SiGe Heterojunction Bipolar Transistor Cavity Base
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Solution Overview
Problem
High collector-base capacitance (Ccb) and base resistance (Rb) in Si/SiGe heterojunction bipolar transistors limit device scaling and performance, particularly in power amplifier applications, due to complex integration schemes and lack of an etch-stop layer for self-aligned emitter-base junctions.
Innovation Solution
The structure includes a collector region with a marker layer, a doped extrinsic base layer, an epitaxial intrinsic base layer within a cavity, and an emitter material over the intrinsic base, utilizing selective epitaxial growth and sidewall spacer structures to control cavity depth and reduce parasitic capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the emitter is formed inside a hole etched into the extrinsic base layer, then the heterojunction bipolar transistor can be formed, but the integration scheme becomes complicated due to lack of an etch-stop layer for self-aligned emitter-base junction
Solution Approach 1:
An etch-stop layer is formed between the extrinsic base and the cavity before emitter formation. This preliminary action enables the subsequent self-aligned etching process to stop precisely at the desired location, achieving self-aligned emitter-base junction without complicated integration schemes.
2Reliability
If conventional integration schemes are used, then the transistor structure can be formed, but high collector-base capacitance and base resistance result which limits device scaling
Solution Approach 1:
The base region is segmented into an extrinsic base layer and an intrinsic base layer within the cavity. This segmentation allows the extrinsic base to provide mechanical support and doping while the intrinsic base in the cavity minimizes parasitic capacitance, enabling device scaling without sacrificing performance.
Solution Approach 2:
The base region is extended into the vertical dimension by forming a cavity that penetrates through the extrinsic base layer. This dimensional change allows the intrinsic base to be positioned deeper, reducing the horizontal overlap between base and collector, thereby minimizing collector-base capacitance and enabling scaling.
3Manufacturing precision
If emitter-first self-aligned schemes are used, then alignment can be achieved, but many challenges including lack of end-point signal for emitter etch, difficult spacer formation, and uneven top surface result
Solution Approach 1:
The etch-stop layer acts as an intermediary between the extrinsic base and the cavity. It provides a clear end-point signal during etching, facilitates easy spacer formation, and creates a planar surface for subsequent processing, thereby solving multiple manufacturing challenges while maintaining self-aligned precision.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach results in lower collector-base capacitance and improved process control, enabling higher cut-off frequencies and breakdown voltage, thus enhancing the performance of SiGe heterojunction bipolar transistors.
Implementation Method 1
at least one marker layer over the collector region... a cavity formed in the layer of doped semiconductor material and extending at least to the at least one marker layer
Implementation Method 2
a plurality of layers of epitaxially semiconductor material over the substrate with at a lower layer of the plurality of layers of epitaxially semiconductor material having a different removal selectivity to at least an upper layer
Data Source
AI summary
The present disclosure relates to semiconductor structures and, more particularly, to heterojunction bipolar transistors and methods of manufacture. The structure includes a collector region composed of semiconductor material; at least one marker layer over the collector region; a layer of doped semiconductor material which forms an extrinsic base and which is located above the at least one marker layer; a cavity formed in the layer of doped semiconductor material and extending at least to the at least one marker layer; an epitaxial intrinsic base layer of doped material located within the cavity; and an emitter material over the epitaxial intrinsic base layer and within an opening formed by sidewall spacer structures.