Polysilicon Stack Collector Structure for Ultra-High Voltage HBTs
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Solution Overview
Problem
The conventional ultra-high voltage SiGe HBT devices have a high series resistance and saturation voltage drop due to the wide collector region, which limits their applications, and reducing dopant concentration in the collector region to lower resistance also decreases the breakdown voltage.
Innovation Solution
A structure for picking up a collector region in a heterojunction bipolar transistor (HBT) is formed with polysilicon stacks, where the first polysilicon layer has a tapered side face and a higher dopant diffusivity or concentration, reducing the series resistance and saturation voltage drop while maintaining high breakdown voltage, and the second polysilicon layer has a lower dopant diffusivity or concentration to prevent dopant diffusion into the upper collector region.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the collector region width is increased to maintain high breakdown voltage, then the breakdown voltage is improved, but the series resistance and saturation voltage drop increase
Solution Approach 1:
The collector region is divided into multiple segments: the upper collector region (between isolation regions) and the lower collector region (extending beneath isolation regions). The polysilicon stacks are positioned to contact both segments, creating multiple current paths that reduce series resistance while maintaining the width needed for high breakdown voltage.
Solution Approach 2:
The invention extends the collector region and polysilicon stacks into the vertical dimension beneath the isolation regions. This allows current to flow through both upper and lower collector regions, effectively reducing resistance without increasing the lateral width of the collector region, thus preserving high breakdown voltage.
2Reliability
If the dopant concentration in the collector region is increased to reduce series resistance, then the series resistance is reduced, but the breakdown voltage decreases
Solution Approach 1:
Different dopant concentrations are applied to different regions: the upper collector region maintains low dopant concentration for high breakdown voltage, while the lower collector region (contacting polysilicon stacks) has higher dopant concentration to reduce series resistance. This local differentiation resolves the contradiction between resistance and breakdown voltage.
Solution Approach 2:
The polysilicon stacks are formed with controlled dopant diffusion in advance, creating a doping profile where the lower collector region is pre-doped at higher concentration before final device assembly. This preliminary doping action enables subsequent low-resistance contact without compromising the upper region's breakdown characteristics.
3Reliability
If polysilicon stacks with high dopant diffusivity are used to reduce series resistance, then the series resistance is reduced, but dopant diffusion into the upper collector region increases
Solution Approach 1:
The polysilicon structure is segmented into multiple layers (first polysilicon layer and second polysilicon layer) with different dopant concentrations and diffusion characteristics. This segmentation allows the lower layers to provide high-diffusivity doping for resistance reduction while upper layers prevent excessive diffusion into the collector region.
Solution Approach 2:
The dopant concentration and diffusivity parameters are changed across different polysilicon layers and regions. The first polysilicon layer has high dopant concentration for resistance reduction, while the second polysilicon layer has lower concentration to limit diffusion. This parameter variation resolves the contradiction between resistance reduction and breakdown voltage maintenance.
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 the series resistance and saturation voltage drop of the collector region while maintaining the high breakdown voltage, allowing for improved device performance without compromising the 'ultra-high voltage' capability.
Implementation Method 1
the first polysilicon layer having a first side face contacting with the collector region, an angle between the first side face and a bottom surface of the first polysilicon layer being smaller than 90 degrees; and a second polysilicon layer located on and in contact with the first polysilicon layer, wherein both the first and second polysilicon layers have an opposite conductivity type to a conductivity type of the substrate, wherein the first polysilicon layer is doped with a dopant having a higher diffusivity or higher concentration than a dopant of the second polysilicon layer
Data Source
AI summary
A structure for picking up a collector region is disclosed. The structure includes a pair of polysilicon stacks formed in the isolation regions and extending below the collector region; and a pair of collector electrodes contacting on the polysilicon stacks, wherein the pair of polysilicon stacks includes: a first polysilicon layer located below the isolation regions, and a second polysilicon layer located on and in contact with the first polysilicon layer, the first polysilicon layer being doped with a dopant having a higher diffusivity or higher concentration than a dopant of the second polysilicon layer, wherein a depth of the polysilicon stacks is greater than a depth of the collector region; the depth of the collector region is greater than a depth of the second polysilicon layer; and the depth of the second polysilicon layer is greater than a depth of the isolation regions.