Insulated Gate Bipolar Transistor
The Insulated Gate Bipolar Transistor (IGBT) is a minority-carrier device with high input impedance and large bipolar current-carrying capability. Many designers view IGBT as a device with MOS input characteristics and bipolar output characteristic that isa voltage-controlled bipolar device. To make use of the advantages of both Power MOSFET and BJT, the IGBT has been introduced.
The basic schematic of a typical N-channel IGBT based upon the DMOS process is shown in Figure. This is one of several structures possible for this device. It is evident that the silicon cross-section of an IGBT is almost identical to that of a vertical Power MOSFET except for the P injecting layer.
It shares similar MOS gate structure and P wells with N source regions. The N layer at the top is the source or emitter andthe P layer at the bottom is the drain or collector. Itis also feasible to make P- IGBTs and for which the doping profile in each layer will be reversed. IGBT has aparasitic thyristor comprising the four-layer NPNP structure. Turn-on of this thyristor is undesirable.
Some IGBTs, manufactured without the N buffer layer, are called non-punch through (NPT) IGBTs whereas those with this layer are called punch-through (PT) IGBTs.
The presence of this buffer layer can significantly improve the performance of the device if the doping level and thickness of this layer are chosen appropriately. Despite physical similarities, the operation of an IGBT is closer to that of a power BJT than a power MOSFET. It is due to the P drain layer (injecting layer) which is responsible for the minority carrier injection into the N- -drift region and the resulting conductivity modulation.
- It has a very low on-state voltage drop due to conductivity modulation and has superior on-state current density. So smaller chip size is possible and the cost can be reduced.
- Low driving power and a simple drive circuit due to the input MOS gate structure. It canbe easily controlled as compared to current controlled devices (thyristor, BJT) in high voltage and high current applications.
- Wide SOA. It has superior current conduction capability compared with the bipolar transistor. It also has excellent forward and reverse blocking capabilities.
- Switching speed is inferior to that of a Power MOSFET and superior to that of a BJT. The collector current tailing due to the minority carrier causes the turnoff speed to be slow.
- There is a possibility of latchup due to the internal PNPN thyristor structure.