What is IGBT-Insulated Gate Bipolar Transistor ?

IGBT- Insulated Gate Bipolar Transistor IGBT is a three terminal switching device. it a is a best combination of BJT and MOSFET. It has low ON state power loss. 

It is also known as metal oxide insulated gate transistor. The three terminal are  Emitter (E) ,Collector(C), and Gate (G).IGBT is free  from breakdown problems which are present in BJT.

•The major difference with the MOSFET, it has cell structure lies in the addition of a p+ injecting layer.

• Injecting layer forms a p-n junction with the drain layer and injects minority carriers.

•The n-type drain layer have two different doping levels:-

• The lightly doped n- region is referred as drain drift region. 

•     It consists of P⁺ layer that forms drain of IGBT  

•     n- drift layer being added to improve break down voltage capacity

•     n⁺ buffer layer is added but is not essential for the operation of the device  

Based on this, layer are two types

•     Symmetric : Doesn’t have n⁺ buffer  layer

•     Asymmetric : n⁺ buffer layer is included

•     It reduces on state volt drop  &  Reduces turn off time

Physical Operation of IGBT:

•When collector is positive w.r.t Emitter and  Gate is +ve w.r.t Emitter.

•At Gate voltage,Vg=0, when Vge is given, Junction J1 is in Forward Bias and J2 is in Reverse bias.

•Current will not flow from C to E because of Junction J2 which is in RB.

•As we increase gate voltage Vg, -ve charge is present at gate side and +ve charge on the side of N+ layer.

•Because of the potential formed because of capacitance, there will be insertion of –ve charge inside the P- layer.

•Therefore a channel is formed and current flows from C to E.

Steady state V-I output  & Transfer Characteristics of  IGBT

•When the gate emitter voltage is below the threshold voltage,very less leakage current flows though the device while the collector – emitter voltage almost equals the supply voltage, during this condition IGBT is  said to be operating in the cut off region.

 • When the gate emitter voltage increases beyond the threshold voltage the IGBT enters into the active region.

•When the gate emitter voltage is increased further collector current (Ic) also increases and for a given load resistance, Vce decreases. 

At the instant when Vce becomes less than Vge,under this situation the driving MOSFET part of the IGBT enters into the ohmic region and drives the output p-n-p transistor to saturation.

•Under this situation the device is said to be in the saturation mode. Voltage drop across the IGBT remains almost constant in saturation mode.

Application of IGBT:

It is used in DC and AC motor drives, UPS systems, power supply and drives for solenoids, relays and contactors.

Advantages of IGBT:

• Combines the advantages of  BJT & MOSFET

•  High input impedance like  MOSFET

• Voltage controlled device like  MOSFET

• Simple gate drive, Lower switching loss

• It has low on state conduction power loss like as  BJT

•Higher current capacity & higher switching speed than a  BJT. ( Switching speed lower than MOSFET)