Normally Off HEMT Transistor Gate Structure for Low On-State Resistance
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Solution Overview
Problem
HEMTs with a normally off type face challenges in achieving a high threshold voltage while maintaining low ON state resistance, as increasing doping concentration in the doped gate region leads to undesired diffusion of dopant impurities, causing increased ON state resistance.
Innovation Solution
Incorporating a diffusion-control region made of gallium nitride between the doped gate region and the heterostructure, and a protection region made of intrinsic gallium nitride, with specific thicknesses and doping concentrations to control dopant impurity diffusion, and using a manufacturing method that includes selective etching and thermal annealing to activate dopants, preventing unwanted magnesium diffusion to the source and drain regions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the doping concentration in the doped gate region is increased to achieve a high threshold voltage, then the threshold voltage is improved, but the ON state resistance increases due to diffusion of dopant impurities to the source and drain regions
Solution Approach 1:
An undoped barrier layer made of gallium nitride is introduced between the doped gate region and the heterostructure. This intermediary layer physically blocks the diffusion of dopant impurities (magnesium) from the doped gate region to the source and drain regions, thereby preventing the increase in ON state resistance while allowing the doped gate region to maintain high doping concentration for achieving high threshold voltage
Solution Approach 2:
The gate structure is segmented into distinct regions: a doped gate region with high magnesium concentration for threshold voltage control, an undoped barrier layer to prevent dopant diffusion, and protected source/drain regions. This segmentation isolates the dopant source from the sensitive channel regions, resolving the contradiction between high threshold voltage and 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 allows for a good compromise between high threshold voltage and reduced ON state resistance by controlling dopant impurity diffusion, thereby preventing an increase in ON state resistance and maintaining efficient transistor performance.
Implementation Method 1
a step of thermal annealing of the wafer to activate the dopant impurities
Implementation Method 2
the dopant impurities spread during the annealing step into the heterostructure only in regions underneath the gate region
Data Source
AI summary
A manufacturing method of an HEMT includes: forming a heterostructure; forming a first gate layer of intrinsic semiconductor material on the heterostructure; forming a second gate layer, containing dopant impurities of a P type, on the first gate layer; removing first portions of the second gate layer so that second portions, not removed, of the second gate layer form a doped gate region; and carrying out a thermal annealing of the doped gate region so as to cause a diffusion of said dopant impurities of the P type in the first gate layer and in the heterostructure, with a concentration, in the heterostructure, that decreases as the lateral distance from the doped gate region increases.