Bipolar Transistor Cavity Structure for Lower Base Resistance
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Solution Overview
Problem
Bipolar transistors with an in-situ doped extrinsic base formed of polycrystalline silicon often exhibit undesirably high resistance, particularly when the structure is on top of a dielectric layer, which affects maximum frequency and signal noise.
Innovation Solution
A bipolar transistor structure is designed with an extrinsic base protruding from an intrinsic base and extending over a cavity, where an insulator is horizontally adjacent to the cavity and below a portion of the extrinsic base, allowing a collector extension region to extend laterally below the insulator and the cavity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If an in-situ doped extrinsic base formed of polycrystalline silicon is used, then the base can be formed on a dielectric layer, but the resistance becomes undesirably high
Solution Approach 1:
The patent introduces a cavity beneath the extrinsic base, transitioning from a planar structure to a three-dimensional structure with vertical separation. This dimensional change allows the base to be suspended over the cavity, reducing the effective resistance path while maintaining the ability to form on a dielectric layer.
Solution Approach 2:
The patent extracts the dielectric material directly beneath the extrinsic base to create a cavity. This removal of the insulating layer beneath the base reduces the parasitic resistance and base-collector capacitance, directly addressing the resistance problem while preserving the overall manufacturing approach.
2Device complexity
If the extrinsic base extends over a dielectric layer, then the structure is simplified, but the base-collector junction capacitance increases
Solution Approach 1:
The dielectric material is extracted from beneath the extrinsic base to form a cavity. This removal eliminates the capacitive coupling between the base and the underlying dielectric, reducing base-collector junction capacitance while maintaining structural simplicity through the suspended base configuration.
Solution Approach 2:
The cavity acts as an intermediary space between the extrinsic base and the underlying structures. This void space provides electrical isolation and reduces parasitic capacitance effects, serving as a mediator that improves performance without adding complex structural elements.
Data Source
AI summary
The disclosure provides bipolar transistor structures with a cavity below an extrinsic base, and methods to form the same. A structure of the disclosure provides a bipolar transistor structure including an extrinsic base protruding from an intrinsic base of a bipolar transistor. The extrinsic base extends over a cavity. An insulator is horizontally adjacent the cavity and below a portion of the extrinsic base. A collector extension region of the bipolar transistor structure extends laterally below the insulator and the cavity.