Tech Briefs

Direct wafer bonding creates ideal channels made of extremely mismatched materials for field effect transistors.

Narrow bandgap semiconductors offer high carrier mobilities and low contact resistances, while wide bandgap semiconductors offer high breakdown voltages. A series of heterojunction transistors has been investigated and proved to be effective for improving both speed and power output. These devices include double heterostructure InP/InGaAs/InP bipolar transistors and composite-channel InAlAs/InGaAs/lnP/InAlAs high-electron-mobility transistors (HEMTs), which have taken full advantage of the matched lattice constant (or pseudomorphic growth).

However, for the most popular wide bandgap semiconductor GaN and SiC, the lattice mismatch between GaN and semiconductors with a reasonably small bandgap (including InGaN) is so large that pseudomorphic growth is very difficult. For instance, the critical thickness of InN on GaN is about one monolayer. To marry the advantages offered by both narrow bandgap and wide bandgap semiconductors, direct wafer bonding was explored for ideal channels made of extremely mismatched materials for field effect transistors.

GaAs was integrated with GaN using wafer fusion, also called direct wafer bonding. GaAs was chosen over InGaAs or InSb because it is binary and easy to obtain. The stable phase for GaAs is a zinc blend. The wafer fusion experiments not only successfully demonstrated dislocation-free integration of extremely lattice-mismatched GaAs and GaN, but also showed the improvement in breakdown voltage by employing a wide bandgap material.

This investigation demonstrated it is feasible to fabricate composite channel transistors; however, more experiments are necessary to understand the effects of the interface between the mismatched materials.

This work was done by Huili (Grace) Xing of the University of Notre Dame for the Office of Naval Research. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Electronics/Computers category. ONR-0019

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Channel Field Effect Transistors (reference ONR-0019) is currently available for download from the TSP library.

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