Tapered Field Plate in Semiconductor Device for Capacitance Control
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Solution Overview
Problem
Conventional vertical insulated gate transistors with field plates face a trade-off between reducing feedback and output capacitance and maintaining low on-resistance, as promoting depletion near the gate electrode to reduce capacitance increases on-resistance, leading to lower saturation currents.
Innovation Solution
The semiconductor device incorporates a tapered field plate in a trench adjacent to the gate electrode, with the distance between the field plate and trench side wall increasing toward the trench base, allowing for controlled depletion expansion near the gate while minimizing depletion in the deep part of the drift layer to reduce on-resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the field plate width in the trench is constant and the electric field uniformly extends to the drift layer, then the depletion layer is expanded in the thickness direction of the drift layer, but the on-resistance increases and saturation current is lowered
Solution Approach 1:
The field plate is designed with a tapered shape where the width varies along the trench depth. Specifically, the field plate width is smaller in the upper portion near the gate electrode and larger in the lower portion near the trench bottom. This local variation in geometry creates a non-uniform electric field distribution that promotes depletion near the gate while maintaining adequate carrier supply from the drift layer, thus resolving the contradiction between achieving high withstand voltage and maintaining low on-resistance for high saturation current.
2Speed
If the depletion layer is promoted in the vicinity of the gate electrode to reduce feedback capacitance and output capacitance, then high frequency characteristic is improved, but on-resistance increases
Solution Approach 1:
The tapered field plate structure creates a locally optimized electric field distribution. The narrower upper portion generates a stronger electric field concentrated near the gate electrode, effectively reducing feedback and output capacitances for improved high-frequency performance. Simultaneously, the wider lower portion ensures adequate depletion extension into the drift layer without excessive resistance increase, maintaining acceptable on-resistance levels.
Solution Approach 2:
The field plate geometry parameter (width) is changed continuously along the trench depth rather than being uniform. This parameter variation allows the electric field strength to be modulated spatially, creating a gradient that optimizes both capacitance reduction near the gate and resistance control in the drift layer, thereby resolving the trade-off between high-frequency characteristics and on-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 design effectively suppresses feedback and output capacitance while maintaining low on-resistance, enhancing high-frequency performance and power amplification by adjusting the electric field strength and depletion layer shape.
Implementation Method 1
a field plate embedded in a first trench formed adjacent to the gate electrode on the surface part of the drift layer with an insulating film interposed between the field plate and the first trench and insulated from the gate electrode
Implementation Method 2
Inclusion of the field plate in the vertical insulated gate transistor allows a depletion layer to be expanded in a thickness direction of a drift layer
Implementation Method 3
with an insulating film interposed between the field plate and the first trench and insulated from the gate electrode
Data Source
AI summary
A semiconductor device is provided with an N−-type drift layer, a P+-type diffusion well region provided on a surface part of the N−-type drift layer, a P-type channel well region, an N+-type diffusion well region, a gate insulating film, a gate electrode laminated on the gate insulating film, a drain trench, a field plate provided in the drain trench with a silicon oxide film and an insulating film interposed therebetween and a field plate electrode formed on the field plate. The field plate is tapered toward a base part of the drain trench. A distance between a side wall of the drain trench and a side face of the field plate is increased toward the base part side.