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Power Transistor with Reverse Biased Second Breakdown Capability

IP.com Disclosure Number: IPCOM000086077D
Original Publication Date: 1976-Jul-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 51K

Publishing Venue

IBM

Related People

Wang, PP: AUTHOR

Abstract

In order to shorten the base current path in power transistors, various configurations have been proposed to maximize the perimeter-to-area ratio of the emitter. One example is a comb-like configuration having laterally extending emitter fingers. Even then, current crowding occurs in the center of the emitter fingers when the power transistor is switched off by reverse base current. This is because of the emitter-base junction de-biasing effect caused by the reverse base current, whereby the emitter-current density is maximum at the center of the emitter areas.

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Power Transistor with Reverse Biased Second Breakdown Capability

In order to shorten the base current path in power transistors, various configurations have been proposed to maximize the perimeter-to-area ratio of the emitter. One example is a comb-like configuration having laterally extending emitter fingers. Even then, current crowding occurs in the center of the emitter fingers when the power transistor is switched off by reverse base current. This is because of the emitter-base junction de-biasing effect caused by the reverse base current, whereby the emitter-current density is maximum at the center of the emitter areas.

When this maximum peak current density exceeds the space-charge limited current density, the electric field inverts in the collector-base region. Avalanche injection occurs when the maximum current density is sufficiently large causing the electric field to approach zero at the (metallurgical or current-induced) base- collector junction, and the maximum field to be located at the high-low (N/+/-N) junction. Under this condition devices exhibit negative resistance and cause reverse-biased second breakdown.

This problem is especially severe in the center of the emitter contact area. This area is designed for wire bonding purposes and is equivalent to a very wide emitter finger. The current is crowded there and can cause failure.

To eliminate this mode of failure, the illustrated transistor configuration is made by masking the center emitter con...