Understanding Power MOSFET Structure

Power MOSFETs are crucial components in modern power electronics, designed to handle high voltages and currents. Let’s explore their unique structural features that enable efficient power handling capabilities.

Basic Structure Overview

    Source Metal
        ║
    ╔═══╩═══╗
    ║ n+ ║ n+ ║      Source
════╝     ╚════
    p+    p      Body
    │
    │  n-        Drift Region
    │
    │
════════════════ n+ Substrate
    ║
    ╨  Drain Metal

Cross-sectional view of a typical Power MOSFET

Vertical Structure

Unlike regular MOSFETs, power MOSFETs employ a vertical structure where current flows from top (source) to bottom (drain), maximizing current handling capacity.

Drift Region

Contains a lightly doped n- region that supports high blocking voltage and manages electric field distribution.

Key Structural Components

Source Metal: Top metal layer for current collection and distribution

n+ Source Regions: Heavily doped regions for carrier injection

p-Body Region: Creates the channel for current flow

n- Drift Region: Supports voltage blocking capability

n+ Substrate: Provides low resistance path to drain

Drain Metal: Bottom metal contact for current flow

Design Consideration

The cell pitch and junction depth are critical parameters that affect the device’s performance characteristics including on-resistance and switching speed.

Cell Structure Details

      Gate
    ┌───┐
 S  │   │  S
┌┴┐ │   │ ┌┴┐
│n+│ │   │ │n+│
└─┘ │   │ └─┘
 p+ └───┘  p+
    p-body
    
     n-
   
     n+

    Drain

Detailed cell structure showing gate, source, and body regions

Key Dimensional Parameters
Cell Pitch: Typically 2-10 μm
Gate Oxide Thickness: 50-100 nm
Junction Depth: 1-3 μm
Drift Region Thickness: 5-50 μm (voltage dependent)

Structural Layer Analysis

Gate Structure

Polysilicon Gate
┌──────────────┐
│ Gate Oxide │
════╝ SiO2 ╚════

Polysilicon material for gate electrode
Optimized gate oxide thickness for reliability
Specialized gate geometry for uniform field distribution
Multiple parallel cells for current handling

Channel Formation
    Gate
    ║
 S  ▼  S
═╗ ═══ ╔═   
 ║ Chr ║    ← Inversion
n+╚═══╝n+      Channel
   p-body     Formation
    │
    │  n-
    │

Channel formation under gate bias

Advanced Structural Features

Trench Structure

    S   G   S
    ┌─╨─┐
    │   │
n+ ═╣   ╠═ n+
    │   │
    │   │
p   │   │   p
    │   │
    └───┘
     n-

Modern trench design for reduced on-resistance

Planar Structure

    S  G  S
    ╨  ╨  ╨
 ═══╗   ╔═══
n+ ═╝   ╚═ n+
    p     p
      n-

Traditional planar design with proven reliability

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