SiC Trench MOSFET Threshold Voltage Control
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Solution Overview
Problem
Conventional silicon carbide semiconductor devices face challenges in freely setting threshold voltage while maintaining sufficient channel mobility, as high doping densities lead to significant electron scattering and decreased channel mobility, making it difficult to achieve a normally-off type operation.
Innovation Solution
A silicon carbide semiconductor device with a substrate having a hexagonal crystal structure and a trench with side walls inclined at 50° to 65° relative to the {0001} plane, featuring a high impurity density body region and a gate insulating film, allows for increased threshold voltage adjustment without compromising channel mobility, utilizing a gate electrode on the insulating film to control the channel region.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the doping density of the body region is increased to adjust the threshold voltage, then the threshold voltage can be positively shifted to enable normally-off type operation, but the channel mobility is significantly decreased due to increased electron scattering
Solution Approach 1:
The patent applies local quality by creating different impurity concentration zones within the body region. Specifically, it introduces a high-concentration impurity region adjacent to the gate insulating film interface while maintaining a lower impurity concentration in the channel formation region. This spatial differentiation allows the high-concentration region to provide sufficient threshold voltage control through charge accumulation, while the low-concentration channel region maintains high electron mobility for fast switching performance.
Solution Approach 2:
The body region is segmented into functionally distinct zones: a high-concentration impurity region near the gate insulating film interface responsible for threshold voltage control, and a low-concentration channel formation region responsible for maintaining high electron mobility. This segmentation resolves the contradiction by assigning different doping levels to different functional areas rather than using a uniform doping concentration throughout the entire body region.
2Reliability
If the impurity density in the body region is set high to achieve normally-off type operation, then the device can turn off automatically, but the on-resistance increases due to reduced channel mobility
Solution Approach 1:
The patent implements local quality by concentrating high impurity density specifically in the region adjacent to the gate insulating film interface, while keeping the channel formation region with lower impurity density. This localized high-concentration region provides sufficient charge to establish the desired threshold voltage for normally-off operation, while the separate low-concentration channel region minimizes electron scattering and maintains low on-resistance, thereby reducing energy loss during conduction.
3Reliability
If the doping concentration is increased to shift threshold voltage, then the device can achieve desired switching characteristics, but the scattering of electrons increases significantly
Solution Approach 1:
The patent segments the body region into a high-concentration impurity region and a low-concentration channel region. The high-concentration region is positioned to provide the necessary charge for threshold voltage control without being part of the main electron transport path. The low-concentration channel region is specifically designed to minimize electron scattering by maintaining low impurity density where electrons flow, thus resolving the contradiction between threshold voltage adjustment capability and electron scattering reduction.
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 configuration enables a high degree of freedom in setting threshold voltage while maintaining high channel mobility, allowing the semiconductor device to operate as a normally-off type with reduced on-resistance and improved switching speed.
Implementation Method 1
a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and an IGBT (Insulated Gate Bipolar Transistor)... controls appearance and disappearance of an inversion layer in a channel region in accordance with a predetermined threshold voltage
Implementation Method 2
Silicon carbide is a wide band gap semiconductor having a band gap larger than that of silicon... the semiconductor device can have a high breakdown voltage, reduced on-resistance
Data Source
Figure 1~3
Figure 4~6
Figure 7~9
AI summary
A silicon carbide layer is epitaxially formed on a main surface of a substrate (1). The silicon carbide layer is provided with a trench having a side wall (6) inclined relative to the main surface. The side wall (6) has an off angle of not less than 50° and not more than 65° relative to a {0001} plane. A gate insulating film (8) is provided on the side wall (6) of the silicon carbide layer. The silicon carbide layer includes: a body region (3) having a first conductivity type and facing a gate electrode (9) with the gate insulating film (8) being interposed therebetween; and a pair of regions (2, 4) separated from each other by the body region (3) and having a second conductivity type. The body region (3) has an impurity density of 5 × 1016 cm-3 or greater. This allows for an increased degree of freedom in setting a threshold voltage while suppressing decrease of channel mobility.