24 August 2017

Insulated Gate Bipolar Transistor ( IGBT )

Introduction

  • The BJT has low power loss but large turn off time therefore the switching speed of the IGBT is slow. 
  • The MOSFET has higher power loss due to high on state resistance but small turn off time therefore the switching speed of the MOSFET is high.
  • The IGBT has combine characteristics of BJT and MOSFET. 
  • The configuration of IGBT consists of BJT and MOSFET in the Darlington configuration. 
  • The input of the IGBT consists of MOSFET whereas the output consists of BJT.
  •  As the current gain of the BJT has low, the voltage rating of the MOSFET should be high for higher power rating. 
  • The input characteristics of the IGBT should be similar to that of power MOSFET whereas the output characteristics similar to that of BJT. 

Structure

Why vertical structure ?
  • The basic structure of the IGBT is shown in the figure A. The structure of the IGBT should be vertical in order to provide maximum area for flow of current. As the on state resistance decreases, the power loss also reduces.
structure of the igbt
  • When we compare the structure of the IGBT with N- channel MOSFET there is additional P+ layer substrate over N+ layer. 
  • This layer makes PN junction diode with drift layer.This layer is called as inject layer because it injects holes in the n- layer. 
  • There are two types of impurities in the n type drift layer.

NON-PUNCH THROUGH IGBT

Lightly doped n- layer : 
  • It is also called as drift layer. The forward blocking voltage in the IGBT depends upon this layer’s doping level and width. It does not affect on state voltage drop due to conductivity modulation.

PUNCH THROUGH IGBT

Heavily doped n+ layer : 
  • It is not necessary for the operation of this layer.It lies between P+ layer and n- layer. 
  • The on state voltage drop , turn off time  and also reverse voltage blocking capacity reduces due to this layer.

Symbol and Equivalent circuit

  • It consist of three terminals namely collector, emitter and gate. 
  • The one cell of IGBT consists of PNP transistor, NPN transistor and driver MOSFET.
PNP transistor
        P+ injecting layer as emitter
        N- drift layer as base
        P body layer as collector
NPN transistor
        N+ region emitter
        P body layer base
        N- drift layer collector


equivalent circuit and symbol of the igbt
Body spreading resistance : 
  • In the NPN transistor base is shorted with emitter but due to lower quality of short, resistance between emitter and base is called body spreading resistance.
  •  If the output current is high, due to high voltage drop across body spreading resistance and pnpn latching process IGBT turns on. The parasitic thyristor is used to solve problem of latching in which doping of the body layer is changed.

Operation

The operation of the IGBT is explained as follows

(A) Inversion layer :

OFF STATE : 
  • When gate-emitter voltage less than the threshold voltage, inversion layer is not created.  
  • The forward voltage between collector to emitter  reverse biased  across junction J2 and only leakage current flow.
ON STATE:  
  • When gate - emitter  voltage greater than threshold voltage inversion layer is created. 
  • Due to this inversion layer conduction channel n+n n- is created, therefore flow of current is possible.
inversion layer in the igbt

(B) Conducting modulation

  • When forward voltage is given to collector to emitter , junction J3 becomes forward biased therefore holes from P+ layer moves towards n+ buffer layer. 
  • The conducting channel is created due to inversion layer resulting resulting there is creation of space charge region in the n- drift layer. 
  • Therefore holes are attracted from n+ buffer layer. The double injection takes place (holes from left and electrons from right) in the n- drift layer therefore its conductivity increases and resistance is decreased. 
  • Therefore due to conducting modulation on state voltage drop across device is reduced.
conductivity modulation in the igbt

Latching problem

  • The drift region (electric field or movement of charge) of the IGBT works as a base of the PNP transistor . 
  • The current gain of the transistor depends upon the width and doping level of the transistor. 
  • If this current gain is kept low, most of the current flow through MOSFET resulting voltage drop across body spreading resistance is decreased and latching problem is solved.  

Static Characteristics  

  • The static characteristics of the IGBT is similar to BJT. 
  • The controlling parameter in the BJT is base current whereas gate to emitter voltage in the IGBT.

Cut off region

  • When the gate-emitter voltage is less than the threshold voltage, the collector to emitter voltage is equal to supply voltage and it is called as cut off region. 
  • The semiconductor device which can withstand forward voltage between collector to emitter is called as forward breaker over voltage BVCE
  • Its value depends upon avalanche breakdown voltage ( high reverse voltage across the junction, it is moderately operated ) of the body – drain  PN junction diode.
  •  There is only leakage current flows through the device. The forward breakdown voltage depends upon collector current in the IGBT.
forward characteristics of the igbt


Active region:

  • When the gate  emitter voltage greater than the threshold voltage, the IGBT operates in the active region. 
  • The collector current depends upon transfer characteristics of the IGBT. 
  • As the gate – emitter voltage increases, the collector current also increases. 
  • The characteristics becomes linear for higher value of collector current. 
  • The ratio of collector current to the gate – emitter voltage is called as forward transconductance.
                  Forward transconductance : Ic / Vge Mho

Saturation region:

  • When the gate  emitter voltage increases, the collector current also increases as shown in the transfer characteristics of the IGBT.  
  • The collector to emitter voltage decreases for a given load resistance RL
  • The collector – emitter voltage ( VCE ) becomes less than the gate – emitter voltage ( VGE ) for a given specific collector current therefore the MOSFET enter in the ohmic region and P+N-P transistor in the saturation region. This is called as saturation region of the device. 
  • The voltage drop across device becomes constant in this region and this voltage drop decreases as the gate – emitter voltage ( VGE ) increases. 
  • The secondary breakdown does not occurs in the IGBT as that of in the BJT. 
  • The reverse voltage blocking capability of the punch through IGBT ( PT IGBT ) is in tens of volt due to heavily doped n+ layer. 
  • The Non punch through IGBT ( NPT IGBT ) can withstand maximum reverse voltage VRB
  • The IGBT always operate in the saturation or cut off region.
transfer characteristics of the igbt

Applications