Bipolar Transistor Protected Collector Edge Reduces Junction Capacitance
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Solution Overview
Problem
Conventional transistor structures for RF applications, such as bipolar junction transistors (BJTs) and heterojunction bipolar transistors (HBTs), often result in increased base-collector junction capacitance due to divot formation at the trench isolation region-collector region interface, leading to reduced maximum oscillation frequency Fmax.
Innovation Solution
A transistor structure with a collector region having a protected outer edge portion is formed by positioning a trench isolation region laterally adjacent to the collector region, with mask layers covering the trench isolation region and extending onto the outer edge of the collector region, preventing divot formation and limiting dopant diffusion from the extrinsic base layer, thereby reducing base-collector junction capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional transistor structures are used without mask layer protection, then the manufacturing process is simpler, but base-collector junction capacitance increases due to divot formation and dopant diffusion
Solution Approach 1:
The mask layers are formed on the trench isolation region before the extrinsic base layer is deposited. This preliminary action prevents divot formation at the isolation region-collector region interface and limits dopant diffusion into the collector region during subsequent processing steps, thereby reducing base-collector junction capacitance and increasing maximum oscillation frequency
Solution Approach 2:
The mask layers serve as an intermediary barrier between the trench isolation region and the collector region. They physically prevent the formation of divots at the interface and block dopant diffusion from the extrinsic base layer into the collector region, particularly at the outer edge portion, thus reducing parasitic capacitance without requiring fundamental changes to the transistor architecture
2Reliability
If mask layers are added to protect the trench isolation region, then base-collector junction capacitance is reduced, but manufacturing complexity increases
Solution Approach 1:
The mask layers are formed during the standard transistor fabrication sequence, specifically before the extrinsic base layer deposition. This timing allows the mask layers to perform their protective function during critical steps (preventing divot formation and limiting dopant diffusion) while integrating into the existing manufacturing flow with minimal disruption
Solution Approach 2:
The mask layers are formed using standard semiconductor fabrication techniques and integrated into the existing manufacturing sequence. They serve as a temporary protective structure that is added and then removed after serving its purpose, allowing the benefits of reduced capacitance to be achieved without permanent structural 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
This approach effectively reduces base-collector junction capacitance, thereby increasing the maximum oscillation frequency Fmax of the transistor.
Implementation Method 1
Mask layer(s) cover the trench isolation region and further extend laterally onto an outer edge portion of the collector region... During processing these mask layer(s) prevent divot formation in the upper corner of the trench isolation region
Implementation Method 2
these mask layer(s) prevent divot formation in the upper corner of the trench isolation region at the isolation region-collector region interface and further limit dopant diffusion from a subsequently formed raised extrinsic base layer into the collector region
Implementation Method 3
Reduction of this base-collector junction capacitance Cbc can result in a higher Fmax... base-collector junction capacitance Cbc is reduced and, consequently, the maximum oscillation frequency Fmax of the transistor is increased
Data Source
AI summary
Disclosed are embodiments of a transistor (e.g., bipolar junction transistor (BJT) or a heterojunction bipolar transistor (HBT)) and a method of forming the transistor with a collector region having a protected upper edge portion for reduced base-collector junction capacitance Cbc. In the embodiments, a collector region is positioned laterally adjacent to a trench isolation region within a substrate. Mask layer(s) cover the trench isolation region and further extend laterally onto the edge portion of the collector region. A first section of an intrinsic base layer is positioned above a center portion of the collector region and a second section of the intrinsic base layer is positioned above the mask layer(s). During processing these mask layer(s) prevent divot formation in the upper corner of the trench isolation region at the isolation region-collector region interface and further limit dopant diffusion from a subsequently formed raised extrinsic base layer into the collector region.