Insulated Gate Bipolar Junction Transistor(IGBT)

Bipolar junction transistor have low power losses but have long switching time.(especially at turnoff).MOSFETs have very fast switching characteristics(low turn on and turn off times)but have higher power losses.IGBT combines the advantages of MOSFET and BJT.Thus an IGBT has low switching times as well as low power losses.Its called as IGBT or GEMET or COMFET(conductivity modulated field effect transistor).

Configuration

The configuration of an IGBT is shown in the figure below

In many respects it is similar to a vertical diffused MOSFET(VDMOS).Main difference is the presence of p+ as injecting layer.Next is n+ layer(buffer layer).There is a p-n junction (j1) between these layers and two more junctions(j2 and j3).Thus this IGBT configurations has  a parasitic SCR.Turn  on  of this SCR is undesirable and the body region of the IGBT is made to avoid turn on.The body source short also helps in minimizing the possible turn on.Some IGBTs do not have n+ layer.If n+ layer is absent ,the device is called non punch through IGBT(NPT-IGBT).If this n+ layer is present its punch through IGBT(PT-IGBT)

VI Characteristics

The drain characteristic of IGBT is given below

It shows the relation between drain current iD and drain source voltage VDS for different values of gate source voltageVGS.

The junction j1 blocks reverse voltage.An IGBT without n+ buffer layer has higher reverse blocking capability.Therefore an IGBT required for blocking high reverse voltage does not have n+ buffer layer..The reverse blocking voltage is shown as the VRM on the VI characteristics.The junction j2 blocks the forward voltage when the IGBT is off.BVDSS denotes the break down voltage in the forward direction.The applied voltage drain and source must be less than BVDSS .

The transfer characteristics of IGBT is shown below

When VGS is less than VGS(Th) ,the device is in off state.This curve is almost linear except when drain current is very low.

Written by John on October 17th, 2008 with 2 comments.
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