Mini Field Plate T-Gate Structure for Low-Capacitance GaN HEMTs
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Solution Overview
Problem
Existing AlGaN/GaN high electron mobility transistors (HEMTs) face challenges in high-frequency operation due to increased gate capacitance and dynamic on-resistance caused by field plate structures, which also affect cutoff-frequency (fT) and maximum frequency (fmax).
Innovation Solution
A mini field plate structure is introduced, comprising a tri-layer gate with a gate foot, neck, and head, where the neck is wider than the foot and supported by a dielectric, reducing parasitic capacitance and enhancing mechanical strength, while maintaining a low gate capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a field plate structure is used to reduce maximum electric field intensity, then breakdown voltage is improved, but gate capacitance increases leading to reduced cutoff-frequency and maximum frequency
Solution Approach 1:
The gate structure is segmented into three distinct layers: gate foot, gate neck, and gate head. This segmentation allows each portion to serve specific functions - the gate foot provides mechanical support, the gate neck transitions the width, and the gate head forms the active gating region. This segmented approach enables reduction of parasitic capacitance while maintaining the field plate's voltage blocking capability.
Solution Approach 2:
The invention transitions from a conventional planar gate structure to a three-dimensional T-shaped gate structure with varying widths in different regions. The gate neck extends over the dielectric passivation layer, creating a vertical dimension that reduces parasitic capacitance between the gate and drain while maintaining effective gate control. This dimensional change allows simultaneous achievement of high breakdown voltage and high frequency performance.
2Reliability
If a longer field plate is used to suppress traps across the gate-drain region, then reliability is improved, but capacitance increases drastically inhibiting high frequency operation
Solution Approach 1:
The field plate structure is applied locally rather than uniformly across the entire gate region. The gate foot provides localized mechanical support and field control at the gate-drain junction, while the gate neck and gate head provide controlled field management over the channel. This localized application of field plate concepts reduces overall capacitance while maintaining trap suppression where most needed.
Data Source
Figure 1A~1D
Figure 1E~1G
Figure 2~3
AI summary
A method of fabricating a gate with a mini field plate includes forming a dielectric passivation layer over an epitaxy layer on a substrate, coating the dielectric passivation layer with a first resist layer, etching the first resist layer and the dielectric passivation layer to form a first opening in the dielectric passivation layer, removing the first resist layer; and forming a tri-layer gate having a gate foot in the first opening, the gate foot having a first width, a gate neck extending from the gate foot and extending for a length over the dielectric passivation layer on both sides of the first opening, the gate neck having a second width wider than the first width of the gate foot, and a gate head extending from the gate neck, the gate head having a third width wider than the second width of the gate neck.