardless of lead polarity.

Source (S) and Drain (D) Identification:

Using R×1k setting, measure inter-pin resistance twice with exchanged leads. The lower resistance value (typically few kΩ to >10 kΩ) indicates forward resistance. Black lead identifies S pole, red indicates D pole.

MOSFET Types and Principles

MOSFETs come in two primary variants:

• N-channel MOSFET (NMOS)
• P-channel MOSFET (PMOS) - featuring lightly doped N-type backgate with P-type source and drain

Both variants operate on the fundamental principle of voltage-controlled current regulation. The gate voltage controls drain current output, distinguishing MOSFETs from traditional transistors by eliminating base current-induced charge storage effects. This results in superior switching speeds.

Technical Foundation

The Field Effect Transistor (FET) derives its name from the electric field projection through an insulating layer, controlling current flow. The gate's minimal current flow through this insulator results in extremely low gate current.

Modern MOSFETs typically employ silicon dioxide as the gate insulator, earning the "metal oxide semiconductor" designation. Their compact size and energy efficiency have made them the preferred choice over bipolar transistors in numerous applications.