Novel buffer engineering: A concept for fast switching and low loss operation of planar IGBT

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Abstract

For the first time, an insulated gate bipolar transistor with a novel buffer is proposed and verified by two-dimensional (2D) mixed device-circuit simulations. The structure of the proposed device is almost identical with that of the conventional IGBT, except for the buffer layer which is formed by employing a three-step, gradually changing doping n+ structure. Compared with the conventional IGBT, the proposed device exhibits better trade-off relation between the conduction and switching losses. The turn-off time is halved from 9.4 μs of the conventional IGBT to 4.5 μs of the proposed device, so the operation speed of the proposed device is greatly improved. Further, the forward blocking voltage is enormously increased from 907 V of the proposed device to 1278 V of the proposed device, which is required for high power operation.

Introduction

The insulated gate bipolar transistor (IGBT) is a promising device for power electronics applications [1], [2], [3], [4], [5] and, therefore, the demand on reducing power loss and increasing operation frequency is rapidly increased. Power loss and operating frequency of the IGBT are decided by the tail-current which occurs when the device is turned off. The major cause of the tail-current is the accumulation of minority carriers in the n drift region in the device. Minority carriers are injected from the collector during on-state and reduce the resistance of the drift region due to the conductivity modulation effect. The tail-current of the IGBT can be suppressed by reducing the minority carrier lifetime in the n drift region of IGBT, which is achieved by electron or neutron irradiation [6], [7]. But this is expensive and damageable.

In this paper, we proposed a new buffer structure for the PT-IGBT which employs a three-step, gradually changing doping n+ structure. Although, the application of such structure to a power diode for fast switch has been reported [8], [9], [10], this is a new proposal of the application of three-step, gradually changing doping structure to the IGBT. Results of two dimensional device simulations demonstrate the proposed IGBT has a better trade-off between on-state voltage and turn-off time, a larger forward blocking voltage and a faster switching speed than the conventional device.

Section snippets

Device structure and operation

Fig. 1(a) and (b) shows the schematic cross-sectional views of the conventional and proposed device structure, respectively. The structure of the proposed device is almost the same as that of the conventional IGBT, except for the buffer layer employing a three-step, gradually changing doping n+ structure. Buffer region (N1-N2-N3) is still 10 μm and W1, W2, W3 is 3, 3 and 4 μm, respectively. The three-step doping concentration increases in the buffer region from the n drift region boundary to the

Device performance

Numerical simulations were performed using the 2D device simulator MEDICI. This simulator performs calculations of the 2D plane structure per unit width (1 μm) in the perpendicular direction. Effective width of the device is an independent parameter. The vertical axis is the collector current flowing through the device area of 30 μm(width)×1 μm(depth). In the actual device, the unit cell shown in Fig. 1 is arrayed. Therefore, when we assume the devices turn on if the collector current density

Discussions and conclusions

Now concerning the disadvantages of IGBT, the major penalty paid by this hybridization of MOS and bipolar properties, is the slower switching of the IGBT compared to that of the power MOSFET. Nevertheless, an interesting feature of IGBT is that its turn-off time can be decreased by using a three-step, gradually changing doping n+ buffer layer at the expenditure of an increase in forward voltage drop. This unique capability endows the IGBT with the valuable opportunity of trading off between

Acknowledgements

The current work is supported by a grant from the National High Technology Research and Development Program of China (863 Program) (2003AA834025) and funded by the national natural science foundation of China (10474076).

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