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LATERAL PROFILED BIPOLAR TRANSISTOR AND IC's

IP.com Disclosure Number: IPCOM000005507D
Original Publication Date: 1983-Mar-01
Included in the Prior Art Database: 2001-Oct-10
Document File: 2 page(s) / 80K

Publishing Venue

Motorola

Related People

Herb Goronkin: AUTHOR

Abstract

In bipolar junction transistors, the base current flowing between the intrinsic base region under the emitter (A) to the extrinsic base region (B) in Figure 1 causes a voltage difference VAB' across the resistor rb.' The magnitude of emitter current is controlled by the emitter-base voltage. Since the voltage VEB' is greater than VEA, current at the emitter edge exceeds current at the emitter center (EA). This can be seen by comparing the two exponentials 'EA - ew WEB. Ig (rb + rb')] lEB' - exP [VEB . lBrb] showing how the current at point B' can greatly exceed the current at point A [l].

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Volume 3 March 1983

LATERAL PROFILED BIPOLAR TRANSISTOR AND ID's

By Herb Goronkin

    In bipolar junction transistors, the base current flowing between the intrinsic base region under the emitter (A) to the extrinsic base region (B) in Figure 1 causes a voltage difference VAB' across the resistor rb.'

   The magnitude of emitter current is controlled by the emitter-base voltage. Since the voltage VEB' is greater than VEA, current at the emitter edge exceeds current at the emitter center (EA). This can be seen by comparing the two exponentials

'EA - ew WEB. Ig (rb + rb')]

lEB' - exP [VEB . lBrb]

showing how the current at point B' can greatly exceed the current at point A [l].

   This "emitter crowding" is important at high injection levels because the current is injected primarily at the edges of the emitter, e.g., in high speed bipolar transistors. In these applications the emitter edge is designed to be long compared to the emitter area. This generally involves difficult lithographic constraints on device layout in order to minimize total emitter area which minimizes emitter-base capacitance. In general, the emitter is designed to be as narrow as possible. The stripe region of emitter located inside the "edges" becomes increasingly parasitic with increasing injection level and results in deterioration of high speed performance.

   The Focused Ion Beam (FIB) can be used to laterally dope the base region to reduce the effects of "emitter crowding." There is presently no other way to provide controlled lateral profiles than by FIB. For example, consider the example [l] where base current varies as

IB (Y) = lBY/L

y being the...