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High Reliability, High Voltage Power Transistor

IP.com Disclosure Number: IPCOM000084732D
Original Publication Date: 1975-Dec-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 3 page(s) / 35K

Publishing Venue

IBM

Related People

Dumke, WP: AUTHOR

Abstract

A high-voltage power transistor's structure is modified so that the transistor can turn off large currents while driving inductive loads, without experiencing damage due to current crowding at the center of the emitter.

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High Reliability, High Voltage Power Transistor

A high-voltage power transistor's structure is modified so that the transistor can turn off large currents while driving inductive loads, without experiencing damage due to current crowding at the center of the emitter.

The greatest stress encountered in a power transistor occurs when the current is in the process of being switched off. The inductive load of the transformer primary attempts to maintain the current through the transistor at a nearly constant rate. With the device turned off, the voltage between the collector and the base becomes very high and the current becomes an avalanche breakdown current in the N/-/ region of the collector.

The electrons from this breakdown continue on to the collector contact and the holes enter the base region. These holes must laterally traverse the base region to arrive at the base contact, and in so doing produce a fairly large voltage difference between the base at the center of the emitter stripe and the base at the edge of the stripe.

For some power transistors, this voltage difference can be as large as 10 volts. This lateral voltage will result in the reverse biasing of the base-emitter junction at the emitter stripe edge. This reverse bias, however, can be sufficient to break down the base-emitter junction, and the consequent electron current into the emitter will be balanced by an injection current concentrated at the center of the emitter stripe. It is this relatively high-density injection current which leads to sudden breakdown in high-voltage transistors.

Fig. 1 is a cross section of a power transistor constructed to avoid the build- up of such high-density injection current. A silicon wafer 2 is provided that...