Vertical BJT Emitter Cavity Structure for Lower Base-Emitter Capacitance
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Solution Overview
Problem
Bipolar junction transistor (BJT) designs that aim to decrease base-emitter capacitance (Cbe) typically result in larger device sizes and degradation of other performance metrics such as maximum oscillation frequency (fmax) or breakdown voltage (BV).
Innovation Solution
A semiconductor structure with a bipolar junction transistor (BJT) featuring a dielectric layer and emitter regions with specific geometries, including a first emitter portion extending vertically through the dielectric layer and a second emitter portion extending laterally, where the dielectric and additional dielectric layers are wider than the first emitter portion, creating cavities that can be filled with dielectric material or left as air/gas pockets to reduce Cbe without affecting device size or other performance metrics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If techniques are used to decrease base-emitter capacitance (Cbe), then cut-off frequency (fT) is improved, but device size increases and other performance metrics (fmax, BV) are degraded
Solution Approach 1:
The emitter region transitions from a conventional planar structure to a three-dimensional structure with vertical and lateral portions. The first emitter portion extends vertically through the dielectric layer, while the second emitter portion extends laterally, creating a multi-dimensional configuration that reduces Cbe without increasing the device footprint.
Solution Approach 2:
The emitter region is divided into distinct segments: a first emitter portion extending vertically and a second emitter portion extending laterally. This segmentation allows each portion to serve specific functions - the vertical portion reduces capacitance while the lateral portion maintains electrical connection, thereby improving fT without sacrificing device performance.
2Speed
If techniques are used to decrease base-emitter capacitance (Cbe), then cut-off frequency (fT) is improved, but other performance metrics (maximum oscillation frequency fmax, breakdown voltage BV) are degraded
Solution Approach 1:
Different portions of the emitter region are given different geometrical properties to optimize local functions. The first emitter portion has a vertical orientation optimized for capacitance reduction, while the second emitter portion has a lateral extension optimized for maintaining electrical connectivity and device performance. This local differentiation allows Cbe reduction without compromising fmax or BV.
3Speed
If the dielectric layer is made wider than the emitter portion, then cavities are created to reduce Cbe, but device complexity increases
Solution Approach 1:
The dielectric layer is merged with the emitter region structure, where the dielectric layer extends wider than the emitter portion to create integrated cavities. This merging approach reduces Cbe by positioning dielectric material strategically around the emitter while maintaining a unified structure that does not significantly increase device complexity.
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
The structure achieves reduced base-emitter capacitance and increased cut-off frequency (fT) without significant changes in device size, maximum oscillation frequency, collector-base capacitance, or breakdown voltage, thereby enhancing the performance of BJTs in RF applications.
Implementation Method 1
To decrease Cbe, these cavities can be filled with dielectric material or dielectric material can block the side openings to the cavities creating, within the cavities, pockets of air, of gas or under vacuum.
Data Source
AI summary
Disclosed are a structure including a transistor and a method of forming the structure. The transistor includes an emitter region with first and second emitter portions. The first emitter portion extends through a dielectric layer. The second emitter portion is on the first emitter portion and the top of the dielectric layer. An additional dielectric layer covers the top of the second emitter portion. The second emitter portion and the dielectric and additional dielectric layers are wider than the first emitter portion. At least a section of the second emitter portion is narrower than the dielectric and additional dielectric layers, thereby creating cavities positioned vertically between edge portions of the dielectric and additional dielectric layers and positioned laterally adjacent to the second emitter portion. The cavities are filled with dielectric material or dielectric material blocks the side openings to the cavities creating pockets of air, of gas or under vacuum.