Conduction Conditions of NMOS Transistors

1. Introduction

NMOS transistors are divided into two types: enhancement-mode and depletion-mode. Their turn-on logic is completely different. Many engineers encounter abnormal heating, failure to turn on, and circuit leakage due to misunderstanding NMOS conduction conditions.

This article uses plain technical language without complex mathematical formulas, fully compatible with all website editors, no garbled characters after pasting.

2. Enhancement-Mode NMOS (E-NMOS)

Most commonly used in power electronics, switching circuits, motor drives

Feature: No built-in conductive channel inside the chip. The transistor is OFF by default at zero gate voltage.

2.1 Basic Conduction Condition

The gate-source voltage must be higher than the threshold voltage.

Threshold voltage (VGS(th)) is a positive value for E-NMOS
If gate voltage is lower than threshold voltage: NMOS keeps OFF, almost no drain current flows
Effective driving rule: Set gate voltage 1V~3V higher than the threshold voltage for stable turn-on

2.2 Two Working States After Turn-On

After the NMOS is turned on, it has two operating regions determined by drain-source voltage:

2.2.1 Linear Region (Fully ON, Recommended for Switches)

Condition: The drain-source voltage is much lower than the effective overdrive voltage.

Extremely low on-resistance (RDS(on))
Very low voltage drop and low power loss
Application: Load switch, DC-DC converter, BLDC motor drive

2.2.2 Saturation Region (Constant Current, For Analog Only)

Condition: The drain-source voltage is greater than or equal to the effective overdrive voltage.

Drain current is fixed and will not increase with voltage
High internal impedance, serious heat generation
Application: Signal amplification, constant current source, current limiting circuit
Do not use for power switching, easy to burn out the MOSFET

3. Depletion-Mode NMOS (D-NMOS)

Special type, mainly for RF and analog circuits, rarely for power switches

Feature: Built-in permanent conductive channel inside the chip. The transistor is ON by default at zero gate voltage.

3.1 Basic Conduction & Cut-off Rule

Default state (0V gate voltage): Fully ON
Always ON when gate voltage is positive or zero
Turn OFF only when gate voltage drops below the negative pinch-off voltage (VGS(off))

3.2 Working Region Classification

Same as enhancement NMOS, divided into linear region (low resistance) and saturation region (constant current) according to drain-source voltage.

Typical application: RF low-noise amplifier, high-impedance analog load

4. Engineering Practical Rules (No Garbled Characters)

4.1 Enhancement NMOS Practical Tips

Gate floating or connected to GND = fully OFF (safe for power systems)
3.3V MCU driving: select low threshold NMOS to ensure full turn-on
10V standard driving: best for low RDS(on) performance
Keep the MOSFET in the linear region during switching operation to avoid overheating

4.2 Depletion NMOS Practical Tips

Cannot be used as a safety power switch, always ON at zero gate voltage
Requires negative voltage power supply to turn off the device
Not recommended for battery management and high-current switching scenarios

5. Core Summary

Enhancement NMOS: OFF at 0V, turn ON when gate voltage exceeds threshold voltage
Depletion NMOS: ON at 0V, only turn OFF with negative gate bias
Power design must use linear region to reduce heat loss
Saturation region is only limited to analog signal and constant current circuits