Stepped Emitter Bipolar Transistor for Better Emitter Utilization
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Solution Overview
Problem
Vertical bipolar transistors with large area emitters suffer from poor emitter utilization due to fringe currents following the least resistive pathway to contacts, resulting in low junction capacitance at peak performance.
Innovation Solution
A bipolar transistor with a stepped emitter structure, featuring a raised mesa and stepped features on its sides, which increases peripheral emitter resistance and directs current flow through the intrinsic emitter, improving emitter utilization and reducing extrinsic base to emitter capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If large area emitters with wide emitter widths are used, then the transistor can be used for high power and high breakdown applications, but emitter utilization is poor due to fringe currents following the least resistive pathway to contacts
Solution Approach 1:
The emitter is segmented into multiple regions at different heights through the stepped configuration. This divides the emitter into an intrinsic region and an extrinsic region, forcing fringe currents to travel through the intrinsic region rather than taking the shortest path to contacts, thereby improving emitter utilization while maintaining high power capability
Solution Approach 2:
The emitter is extended into the vertical dimension with stepped heights, creating a three-dimensional structure. This vertical segmentation adds a new dimension to current flow paths, forcing currents to traverse through the intrinsic emitter region vertically before reaching the base, thus improving emitter utilization without reducing the horizontal emitter area needed for high power applications
2Power
If large area emitters are used, then the transistor can handle high power applications, but junction capacitance at peak performance is low
Solution Approach 1:
The emitter is segmented vertically into stepped regions, creating distinct intrinsic and extrinsic zones. This segmentation allows the intrinsic region to contribute to junction capacitance while the extrinsic region provides low-resistance contact paths, thus improving junction capacitance without sacrificing the large emitter area needed for high power capability
3Reliability
If stepped emitter features are added, then emitter utilization is improved and current flow is directed through the intrinsic emitter, but device complexity increases
Solution Approach 1:
The emitter is segmented into stepped regions that can be formed using standard semiconductor fabrication processes such as selective epitaxial growth or selective deposition. This segmentation improves emitter utilization by directing current through the intrinsic region while maintaining compatibility with existing manufacturing techniques, thus limiting the increase in device complexity
Solution Approach 2:
Different regions of the emitter are given different heights and properties - the intrinsic region has greater height to improve utilization, while the extrinsic region has reduced height for low-resistance contacts. This local differentiation achieves improved emitter utilization without requiring complete redesign of the entire device structure
Data Source
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AI summary
A structure comprising: a collector; a base over the collector; and an emitter over the base, the emitter comprising at least one stepped feature over the base.