Publications

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Photodetectors sensitive to the ultra-violet spectrum were demonstrated using an AlGaN high electron mobility transistor with an GaN nanodot optical floating gate. Peak responsivity of 2 x 10⁹ A/W was achieved with a gain-bandwidth product > 1 GHz at a cut-on energy of 4.10 eV. Similar devices exhibited visible-blind rejection ratios > 10⁶. The photodetection mechanism for $β$-Ga₂O₃ was also investigated. It was concluded that Schottky barrier lowering by self-trapped holes enables photodetector gain.

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AlGaN-channel high electron mobility transistors (HEMTs) are among a class of ultra wide-bandgap transistors that are promising candidates for RF and power applications. Long-channel Al_xGa_1-xN HEMTs with x = 0.7 in the channel have been built and evaluated across the -50°C to +200°C temperature range. These devices achieved room temperature drain current as high as 46 mA/mm and were absent of gate leakage until the gate diode forward bias turn-on at ~2.8 V, with a modest -2.2 V threshold voltage. A very large I_on/I_off current ratio, of 8 × 10⁹ was demonstrated. A near ideal subthreshold slope that is just 35% higher than the theoretical limit across the temperature range was characterized. The ohmic contact characteristics were rectifying from -50°C to +50°C and became nearly linear at temperatures above 100°C. An activation energy of 0.55 eV dictates the temperature dependence of off-state leakage.

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Here, we present a low resistance, straightforward planar ohmic contact for Al_0.45Ga_0.55N/Al_0.3Ga_0.7N high electron mobility transistors. Five metal stacks (a/Al/b/Au; a = Ti, Zr, V, Nb/Ti; b = Ni, Mo, V) were evaluated at three individual annealing temperatures (850, 900, and 950°C). The Ti/Al/Ni/Au achieved the lowest specific contact resistance at a 900°C anneal temperature. Transmission electron microscopy analysis revealed a metal-semiconductor interface of Ti-Al-Au for an ohmic (900°C anneal) and a Schottky (850°C anneal) Ti/Al/Ni/Au stack. HEMTs were fabricated using the optimized recipe with resulting contacts that had room-temperature specific contact resistances of ρ_c = 2.5 × 10^-5 Ω cm², sheet resistances of R_SH = 3.9 kΩ/$\blacksquare$, and maximum current densities of 75 mA/mm (at VGATE of 2 V). Electrical measurements from -50 to 200°C had decreasing specific contact resistance and increasing sheet resistance, with increasing temperature. These contacts enabled state-of-the-art performance of Al_0.45Ga_0.55N/Al_0.3Ga_0.7N HEMTs.

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Emerging ultrawide bandgap semiconductor materials are logical candidates for applications that exploit the large critical electric field (EC) associated with these devices. For semiconductor devices, EC scales approximately as Eg2 5 [1]. With a 25% larger bandgap and an approximately 73% larger EC than GaN, Al0.3Ga0.7N-channel high election mobility transistors (HEMTs) might be viable candidates for harsh environment electronics or short wavelength photo-transistors. In either case, transistor quality factors such as minimal off-state leakage currents and subthreshold slope factor are important metrics.

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Due to the ultra-wide bandgap of Al-rich AlGaN, up to 5.8 eV for the structures in this study, obtaining low resistance ohmic contacts is inherently difficult to achieve. A comparative study of three different fabrication schemes is presented for obtaining ohmic contacts to an Al-rich AlGaN channel. Schottky-like behavior was observed for several different planar metallization stacks (and anneal temperatures), in addition to a dry-etch recess metallization contact scheme on Al0.85Ga0.15N/Al0.66Ga0.34N. However, a dry etch recess followed by n+-GaN regrowth fabrication process is reported as a means to obtain lower contact resistivity ohmic contacts on a Al0.85Ga0.15N/Al0.66Ga0.34N heterostructure. Specific contact resistivity of 5 × 10−3 Ω cm2 was achieved after annealing Ti/Al/Ni/Au metallization.

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AlGaN-channel high electron mobility transistors (HEMTs) are among a class of ultra wide-bandgap transistors that have a bandgap greater than ∼3.4 eV, beyond that of GaN and SiC, and are promising candidates for RF and power applications. Long-channel AlxGa1-xN HEMTs with x = 0.3 in the channel have been built and evaluated across the −50◦C to +200◦C temperature range. Room temperature drain current of 70 mA/mm, absent of gate leakage, and with a modest −1.3 V threshold voltage was measured. A very large Ion/Ioff current ratio, greater than 108 was demonstrated over the entire temperature range, indicating that off-state leakage is below the measurement limit even at 200◦C. Combined with near ideal subthreshold slope factor that is just 1.3× higher than the theoretical limit across the temperature range, the excellent leakage properties are an attractive characteristic for high temperature operation.