Although significant progress has been achieved in the GaN-based high-power/high-frequency electronic devices such as AlGaN/GaN heterostructure field effect transistors (HFETs) [1][2][3], it is necessary to use thinner AlGaN layer for achieving higher transconductance (g m ) and more precise control of threshold voltage in HFETs. To develop normally-off (enhancement) mode devices which are attractive for gaining in flexibility of circuit and/or system design, in addition, very thin AlGaN barrier thickness less than 10nm is required. A gate-recessing process is one of the actual approaches to reduce the effective thickness of a barrier layer.However, Schottky contacts fabricated on GaN and AlGaN still suffer from serious leakage problems [4][5][6][7][8][9]. Although some models associated with the trap-assisted tunneling [10], the defect-related thin surface barrier (TSB) [8] and the dislocation-related hopping transport [11] have been proposed, the leakage mechanism through GaN and AlGaN Schottky interfaces has not yet been clarified, and thereby there is still no solution to suppress leakage currents. For Schottky-gate (SG) structures on AlGaN/GaN HEMTs with thinner AlGaN barrier layers or recessed-gate structures, leakage problems can be enhanced, making the gate control of drain current very difficult.An FET device having an insulated gate (IG) structure is expected to suppress the gate leakage. Moreover, an insulator film can act as a passivation layer, making the surface more stable in the device. An Al 2 O 3 IG structure is very attractive for the application to AlGaN/GaN HFETs [12][13][14], since it has relatively high dielectric constant (~ 9) and a large conduction-band offset at the Al 2 O 3 /AlGaN interface [12]. In fact, the Al 2 O 3 IG AlGaN/GaN HFETs exhibited good gate control of drain current with low leakage currents, and suppressed current collapse under both drain stress and gate stress [12].In this letter, we demonstrate the controllability of an Al 2 O 3 insulated-gate structure in the AlGaN/GaN HFETs having a thin AlGaN barrier layer (7 nm).The Al 0.2 Ga 0.8 N/GaN heterostructures was grown by metal organic vapor phase epitaxy on n-type 6H-SiC substrates, as schematically shown in Fig.1. A very thin AlGaN layer (7 nm) was grown with doping of Si (2 x 10 18 cm -3 ). The electron concentration and mobility of the sample at room temperature (RT) were 4.0 x 10 12 cm -2 and 830 cm 2 /Vs, respectively. The device isolation was carried out by an electron-cyclotron-resonance (ECR) assisted reactive ion beam etching using a gas system consisting of CH 4 , H 2 , Ar and N 2 [15]. As an ohmic contact, a Ti/Al/Ti/Au layered structure was formed on the surface of AlGaN/GaN followed by annealing at 800 o C for 1 min in N 2 ambient. A Ni/Au contact was used as a Schottky gate.The Al 2 O 3 -based surface passivation structure was fabricated through the following in-situ steps [16]: The AlGaN surface was treated in ECR-N 2 plasma at 280 o C for 30 s. Then an Al layer with a nominal thickness of 3 nm was depo...
Exposure of Pt/GaN and Pt/AlGaN/GaN Schottky diodes to H 2 gas at moderately high temperatures around 100 o C resulted in marked increase of forward and reverse currents. Increase was much larger in the Pt/AlGaN/GaN diode than in the Pt/GaN diode. Rapid turn-on responses and somewhat slower turn-off responses were observed with reproducible response magnitudes. A rigorous computer simulation of I-V curves indicated that current changes are due to changes in the Schottky barrier height caused either by H-induced formation of interfacial dipole or by hydrogen passivation of interface states.
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A normally-off AlGaN/GaN MOS heterojunction field-effect transistor (MOS-HFET) with a recessed gate structure formed by selective area regrowth is demonstrated. The fabricated MOS-HFET exhibits a threshold voltage of 1.7 V with an improved hysteresis of 0.5 V as compared with a device fabricated by a conventional dry etching process. An analysis of capacitance–voltage (C–V) characteristics reveals that the dry etching process increases interface state density and introduces an additional discrete trap. The use of the selective area regrowth technique effectively suppresses such degradation, avoiding the MOS interface from being exposed to dry etching. The results presented in this paper indicate that the selective area regrowth technique is promising for the fabrication of normally-off AlGaN/GaN MOS-HFETs.
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