Bipolar Transistor Base Contact via Lateral Epitaxy
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Solution Overview
Problem
Forming a bipolar transistor with low resistance access to the base region and low stray capacitance between the base and collector is challenging due to difficulties in creating a base contact region effectively in existing manufacturing methods.
Innovation Solution
A method involving a single-crystal silicon substrate with a stack of layers, including a sacrificial layer, where a collector and base region are formed by selective epitaxy, and a base contact region is created through epitaxy from the side of the base semiconductor region, with a subsequent etching process to form a conductive path, reducing resistance and capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a base contact region is formed to reduce access resistance, then the resistance to the base region is reduced, but the stray capacitance between the base and collector increases
Solution Approach 1:
The base contact region is formed by lateral epitaxial growth from the side wall of the base region rather than from the top surface. This side-wall formation approach allows the contact region to extend vertically along the base region's side, creating a three-dimensional contact structure that reduces resistance without increasing the horizontal overlap area with the collector, thus minimizing stray capacitance.
Solution Approach 2:
An insulating layer is introduced between the base contact region and the collector region. This intermediary insulating layer electrically isolates the base contact from the collector, preventing direct capacitive coupling while allowing the base contact region to maintain low resistance access to the base region through lateral and vertical epitaxial growth.
2Ease of manufacture
If the base contact region is formed from the top surface, then the manufacturing process is simpler, but the stray capacitance between base and collector increases
Solution Approach 1:
The insulating layer is deposited on the base region before forming the base contact region. This preliminary action prepares the surface for selective epitaxial growth, ensuring that the base contact will form only in the desired lateral direction from the side wall, preventing vertical growth that would increase capacitance while maintaining manufacturing feasibility through standard epitaxial processes.
Solution Approach 2:
The epitaxial growth conditions are controlled to enable lateral growth preferentially at the side wall of the base region where the insulating layer is present, while suppressing vertical growth. This local quality control ensures the base contact region forms with the optimal geometry for low resistance and low capacitance, with different growth characteristics at different locations.
3Reliability
If a complex stack of layers with sacrificial material is used, then the base contact region can be formed with low resistance and low capacitance, but the device complexity increases
Solution Approach 1:
A sacrificial layer is removed from the stack of layers to create a localized opening or modified region that enables selective epitaxial growth of the base contact region. By extracting this sacrificial material, the process achieves precise control over where the base contact forms, enabling low resistance and low capacitance characteristics without requiring the entire complex stack to remain in place, as the sacrificial layer is removed after serving its purpose.
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 method enables the formation of a bipolar transistor with reduced base-collector capacitance and access resistance, improving the transistor's performance for high-frequency applications by minimizing stray capacitance and optimizing the base contact region.
Implementation Method 1
a stack of layers comprising a sacrificial layer made of a first material arranged between two insulating layers made of a second material selectively etchable over the first material
Implementation Method 2
in the opening, forming by selective epitaxy, all the way to a level between the upper level of the first silicon layer and the lower level of the sacrificial layer, a collector region made of doped silicon
Implementation Method 3
forming a base contact region by epitaxy from the side of the base semiconductor region
Data Source
AI summary
A bipolar transistor is supported by a single-crystal silicon substrate including a collector contact region. A first epitaxial region forms a collector region of a first conductivity type on the collector contact region. A second epitaxial region forms a base region of a second conductivity type. Deposited semiconductor material forms an emitter region of the first conductivity type. The collector region, base region and emitter region are located within an opening having sidewalls lined with an insulating sheath. A portion of the insulating sheath adjacent the base region is removed and a base contact region is formed by epitaxial material grown from a portion of the base region exposed by removal of the portion of the insulating sheath.