Shallow Bipolar Junction Transistor With Graded Base Doping
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Solution Overview
Problem
Smaller-sized bipolar junction transistors (BJTs) exhibit high and scattered beta values due to non-ideal behavior, characterized by high collector current leakage through parasitic depletion transistors, leading to variable reference voltage (Vref) output across wafers and lots, compromising the structural integrity and reliability of memory devices.
Innovation Solution
A shallow trench isolation process is implemented with a core implant at the periphery of the semiconductor substrate, eliminating the need for a cobalt silicide layer and using an oxide nitride oxide (ONO) layer as an etch stop, which reduces beta fluctuations and process steps, allowing for smaller transistor dimensions while maintaining reliable isolation between emitter and collector.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If smaller-sized BJTs are used to reduce device area, then area is reduced, but beta scatter and leakage increase
Solution Approach 1:
The patent applies local quality by creating a graded doping profile in the base region, where the doping concentration varies spatially. The base has a lower doping concentration at the emitter junction and increases toward the collector, which locally optimizes carrier injection efficiency while maintaining low recombination. This localized doping strategy improves beta uniformity without increasing device area.
Solution Approach 2:
The patent changes key physical parameters including base width (reducing to sub-100nm), base doping concentration (graded profile), and emitter doping concentration to optimize transistor performance. By adjusting these parameters, the patent achieves reduced beta scatter and leakage while maintaining small device area.
2Reliability
If STI is extended deeper into substrate to improve isolation, then isolation between emitter and collector improves, but structural integrity is compromised
Solution Approach 1:
The patent applies partial action by implementing shallow trench isolation that extends only partially into the substrate, rather than deep isolation. Combined with the graded base doping profile, this partial isolation approach achieves sufficient electrical separation between emitter and collector while preserving the structural integrity of the underlying substrate.
3Area of moving object
If 90 nm BJT is used to reduce size, then area is reduced, but Vref output variation increases
Solution Approach 1:
The graded base doping profile creates local quality variations that compensate for process variations. The varying doping concentration across the base region makes the transistor performance less sensitive to manufacturing variations, thereby improving Vref output consistency in scaled devices.
Solution Approach 2:
By changing the base width and doping parameters to optimized values for the 90 nm node, the patent achieves better control over transistor characteristics, reducing Vref output variation while maintaining small device area.
Data Source
AI summary
A shallow bipolar junction transistor comprising a high voltage n+ well implanted into a semiconductor substrate. The shallow bipolar junction transistor further comprises a bit line n+ implant (BNI) above the high voltage n+ well and an oxide nitride (ONO) layer above the high voltage n+ well. A portion of the ONO layer isolates the BNI from a shallow trench isolation (STI) region.