GaN HFET Interface Resistance Reduction via Selective Regrowth
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Solution Overview
Problem
AlXGa1-XN/GaN Heterojunction Field Effect Transistors (HFETs) face high contact and access resistance due to the resistive nature of wide bandgap materials, limiting their high-frequency and power performance, and existing ohmic contact techniques result in inconsistent contact resistance and limited device design flexibility.
Innovation Solution
Exposing the Barrier Layer on one side of the gate and regrowing a doped Cap layer of crystalline Group III-Nitride to increase the 2DEG charge density at the source/drain interface, while maintaining charge density below the gate, thereby reducing interface resistance through bandgap engineering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If ohmic contacts are made by annealing a Ti/Al-containing metal stack to drive metal through the wide bandgap barrier layer, then contact is achieved to the 2DEG, but the contact resistance is high and inconsistent
Solution Approach 1:
The patent changes the fundamental approach from metal annealing to semiconductor regrowth, altering the material state and formation process. By regrowing n+-doped GaN cap layer instead of annealing metal stacks, the contact resistance is reduced and made more consistent through controlled epitaxial growth parameters
Solution Approach 2:
The patent replaces the mechanical/thermal process of metal annealing and diffusion with a chemical epitaxial regrowth process. Instead of driving metal through the barrier layer using thermal energy, the invention uses selective area regrowth of doped GaN to create low-resistance contacts
2Speed
If the effective source-drain distance is reduced for improved high-frequency performance, then frequency response improves, but the interface resistance increases
Solution Approach 1:
The patent applies local quality by creating n+-doped GaN cap layer specifically in the contact regions through selective area regrowth. This localized doping increases charge density and reduces interface resistance at the source/drain contacts without affecting the overall device dimensions or channel length, thereby maintaining high-frequency performance
3Ease of manufacture
If conventional ohmic contact techniques are used, then contact formation is achieved, but device design flexibility is limited
Solution Approach 1:
The patent introduces dynamics to the contact formation process by using selective area regrowth that can be dynamically controlled through masking patterns and growth conditions. This allows flexible device design including different contact configurations, scaled dimensions, and integrated contact/active region definition in a single epitaxial process
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
Significantly reduces interface resistance between the source/drain and channel layer, enhancing high-frequency performance and device design flexibility by increasing charge density at the interface without perturbing the charge density below the gate.
Implementation Method 1
By bandgap engineering through the use of particular thicknesses and compositions of the Barrier Layer and the doped Cap layer, the 2DEG charge density can be increased in the interface area between the source/drain contacts and the channel layer
Data Source
AI summary
The interface resistance between the source/drain and gate of an HFET may be significantly reduced by engineering the bandgap of the 2DEG outside a gate region such that the charge density is substantially increased. The resistance may be further reduced by using an n+GaN Cap layer over the channel layer and barrier layer such that a horizontal surface of the barrier layer beyond the gate region is covered by the n+GaN Cap layer. This technique is applicable to depletion and enhancement mode HFETs.