that will cause a large enough voltage drop to activate the thyristor. Hence, the device manufacturer specifies the peak allowable collector current that can flow without latch-up occurring. There is also a corresponding gate source voltage that permits this current to flow that should not be exceeded. Like the power MOSFET, the IGBT does not exhibit the secondary breakdown phenomenon common to bipolar transistors. However, care should be taken not to exceed the maximum power dissipation and specified maximum nction temperature of the device under all conditions for guaranteed reliable operation. The on- tate voltage of the IGBT is heavily dependent on the gate voltage. To obtain a low on-state voltage, a sufficiently high gate voltage must be applied In general, IGBTs can be classified as punch EMITTERN GATE EMITTER N+ GATE through(PT) and nonpunch-through(NPT) struc- Ires, as shown in Fig. 30.6. In the PT IGBT, an N uffer layer is normally introduced between the P+ substrate and the N-epitaxial layer, so that the whole N- drift region is depleted when the device is blocking N-Base or Epitaxial Drift Regis the off-state voltage, and the electrical field shape Base or Epitaxial Drift Regi gion is close to a rectangular punch-through IGBT, a better trade-off between the Substrate forward voltage drop and turn-off time can be achieved. PT IGBTs are available up to about 1200 V. COLLECTOR High voltage IGBTs are realized through non punch-through process. The devices are built ona n- wafer substrate which serves as the n- base drift region. Experimental NPT IGBTs of up to about 4 KV have been reported in the literature. NPT IGBTs more robust than PT IGBTs particularly under short circuit conditions. But NPT IGBTs have a higher for ward voltage drop than the PT IGBTs The PT IGBTs cannot be as easily paralleled as MOSFETs. The factors that inhibit current sharing of parallel-connected IGBTs are (1)on-state current unbalance, caused by Va(sat) distribution and main FIGURE 30.6 Nonpunch-through IGBT,(b)Punch- circuit wiring resistance distribution, and(2)current through IGBT, (c)IGBT equivalent circuit unbalance at turn-on and turn-off, caused by the switching time difference of the parallel connected devices and circuit wiring inductance distribution. The NPT IGBTs can be paralleled because of their positive temperature coefficient property MOS-Controlled Thyristor (MCT The MCT is a new type of power semiconductor device that combines the capabilities of thyristor voltage and current with MOS gated turn-on and turn-off. It is a high power, high frequency, low conduction drop and ugged device, which is more likely to be used in the future for medium and high power applications. A cross sectional structure of a p-type MCT with its circuit schematic is shown in Fig. 30.7. The MCT has a thyristor type structure with three junctions and PNPn layers between the anode and cathode In a practical MCT, about 00,000 cells similar to the one shown are paralleled to achieve the desired current rating. MCT is turned on by a negative voltage pulse at the gate with respect to the anode, and is turned off by a positive voltage pulse The MCT was announced by the General Electric R D Center on November 30, 1988. Harris Semiconductor Corporation has developed two generations of p-MCTs Gen-1 p-MCTs are available at 65 A/1000 V and 75A/600 V with peak controllable current of 120 A Gen-2 p-MCTs are being developed at similar current and voltage ratings, with much improved turn-on capability and switching speed. The reason for developing p-MCT is the fact that the current density that can be turned off is 2 or 3 times higher than that of an n-MCT; but n-MCTs are the ones needed for many practical applications. Harris Semiconductor Corporation is in the process of developing n-MCTS, which are expected to be commercially available during the next one to two years C 2000 by CRC Press LLC© 2000 by CRC Press LLC that will cause a large enough voltage drop to activate the thyristor. Hence, the device manufacturer specifies the peak allowable collector current that can flow without latch-up occurring. There is also a corresponding gate source voltage that permits this current to flow that should not be exceeded. Like the power MOSFET, the IGBT does not exhibit the secondary breakdown phenomenon common to bipolar transistors. However, care should be taken not to exceed the maximum power dissipation and specified maximum junction temperature of the device under all conditions for guaranteed reliable operation. The on￾state voltage of the IGBT is heavily dependent on the gate voltage. To obtain a low on-state voltage, a sufficiently high gate voltage must be applied. In general, IGBTs can be classified as punch￾through (PT) and nonpunch-through (NPT) struc￾tures, as shown in Fig. 30.6. In the PT IGBT, an N+ buffer layer is normally introduced between the P+ substrate and the N– epitaxial layer, so that the whole N– drift region is depleted when the device is blocking the off-state voltage, and the electrical field shape inside the N– drift region is close to a rectangular shape. Because a shorter N– region can be used in the punch-through IGBT, a better trade-off between the forward voltage drop and turn-off time can be achieved. PT IGBTs are available up to about 1200 V. High voltage IGBTs are realized through non￾punch-through process. The devices are built on a N– wafer substrate which serves as the N– base drift region. Experimental NPT IGBTs of up to about 4 KV have been reported in the literature. NPT IGBTs are more robust than PT IGBTs particularly under short circuit conditions. But NPT IGBTs have a higher for￾ward voltage drop than the PT IGBTs. The PT IGBTs cannot be as easily paralleled as MOSFETs. The factors that inhibit current sharing of parallel-connected IGBTs are (1) on-state current unbalance, caused by VCE(sat) distribution and main circuit wiring resistance distribution, and (2) current unbalance at turn-on and turn-off, caused by the switching time difference of the parallel connected devices and circuit wiring inductance distribution. The NPT IGBTs can be paralleled because of their positive temperature coefficient property. MOS-Controlled Thyristor (MCT) The MCT is a new type of power semiconductor device that combines the capabilities of thyristor voltage and current with MOS gated turn-on and turn-off. It is a high power, high frequency, low conduction drop and a rugged device, which is more likely to be used in the future for medium and high power applications. A cross sectional structure of a p-type MCT with its circuit schematic is shown in Fig. 30.7. The MCT has a thyristor type structure with three junctions and PNPN layers between the anode and cathode. In a practical MCT, about 100,000 cells similar to the one shown are paralleled to achieve the desired current rating. MCT is turned on by a negative voltage pulse at the gate with respect to the anode, and is turned off by a positive voltage pulse. The MCT was announced by the General Electric R & D Center on November 30, 1988.Harris Semiconductor Corporation has developed two generations of p-MCTs. Gen-1 p-MCTs are available at 65 A/1000 V and 75A/600 V with peak controllable current of 120 A. Gen-2 p-MCTs are being developed at similar current and voltage ratings, with much improved turn-on capability and switching speed. The reason for developing p-MCT is the fact that the current density that can be turned off is 2 or 3 times higher than that of an n-MCT; but n-MCTs are the ones needed for many practical applications. Harris Semiconductor Corporation is in the process of developing n-MCTs, which are expected to be commercially available during the next one to two years. FIGURE 30.6 Nonpunch-through IGBT, (b) Punch￾through IGBT, (c) IGBT equivalent circuit