Igbt

AN-983 (v.Int) (HEXFET® is a trademark of International Rectifier)

IGBT Characteristics

Topics covered: How the IGBT complements the MOSFET Silicon structure and equivalent circuit Conduction characteristics and “switchback” Switching characteristics Latching Safe Operating Area Transconductance How to read the data sheet Families of IGBTs

1. HOW THE IGBT COMPLEMENTS THE POWER MOSFETSwitching speed, peak current capability, ease of drive, wide SOA, avalanche and dv/dt capability have made power MOSFETs the logical choice in new power electronic designs. These advantages, a natural consequence of being majority carrier devices, are partly mitigated by their conduction characteristics which are strongly dependent on temperature and voltage rating. Furthermore, as the voltagerating goes up, the inherent reverse diode displays increasing Qrr and Trr which leads to increasing switching losses. IGBTs on the other hand, being minority carrier devices, have superior conduction characteristics, while sharing many of the appealing features of power MOSFETs such as ease of drive, wide SOA, peak current capability and ruggedness. Generally speaking, the switching speed of an IGBTis inferior to that of power MOSFETs. However, as detailed in INT-990 Sec VIII, a new line of IGBTs from International Rectifier has switching characteristics that are very close to those of power MOSFETs, without sacrificing the much superior conduction characteristics. The absence of the integral reverse diode gives the user the flexibility of choosing an external fast recovery diode to match aspecific requirement or to purchase a “co-pak”, i.e. an IGBT and a diode in the same package. The lack of an integral diode can be an advantage or a disadvantage, depending on the frequency of operation, cost of diodes, current requirement, etc.

GATE POLYSILICON

OXIDE

EMITTER N+ PP+ rb N- EPI N+ BUFFER LAYER G P+ SUBSTRATE COLLECTOR (a) DEVICE STRUCTURE E EMITTER rb N+ C COLLECTOR

rb(b) Device Symbol

(c) EQUIVALENT CIRCUIT

Figure 1. Silicon cross-section of an IGBT with its equivalent circuit and symbol (N-Channel, enhancement mode). The terminal called collector is, actually, the emitter of the PNP. In spite of its similarity to the cross-section of a power MOSFET, operation of the two transistors is fundamentally different, the IGBT being a minority carrier device.AN-983 (v.Int)

2. SILICON STRUCTURE AND EQUIVALENT CIRCUIT
Except for the P+ substrate, the silicon cross-section of an IGBT (Figure 1) is virtually identical to that of a power MOSFET. Both devices share a similar polysilicon gate structure and P wells with N+ source contacts. In both devices the N- type material under the P wells is sized in thickness and resistivity to sustain the fullvoltage rating of the device. However, in spite of the many similarities, the physical operation of the IGBT is closer to that of a bipolar transistor than to that of a power MOSFET. This is due to the P+ substrate which is responsible for the minority carrier injection into the N-region and the resulting conductivity modulation. In a power MOSFET, which does not benefit from conductivitymodulation, a significant share of the conduction losses occur in the N-region, typically 70% in a 500V device. As shown in the equivalent circuit of Figure 1, the IGBT consists of a PNP driven by an N-Channel MOSFET in a pseudoDarlington configuration. The JFET has been included in the equivalent circuit to represent the contriction in the flow of curr ent between adjacent P-wells. The cell density ofthe MOSFET structure is higher than that of a high-voltage, comparable technology MOSFET and, consequently, has better Resistance-Area product. The base region of the PNP is not brought out and the emitter-base PN junction, spanning the entire extension of the wafer cannot be terminated nor passivated. This influences the turn-off and reverse blocking behavior of the IGBT, as will be explained...