Diluted Drain MOS Transistor Drift Region Doping
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Solution Overview
Problem
Forming extended drain MOS transistors in integrated circuits requires tradeoffs that increase fabrication costs or complexity, or decrease performance, due to the need for optimizing drain operating voltage, breakdown voltage, and transistor area.
Innovation Solution
A process sequence involving a drift region implant mask with alternating exposed and masked areas is used to ion implant dopants into the substrate, allowing dopants to diffuse and form a continuous drift region with varying doping densities, which reduces the average doping density under the gate compared to the drain contact active area, thereby optimizing transistor performance and reducing fabrication complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the drain drift region is formed with uniform doping density to achieve desired breakdown voltage, then the transistor area increases, but the fabrication cost and complexity increase
Solution Approach 1:
The patent applies local quality by creating a drain drift region with non-uniform doping density, where the doping concentration varies laterally from a first value near the drain contact to a second value under the gate. This allows different regions of the drift region to have optimized properties: higher doping near the drain for breakdown voltage control and lower doping under the gate for reduced area, eliminating the need for complex fabrication processes required for uniform doping.
Solution Approach 2:
The patent changes the doping density parameter across the drain drift region, transitioning from a first doping density near the drain contact active area to a second doping density under the gate. This parameter variation allows simultaneous optimization of breakdown voltage (BVDSS and BVDII) and transistor area without increasing fabrication complexity, as the doping profile is achieved through controlled implantation processes.
2Reliability
If the drain drift region is optimized for high breakdown voltage, then the transistor area increases, but the device performance decreases
Solution Approach 1:
The patent creates different doping densities in different regions of the drain drift region to simultaneously achieve high breakdown voltage and good device performance. The first doping density near the drain contact optimizes for breakdown voltage (BVDSS), while the second doping density under the gate optimizes for device performance metrics such as on-state current and switching characteristics, eliminating the need to increase overall transistor area.
Solution Approach 2:
By varying the doping density parameter across the drain drift region from a higher value near the drain contact to a lower value under the gate, the patent achieves optimal breakdown voltage characteristics without sacrificing device performance. This parameter gradient allows the drift region to fulfill multiple functional requirements within a compact area.
3Reliability
If the doping density in the drain drift region is increased to improve breakdown voltage, then the fabrication cost increases, but the transistor area increases
Solution Approach 1:
The patent applies local quality by concentrating higher doping density only in specific regions (near the drain contact active area) where it is most needed for achieving desired breakdown voltage (BVDSS and BVDII). The regions under the gate have lower doping density, reducing the overall area requirement while maintaining adequate breakdown performance through localized high-doping zones.
Solution Approach 2:
The patent changes the doping density parameter across the drain drift region, creating a gradient from higher doping near the drain contact to lower doping under the gate. This parameter variation achieves the desired breakdown voltage without requiring uniformly high doping throughout the entire drift region, thereby reducing the transistor area compared to conventional uniform doping approaches.
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
This approach reduces the area of the MOS transistor, lowers fabrication costs, and improves breakdown voltage values by allowing for a balanced tradeoff between BVDSS and BVDII, while maintaining efficient operation.
Implementation Method 1
A drift region ion implant process is performed which implants dopants for the drift region into a substrate of the integrated circuit under the area exposed by the drift region implant mask
Implementation Method 2
During one or more subsequent anneal processes, the implanted dopants diffuse and become activated
Data Source
AI summary
An integrated circuit containing an extended drain MOS transistor may be formed by forming a drift region implant mask with mask fingers abutting a channel region and extending to the source/channel active area, but not extending to a drain contact active area. Dopants implanted through the exposed fingers form lateral doping striations in the substrate under the mask fingers. An average doping density of the drift region under the gate is at least 25 percent less than an average doping density of the drift region at the drain contact active area. In one embodiment, the dopants diffuse laterally to form a continuous drift region. In another embodiment, substrate material between lateral doping striations remains an opposite conductivity type from the lateral doping striations.