MOSFET Super Junction Charge Density Optimization
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Solution Overview
Problem
Conventional MOSFETs experience significant irregularities in switching characteristics when turned off, particularly due to charge balance irregularities around the gate, leading to increased switching times and reduced performance.
Innovation Solution
The MOSFET design incorporates a super junction structure with a specific arrangement of n-type and p-type column regions, where the average positive charge density is expressed by an upward convex curve, optimizing the depth and width profiles to minimize charge imbalance and enhance depletion characteristics, thereby reducing feedback capacitance and improving switching efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the n-type column region and p-type column region are formed with equal total dopant amounts to achieve charge balance, then the MOSFET achieves low ON resistance and high withstand voltage, but irregularities in charge balance around the gate cause significant irregularities in switching characteristics when the MOSFET is turned off
Solution Approach 1:
The patent applies local quality by making the dopant concentration in the n-type column region non-uniform: higher concentration near the gate electrode and lower concentration toward the drain. This local variation in dopant concentration compensates for charge balance irregularities around the gate, reducing switching characteristic irregularities while maintaining overall charge balance between n-type and p-type column regions.
Solution Approach 2:
The patent changes the dopant concentration parameter within the n-type column region, transitioning from a uniform concentration to a depth-dependent concentration profile. Specifically, the dopant concentration is set to decrease with increasing depth from the surface, which optimizes the charge distribution to reduce switching irregularities caused by gate-related charge balance variations.
2Stability of the object's composition
If the dopant concentration in n-type and p-type column regions is set to fixed values, then the super junction structure achieves good charge balance, but the switching characteristics show large irregularities when the MOSFET is turned off due to charge balance irregularities around the gate
Solution Approach 1:
The patent introduces local quality by creating a non-uniform dopant concentration profile within the n-type column region, where the concentration varies with depth. This local variation compensates for the charge balance irregularities that occur around the gate electrode during switching, thereby improving switching characteristics while maintaining overall charge balance stability.
Solution Approach 2:
The patent applies dynamics by making the dopant concentration in the n-type column region variable rather than fixed. The concentration profile is designed to change with depth, creating a dynamic charge distribution that adapts to the electric field conditions during different switching states, thereby reducing switching irregularities.
3Ease of manufacture
If the side walls of the n-type column region are formed in a tapered shape narrowed toward the first main surface side, then the device structure is optimized for manufacturing, but the charge balance irregularities around the gate increase, leading to larger irregularities in switching characteristics
Solution Approach 1:
The patent compensates for the tapered side wall geometry by changing the dopant concentration parameter within the n-type column region. The concentration is designed to be higher near the surface and decrease with depth, which counteracts the charge balance irregularities introduced by the tapered shape, thereby maintaining good switching characteristics despite the manufacturing-optimized geometry.
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 results in reduced switching irregularities, lower feedback capacitance, and increased avalanche breakdown resistance, maintaining low ON resistance and high withstand voltage while minimizing the impact of charge balance irregularities on switching characteristics.
Implementation Method 1
using a deepest position of a surface of a depletion layer on the first main surface side when the depletion layer extends most in the super junction structure by turning off the MOSFET as a reference
Implementation Method 2
the average positive charge density ρ(x) at the predetermined depth position in the super junction structure expressed by a following formula (1) when the super junction structure is depleted by turning off the MOSFET is taken on an axis of ordinates, the average positive charge density ρ(x) is expressed by an upward convex curve projecting in a right upward direction
Implementation Method 3
increased avalanche breakdown resistance
Data Source
AI summary
A MOSFET includes: a semiconductor base substrate having a super junction structure; and a gate electrode formed on a first main surface side of the semiconductor base substrate by way of a gate insulation film. In a graph where a depth x at a predetermined depth position in the super junction structure is taken on an axis of abscissas, and an average positive charge density ρ(x) at the predetermined depth position in the super junction structure is taken on an axis of ordinates, the average positive charge density ρ(x) at a predetermined depth position of the super junction structure when the super junction structure is depleted by turning off the MOSFET is expressed by an upward convex curve projecting in a right upward direction.