---
title: MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)
url: https://doc.liz6.com/en/hardware/03-semiconductor/05-MOSFET
locale: en
area: hardware
tags:
- hardware
- semiconductor
date: 2026-06-30
modified: 2026-07-16
description: MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) Basic Concepts MOSFET — A semiconductor device that controls current using an electric field, serving…
---

# MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)

## Basic Concepts

**MOSFET** — A semiconductor device that controls current using an electric field, serving as a fundamental component in modern digital circuits and power electronics.

```
       G (Gate)
       │
       │ ← SiO2 Insulating Layer
       │
┌──────┴───────┐
│    P-type Substrate    │
│   ┌──┐  ┌──┐ │
│   │N │  │N │  ← Two N-regions: Source and Drain
│   └──┘  └──┘ │
│      ↓        │
│    Channel
└───────────────┘
       │
       S (Source)
```

### Types

| Type | Symbol | Turn-on Condition | Application |
|------|--------|-------------------|-------------|
| N-channel Enhancement | NMOS | Vgs > Vth | Mainstay of digital circuits |
| P-channel Enhancement | PMOS | Vgs < -Vth | Complementary circuits |
| N-channel Depletion | - | Conducts when Vgs < 0 | Specific applications |
| P-channel Depletion | - | Conducts when Vgs > 0 | Specific applications |

## Working Principle (N-channel Enhancement Mode)

### Cutoff State (Vgs < Vth)
```
       G
       │
       │ ═══ (No electric field, no channel formed)
       │
┌──────┴──────┐
│    P-type Substrate   │
│   ┌──  ──┐  │
│   │     │  │  ← No conductive channel
│   └──  ──┘  │
└─────────────┘
```

### On State (Vgs > Vth)
```
       G ─── +++ (Electric field attracts electrons)
       │
       │ ═══════════════
       │ SiO2 Insulating Layer
       │ ═══════════════
┌──────┴──────┐
│    P-type Substrate   │
│   ┌──┐  ┌──┐│
│   │N │══│N ││ ← Electrons accumulate to form an N-type channel
│   └──┘  └──┘│
└─────────────┘
```

### Current Formation (Vds > 0)
- The channel connects the source and drain.
- Electrons flow from source to drain.
- **Id ∝ (Vgs - Vth)** (Saturation region)

## Four Operating Regions

```
Id
│        ┌─ Saturation Region (Constant Current)
│       /
│      /
│     / ← Linear Region (Variable Resistance Region)
│    /
│   /
│  /
│ /
└────────────────────── Vds
   │
   Vgs > Vth
```

### 1. Cutoff Region
```
Vgs < Vth
Id = 0 (In reality, there is a tiny leakage current)
```

### 2. Linear Region (Linear / Ohmic)
```
Vgs > Vth, Vds < Vgs - Vth

Id = μn × Cox × (W/L) × [(Vgs - Vth) × Vds - Vds²/2]

Characteristics: Behaves like a variable resistor
```

### 3. Saturation Region
```
Vgs > Vth, Vds > Vgs - Vth

Id = ½ × μn × Cox × (W/L) × (Vgs - Vth)²

Characteristics: Constant current, device acts as an amplifier
```

### 4. Breakdown Region
```
Vds too high → Breakdown → Id increases sharply
```

## Key Parameters

| Parameter | Symbol | Description |
|------|------|------|
| Threshold Voltage | Vth | Minimum Vgs required to turn on (approx. 0.5~3V) |
| Drain Current | Id | Operating current |
| On-Resistance | Rds(on) | On-state resistance (lower is better) |
| Cgs/Cgd/Cds | - | Inter-electrode capacitance (important for high frequency) |
| Vds(max) | - | Maximum Drain-Source Voltage |
| Qg | - | Gate Charge (switching speed) |

## Parasitic Elements

```
       G
       │ Cgd
       ├──────┐
       │      │
    Cgs    ┌──┴──┐
   ┌─┴─┐    │     │
   │   │    │ Id  │
   └───┘    └──┬──┘
    │          │
   Body      Drain
    └──────────┘
         Rds(on)
```

### Parasitic Diode
- A parasitic diode exists between the Body and Source.
- **Body Diode**: Provides a reverse current path between S and D.
- Pay attention to this when using high-side switches.

## Selection Guidelines

### 1. Vds(max)
- Choose > 2x the operating voltage (with margin).

### 2. Rds(on)
- Conduction loss P = I² × Rds(on)
- Lower is better, but often comes with higher Qg.

### 3. Vth
- Logic level matching: Choose low Vth for 3.3V/5V systems.

### 4. Qg (Gate Charge)
- Affects switching speed.
- Affects drive loss.

### 5. Thermal Management
- P = I²R or P = Vds × Id
- Check thermal resistance if a heatsink is needed.

## Drive Circuits

### Basic Drive
```
  ┌─────┐
  │ GPIO│──┐
  └─────┘  │
           │ Rg (Gate Resistor)
           │      ┌──┐
           └──────┤G │
                  └──┘
                 ┌─────────┐
                 │  MOSFET │
                 └─────────┘
```

### Why use a Gate Resistor?
- Limits charging/discharging current.
- Eliminates oscillation.
- Controls switching speed (EMI).

## Comparison with BJT

| Feature | MOSFET | BJT |
|------|--------|-----|
| Control Method | Voltage (Vgs) | Current (Ib) |
| Input Impedance | Extremely High (∞) | Low |
| Drive Power Consumption | Extremely Low (except during switching) | Continuous consumption |
| Switching Speed | Fast | Slow |
| Rds(on) | mΩ range | Vce(sat) |
| Charge Carriers | Electrons (N) or Holes (P) | Both participate |

---
*Keywords: MOSFET, Vgs, Vth, Rds(on), Saturation Region, Linear Region, Body Diode, N-channel, P-channel*