SiC MOSFET Drift Layer Segmentation for Breakdown Voltage and On-Resistance
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Solution Overview
Problem
Existing MOSFETs with silicon carbide drift layers face challenges in achieving high breakdown voltage and high channel mobility while maintaining a normally OFF state, as they often require complex impurity concentration control to avoid accumulation mode operation.
Innovation Solution
A MOSFET structure with a silicon carbide drift layer having a first region with a lower impurity concentration near the surface and a second region with a higher impurity concentration deeper within, along with corresponding base regions, is fabricated using a multi-step epitaxial growth and ion implantation process to control impurity concentrations accurately.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the drift layer is made with high impurity concentration to reduce ON-state resistance, then channel mobility improves, but breakdown voltage decreases
Solution Approach 1:
The drift layer is segmented into multiple regions with different impurity concentrations: a first drift layer region with lower impurity concentration (1×10^16 to 1×10^18 atoms/cm³) and a second drift layer region with higher impurity concentration (1×10^18 to 1×10^20 atoms/cm³). This segmentation allows each region to fulfill different functions - the first region maintains high breakdown voltage while the second region provides low ON-state resistance through high channel mobility.
Solution Approach 2:
Different regions of the drift layer are assigned different impurity concentrations tailored to their specific functional requirements. The first drift layer region (deeper region) has lower impurity concentration optimized for breakdown voltage, while the second drift layer region (shallower region) has higher impurity concentration optimized for channel mobility and low ON-state resistance.
2Strength
If the drift layer has low impurity concentration to achieve high breakdown voltage, then breakdown voltage improves, but ON-state resistance increases
Solution Approach 1:
The drift layer is divided into first and second regions with different impurity concentrations. The first region extends from the surface to a first depth with lower impurity concentration for high breakdown voltage, while the second region extends from the surface to a second depth (deeper than first region) with higher impurity concentration for low ON-state resistance.
Solution Approach 2:
The drift layer exhibits spatially varying impurity concentration where the first region has lower concentration optimized for voltage blocking and the second region has higher concentration optimized for current conduction, allowing simultaneous achievement of high breakdown voltage and low ON-state resistance.
3Loss of energy
If accumulation mode structure is used to achieve high channel mobility, then ON-state resistance decreases, but normally OFF operation becomes difficult to achieve
Solution Approach 1:
Instead of using the conventional accumulation mode where high impurity concentration in the drift layer provides high channel mobility, this invention inverts the approach by using a low impurity concentration drift layer combined with a high impurity concentration base region. This inversion achieves high channel mobility through the base region rather than the drift layer, enabling normally OFF operation while maintaining low ON-state resistance.
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 enables the MOSFET to achieve high breakdown voltage, low on-state loss, and low gate threshold voltage, while ensuring easy realization of a normally OFF state without the accumulation mode, thereby improving device performance and reliability.
Implementation Method 1
the drift layer has a first region extending from the surface to a first given depth and a second region formed in a region deeper than the first given depth, and the first region has an impurity concentration lower than that of the second region
Implementation Method 2
implanting impurity ions of a second conductivity type with a relatively high concentration into the relatively-high-impurity-concentration drift layer, so as to form a base region having a relatively high impurity concentration
Data Source
AI summary
The present invention provides a MOSFET and so forth that offer high breakdown voltage and low on-state loss (high channel mobility and low gate threshold voltage) and that can easily achieve normally OFF. A drift layer 2 of a MOSFET made of silicon carbide according to the present invention has a first region 2a and a second region 2b. The first region 2a is a region from the surface to a first given depth. The second region 2b is formed in a region deeper than the first given depth. The impurity concentration of the first region 2a is lower than the impurity concentration of the second region 2b.