Construction, Operating Principle, and Switching Characteristics of Power MOSFET and IGBT: -
In the field of power electronics, semiconductor devices
play a vital role in efficiently controlling and converting electrical energy.
Among them, power MOSFETs (metal–oxide–semiconductor field-effect transistors) and IGBTs (insulated gate bipolar transistors) are two of the most widely used
devices. Both are designed to handle high voltage and current, yet they differ
in construction, operating principles, and switching characteristics.
Understanding these devices is essential for engineers working in motor drives,
inverters, converters, and modern power supplies.
In this blog, we will explore the construction, working
principle, and switching behavior of power MOSFETs and IGBTs in detail,
supported by diagrams and illustrations.
1. Power MOSFET
1.1 Construction of Power MOSFET
A power MOSFET is an advanced form of the conventional
MOSFET, designed to handle high power levels. It has three terminals: Gate (G),
Drain (D), and Source (S).
The construction includes:
- Source Region: Heavily doped n+ region for current entry.
- Body/Channel Region: A p-type region beneath the gate, which forms the conduction channel when voltage is applied.
- Drain Region: Connected to the n+ substrate or drift region for current flow out.
- Gate: A thin insulating oxide layer separates the gate terminal from the channel, ensuring negligible gate current.
The drift region thickness determines the voltage-blocking
capability, while the channel width defines the current-carrying capacity.
1.2 Operating Principle of Power MOSFET
The operation is voltage-controlled. When a positive voltage
is applied between gate and source (VGS > Vth), an inversion layer
(n-channel) is formed in the p-body region. This allows current to flow between the drain and source.
Key operating modes:
- Cut-off Region: VGS < Vth → No conduction.
- Ohmic Region: VGS > Vth, VDS small → MOSFET behaves like a resistor.
- Saturation/Active Region: VGS > Vth, VDS large → Current is controlled mainly by VGS.
Because the gate is insulated, the input current is almost
zero, giving the MOSFET a high input impedance and making it highly efficient.
1.3 Switching Characteristics of Power MOSFET
Power MOSFETs are known for their fast switching speed,
making them suitable for high-frequency applications.
Switching behavior involves:
Turn-ON: When the gate voltage rises, the capacitances (input
capacitance, gate-to-drain capacitance, and gate-to-source capacitance) must be
charged. Once VGS crosses the threshold, the device conducts rapidly.
Turn-OFF: When the gate voltage is removed, these capacitances
discharge. The speed of turn-off depends on gate resistance and capacitance
values.
Switching Losses: Due to the overlap of voltage and current
during transitions. However, MOSFETs generally have low switching losses.
Important parameters:
- Rise Time (tr): Time for drain current to rise.
- Fall Time (tf): Time for drain current to fall.
- Delay Time (td(on)/td(off)): Delay before switching begins.
2. IGBT (Insulated Gate Bipolar Transistor)
2.1 Construction of IGBT
An IGBT combines the features of a MOSFET (voltage control,
high input impedance) and a BJT (low conduction losses, high current
capability). It has three terminals: Gate (G), Collector (C), and Emitter (E).
The structure includes:
- Emitter Region: Heavily doped n+ region.
- Body Region: p-type region where the channel is formed.
- Drift Region: The n-drift layer is responsible for voltage blocking.
- Collector Region: p+ substrate at the bottom.
[Photo Suggestion: Cross-sectional diagram of IGBT showing
gate, emitter, body, drift region, and collector.]
The added p+ collector layer injects holes into the drift
region, reducing resistance and improving conduction, which is the main reason
for IGBT’s low on-state losses.
2.2 Operating Principle of IGBT
The gate operation is similar to that of MOSFETs. A positive voltage
on the gate relative to the emitter induces an n-channel in the p-body region.
Electrons flow from the emitter to the drift region, while holes are injected from the
collector side, enhancing conductivity.
Modes of operation:
- Off State: VGE < Vth, no conduction.
- On State: VGE > Vth, conduction occurs due to combined electron and hole flow.
- Forward Blocking: High collector-emitter voltage is supported by the drift region.
Because of the minority carrier injection, the IGBT offers
lower conduction losses compared to the MOSFET, especially at higher voltages.
2.3 Switching Characteristics of IGBT
IGBTs have slower switching speeds than MOSFETs due to
minority carrier effects, but they handle higher voltages (up to several kV).
Switching behavior:
Turn-ON: Gate capacitances are charged, forming a conduction
channel. Holes from the collector flood the drift region, leading to “tail
current.”
Turn-OFF: During switching off, the stored charge in the drift
region causes a tail current that prolongs the turn-off time.
Switching Losses: Higher than MOSFETs at high frequency, but
conduction losses are lower.
 |
| Switching the waveform of the IGBT |
3. Comparison Between Power MOSFET and IGBT
|
Feature |
Power MOSFET |
IGBT |
|
Control |
Voltage-controlled |
Voltage-controlled |
|
Conduction |
Majority carrier (electrons) |
Both majority & minority carriers |
|
Switching Speed |
Very fast (suitable for MHz) |
Slower (tens of kHz) |
|
Conduction Loss |
Higher (due to channel resistance) |
Lower (due to conductivity modulation) |
|
Voltage Rating |
Up to ~1 kV |
Up to ~6 kV |
|
Applications |
SMPS, DC-DC converters, RF circuits |
Motor drives, inverters, induction heating, UPS |
4. Applications
Power MOSFET:
- Switching mode power supplies (SMPS)
- DC-DC converters
- Battery management systems
- High-frequency inverters
IGBT:
- Induction motor drives
- Traction systems (electric trains, EVs)
- Renewable energy inverters (solar, wind)
- UPS systems
5. Conclusion
Power MOSFETs and IGBTs are both indispensable in power
electronics, yet their selection depends on the application. If high-frequency,
low-to-medium voltage switching is needed, MOSFETs are the better choice. On
the other hand, for high-voltage, medium-frequency applications requiring
efficient conduction, IGBTs dominate.
By understanding their construction, working, and switching
behavior, engineers can design energy-efficient and reliable systems. With the
increasing demand for renewable energy and electric vehicles, both devices will
continue to play a crucial role in shaping the future of power electronics.
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