Ohmic Electrode Structure for GaN Transistors
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Solution Overview
Problem
Gallium nitride field effect transistors (GaNFETs) face issues with unstable ohmic properties due to the use of Ti/Al electrodes, which form Schottky barriers, experience electromigration, and surface roughening at high temperatures, and are prone to corrosion in humid environments due to exposed aluminum.
Innovation Solution
A Ta/Al stacked structure with an additional metal layer of tantalum, nickel, palladium, or molybdenum is used for the ohmic electrodes, forming a compound layer that enhances thermal stability and mechanical strength, while a cap layer provides moisture resistance to prevent corrosion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Ti/Al electrode is used to form ohmic contact, then contact resistivity is reduced, but Schottky barrier is formed due to work function mismatch
Solution Approach 1:
The patent uses a composite electrode structure consisting of multiple metal layers (Ti/Al/Ti or Ti/Al/Pt) instead of a single Ti/Al layer. This composite structure combines materials with different work functions to achieve both low contact resistivity and prevent Schottky barrier formation, resolving the contradiction between ohmic contact stability and Schottky barrier prevention.
2Reliability
If Ti/Al electrode is annealed to obtain ohmic properties, then contact quality improves, but electrode surface becomes coarse and irregularities form
Solution Approach 1:
The multi-layer composite electrode structure (Ti/Al/Ti or Ti/Al/Pt) maintains surface flatness during annealing because the outer Ti or Pt layers have higher melting points and better thermal stability than Al alone. This prevents the surface coarsening that occurs with simple Ti/Al structures while still achieving the desired ohmic properties through controlled annealing.
Solution Approach 2:
The patent applies a protective outer layer (Ti or Pt) over the Al layer before annealing. This outer layer acts as a cushion that prevents direct surface degradation during the annealing process, maintaining electrode surface flatness while allowing the internal Al layer to form the necessary ohmic contact with the semiconductor.
3Reliability
If Al is used as electrode material, then ohmic contact is achieved, but Al atom moves during high temperature operation causing electromigration
Solution Approach 1:
The patent creates a composite electrode where Al is sandwiched between Ti or Pt layers. The Al layer provides the necessary ohmic contact properties, while the outer Ti/Pt layers with higher melting points and lower diffusivity prevent Al atom migration during high-temperature operation, thus preventing electromigration and maintaining compositional stability.
Solution Approach 2:
The patent transitions from a single-layer Al electrode to a multi-layer vertical structure (Ti/Al/Ti or Ti/Al/Pt). This dimensional change creates a confined geometry where the Al layer is sandwiched between stabilizing layers, physically constraining Al atom movement and preventing electromigration while maintaining the ohmic contact function.
4Reliability
If Al layer is exposed on electrode surface, then ohmic contact is optimized, but electrode surface is corroded in humid environment
Solution Approach 1:
The patent uses a composite electrode structure where Ti or Pt outer layers cover the Al layer. This composite structure maintains optimized ohmic contact through the Al layer while the Ti/Pt outer layers provide corrosion resistance in humid environments, preventing moisture from reaching and corroding the Al surface.
Solution Approach 2:
The Ti or Pt outer layers act as intermediary protective barriers between the Al layer and the humid environment. These intermediary layers allow the Al layer to maintain its optimized ohmic contact function while protecting it from moisture corrosion, effectively mediating between the conflicting requirements of surface exposure for contact optimization and surface protection for corrosion prevention.
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 solution improves the reliability of ohmic electrodes in both high temperature and high humidity environments by suppressing electromigration and surface roughening, ensuring stable and durable electrode performance.
Implementation Method 1
a metal layer formed on the aluminum layer and made of any one material of tantalum, nickel, palladium, and molybdenum
Implementation Method 2
annealing at temperatures lower than 600° C., and thus forming an ohmic electrode
Data Source
AI summary
A semiconductor device is configured so as to comprise a substrate, an n-type semiconductor layer or an undoped semiconductor layer on the substrate, and an ohmic electrode on the n-type semiconductor layer or the undoped semiconductor layer, and the ohmic electrode is configured so as to comprise a tantalum layer formed on the n-type semiconductor layer or the undoped semiconductor layer, an aluminum layer formed on the tantalum layer, and a metal layer formed on the aluminum layer and made of any one material of tantalum, nickel, palladium, and molybdenum.