Effective Treatment on AlGaN/GaN MSM-2DEG Varactor with (NH[sub 4])[sub 2]S/P[sub 2]S[sub 5] Solution

Abstract

The effect of surface passivation using ͑NH 4 ͒ 2 S and ͑NH 4 ͒ 2 S/P 2 S 5 on a AlGaN/GaN-based metal-semiconductor-metal diode above a two-dimensional electron gas ͑MSM-2DEG͒ varactor was investigated. The surface property, capacitance ratio ͑C max /C min ͒, and leakage current of the prepared samples were studied before and after treatments using X-ray photoelectron spectroscopy and capacitance-voltage and current-voltage analyses. It showed that the ͑NH 4 ͒ 2 S/P 2 S 5 -treated sample had the most excellent surface state and C max /C min and the least leakage current because of either reduced native oxide or deposited phosphorus compounds only provided by ͑NH 4 ͒ 2 S/P 2 S 5 and sulfide upon the surface, also validated by having the highest sheet carrier density. Hence, these promising results promote further potential for varactor applications.The metal-semiconductor-metal diode above a two-dimensional electron gas ͑MSM-2DEG͒ has shown its potential as a varactor that can be easily integrated with high electron mobility transistor devices. In addition, its voltage-dependent capacitance ratio is much larger than that of conventional varactor diodes and can be tuned by electrode geometry in contrast to the conventional p-n, Schottky, or heterostructure diodes where the ratio is only defined by the layer structure. 1-3 After the development of SiO 2 /AlGaN/GaN-based double metal-oxide-semiconductor heterojunction capacitors with reduced leakage current, the MSM-2DEG based on this layer structure was proposed as a robust radio-frequency switch. 4 Most of these applications seek the large capacitance ratio. However, except by tuning the electrode geometry, few investigations on the improvement of capacitance ratio obtaining the superior varactor performance are published. 5,6 Marso et al. 5 reported that metal-oxidesemiconductor heterojunction field effect transistor ͑MOSHFET͒ MSM with an oxide layer between the metal and the semiconductor decreased the C max and made the capacitance-voltage ͑C-V͒ characteristic asymmetric even though it could reduce the leakage current. Besides, they also indicated that better controllable capacitance ratio and stable C-V properties could be obtained in the heterostructure field effect transistor ͑HFET͒ MSM without an oxide layer between the metal and the semiconductor, but it was with a worse leakage current. It was reported that sulfur treatment is very effective in reducing the surface states and surface recombination velocity in III-V compound semiconductors. 7-12 Of the various sulfur treatments, only the ͑NH 4 ͒ 2 S x treatment achieved promising results due to its capacity to etch the native oxide and the GaAs surface and to tie up the dangling bonds with sulfur on a freshly exposed prismatic GaAs surface. Although increasing immersion time promotes the performance, longer immersion time causes a higher surface roughness and a decline in the mobility of electrons by high sulfide contamination and chemical reaction. Hence, hot ͑NH 4 ͒ 2 S x treatment or ͑NH 4 ͒ 2 S x + UV ...

Improvement of Surge Protection by Using an AlGaN/GaN-Based Metal–Semiconductor–Metal Two-Dimensional Electron Gas Varactor

“…And then the serial equivalent capacitance is dominated by the relative low value 2DEG capacitance. Due to the (NH 4 ) 2 S/P 2 S 5 -treated case has a better surface state by reducing the native oxide and increasing the extra stable phosphorus compounds and sulfide on the grown surface, as demonstrated in our previous study, 15) the improvement of nonlinear behaviors, including the improved leakage current and the enlarged capacitance swing, are obtained. Furthermore, an improved Schottky barrier height (0.66 eV) of (NH 4 ) 2 S/P 2 S 5 -treated sample, by calculating from the measured I-V curves and using the thermionic emission model, 18) is obtained as compared to that (0.65 eV) of the untreated sample.…”

“…More details can be found in our previous report. 15) The DC properties, including current-voltage (I-V ) and capacitance-voltage (C-V ), were measured by using a Keithley 2430 pulse mode source meter and an HP 4285A precision LCR meter, respectively. In addition, the frequency response was taken by an Agilent E8362C network analyzer.…”

Enhanced capacitance ratio and low minimum capacitance of varactor devices based on depletion-mode Ga-doped ZnO TFTs with a drain-offset structure

“…varactors, are essential in various RF circuit applications such as voltage-controlled oscillators, tunable filters and frequency multipliers [22]. For applications involving high frequencies and a wide frequency range, varactors having both small capacitances and high capacitance tuning ratios are required [23]. Also, it is critically important that varactors need to be integrated together with high-speed devices on the same substrate [23].…”

“…For applications involving high frequencies and a wide frequency range, varactors having both small capacitances and high capacitance tuning ratios are required [23]. Also, it is critically important that varactors need to be integrated together with high-speed devices on the same substrate [23]. In developing ZnO TFT-based RF circuits, varactors based on ZnO TFTs are highly desirable as these can be easily integrated with ZnO TFT circuits.…”