BJT Base Conductor Pullback Reduces Junction Resistance
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Solution Overview
Problem
Conventional vertical bipolar junction transistors (BJTs) have a small contact area between the base region and the base polysilicon layer, leading to high contact resistance and limited gain due to a rounded recess surface, which restricts the collector/base junction area and increases resistance.
Innovation Solution
The recess in which the base region is grown extends under and laterally into the base polysilicon layer, increasing the contact area between the base region and the conductive base layer, and forming a planar bottom surface to enhance the contact area between the base and collector regions, thereby reducing resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional vertical BJT structure is used with a recess in the base polysilicon layer, then the base region can be formed, but the contact area between the base region and base polysilicon layer is small leading to high contact resistance
Solution Approach 1:
The recess is extended not only vertically but also laterally under the base polysilicon layer, transforming a one-dimensional vertical contact into a three-dimensional contact volume. This dimensional expansion significantly increases the contact area between the base region and base polysilicon layer, thereby reducing contact resistance without increasing the device footprint.
Solution Approach 2:
The base region is nested within the recess that extends under the base polysilicon layer, creating a nested structure where the base region sits within the recessed area. This nesting arrangement maximizes the contact interface between the base region and the conductive base layer while maintaining a compact structure.
2Reliability
If a conventional recess structure is used, then the base region can be formed, but the rounded recess surface limits the collector/base junction area and increases resistance
Solution Approach 1:
The invention intentionally creates a planar (flat) bottom surface in the recess rather than a rounded surface. This planar geometry maximizes the collector/base junction area by providing a flat interface for the base region to contact the conductive base layer, thereby reducing resistance at this critical junction.
3Area of stationary object
If a dielectric layer is placed below the conductive base layer, then isolation is provided, but it reduces the contact area between the base region and conductive base layer
Solution Approach 1:
The invention extracts (removes) the dielectric layer that was previously placed below the conductive base layer. By eliminating this intermediate dielectric layer, the base region can directly contact the conductive base layer over a larger area, significantly reducing contact resistance. The recess structure provides sufficient isolation without requiring the additional dielectric layer.
Data Source
AI summary
Some embodiments are directed to a bipolar junction transistor (BJT) with a collector region formed within a body of a semiconductor substrate, and an emitter region arranged over an upper surface of the semiconductor substrate. The BJT includes a base region arranged over the upper surface of the semiconductor substrate, which vertically separates the emitter and collector regions. The base region is arranged within, and in contact with, a conductive base layer, which delivers current to the base region. The base region includes a planar bottom surface, which increases contact area between the base region and the semiconductor substrate, thus decreasing resistance at the collector/base junction, over some conventional approaches. The base region can also include substantially vertical sidewalls, which increases contact area between the base region and the conductive base layer, thus improving current delivery to the base region.