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

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

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

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