High Voltage Bipolar Transistor Extended Base Region
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Solution Overview
Problem
High voltage bipolar transistors in integrated circuits face the Kirk effect, leading to a widened base region and increased base current due to recombination sites, which reduces transistor gain and increases base-emitter capacitance when operating at high collector current densities.
Innovation Solution
The fabrication of a high voltage bipolar transistor with an extended base extrinsic region using a silicide block layer or an extended emitter-base dielectric layer, which maintains constant gain without significantly increasing base-emitter capacitance, by laterally separating the emitter-base junction from recombination sites.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the lateral width of the emitter overlap of the base layer is increased to remove recombination sites from the collector current path, then the transistor gain is maintained, but the base-emitter capacitance increases unacceptably
Solution Approach 1:
The patent extends the base layer laterally in a third dimension beyond the emitter edges, creating an extrinsic base region that removes recombination sites from the collector current path without increasing the vertical overlap between emitter and base. This dimensional extension maintains transistor gain while avoiding increased base-emitter capacitance.
2Strength
If lower doping densities are used in the collector to prevent breakdown of base-collector junctions, then high voltage operation is achieved, but the threshold for Kirk effect onset is reduced
Solution Approach 1:
The patent extracts the recombination site region from the collector current path by laterally extending the base layer beyond the emitter edges. This creates an extrinsic base region that collects carriers before they reach high-field regions, preventing Kirk effect onset even at lower collector doping densities required for high voltage operation.
Data Source
AI summary
High voltage bipolar transistors built with a BiCMOS process sequence exhibit reduced gain at high current densities due to the Kirk effect. Threshold current density for the onset of the Kirk effect is reduced by the lower doping density required for high voltage operation. The widened base region at high collector current densities due to the Kirk effect extends laterally into a region with a high density of recombination sites, resulting in an increase in base current and drop in the gain. The instant invention provides a bipolar transistor in an IC with an extended unsilicided base extrinsic region in a configuration that does not significantly increase a base-emitter capacitance. Lateral extension of the base extrinsic region may be accomplished using a silicide block layer, or an extended region of the emitter-base dielectric layer. A method of fabricating an IC with the inventive bipolar transistor is also disclosed.