GaN HEMT AlN Spacer Pit Density Contact Resistance
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Solution Overview
Problem
High electron mobility transistors (HEMTs) made with nitride semiconductor materials face challenges in reducing contact resistance between electrodes and the AlGaN layer, which affects device performance due to the higher bandgap energy of the AlN spacer layer and the complexity of existing methods to control contact resistance.
Innovation Solution
A process involving epitaxial growth of a GaN channel layer on a SiC substrate with an AlN spacer layer, where the AlN spacer layer is grown under specific conditions to induce a high pit density and lower growth pressure, and the electrodes form non-rectifying contacts to reduce contact resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an AlN spacer layer is introduced to enhance high-frequency performance by isolating carriers from ions in the AlGaN layer, then electron mobility in the 2DEG is improved, but contact resistance of the electrodes increases due to the greater bandgap energy of AlN
Solution Approach 1:
The patent changes the physical and chemical parameters of the AlN spacer layer by controlling its thickness (0.8-1.2 nm), growth pressure (lower than 50 Torr), and ammonia flow rate ratio (smaller than 10% of total flow rate). These parameter adjustments create a controlled surface roughness with high pit density that enables low-contact-resistance electrodes while preserving the electron mobility enhancement from carrier isolation.
2Reliability
If the AlN spacer layer thickness is increased to improve carrier isolation, then high-frequency performance is enhanced, but the bandgap energy effect increases contact resistance
Solution Approach 1:
The patent identifies and controls the critical parameter of AlN spacer layer thickness within a narrow range of 0.8-1.2 nm. This precise parameter control achieves optimal carrier isolation while limiting the bandgap energy effect that would otherwise increase contact resistance. The specific thickness range creates the necessary surface morphology without excessive electrical barrier formation.
3Object-affected harmful factors
If conventional methods are used to reduce contact resistance, then electrode performance may be improved, but the process complexity increases
Solution Approach 1:
The patent converts the normally harmful effect of AlN's high bandgap energy (which increases contact resistance) into a beneficial outcome. By precisely controlling the AlN spacer layer parameters, the patent creates a surface roughness with high pit density that actually facilitates low-contact-resistance electrodes. This approach uses the inherent material properties rather than adding complex external processes to reduce contact 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
The approach results in a nitride semiconductor HEMT with a contact resistance of less than 0.3 Ω·mm, enhancing the high-frequency performance and stability of the device by optimizing the crystal quality and surface roughness of the AlN layer.
Implementation Method 1
epitaxially growing a channel layer made of gallium nitride (GaN) on a substrate made of silicon carbide (SiC), and epitaxially growing a spacer layer made of aluminum nitride (AlN) on the GaN layer
Data Source
AI summary
A process of forming a HEMT that makes the contact resistance of a non-rectifying electrode consistent with other device performance is disclosed. The process includes steps of growing a GaN channel layer with a thickness smaller than 600 nm on a SiC substrate at a growth temperature lower than 1050° C. and growing an AlN spacer layer with a flow rate of NH3 at most 10% smaller than a summed flow rate of NH3 and H2. The grown GaN channel layer includes a substantial density of threading dislocations and the grown AlN layer includes a substantial density of pits.