Browse Prior Art Database

DOUBLE BALLASTED EPI BASE TRANSISTOR CELL

IP.com Disclosure Number: IPCOM000005531D
Original Publication Date: 1984-Apr-01
Included in the Prior Art Database: 2001-Oct-12
Document File: 2 page(s) / 122K

Publishing Venue

Motorola

Related People

Robert S. Wrathall: AUTHOR

Abstract

Epitaxial base power transistors are fabricated on starting material with a thick, 7-15 micron, epitaxial layer on the front surface. The epitaxial layer is of opposite doping from the substrate which forms the collector of the device. The emitter is diffused from the top surface as is a base contact diffusion in the case of a PNP transistor. (NPN transistors with a P type epitaxial layers do not generally need such a contact diffusion.) A substantial problem in bipolar transistors is the development of hot spots under high power dissipation. The negative temperature coefficient of the emitter-base junction forward voltage can produce hot spots. These hot spots can produce a thermal runaway problem ending in destruction of the device. In order to minimize this adverse device characteristic, resistor ballasting has been traditionally employed in conjunction with the parasitic, naturally occurring ballasting associated with the contacts and diffusions of the transistor.

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:'$$ MOTOROLA Technical Developments

/ Volume4 April 1984

DOUBLE BALLASTED EPI BASE TRANSISTOR CELL

by Robert S. Wrathall

Semiconductor Research and Development Laboratories

5005 E. McDowell Rd, Mail Stop A170

   Epitaxial base power transistors are fabricated on starting material with a thick, 7-15 micron, epitaxial layer on the front surface. The epitaxial layer is of opposite doping from the substrate which forms the collector of the device. The emitter is diffused from the top surface as is a base contact diffusion in the case of a PNP transistor. (NPN transistors with a P type epitaxial layers do not generally need such a contact diffusion.)

   A substantial problem in bipolar transistors is the development of hot spots under high power dissipation. The negative temperature coefficient of the emitter-base junction forward voltage can produce hot spots. These hot spots can produce a thermal runaway problem ending in destruction of the device. In order to minimize this adverse device characteristic, resistor ballasting has been traditionally employed in conjunction with the parasitic, naturally occurring ballasting associated with the contacts and diffusions of the transistor.

   The resistive ballasting provides some negative feedback to partially oppose the positive tempera- ture coefficient of the emitter-base junction. As a hot spot develops and conducts more current, the added current causes a larger voltage drop across the ballast resistor, resulting in a more evenly distributed current over the area of the power transistor.

   Figure 1 shows a transistor layout which distributes the ballast resistors around each individual emitter cell. Two types of ballasting are designed into the device, base ballasting, represented by the resi...