pHEMT Ohmic Contact Layer Reduces Resistance
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Solution Overview
Problem
Pseudomorphic high electron mobility transistors (pHEMTs) face performance limitations due to high source and drain contact resistances, which affect maximum output current, transconductance, and high-frequency performance.
Innovation Solution
Incorporating a graded low bandgap Ohmic contact layer, such as InxGa1-xAs, between the source/drain metal contacts and cap layers, with increasing Indium content from the bottom to the top to reduce lattice mismatch and strain, and forming a gate metal stack between the source and drain terminals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional metal contact structure is used directly on the cap layer, then the device structure is simple, but the source and drain contact resistances are high, limiting performance
Solution Approach 1:
An Ohmic contact layer with smaller bandgap than the cap layer is introduced as an intermediary between the metal contact and the cap layer. This intermediate layer facilitates better electrical contact by reducing the Schottky barrier height, thereby lowering contact resistance without significantly complicating the overall device structure.
Solution Approach 2:
The bandgap parameter of the contact structure is changed by selecting materials with progressively smaller bandgaps from the cap layer down to the Ohmic contact layer. This parameter change enables more efficient charge carrier transport and reduces contact resistance at the metal-semiconductor interface.
2Reliability
If materials with different lattice constants are used for heterojunction, then larger bandgap differences are achieved improving transistor performance, but crystal defects increase due to lattice mismatch
Solution Approach 1:
The heterostructure employs layers with locally optimized material compositions and bandgaps. Each layer is specifically designed with particular material properties to achieve the desired bandgap differences for high-performance operation while managing lattice mismatch through localized composition adjustments.
Solution Approach 2:
The device utilizes composite material structures with multiple semiconductor layers having different compositions and properties. This composite approach allows optimization of electrical performance through bandgap engineering while managing structural integrity through careful selection of lattice-matched or graded material combinations.
3Reliability
If the Ohmic contact layer has the same bandgap as the cap layer, then the material system is simple, but the contact resistance remains high
Solution Approach 1:
The key parameter changed is the bandgap of the contact layer. By selecting materials with progressively smaller bandgaps from the cap layer to the Ohmic contact layer, the structure enables more efficient electrical contact. This parameter change directly addresses the high contact resistance issue without requiring overly complex material systems.
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 configuration minimizes lattice mismatch, reduces strain, and lowers source and drain contact resistances, thereby enhancing the high-frequency performance and current gain of pHEMTs.
Implementation Method 1
an Ohmic contact layer on the cap layer... The Ohmic contact layer has a smaller bandgap than the cap layer
Implementation Method 2
Incorporating a graded low bandgap Ohmic contact layer, such as InxGa1-xAs, between the source/drain metal contacts and cap layers, with increasing Indium content from the bottom to the top to reduce lattice mismatch and strain
Data Source
AI summary
In certain aspects, a pseudomorphic high electron mobility transistor (pHEMT) comprises a substrate layer, a bottom barrier layer on the substrate layer, a channel layer on the bottom barrier layer, an upper barrier on the channel layer, and a source and a drain on the upper barrier layer. The source and the drain each has a cap layer, an Ohmic contact layer on the cap layer, and a metal contact layer on the Ohmic contact layer. The Ohmic contact layer has a smaller bandgap than the cap layer. The pHEMT further comprises a gate metal stack on the upper barrier layer.