Ring-Shaped Collector Implant for Heterojunction Bipolar Transistors
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Solution Overview
Problem
Heterojunction bipolar transistors (HBTs) face challenges in reducing parasitic base-collector capacitance and delaying the Kirk effect, which negatively impact high-frequency performance due to increased collector current density and current crowding, leading to increased collector resistance and capacitance.
Innovation Solution
A ring-shaped collector implant structure with higher dopant concentration is introduced beneath the perimeter of the emitter, self-aligned to the emitter, reducing collector resistance and minimizing base-collector capacitance increase, while maintaining low dopant concentration under the emitter's center to delay the Kirk effect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If vertical dimension of transistor is reduced to increase operation speed, then transit time decreases, but base-collector capacitance increases
Solution Approach 1:
The ring-shaped collector implant structure enables localized reduction of base-collector capacitance at the perimeter region without compromising the overall transistor performance. By concentrating the high doping in a ring structure rather than uniformly throughout, the patent achieves better control over the base-collector capacitance while maintaining the reduced vertical dimension benefits for high-speed operation.
2Speed
If collector current density is increased to improve AC performance, then frequency response improves, but Kirk effect is accelerated
Solution Approach 1:
The ring-shaped implant structure addresses current crowding at the perimeter by providing localized high doping concentration in the collector region beneath the emitter perimeter. This reduces the electric field strength and space charge region width at the perimeter where current crowding occurs, thereby delaying the Kirk effect and improving high-frequency performance.
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 solution effectively reduces base-collector capacitance and delays the Kirk effect, enhancing high-frequency performance by localizing increased collector doping at the emitter's perimeter, thereby improving AC performance with minimal increase in base-collector capacitance.
Implementation Method 1
A ring shaped collector implant structure is formed within an upper portion of the collector layer, wherein the ring shaped collector implant structure is disposed so as to be aligned beneath a perimeter portion of the emitter. The lower collector implant structure and the ring shaped collector structure have a higher dopant concentration with respect to the collector layer.
Implementation Method 2
The principal difference between the BJT and HBT is the use of differing semiconductor materials (e.g., silicon, germanium) for the emitter and base regions, thus creating a heterojunction. The effect is to limit the injection of holes into the base region, since the potential barrier in the valance band is so large.
Data Source
AI summary
A bipolar transistor structure includes an intrinsic base layer formed over a collector layer, an emitter formed over the intrinsic base layer, and an extrinsic base layer formed over the intrinsic layer and adjacent the emitter. A ring shaped collector implant structure is formed within an upper portion of the collector layer, wherein the ring shaped collector implant structure is disposed so as to be aligned beneath a perimeter portion of the emitter.