AN-7505| Application Note

Improved IGBTs with Fast Switching Speed And High-Current Capability
Application Note December 1993 AN-7505

Abstract /Title AN75 5) Subect Impro ed GBTs ith ast witch ng peed nd ighurent apaility) Autho () Keyords Interil orpoation, emionuctor, Cretor () DOCI FO dfark
Conventional vertical power MOSFETs are limited at high voltages (>500V) by the appreciable resistance of their epitaxial drain region. In a new MOS-gate controlled device called an IGBT, this limitation is overcome by modulating the conductivity of the resistive drain region, thereby reducing the on-resistance of the device by a factor of at least 10. However, the device previously described is slow in turnoff, having a fall time in the range 8 to 40us. The purpose of our present work has been to reduce the fall time significantly and to increase the latching current level of the IGBTs, while retaining its desirable features. By modification of the epitaxial structure and addition of recombination centers, we have achieved fall times as low as 0.1us and latching currents as high as 50A, while retaining on-resistance values <0.2 for a 0.09cm2 chip area. The techniques used for the introduction of recombination centers include electron, gamma-ray, and neutron irradiation, as well as heavy metal doping. For a
series of IGBTs (with forward-blocking voltage capabilities of 400-600V), the fall time can be reduced by more than one order of magnitude with a penalty of less than a 20% increase in on-resistance. The maximum operating current is limited by latchup of the parasitic thyristor that is inherent in the device structure. Typical latching current levels of IL, < 10A were observed in 0.09cm2 area devices when the gate voltage was turned off rapidly (<1ms); for slower gate voltage turnoff (~10ms), IL values as high as ~30A were observed. The purpose of the present work has been to reduce tF and to increase IL while retaining the desirable features of the device. By modifying the epitaxial structure and adding recombination centers to the epitaxial drain region, we have achieved tF values as low as 100ns and lL values as high as 50A with rapid gate voltage turnoff.

Modified Structure
A schematic diagram of the original IGBT structure[4] is shown in Figure 1(a), and the equivalent circuit is shown in Figure 1(b); they are similar to those of an MOS-gated thyristor except for the presence of the shunting resistance Rs in each unit cell. The fabrication is like that of a standard n-channel power MOSFET, except that the n-epitaxial layer is grown on a p+ substrate instead of an n+ substrate. The heavily doped p+ region in the center of each unit cell, combined with the aluminum contact shorting the n+ and p+ regions, provides the shunting resistance RS. This has the effect of lowering the current gain of the n-p-n transistor in the equivalent circuit so that npn + pnp <1, thereby preventing latching over a large operating range of anode voltage VA and anode current iA. However, for sufficiently large iA, emitter injection in the n-p-n transistor will increase, accompanied by an increase in npn. When npn + pnp increases to 1, the four-layer device will latch; the level of i A at which this oc
curs is the latching current level, IL. Thus, it can be seen that a structure modification that lowers pnp will allow a greater range of iA (and npn) without latching; that is, a reduction in pnp corresponds to an increase in IL. The modified structure shown in Figure 1(C) differs from that in Figure 1(A) by the addition of a thin (~10mm) layer of n+ silicon in the epitaxial structure between the n- region and the p+ substrate. This n+ layer lowers the emitter injection efficiency of the p-n-p transistor in the equivalent circuit, and results in an increase in IL by a factor of 2 to 3. In addition, there is also a reduction in t F. These results are illustrated in Figure 2, in which tF, is plotted versus iA for each device structure. It should be noted that IGBTs with the modified structure can block high voltage only in the forward voltage direction since the emitter junction (p+ - n+) of the p-n-p transistor breaks down at a low level when the polarity of the applied voltage is reversed.

Introduction
Vertical MOSFETs have become increasingly important in discrete power device applications due primarily to their high input impedance, rapid switching times and low on-resistance. However, the on-resistance of such devices increases with increasing drain-source voltage capability,[1-3] thereby limiting the practical value of power MOSFETs to applications below a few hundred volts. This limitation has been effectively overcome by the development of a new MOS power device in which the conductivity of the n-type epitaxial drain region is greatly increased (modulated) by the injection of minority carriers from a p-type substrate. We have called this device a COMFET-an acronym for COnductivity Modulated Field Effect Transistor;[4] the device has also been called an IGBT or Insulated Gate Bipolar Transistor. The devices, as originally described, had most of the advantages of conventional power MOSFETs; in addition, they exhibited more than an order-of-magnitude reduction in high current on-resistance values, permi
tting improved utilization of silicon chip area. However, they also had two disadvantages: When a IGBT is turned off, the injected minority carriers that remain in the epitaxial drain region decay by recombination with majority carriers at a rate determined by the minority-carrier lifetime, F. Large values of resulted in anode-current fall time, tF, in the range 8-40ms. [4,5]

2002 Fairchild Semiconductor Corporation

Application Note 7505 Rev. A1