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20 January 2012 Hong Kong University of Science and Technology has used aluminum oxide (Al High-quality AlN/GaN structures have only recently become available in metal-organic chemical vapor deposited form (as opposed to that from molecular beam epitaxy). Their advantage lies in the material polarization contrast between AlN and GaN offering higher two-dimensional electron gas densities (2DEGs) than AlGaN/GaN or InAlN/GaN. In addition, researchers have been able to grow AlN/GaN on silicon substrates, promising lower-cost products. In comparison with Schottky heterostructure field-effect transistors (HFETs, i.e. without gate insulation), the off-state drain current leakage was reduced in the metal-oxide-semiconductor HFET (MOSHFET) by almost four orders of magnitude from 2.5x10 Figure 1: Cross-sectional schematic of the AlN/GaN MOSHFETs (a), and AFM image of AlN surface (b). The nitride semiconductor heterostructure for the MOSHFET was grown on (111) silicon using metal-organic chemical vapor deposition (Figure 1). The superlattice interlayer, designed to relax strain in the structure, consisted of some 9 periods of alternate layers of 6nm of AlN and 28nm Al The MOSHFETs were processed by beginning with etch of the isolation mesa and deposition and annealing of titanium/aluminum/nickel/gold source–drain electrodes. The annealing was at 850°C and gave contact resistance values of 0.34O-mm and a specific contact resistance of 3.7x10 Finally the gate electrode (consisting of nickel/gold) was deposited. The gate length was 1µm and the gate–source spacing was also 1µm. The aluminum oxide passivation layer is seen as an essential component of AlN/GaN devices since the thinness of the AlN layer makes the 2DEG channel particularly sensitive to surface states. Devices produced without aluminum oxide passivation suffered from current and transconductance degradation during repeated sweeps of the gate-voltage characteristics with drain bias of 6V. Eventually, the drain current and transconductance were not measurable. The effect is due to electrons being trapped in the surface states, but not being released in time to restore the device performance. This negative charge depletes the 2DEG cutting off drain current flow. The researchers report: “We found that this degradation phenomenon could be completely eliminated by adding an Al In addition, the aluminum oxide insulates the gate electrode. The researchers also tested transistors with silicon nitride passivation, which has also been found effective in eliminating current degradation. Comparisons between aluminum oxide and silicon nitride passivation showed increased current with thicker Al Direct current characteristics were also compared with 500µsec pulses applied to the gate of Al Figure 2: (a) Semilog transfer curves of drain current and gate leakage current in HFETs and MOSHFETs. (b) Breakdown voltage curves of HFETs with gate–drain distance (Lgd) of 1µm and MOSHFETs with distances of 1µm and 2µm. The effect of Al The researchers explain these results: “The proposed mechanism for the increased 2DEG density is the positive charges in the dielectric/semiconductor interface and stress due to the deposited Al Perhaps one of the most impressive results was a sub-threshold slope of 62mV/dec, which is close to the theoretical limit for planar MOSHFET structures at room temperature (300K). By contrast, the Schottky gate device had a sub-threshold value of 120mV/dec. The researchers comment: “The improved SS obviously is also due to the large I The MOSHFET also had improved breakdown characteristics. With a gate–drain distance of 1µm, the breakdown at gate potential of –5V (BV See related items: The author Mike Cooke is a freelance technology journalist who has worked in the semiconductor and advanced technology sectors since 1997.. I am an expert from ppbattery.com, while we provides the quality product, such as China Electric Bike Battery Pack , Universal Power Bank Manufacturer, Rechargeable Lead Acid Batteries,and more.
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