IGBT working principle types circuit
An IGBT, or insulated gate bipolar transistor, is a three-terminal semiconductor device that is primarily used as an electronic switch. It combines the simple gate drive characteristics of a MOSFET with the high current and low saturation voltage capabilities of a BJT (bipolar junction transistor). This makes it ideal for high-performance, high-speed switching applications.
In the world of modern power electronics, the IGBT (Insulated Gate Bipolar Transistor) stands as a key component in the efficient conversion and control of power. From electric vehicles and renewable energy systems to industrial automation and consumer electronics, IGBTs have revolutionized how power is handled in high-voltage, high-current applications.
What is an insulating gate in IGBT?
In the world of power electronics, IGBTs (Insulated Gate Bipolar Transistors) have become the cornerstone of high-efficiency power conversion. They are commonly used in applications such as electric vehicles, inverters, motor drives, and power supplies. One of the key components of an IGBT is the insulated gate, which plays a critical role in how the device works. In this article, we will explore the function of the insulated gate in an IGBT, how it controls current flow, and why it is important for efficiency and performance.
An IGBT is a semiconductor device that combines the high input impedance of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) with the low on-state power loss of a BJT (Bipolar Junction Transistor). This hybrid design offers the advantages of both devices, making IGBTs ideal for converting and amplifying electrical power in a wide range of applications
Basic Structure of IGBT
Before we dive into the insulating gate, it is important to understand the basic layers of an IGBT:
Collector: Bottom layer (P+ substrate)
Drain region: N- layer to block voltage
Base (P-region): Part of the BJT structure
Emitter: Top N+ layer
Gate: A layer of insulating material, often silicon dioxide, separating the metal contact from the semiconductor.
The insulating gate is the heart of the IGBT’s control mechanism. It is essentially a gate terminal separated from the underlying semiconductor layers by a thin oxide layer, usually SiO₂ (silicon dioxide). This configuration is borrowed from the MOSFET design, which gives the IGBT its high input impedance and low gate drive power requirement.
Key features of an insulating gate:
High impedance: Very little current is required to control the gate.
Capacitance behavior: Gate control depends on voltage, not current.
Electrical isolation: Prevents direct current flow between the gate and the rest of the device.
How the insulating gate works in an IGBT
The voltage applied to the gate terminal of an IGBT has a significant effect on how well it conducts. Here is how the insulating gate affects the behavior of the device:
1-Gate off
When no voltage is applied between the gate and the emitter, the insulating gate does not form a conductive channel in the underlying semiconductor.
As a result, the device remains off, and no current flows from the collector to the emitter.
2-Gate on
When a positive voltage is applied to the gate (typically 15V for power applications), an inversion layer forms at the interface between the gate oxide and the P-base region.
This inversion layer acts as a channel that allows electrons to flow from the emitter to the N-biased region.
Once the electrons reach the N- region, they forward bias the P-N junction between the P-Base and N-Dift regions.
This forward bias switches the BJT portion of the device, allowing majority carrier (hole) injection from the P+ collector.
The result is a large current flow from the collector to the emitter with a low voltage drop, thanks to the conductivity modulation.
3-Controlled Switching
The conductive gate allows the IGBT to switch rapidly due to its capacitive nature. Since the gate is isolated and draws minimal current, it can be turned on and off rapidly by varying the gate-emitter voltage. This is especially important in applications such as PWM (pulse width modulation), where switching speed affects overall efficiency and thermal efficiency.
What is an Bipolar Transistor in IGBT?
In contemporary power electronics, insulated gate bipolar transistors (IGBTs) are essential components. Combining the advantages of both MOSFETs and BJTs (Bipolar Junction Transistors), IGBTs offer high efficiency and fast switching characteristics that make them ideal for applications ranging from electric vehicles to industrial inverters. A key part of the functionality of an IGBT lies in its internal bipolar transistor. In this article, we will take a closer look at the operation of the bipolar transistor in an IGBT, helping you understand its role and how it contributes to the overall performance of the device.
The input characteristics of a MOSFET (metal-oxide-semiconductor field-effect transistor) and the output characteristics of a BJT are combined in an IGBT, a three-terminal power semiconductor device. Its terminals are:
Gate (G)
Collector (C)
Emitter (E)
The IGBT is often described as a “voltage-controlled bipolar device” because it uses the MOS gate structure to control the bipolar transistor.
Let’s quickly review the structure of a bipolar transistor before learning about how it works inside an IGBT. Four semiconductor layers make up the NPNP structure of an IGBT. Here is a summary:
Emitter side: N+ (buffer layer) and P+ (emitter layer)
Enhanced region: N- (lightly doped region)
Collector side: P+ substrate (collector layer)
The gate terminal is insulated from the structure of the device by a thin oxide layer, as in a MOSFET.
This layered structure forms a MOSFET at the top (between the gate, N-channel, and N-drift region) and a PNP transistor at the bottom (P+, N-, P substrate), forming a hybrid system.
Role of the bipolar transistor in IGBT operation
1-Initiation of conduction (turn-on phase)
The N-drift region below the gate is reversed when a positive voltage is applied to the gate terminal (with respect to the emitter), forming a conductive N-channel. From the N+ emitter, electrons begin to flow through the N-channel into the N-drift zone.
This electron injection activates the bipolar PNP transistor inside the IGBT:
The electrons move from the emitter to the collector.
As they move, they lower the potential barrier for holes in the P+ collector.
This enables holes to be injected from the P+ collector into the N- drift region.
The result is a bipolar current flow—electrons from emitter to collector and holes from collector to emitter.
This hole-electron pair injection causes conductivity modulation of the N-drift region, greatly reducing its resistance. This is one reason why IGBTs have a lower on-state voltage than MOSFETs in high-voltage applications.
2-During conduction
Once the device is fully turned on, the bipolar transistor continues to conduct holes, and the enhanced region is heavily modulated with charge carriers. This state offers very low conduction losses, which is a major advantage of IGBTs over conventional power MOSFETs, especially in high-current, high-voltage environments.
In this phase, the IGBT behaves like a saturated bipolar transistor but retains the advantage of voltage control through the insulating gate.
3-Turn-off phase
When the gate voltage is removed or reversed (to 0V or negative), the N-channel turns off. Electron injection stops immediately, and the bipolar process stops. However, due to the stored charge (holes) in the enhanced region, turn-off is slower than in a pure MOSFET.
This stored charge must be recombined or removed, which leads to tail current – a characteristic drawback of the bipolar nature of IGBTs. However, advances such as field-stop and trench gate designs help to mitigate this problem.
IGBT working principle
The IGBT is turned on when a voltage is applied between the gate and emitter terminals, causing current to flow between the collector and emitter. It is turned off when this gate voltage is removed or brought below a threshold.
It effectively behaves like a switch:
1-Gate on
The insulated gate bipolar transistor (IGBT) plays a key role in high-efficiency and high-speed switching applications. From electric vehicles to industrial drives and solar inverters, IGBTs are the go-to semiconductor devices for handling large amounts of power with precision.
But how does the gate of an IGBT control its operation?
The insulated gate bipolar transistor (IGBT) is a three-terminal power semiconductor device that combines the high input impedance of a MOSFET with the low on-state power loss of a BJT (bipolar junction transistor). The terminals are:
Gate (G)
Collector (C)
Emitter (E)
The IGBT is a voltage-controlled device where the gate terminal plays a central role in turning the device on and off.
Internal structure of an IGBT
To understand the role of the gate, it is important to understand the internal configuration. The IGBT has:
A MOSFET structure on the gate side.
A p-type layer that forms a junction with the n-drift region, enabling bipolar conduction.
When the gate is turned on, the MOSFET part allows electrons to flow, which energizes the BJT section, causing a very large current to flow from the collector to the emitter.
How does the gate work in an IGBT?
1-Gate control is voltage-based.
Unlike BJTs (which are current-controlled), the gate of an IGBT is voltage-controlled, similar to a MOSFET. Applying a positive voltage between the gate and emitter (V_GE) creates an inverted layer (n-channel) that allows current to flow.
2-Threshold voltage
To turn on an IGBT, the gate-emitter voltage must be greater than a threshold voltage (V_GE(th)), typically around 4V to 8V depending on the device.
V_GE < V_GE(th) → IGBT is off.
V_GE > V_GE(th) → IGBT starts to conduct.
3-Gate Capacitance and Switching Time
Due to its capacitance, the gate needs to be charged or discharged before switching. Gate drivers are used to supply the current required for quick switching.
Turn-on delay time (td(on)) – Time for the gate to charge
Rise time (tr) – Time to reach full conduction
Turn-off delay time (td(off)) – Time to start switching off
Fall time (tf) – Time to completely turn off
These times are essential in high-speed applications.
Gate drive requirements
Since the gate does not draw a continuous current (like a BJT), the power loss in the gate is low. However, for the IGBT to switch quickly, the gate driver circuit needs to:
Provide enough peak current to overcome the gate capacitance.
Control the switching speed and avoid excessive dv/dt or di/dt.
Provide a negative gate voltage during turn-off to prevent false turn-on due to Miller capacitance
Miller effect in IGBTs
Miller capacitance (gate-collector capacitance) can feed voltage spikes into the gate during switching, causing unintended turn-on. To mitigate this:
A negative gate bias (such as -5V) is used during turn-off.
Gate resistors are optimized.
Clamp circuits or Miller clamps are incorporated into drivers.
Gate resistor selection
The gate resistor controls the charging and discharging rate of the gate. Choosing the appropriate value involves trade-offs:
Low resistance → fast switching, but high EMI and possible ringing.
High resistance → slow switching, low EMI, but high power loss
2- Gate off
Insulated Gate Bipolar Transistors (IGBTs) are widely used in power electronics for high-efficiency and high-speed switching applications, such as inverters, motor drives, and power supplies. One of the key aspects of operating an IGBT safely and efficiently is understanding how the “gate-off” function works.
An IGBT (Insulated Gate Bipolar Transistor) is a three-terminal semiconductor device that combines the high input impedance of a MOSFET with the low saturation voltage of a BJT (Bipolar Junction Transistor). Its terminals are:
Collector (C)
Emitter (E)
Gate (G)
The voltage applied to the gate terminal controls the IGBT. Unlike BJTs, which are current-controlled, this device is voltage-controlled.
Understanding Gate-Off Operation in IGBT
1-Role of the Gate
The gate in an IGBT acts like a switch. To turn on the IGBT, a positive voltage (typically +15V to +20V) is applied between the gate and the emitter. This increases the MOSFET channel and allows current to flow from the collector to the emitter.
To turn off the IGBT, this gate-emitter voltage needs to be removed or made negative, typically pulling it down to 0V or even -5V depending on the design.
2-Gate-Off Process – Step by Step
Let’s go step by step to understand what happens when an IGBT is turned off:
a Removing the Gate Voltage
The gate driver circuit pulls the gate voltage down to 0V (or a negative voltage). This process closes the MOS channel that connects the p-type body to the emitter.
b Blocking the inversion layer
The inversion layer, which forms under the gate oxide and allows electrons to pass through, is removed when the gate voltage is removed. This indicates that the base drive of the PNP BJT internal to the IGBT structure is stopped, resulting in the n-channel MOSFET component being turned off.
c Current flow stops
The flow of collector-to-emitter current stops because the BJT component is no longer receiving base current. By doing so, the current flowing from the collector to the emitter is essentially stopped, keeping the IGBT in its off state.
d Charge removal
The IGBT still has some stored charge due to minority carriers (holes in the n-drift region). These charges need to be recombined or removed to completely turn off the device. This process is known as tail current deactivation, and it slightly delays the complete turn-off of the IGBT.
3-Importance of Negative Gate Voltage
In high-speed or high-voltage switching applications, a negative gate voltage (usually -5V) is applied during turn-off. This ensures:
Rapid removal of channel charge.
Avoids false turn-on due to high dV/dT (rate of change of voltage).
Increases noise immunity.
4-Role of Gate Driver in Gate-Off
The gate driver circuit is critical in the gate-off process. It must be able to:
Sink the gate charge quickly.
Provide a channel with low impedance for the gate to discharge.
Protect the gate from overvoltage conditions.
Some drivers include features such as desaturation detection, soft turn-off, or active clamping to increase safety during fault conditions such as short circuits.
Why is gate-off important in practical applications?
A well-designed gate-off mechanism is important for:
Safe turn-off of the IGBT during fault conditions.
Minimizing switching losses.
Preventing device failure due to shoot-through current or cross-conduction.
Increasing the overall reliability of power converters and inverters.
The IGBT maintains high input impedance and low power loss in the on state, making it highly efficient.
Frequently Asked Questions
1. What is an IGBT?
Answer:
Response: The term IGBT refers to Insulated Gate Bipolar Transistor. It is a semiconductor component employed for efficient electrical power switching or amplification. It brings together the benefits of both MOSFETs (high input impedance) and BJTs (low power loss in the on state).
2. Where is IGBT used?
Answer:
IGBTs are commonly used in:
Inverters for motor drives
Electric vehicles (EVs)
Renewable energy systems (e.g., solar, wind)
Uninterruptible power supplies (UPS)
Welding machines
Induction heating
3. What are the main advantages of IGBT?
Answer:
Great efficiency and rapid switching
Management of high voltage and current
On-state voltage drop is low
Elevated input impedance (easy to drive)
4. How does an IGBT work?
Answer:
To turn ON an IGBT, a positive voltage is applied to the gate; to turn it OFF, this voltage is removed. When it is on, it allows current to flow from collector to emitter; when it is off, it blocks this flow. It combines a bipolar transistor output with a MOSFET gate structure on the inside.
5. What are the limitations of IGBT?
Answer:
Slower switching speed compared to MOSFETs
Possibility of latch-up
More expensive
Not ideal for very high-frequency applications
6. How to protect IGBT in a circuit?
Answer:
Methods of protection include:
Snubber circuits for absorbing voltage spikes
Gate resistors for regulating switching speed
Protection against overcurrent (such as circuit breakers, fuses)
Gate driver desaturation detection
7. What is the typical gate voltage for an IGBT?
Answer:
The gate voltage usually varies from +15V (ON) to either 0V or -5V (OFF), based on the specific design and application.
8. Can IGBT be used for AC applications?
Answer:
Indeed. AC inverters, motor control systems, and various AC power conversion applications commonly utilize IGBTs.
9. What causes an IGBT to fail?
Answer:
Overvoltage or overcurrent
Thermal stress (overheating)
Improper gate drive
Latch-up or secondary breakdown
Inadequate heat sinking
10. What are the different types of IGBT?
Answer:
NPT (Non-Punch Through): Offers high ruggedness, suitable for low-frequency applications.
PT (Punch Through): Has a thinner wafer for faster switching, used in high-frequency circuits.
Field Stop IGBT: Improved performance with fast switching and low losses, commonly used in modern power systems.
Trench IGBT: Uses a trench gate structure for higher current density and lower losses.
