Comparison of AlGaN/GaN high electron mobility transistors grown on AlN/SiC templates or sapphire
Introduction
AlGaN/GaN high-electron mobility transistors (HEMTs) are very promising for high-voltage and high-temperature microwave applications. In these heterostructures grown on substrates producing the wurtzite crystal structure, polarization effects are present which strongly influence the electron density and potential profile [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. The AlGaN/GaN devices can operate at drain voltages more than an order of magnitude higher than comparable AlGaAs/GaAs HEMTs, even for very short channel lengths (0.07 μm) [3]. Currently, most devices are fabricated on heteroepitaxial material grown on sapphire substrates because of the relatively low cost and general availability of these substrates. For power applications, however, it is desirable to have a substrate with higher thermal conductivity, such as SiC [14], [17]. Additional research is needed to optimize the growth and device performance of nitride-based HEMTs on SiC substrates. One way to reduce lattice mismatch between the device structure and SiC substrate is to grow devices on an AlN/SiC template consisting of a thin insulating AlN layer grown on SiC. In this paper, we report a detailed investigation of the dc performance of AlGaN/GaN HEMTs grown side-by-side on either sapphire or AlN/SiC substrates. The gate length dependence of the dc parameters as well as the temperature dependence of transconductance, drain current and forward and reverse gate current characteristics were measured for both types of devices.
Section snippets
Experimental
The Al2O3 and AlN/SiC template were first deposited with a thin AlN buffer, and then the layer structure shown in Fig. 1 was grown by Metal Organic Chemical Vapor Deposition at ∼1040 °C using conventional precursors. The AlN/SiC template was fabricated by hydride vapor phase epitaxy (HVPE) of ∼100 nm AlN on the (0 0 0 1)Si face of on-axis 6H-SiC substrate (conducting n∼1017 cm−3). Such AlN layers are single crystal and display electrical resistivity of about 109 at room temperature and 106
Gate length dependence
Fig. 4 shows ID–VDS characteristics for 0.75 μm gate length HEMTs on both types of substrates. Higher drain currents were obtained with the devices on sapphire but for both types of HEMTs there was no current collapse observed due to hot electron injection and trapping in the buffer layer.
Similar behavior was obtained with shorter gate length devices, as shown in Fig. 5 for 0.25×100 μm2 devices on both types of substrates. There is excellent pinch-off for both types of devices and slightly
Summary and conclusions
A direct comparison of the dc performance of AlGaN/GaN HEMTs grown on sapphire substrates or AlN/SiC templates shows the following main points:
- 1.
The devices on AlN/SiC do not show any self-heating effects even for relatively high-voltage (40 V) operation. By sharp contrast, the HEMTs on sapphire show pronounced self-heating effects on the I–V characteristics for measurement temperatures up to ∼200 °C.
- 2.
The structures grown on AlN/SiC show significantly lower defect densities measured by TEM than
Acknowledgements
The work at UF is partially supported by NSF grants CTS-991173 and DMR-0101438. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin company, for the United States Department of Energy under contract DE-AC04 94 AL 85000. The work at TDI was supported by Ballistic Missiles Defense/Innovative Science and Technology and managed by the office of Naval Research (contract manager Colin Wood).
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