Features of IGBT
Features of IGBT:
- The introduction of Power MOSFET was originally regarded as a major threat to the power bipolar transistor. However, initial claims of infinite current gain for the power MOSFETs were diluted by the need to design the gate drive circuit capable of supplying the charging and discharging current of the device input capacitance.
- This is especially true in high frequency circuits where the power MOSFET is particularly valuable due to its inherently high switching speed.
- On the other hand, MOSFETs have a higher on state resistance per unit area and consequently higher on state loss. This is particularly true for higher voltage devices (greater than about 500 volts) which restricted the use of MOSFETs to low voltage high frequency circuits (eg. SMPS).
- With the discovery that power MOSFETs were not in a strong position to displace the BJT, many researches began to look at the possibility of combining these technologies to achieve a hybrid device which has a high input impedance and a low on state resistance.
- The obvious first step was to drive an output npn BJT with an input MOSFET connected in the Darlington configuration. However, this approach required the use of a high voltage power MOSFET with considerable current carrying capacity (due to low current gain of the output transistor).
- Since no path for negative base current exists for the output transistor, its turn off time also tends to get somewhat larger. An alternative hybrid approach was investigated at GE Research center where a MOS gate structures was used to trigger the latch up of a four layer thyristor. However, this device was also not a true replacement of a BJT since gate control was lost once the thyristor latched up.
- After several such attempts it was concluded that for better results MOSFET and BJT technologies are to be integrated at the cell level. This was achieved by the GE Research Laboratory by the introduction of the device IGT and by the RCA research laboratory with the device COMFET.
- The IGT device has undergone many improvement cycles to result in the modern Insulated Gate Bipolar Transistor (IGBT).
- These devices have near ideal characteristics for high voltage (> 100V) medium frequency (< 20 kHZ) applications.
- These devices along with the MOSFET (at low voltage high frequency applications) have the potential to replace the BJT completely.