Trench MOSFET Channel Doping to Cut Switching Losses
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Solution Overview
Problem
Field-effect transistors, particularly trench MOSFETs, face high switching losses and crystal defects due to the bipolar nature of the pn diode at the transition between the p-doped shielding region and the n-doped drain or drift layer, especially at high current densities.
Innovation Solution
The channel layer is doped with lower p-doping or higher n-doping in regions more than 10 nm away from the gate trench, creating a unipolar current path that reduces switching losses and current density in the pn diode by connecting in parallel with the conventional current path.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a pn diode is formed at the transition between the p-doped shielding region and the n-doped drain layer, then the transistor can be produced with conventional doping methods, but high switching losses and crystal defects occur at high current densities
Solution Approach 1:
The patent applies local quality by creating a laterally differentiated doping profile in the channel layer. Specifically, the channel layer has different doping concentrations in different lateral regions: a first doping concentration in the region laterally adjacent to the gate trench, and a second doping concentration in the region laterally spaced from the gate trench. This local differentiation allows the formation of a unipolar n-type channel region that conducts current without forming a pn diode, thereby reducing switching losses while maintaining manufacturability through selective doping processes.
Solution Approach 2:
The patent changes the doping parameter (doping concentration) as a function of lateral position in the channel layer. By varying the doping concentration from one region to another, the patent transforms the electrical characteristics of the channel, enabling unipolar conduction in the region spaced from the gate trench. This parameter change eliminates the formation of a pn diode and its associated switching losses, while still allowing conventional doping methods to be used.
2Device complexity
If a pn diode is formed at the transition between the p-doped shielding region and the n-doped drain layer, then the transistor structure is simplified, but crystal defects are generated in the substrate at high current densities
Solution Approach 1:
The patent applies local quality by creating a laterally differentiated doping profile in the channel layer. Specifically, the channel layer has different doping concentrations in different lateral regions: a first doping concentration in the region laterally adjacent to the gate trench, and a second doping concentration in the region laterally spaced from the gate trench. This local differentiation allows the formation of a unipolar n-type channel region that conducts current without forming a pn diode, thereby reducing switching losses while maintaining manufacturability through selective doping processes.
Solution Approach 2:
The patent changes the doping parameter (doping concentration) as a function of lateral position in the channel layer. By varying the doping concentration from one region to another, the patent transforms the electrical characteristics of the channel, enabling unipolar conduction in the region spaced from the gate trench. This parameter change eliminates the formation of a pn diode and its associated switching losses, while still allowing conventional doping methods to be used.
Data Source
AI summary
A field-effect transistor. The field-effect transistor includes: a source layer doped according to a first type, a drain layer doped according to a first type, a channel layer located vertically between the source layer doped according to the first type and the drain layer doped according to the first type, and a gate trench which extends vertically from the source layer doped according to the first type to the drift layer doped according to the first type and adjoins the channel layer. The channel layer has, at least on average, a lower doping of the second type and a higher doping of the first type in a region that is more than a specified distance from the gate trench than in a region that is less than the specified distance from the gate trench. Methods for production are also described.