LDMOS Step-Like Drift Region for Low On-Resistance and High Breakdown Voltage
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Solution Overview
Problem
Existing LDMOS devices face a trade-off between low on-resistance and high breakdown voltage, as increasing doping concentration reduces breakdown voltage and decreasing drift region length increases on-resistance.
Innovation Solution
The LDMOS device features a step-like drift region with progressively decreasing thickness from the channel to the drain, allowing complete depletion and higher doping concentration to reduce on-resistance while maintaining high breakdown voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the doping concentration of the drift region is increased to reduce on-resistance, then the on-resistance decreases, but the breakdown voltage decreases
Solution Approach 1:
The drift region is divided into multiple regions with different doping concentrations along the current flow direction. The first drift region has a lower doping concentration to maintain high breakdown voltage, while the second drift region has a higher doping concentration to reduce on-resistance. This local differentiation allows each region to optimize for its specific function, resolving the contradiction between low on-resistance and high breakdown voltage.
Solution Approach 2:
The drift region is segmented into at least two distinct drift regions with different doping concentrations. The first drift region (closer to the channel) has lower doping concentration to ensure complete depletion and high breakdown voltage, while the second drift region (closer to the drain) has higher doping concentration to reduce series resistance. This segmentation enables the device to achieve both low on-resistance and high breakdown voltage simultaneously.
2Loss of energy
If the physical length of the drift region is decreased to reduce on-resistance, then the on-resistance decreases, but the breakdown voltage decreases
Solution Approach 1:
Different segments of the drift region are assigned different doping concentrations to optimize local properties. The first drift region has lower doping concentration extending closer to the channel to maintain high breakdown voltage, while the second drift region has higher doping concentration to reduce on-resistance. This local quality differentiation allows the drift region to achieve both low on-resistance and high breakdown voltage without compromising either parameter.
Solution Approach 2:
The doping concentration parameter is changed along the drift region to resolve the contradiction. By implementing a doping concentration gradient where the first drift region has lower concentration and the second drift region has higher concentration, the device achieves optimal balance between breakdown voltage (requiring low doping and long length) and on-resistance (requiring high doping and short length).
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 step-like drift region design enables complete depletion and increased doping concentration, achieving both low on-resistance and high breakdown voltage, improving device performance.
Implementation Method 1
When a high voltage is applied to the drain region 20 of the existing n-type LDMOS device shown in FIG. 1a, the channel of the device will cause a depletion region horizontally extend towards the drain region 20. Moreover, a PN junction formed between the n-type drift region 12 and the p-type substrate 10 will cause the depletion region vertically extend towards the p-type substrate 10.
Implementation Method 2
As both the horizontal and vertical dimensions of the depletion region are determined by, and reversely proportional to, the doping concentration of the drift region 12, a heavily doped drift region 12 will not be completely depleted even upon the occurrence of the device's avalanche breakdown.
Implementation Method 3
a doped region having a first type of conductivity and being formed in the substrate, the doped region being located at a first end of the drift region and laterally adjacent to the drift region; and a heavily doped drain region having a second type of conductivity and being formed in the substrate
Data Source
AI summary
An LDMOS device is disclosed. The LDMOS device includes: a substrate having a first type of conductivity; a drift region having a second type of conductivity and being formed in the substrate; a doped region having the first type of conductivity and being formed in the substrate, the doped region being located at a first end of the drift region and laterally adjacent to the drift region; and a heavily doped drain region having the second type of conductivity and being formed in the substrate, the heavily doped drain region being located at a second end of the drift region, wherein the drift region has a step-like top surface with at least two step portions, and wherein a height of the at least two step portions decreases progressively in a direction from the doped region to the drain region. A method of fabricating LDMOS device is also disclosed.