SiGe Emitter Graded Profile for HBT Frequency
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
As silicon germanium (SiGe) hetero-junction bipolar transistors (HBTs) approach high switching speeds, the emitter delay time becomes a significant portion of the overall forward transmit time, potentially limiting AC performance, necessitating a reduction in emitter transmit time for improved switching speed and frequency performance.
Innovation Solution
A hetero-junction bipolar transistor structure with a silicon-germanium emitter layer featuring a graded germanium concentration profile, creating a quasi-drift field that accelerates minority carriers, thereby reducing emitter delay time and increasing cut-off frequency (fT) and maximum oscillation frequency (fMAX).
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If base size is scaled down to reduce base transmit time, then base transmit time decreases, but emitter transmit time becomes a larger portion of overall forward transmit time
Solution Approach 1:
The emitter layer is divided into regions with different germanium concentrations - a first region with lower germanium concentration adjacent to the base and a second region with higher germanium concentration. This local variation in composition creates a quasi-drift field in the neutral region that accelerates carrier transport, reducing emitter delay time while maintaining the scaled-down base dimensions
Solution Approach 2:
The germanium concentration parameter is varied throughout the emitter layer to optimize performance. By grading the germanium concentration from lower values near the base to higher values at the emitter contact, the patent creates beneficial electric fields that reduce emitter transit time without requiring further base scaling
2Speed
If emitter delay time is reduced to improve switching speed, then cut-off frequency increases, but device complexity increases due to graded concentration profile
Solution Approach 1:
The patent employs parameter changes by varying the germanium concentration throughout the emitter layer. This graded concentration profile creates a quasi-drift field that reduces emitter delay time and increases cut-off frequency. The complexity is managed through controlled epitaxial growth processes that can precisely regulate composition gradients
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 graded germanium concentration in the emitter layer reduces emitter delay time, enhancing the cut-off frequency and maximum oscillation frequency, thereby improving the overall switching speed and AC performance of the transistor.
Implementation Method 1
By incorporating the germanium in graded concentrations through the emitter layer a quasi-drift field is created in the neutral region of the emitter layer. This quasi-drift field lowers the emitter delay time for the neutral region so as to increase cut-off frequency (fT). The quasi-drift field is induced by the valence bandgap grading across the neutral region so as to accelerate movement of minority carriers (e.g., holes) from the base layer through the emitter layer
Implementation Method 2
The quasi-drift field is induced by the valence bandgap grading across the neutral region so as to accelerate movement of minority carriers (e.g., holes) from the base layer through the emitter layer which increases the cut-off frequency (fT)
Data Source
AI summary
Disclosed are an improved hetero-junction bipolar transistor (HBT) structure and a method of forming the structure that incorporates a silicon-germanium emitter layer with a graded germanium profile. The graded germanium concentration creates a quasi-drift field in the neutral region of the emitter layer. This quasi-drift field induces valence bandgap grading within the emitter layer so as to accelerate movement of holes from the base layer through the emitter layer. Accelerated movement of the holes from the base layer through the emitter layer reduces emitter delay time and thereby, increases the cut-off frequency (fT) and the maximum oscillation frequency (fMAX) of the resultant HBT.