Ya-Lan Chiou

AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistors with ZnO gate layer and (NH4)2Sx surface treatment

The AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) with ZnO gate insulator deposited using a vapor cooling condensation system were fabricated. The AlGaN surface treatment using (NH4)2Sx was performed to improve the quality of the interface between the ZnO layer and AlGaN layer. The (NH4)2Sx-treated MOS-HEMTs exhibited a higher saturation drain-source current of 0.74 A/mm, a maximum extrinsic transconductance of 200 mS/mm, an unit gain cutoff frequency of 9.1 GHz, a maximum frequency of oscillation of 17.1 GHz, and the Hooge’s coefficient of 8.28×10−6. The improved performances of the (NH4)2Sx-treated MOS-HEMTs were attributed to the reduction in surface states.

AlGaN/GaN MOS-HEMTs With Gate ZnO Dielectric Layer

The vapor cooling condensation system is used to grow ZnO insulator films of low carrier concentration and high resistivity as the gate dielectrics for AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs). The saturation drain-source current and the maximum extrinsic transconductance are measured as 0.61 A/mm and 153 mS/mm, respectively. The gate leakage currents, determined with the forward gate bias of VGS = 3.5 V and the reverse gate bias of VGS = -12 V, applied are 1.21 × 10-4 A/mm and 7.16 × 10-6 A/mm, respectively. The unit gain cutoff frequency and maximum frequency of the oscillation are also measured as 7.2 and 11.5 GHz, respectively. The low-frequency noise obtained is well fitted with a 1/f function in the linear region. Hooges coefficient α is extracted as 9.74 × 10-5 when the MOS-HEMTs operate at 100 Hz and VGS = -4 V . The current recoveries of the gate and drain lags are determined to be 61% and 47% for the MOS-HEMTs, respectively.

Photoelectrochemical Function in Gate-Recessed AlGaN/GaN Metal–Oxide–Semiconductor High-Electron-Mobility Transistors

Photoelectrochemical (PEC) wet etching and oxidation methods were used for fabricating gate-recessed AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs). The AlGaN layer was recessed by the PEC wet etching method. The PEC oxidation method was then performed to directly grow an oxide film on the recessed surface of the AlGaN layer as gate dielectric film and passivation of the surface. The gate-recessed AlGaN/GaN MOS-HEMTs exhibited a saturation drain-source current of 642 mA/mm at VGS = 0 V, a maximum extrinsic transconductance of 86 mS/mm, and an off-state breakdown voltage of larger than -100 V.

Band Alignment and Performance Improvement Mechanisms of Chlorine-Treated ZnO-Gate AlGaN/GaN Metal–Oxide–Semiconductor High-Electron Mobility Transistors

The intrinsic ZnO (i-ZnO) film deposited by a vapor cooling condensation system was used as the gate dielectric layer of the AlGaN/GaN MOS-HEMTs. The chlorine surface treatment was utilized to obtain a high-quality i-ZnO/AlGaN interface due to the reduced surface state density. The chlorine-treated MOS-HEMTs showed the better direct current and pulsed output performances than those of the untreated MOS-HEMTs. The resulting unit gain cutoff frequency and the maximum frequency of oscillation were 9.5 and 19.4 GHz, respectively. The Hooges coefficient was 7.23 × 10-6, when the chlorine-treated ZnO-gate MOS-HEMTs operated at 100 Hz and the gate-source voltage of -4 V. Compared with the untreated MOS-HEMTs, the chlorine-treated MOS-HEMTs revealed better performances. The valence-band offset of i-ZnO/AlGaN was measured by X-ray photoelectron spectroscopy. The valence-band offset of the i-ZnO film on the untreated and the chlorine-treated AlGaN was 1.53 and 2.05 eV, respectively. The conduction-band offset of the i-ZnO film on the untreated AlGaN and the chlorine-treated AlGaN was deduced to be 0.77 and 1.29 eV, respectively. The improved performances of the chlorine-treated MOS-HEMTs and the enhanced conduction-band offset of the i-ZnO/AlGaN interface were attributed to the decrease of Ga dangling bonds and the passivation of N vacancies on the AlGaN surface by using the chlorine surface treatment.

GaN-based p-type metal-oxide–semiconductor devices with a gate oxide layer grown by a bias-assisted photoelectrochemical oxidation method

In this work, a bias-assisted photoelectrochemical (PEC) oxidation method was used to form an oxide insulator for GaN-based p-type metal-oxide?semiconductor (MOS) devices. The inversion breakdown and accumulation breakdown fields of the resulting GaN p-type MOS devices were 11.6 MV cm?1 and 3.7 MV cm?1, respectively. The interface-state density of the GaN p-type MOS devices was 4.18 ? 1011 cm?2 eV?1 obtained by a photo-assisted capacitance?voltage measurement method. In addition, the negative fixed oxide charge of 2.4 ? 1012 cm?2 eV?1 was also estimated.

Frequency and Noise Performances of Photoelectrochemically Etched and Oxidized Gate-Recessed AlGaN/GaN MOS-HEMTs

The gate-recessed AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistors (MOS-HEMTs) were fabricated using the combination technology of the photoelectrochemical (PEC) wet etching method and the PEC oxidation method. The gate-recessed structure on the AlGaN layer was first carried out using the PEC wet etching method followed by the direct growth of the gate oxide layer on the recessed surface using the PEC oxidation method. According to the measured pulsed output characteristics, the low frequency noise results and the Hooges coefficient, the performances of the gate-recessed MOS-HEMTs are better than those of the planar gate MOS-HEMTs. The improved performances of the gate-recessed MOS-HEMTs are attributed to the removal of original damages and native defects using the PEC etching process and the passivation function using the PEC oxidation process.

Flicker Noises of AlGaN/GaN Metal-Oxide-Semiconductor High Electron Mobility Transistors

AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) with gate insulators grown using the photoelectrochemical oxidation method were fabricated in this work. The pinch-off voltage, maximum extrinsic transconductance, and drain-source saturation current at V GS = 0 V were -9 V, 88.20 mS/mm, and 665 mA/mm, respectively. When the MOS-HEMTs operated at V GS = -20 and 20 V, the gate leakage current was only 31 and 960 nA, respectively. The normalized noise power spectra of MOS-HEMTs operated in the linear region and the saturation region were fitted well by 1/f γ law from 4 Hz to 10 kHz. The exponent γ values were all closed to unity and independent of V GS . In the linear region (V DS = 2 V) at V GS = -8 V and V GS = 2 V, the α ch and α s estimated at a frequency of 100 Hz were 8.69 × 10 -6 and 9.29 × 10- 5 , respectively. The α ch and α s estimated in the saturation region (V DS = 10 V) at V GS = -8 V and V GS = 2 V at a frequency of 100 Hz were 1.61 × 10- 4 and 2.08 × 10 -3 , respectively. The normalized noise power density was a function of V -1 G , V -3 G , and V 0 G corresponded to the three regions of V G ≤ 3 V (V GS ≤ -6 V), 3 V ≤ V G ≤ 9 V (-6 V ≤ V GS ≤ 0 V), and V G ≤ 9 V (V GS ≤ 0 V), respectively, where V G was the effective gate bias defined as (V GS - V off ).

( NH4 ) 2 S x -Treated AlGaN ∕ GaN MOS-HEMTs with ZnO Gate Dielectric Layer

The function of the (NH 4 ) 2 S x surface treatment on the AIGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) was investigated by using the pulsed output characteristics and the low frequency noise measurements. The low carrier concentration and high resistivity 30-nm-thick ZnO film was deposited using the designed vapor cooling condensation system and utilized as the gate dielectric layer of the AlGaN/GaN MOS-HEMTs. The significant improvement of the pulsed output performance and low frequency noise behavior of the (NH 4 ) 2 S x -treated AlGaN/GaN MOS-HEMTs showed that the (NH 4 ) 2 S x surface treatment was an effective technique to reduce the surface state density and to obtain high quality interface between the ZnO gate dielectric layer and the AlGaN layer. The decrease of the surface states is attributed to the reduction of Ga dangling bonds and passivation of N vacancies by the formation of Ga-S bond on the AlGaN surface.

Photoelectrochemical oxidation-treated AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistors with oxidized layer/Ta2O5/Al2O3 gate dielectric stack

Photoelectrochemical (PEC) oxidation method was used to directly oxidize AlGaN layer as the oxide layer of AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistors (MOS-HEMTs). High-k Ta2O5 layer and wide bandgap Al2O3 layer were sequentially deposited on the PEC-oxidized layer as the gate dielectric stack of the MOS-HEMTs. Comparing with the Al2O3/Ta2O5/Al2O3 gate dielectric stack, the resulting MOS-HEMTs exhibited improved performances, including a maximum extrinsic transconductance of 134 mS/mm, a Hooges coefficient of 1.32 × 10−4, and a maximum output power of 3.44 W/mm. These experimental results verified that high performance gate dielectric stack/AlGaN interface was achieved using the PEC oxidation method.

AlGaN/GaN MOS-HEMTs with ZnO gate insulator and chlorine surface treatment

AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) were fabricated with ZnO gate insulator and chlorine surface treatment. It is revealed that the chlorine treatment reduced the gate lag phenomenon and enhanced the device performance. The gate leakage current was also reduced about one order of magnitude in comparison to the conventional one. The chlorine-treated MOS-HEMTs exhibited a saturation drain-source current of 0.85 A/mm, a peak extrinsic transconductance of 207 mS/mm, and an off-state breakdown voltage larger than 100V. This significant improvement was owing to the reduction in surface state density, which was resulted from the decrease of Ga dangling bonds and the passivation of N vacancies on the AlGaN surface.