Power Semiconductor Devices: Principles and Applications

Posted on May 1, 2026 in Other university degrees and diplomas

Lecture 1: Power Semiconductor Devices — The Big Picture

What Power Semiconductor Devices Do

  • These devices work as switches without mechanical movement.
  • They control large power flow using a small control signal.

Main Device Groups

  • Diodes: Turned on and off by the power circuit.
  • Thyristors: Can be latched ON by the control signal, but OFF by the power circuit.
  • Controllable switches: Can be turned ON and OFF by the control signal.

Common Power Devices

  • Power diode
  • Power MOSFET
  • Power BJT
  • IGBT
  • Thyristor family: SCR, GTO, MCT, IGCT

Ideal Switch vs. Practical Switch

Ideal Switch

  • Blocks any forward and reverse voltage when OFF.
  • Carries any current with zero voltage drop when ON.
  • Switches instantly.
  • Needs almost no control power.

Practical Switch

  • Has finite blocking voltage.
  • Has small leakage current when OFF.
  • Has voltage drop when ON.
  • Needs finite turn-on and turn-off time.
  • Needs some gate/base drive power.

Carrier-Based Classification

  • Majority carrier devices: MOSFET, Schottky diode.
    • Very fast switching.
    • Switching is controlled mainly by charging/discharging capacitances.
    • Forward drop increases quickly when breakdown voltage increases.
  • Minority carrier devices: BJT, IGBT, thyristor family.
    • Can handle high voltage with lower forward voltage drop.
    • Slower switching because stored charge must be added or removed.

Switching Loss

  • During switching, voltage and current exist at the same time.
  • Energy is lost every time the device switches.
  • Switching loss = energy lost per transition × switching frequency.
  • This limits the maximum practical switching frequency of converters.

Lecture 2: Power Diodes and Reverse Recovery

Basic PN Junction Idea

  • A diode is made by joining P-type and N-type semiconductors.
  • At the junction, a depletion region or space charge region forms.
  • Under equilibrium, carrier movement is balanced.
  • Under forward bias, the barrier reduces and current flows.
  • Under reverse bias, the barrier increases and current is blocked.

Diode Equation

  • The diode current follows the Shockley equation: I_D = I_S(e^(V_D / ηV_T) – 1)
  • Where:
    • I_S = reverse saturation current
    • V_D = applied diode voltage
    • V_T = kT/q = thermal voltage
    • η = emission coefficient
  • At room temperature, V_T is about 26 mV.

What Makes a Power Diode Different

  • A power diode is the power version of a signal diode.
  • It must:
    • Carry very large forward current.
    • Block very high reverse voltage.
  • To support high reverse voltage, the diode needs a wide depletion region.
  • If you increase doping too much, forward loss reduces but reverse breakdown voltage also reduces.
  • Power diodes use a special structure with a lightly doped drift region.

Power Diode Structure

  • n+ substrate at the cathode side.
  • Lightly doped n− drift region.
  • Heavily doped p+ region as the anode.
  • The thickness of the n− drift region strongly affects the breakdown voltage.

Forward and Reverse Behavior

  • In forward bias, the diode has a small drop, usually less than about 1 V for ideal discussion; in practical power devices, it may be a few volts.
  • In reverse bias, only a small leakage current flows until breakdown.

Switching Characteristics

Turn-on

  • When a diode starts conducting, the current may rise gradually because of circuit inductance.
  • The forward voltage can briefly rise to a high value called forward recovery voltage.
  • Forward recovery time is the time needed for the voltage to settle after forward current begins.

Turn-off

  • When a forward-conducting diode is suddenly reverse biased, it does not stop instantly.
  • Stored minority carriers must be removed first.
  • A reverse current appears for a short time, called reverse recovery.
  • The peak reverse current is called Irr.

Why Reverse Recovery Matters

  • Reverse recovery can cause:
    • Overvoltage.
    • Overcurrent.
    • Extra switching loss.

Types of Power Diodes

  • Standard recovery diode: Suitable for 50/60 Hz line frequency.
  • Fast recovery diode: Low reverse recovery time; used in high-frequency applications.
  • Ultra-fast recovery diode: Faster than fast recovery; used for high-speed switching.
  • Schottky diode: Majority carrier device; nearly no reverse recovery; low forward drop (~0.3 V); limited to 50–100 V.
  • Line-frequency diode: Designed for very low on-state loss; high reverse recovery time.

Lecture 3: Power MOSFET

What It Is

  • MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor.
  • It is a 3-terminal device: Gate, Drain, Source.
  • It is a majority carrier device.

Why MOSFET Is Important

  • Very fast switching.
  • Very high input impedance.
  • Easy to drive.
  • No recombination delay.

Structure and Working

  • Power MOSFET has a vertical structure.
  • OFF state: No gate voltage above threshold; drain current is zero.
  • ON state: Gate voltage above threshold creates a channel; device behaves like a low resistance switch.

Key Characteristics

  • RDS(on) is critical; it rises rapidly as voltage rating increases.
  • MOSFET has a positive temperature coefficient, making paralleling easier.
  • No secondary breakdown area.

Ratings and Use

  • Voltage: up to 500–600 V.
  • Switching frequency: can exceed 100 kHz.
  • Best for lower voltage and higher frequency power conversion.

Lecture 4: IGBT

What It Is

  • IGBT stands for Insulated Gate Bipolar Transistor.
  • Combines MOSFET-like gate control with BJT-like low conduction loss.

Working and Structure

  • When gate-emitter voltage is above threshold, an induced channel forms, triggering the PNP part.
  • Terminals: Gate, Collector, Emitter.

Switching Times

  • Turn-on time: Delay time + Rise time.
  • Turn-off time: Delay time + Fall times.
  • Current tailing: A tail current remains during turn-off due to stored charge, slowing the process compared to MOSFETs.

Ratings and Use

  • Commonly used in 500–1700 V range.
  • Typical switching frequency: 3–30 kHz.
  • Best for medium to high voltage and power.

Gate Driver Notes

  • Gate drivers provide protection features: short-circuit, Miller clamp, shoot-through, and overcurrent protection.

Lecture 5: Thyristor Family

SCR (Silicon Controlled Rectifier)

  • 4-layer PNPN device.
  • Latching behavior: stays ON until current falls below holding current.
  • Highest-rated, lowest-cost thyristor; passive turn-off.

GTO (Gate Turn-Off Thyristor)

  • Can be turned OFF by a reverse gate current.
  • Requires very large reverse gate current (1/5th of anode current).
  • High power ratings (up to 5 kV, 5 kA).

IGCT (Insulated Gate-Commutated Thyristor)

  • Integrates the power switch with the gate-drive unit.
  • Very low on-state voltage.
  • Voltage up to 6.5 kV.

Lecture 6: HEMT / AlGaN-GaN HEMT

What HEMT Means

  • High Electron Mobility Transistor.
  • Uses a heterojunction channel between two different semiconductors.

2DEG (2-D Electron Gas)

  • The heart of the HEMT; electrons move in a thin sheet with very low scattering.
  • Results in higher mobility, lower noise, and higher speed.

Advantages

  • Wide bandgap.
  • Low on-state resistance.
  • High converter efficiency.
  • Excellent for high-frequency operation.

Final Exam Revision Sheet

  • Diode: 1-way current; watch reverse recovery.
  • MOSFET: Majority carrier, voltage controlled, very fast.
  • IGBT: MOS gate with bipolar conduction, good for medium/high voltage.
  • SCR: Latching thyristor, cannot be turned off by gate.
  • GTO: Thyristor that can be turned off by negative gate current.
  • IGCT: Advanced thyristor, very low on-state loss.
  • HEMT: Heterojunction device using 2DEG for low noise and high frequency.