Structure Operation and Characteristics of IGBT

INSULATED GATE BIPOLAR TRANSISTOR(IGBT)-

IGBT is Insulated Gate Bipolar Transistor. It is MOSFET cascaded with a BJT.  IGBT is the hybrid device could be gated like a power MOSFET. It has  low on-state resistance because the majority of the output current is handled by the BJT. IGBT is also known as conductivity modulated FET (COMFET),  insulated gate transistor (IGT) and bipolar-mode MOSFET. 
 
BASIC CONSTRUCTION-

The n+ layer substrate at the drain in the power MOSFET is substituted by p+ layer substrate and called as collector. When gate to emitter voltage is positive,n- channel is formed in the p- region.This n- channel short circuit the n- and n+ layer and an electron movement in n channel cause hole injection from p+subtrate layer to n- layer.

Basic Structure  

Its structure resembles an n-channel power MOSFET constructed on a p-type substrate. IGBT is a three-terminal power semiconductor switch used to control the electrical energy. The three terminals are Emitter, Gate and Collector. This p+ layer nearer to n-drift forms the IGBT collector. The p+ substrate, n− drift layer, and p+ emitter constitute a BJT. It has small current gain. Base current of BJT is controlled by the voltage applied to the MOS gate. It is assumed that the emitter terminal is connected to the ground potential.

Operation

Conductivity modulation

Conductivity modulation occurs by injecting minority carriers into the drain drift region.  

Reverse blocking state.

By applying a negative voltage to the collector, the pn junction between the p+ substrate and the n− drift region is reverse biased which prevents current flow and the device is in its reverse blocking state.

Forward blocking state

If the gate terminal is kept at ground potential but a positive potential is applied to the collector, the pn junction between the p-base and n− drift region is reverse biased. This prevents any current flow and the device is in its forward blocking state until the open base breakdown of the pnp transistor is reached.

V_G- Supply voltage to Gate

V_GE- Voltage Drop across gate and emitter

V_CE- Voltage Drop across collector and emitter

V_CC- Supply voltage to collector

R₁,R₂,R- Resistors

Formation of inversion layer

When a positive potential greater than threshold voltage is applied to the gate, the MOS region is inverted under the gate.  n channel is formed. This channel provides a path for electrons to flow into the n− drift region. The pn junction between the p+ substrate and n− drift region is forward biased and holes are injected into the drift region. The electrons in the drift region recombine with these holes to maintain space charge neutrality and the remaining holes are collected at the emitter, causing a vertical current flow between the emitter and collector. This conductivity modulation decreases the resistance of the drift region. Therefore, IGBT has a much greater current density than a power MOSFET with reduced forward voltage drop. The base–collector junction of the pnp BJT cannot be forward biased, and therefore this transistor will not operate in saturation.

pinch-off

 When the potential drop across the inversion layer becomes comparable to the difference between the gate voltage and threshold voltage, channel pinch-off occurs. The pinch-off limits the electron current and as a result the holes injected from the p+ layer. Therefore, base current saturation causes the collector current

Characteristics

The IGBT has MOSFET input characteristics and BJT output characteristics

Forward Characteristics
 

This characteristic is Collector current Vs Collector voltage and gate emitter voltage is kept at constant value

Transfer characteristics 
 

This characteristic is Collector current Vs Gate voltage. If gate voltage is greater than threshold voltage, collector current increases.

Advantages

1.     It has superior on-state characteristics

2.     It has good switching speed

3.     It has excellent safe operating area.

4.     IGBT has high current density

Application

1.     It is used in high power converters

2.     It is used as a high speed switch.