DOUBLE BALLASTED EPI BASE TRANSISTOR CELL
Original Publication Date: 1984-Apr-01
Included in the Prior Art Database: 2001-Oct-12
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.