Insulated Gate Bipolar Transistor (IGBT)

Introduction

What is an IGBT?

Definition

IGBT = Insulated Gate Bipolar Transistor

  • A hybrid power semiconductor device

  • Combines the best features of:

    • MOSFET: Easy to drive, fast switching

    • BJT: Low voltage drop, high current capability

  • Three terminals: Gate (G), Collector (C), Emitter (E)

  • Voltage-controlled device (like MOSFET)

  • Used in medium to high power applications

Key Point

IGBT = MOSFET input characteristics + BJT output characteristics

Why Do We Need IGBT?

Power Device Limitations:

  • MOSFETs: Good for low voltage only

  • BJTs: Need complex base drive circuits

  • Thyristors: Cannot turn OFF easily

IGBT Solution:

  • High voltage capability (600V - 6.5kV)

  • Simple gate drive (like MOSFET)

  • High current capability (up to kA range)

  • Full control (both ON and OFF)

Applications

  • Motor drives

  • UPS systems

  • Welding machines

  • Electric vehicles

  • Solar inverters

Structure and Symbol

IGBT Structure

Basic Structure:

  • Four-layer semiconductor (P-N-P-N)

  • Gate isolated by thin oxide layer

  • Similar to MOSFET gate structure

  • Vertical power flow design

Key Features:

  • Gate controls electron flow

  • Channel formation under gate

  • Hole injection from collector

  • High current density capability

n-channel IGBT cross-section
n-channel IGBT cross-section

IGBT Symbol and Types

n-channel IGBT symbol
n-channel IGBT symbol

  • Most common type

  • Arrow shows current direction

  • Gate controls the device

p-channel IGBT symbol
p-channel IGBT symbol

  • Less commonly used

  • Reverse current direction

  • Same operating principle

Remember

Focus on n-channel IGBTs as they are most widely used.

Basic Operation

How IGBT Works - Simple Explanation

Turn-ON Process:

  1. Apply positive voltage to gate (V\(_{GE}\) \(>\) V\(_{th}\))

  2. Creates electron channel (like MOSFET)

  3. Electrons flow from emitter to drift region

  4. Holes injected from collector

  5. Device turns ON

Turn-OFF Process:

  1. Remove gate voltage (V\(_{GE}\) \(<\) V\(_{th}\))

  2. Electron channel disappears

  3. Stored holes must be removed

  4. Device turns OFF

Carrier flow in IGBT
Carrier flow in IGBT

Key Concept

Gate voltage controls electron flow, which controls hole injection

Conductivity Modulation - Why IGBT is Special

What is Conductivity Modulation?

When holes from collector enter the drift region, they increase conductivity significantly

Benefits:

  • Low ON-state voltage drop

  • High current handling capability

  • Better than MOSFET for high currents

Trade-off:

  • Slower switching (due to stored charge)

  • Turn-off time longer than MOSFET

Analogy

Think of it like adding salt to water - increases conductivity!

Remember

Lower V\(_{CE(sat)}\) but slower switching

Equivalent Circuits

IGBT Equivalent Circuits - Simplified Models

Simple model: MOSFET + Diode
Simple model: MOSFET + Diode

Simple Model:

  • Easy to understand

  • Good for basic analysis

  • Shows unidirectional conduction

Accurate model: MOSFET + BJT
Accurate model: MOSFET + BJT

Better Model:

  • More accurate for calculations

  • Shows current amplification

  • MOSFET drives BJT base

Note

Use the MOSFET + BJT model for circuit analysis and calculations

Parasitic Thyristor - Important Concept

The Problem:

  • Four-layer structure can form thyristor

  • If activated, causes latch-up

  • Gate loses control

  • Device may be damaged

Prevention:

  • Keep emitter resistance low

  • Proper device layout

  • Stay within Safe Operating Area

  • Use appropriate gate drive

Parasitic thyristor in IGBT
Parasitic thyristor in IGBT

Note

Always mention parasitic thyristor as a limitation of IGBT and its prevention methods

Characteristics

Transfer Characteristics

Key Parameters:

  • Threshold voltage (V\(_{th}\)): 3-6V typically

  • Transconductance (g\(_m\)): Controls current gain

  • Similar to MOSFET transfer curve

Temperature Effects:

  • V\(_{th}\) decreases with temperature

  • Current increases with temperature

  • Important for parallel operation

I\(_C\) vs V\(_{GE}\) characteristics
I\(_C\) vs V\(_{GE}\) characteristics

Design Point

Choose gate voltage 2-3 times higher than V \(_{th}\) for reliable operation

Output Characteristics

I\(_C\) vs V\(_{CE}\) characteristics
I\(_C\) vs V\(_{CE}\) characteristics

Key Features:

  • Forward blocking capability

  • No reverse blocking (needs antiparallel diode)

  • Saturation region operation

  • V\(_{CE(sat)}\) = 1.5-3V typically

Comparison:

  • Lower V\(_{CE(sat)}\) than MOSFET at high currents

  • Higher V\(_{CE(sat)}\) than BJT

Switching Characteristics

IGBT Switching Behavior

Key Difference from MOSFET

IGBT has tail current during turn-off due to stored minority carriers

Turn-ON:

  • Similar to MOSFET

  • Gate capacitance charging

  • Current rises quickly

  • Voltage falls

Turn-OFF:

  • Gate capacitance discharging

  • Current falls in two stages

  • Tail current - slow decay

  • Limits switching frequency

IGBT switching waveforms
IGBT switching waveforms

Important

Tail current is the main limitation for high-frequency applications

Gate Drive Requirements

Gate Drive Specifications

IGBTs need proper gate drive circuits for reliable operation

Voltage Requirements:

  • Turn-ON: +12V to +15V

  • Turn-OFF: -5V to -15V (for fast turn-off)

  • Maximum rating: \(\pm 20~\mathrm{V}\) typically

Current Requirements:

  • Peak current: few amperes

  • Low source impedance preferred

  • Gate resistor for controlling switching speed

Protection:

  • Zener diode across gate-emitter

  • Gate resistor for dv/dt immunity

  • Isolated power supply

Typical Values

V\(_{GE}\) = +15V (ON)
V \(_{GE}\) = -8V (OFF)
R \(_G\) = 10-47\(\Omega\)

Types and Applications

IGBT Types - Simplified Classification

Two Main Types

Classification based on internal structure and performance

1. Fast IGBTs:

  • Lower switching losses

  • Higher switching frequency (\(>\) 20 kHz)

  • Slightly higher V\(_{CE(sat)}\)

  • Used in: High-frequency converters

Applications:

  • Switch-mode power supplies

  • Motor drives (high speed)

  • Induction heating

2. Low-loss IGBTs:

  • Lower V\(_{CE(sat)}\)

  • Higher conduction efficiency

  • Slower switching

  • Used in: Low-frequency applications

Applications:

  • Traction drives

  • UPS systems

  • Large motor drives

Selection Criteria

Choose based on switching frequency and power loss requirements

Major Applications

Industrial Applications:

  • Motor drives (most common)

  • Variable frequency drives (VFDs)

  • Uninterruptible Power Supplies (UPS)

  • Welding equipment

  • Induction heating

Power Range:

  • Few kW to MW

  • Voltage: 600V to 6.5kV

  • Frequency: 1 kHz to 100 kHz

Renewable Energy:

  • Solar inverters

  • Wind power converters

  • Energy storage systems

Transportation:

  • Electric vehicle inverters

  • Train traction systems

  • Ship propulsion

Why IGBT is Preferred

Good compromise between switching speed, voltage rating, and drive simplicity

Advantages and Limitations

IGBT - Pros and Cons

Advantages:

  • Simple voltage-controlled operation

  • High input impedance (minimal drive power)

  • High current density

  • Good overload capability

  • Wide Safe Operating Area

  • Available in high voltages

  • Cost-effective for medium power

  • Mature and reliable technology

Limitations:

  • Limited switching frequency

  • Tail current causes turn-off delay

  • No reverse conduction

  • Temperature-dependent

  • Latch-up possibility

  • Gate voltage sensitivity

  • Higher voltage drop than MOSFET at low currents

Key Takeaway

IGBT is excellent for medium-power, medium-frequency applications where simple drive and high power capability are needed

Design Considerations

Important Design Considerations

1. Protection Circuits:

  • Gate protection: Zener diode (15V rating)

  • Overcurrent protection: Current sensing + soft turn-off

  • Overvoltage protection: Snubber circuits

2. Thermal Management:

  • Junction temperature \(<\) 150°C

  • Proper heat sink design

  • Thermal interface materials

  • Temperature monitoring

3. Gate Drive Design:

  • Isolated power supply

  • Appropriate gate resistance

  • Short connection paths

  • EMI considerations

Comparison

Power Device Comparison - Summary Table

Parameter IGBT MOSFET BJT Thyristor
Drive Power Low Low High Low
Switching Speed Medium Fast Slow Slow
On-state Loss Medium High Low Low
Voltage Rating High Medium High Very High
Current Rating High Medium High Very High
Control Full Full Full Partial
Cost Medium Low Low Medium
Best for Medium Power High Freq Linear Apps High Power

Selection Guide

Choose IGBT when: Medium power (kW range), moderate frequency (\(<\) 100 kHz), high voltage (\(>\) 600V)

Summary

Key Points to Remember

  1. IGBT combines MOSFET input + BJT output characteristics

  2. Advantages: Easy to drive, high power capability, good SOA

  3. Main limitation: Tail current limits switching frequency

  4. Applications: Motor drives, UPS, welding, electric vehicles

  5. Protection needed: Gate, overcurrent, and overvoltage protection

  6. Parasitic thyristor: Must be prevented from latching

  7. Best for: Medium power, medium frequency applications

  8. Gate drive: +15V ON, -8V OFF, with proper isolation