Hiroshi OKADA Mao FUKINAKA Yoshiki AKIRA
Effects of Al thickness in Ti/Al/Ti/Au ohmic contact on AlGaN/GaN heterostructures are studied. Samples having Al thickness of 30, 90 and 120 nm in Ti/Al/Ti/Au have been investigated by electrical and X-ray photoelectron spectroscopy (XPS) depth profile analysis. It is found that thick Al samples show lower resistance and formation of Al-based alloy under the oxidized Al layer.
Akio WAKEJIMA Arijit BOSE Debaleen BISWAS Shigeomi HISHIKI Sumito OUCHI Koichi KITAHARA Keisuke KAWAMURA
A detailed investigation of DC and RF performance of AlGaN/GaN HEMT on 3C-SiC/low resistive silicon (LR-Si) substrate by introducing a thick GaN layer is reported in this paper. The hetero-epitaxial growth is achieved by metal organic chemical vapor deposition (MOCVD) on a commercially prepared 6-inch LR-Si substrate via a 3C-SiC intermediate layer. The reported HEMT exhibited very low RF loss and thermally stable amplifier characteristics with the introduction of a thick GaN layer. The temperature-dependent small-signal and large-signal characteristics verified the effectiveness of the thick GaN layer on LR-Si, especially in reduction of RF loss even at high temperatures. In summary, a high potential of the reported device is confirmed for microwave applications.
Masaru SATO Yoshitaka NIIDA Atsushi YAMADA Junji KOTANI Shiro OZAKI Toshihiro OHKI Naoya OKAMOTO Norikazu NAKAMURA
This paper presents recent progress on high frequency and wide bandwidth GaN high power amplifiers (PAs) that are usable for high-data-rate wireless communications and modern radar systems. The key devices and design techniques for PA are described in this paper. The results of the state-of-the art GaN PAs for microwave to millimeter-wave applications and design methodology for ultra-wideband GaN PAs are shown. In order to realize high output power density, InAlGaN/GaN HEMTs were employed. An output power density of 14.8 W/mm in S-band was achieved which is 1.5 times higher than that of the conventional AlGaN/GaN HEMTs. This technique was applied to the millimeter-wave GaN PAs, and a measured power density at 96 GHz was 3 W/mm. The modified Angelov model was employed for a millimeter-wave design. W-band GaN MMIC achieved the maximum Pout of 1.15 W under CW operation. The PA with Lange coupler achieved 2.6 W at 94 GHz. The authors also developed a wideband PA. A power combiner with an impedance transformation function based on the transmission line transformer (TLT) technique was adopted for the wideband PA design. The fabricated PA exhibited an average Pout of 233 W, an average PAE of 42 %, in the frequency range of 0.5 GHz to 2.1 GHz.
Taketoshi TANAKA Norikazu ITO Shinya TAKADO Masaaki KUZUHARA Ken NAKAHARA
TCAD simulation was performed to investigate the material properties of an AlGaN/GaN structure in Deep Acceptor (DA)-rich and Deep Donor (DD)-rich GaN cases. DD-rich semi-insulating GaN generated a positively charged area thereof to prevent the electron concentration in 2DEG from decreasing, while a DA-rich counterpart caused electron depletion, which was the origin of the current collapse in AlGaN/GaN HFETs. These simulation results were well verified experimentally using three nitride samples including buffer-GaN layers with carbon concentration ([C]) of 5×1017, 5×1018, and 4×1019 cm-3. DD-rich behaviors were observed for the sample with [C]=4×1019 cm-3, and DD energy level EDD=0.6 eV was estimated by the Arrhenius plot of temperature-dependent IDS. This EDD value coincided with the previously estimated EDD. The backgate experiments revealed that these DD-rich semi-insulating GaN suppressed both current collapse and buffer leakage, thus providing characteristics desirable for practical usage.
Tadayoshi DEGUCHI Hideshi TOMITA Atsushi KAMADA Manabu ARAI Kimiyoshi YAMASAKI Takashi EGAWA
Current collapse of AlGaN/GaN heterostructure field-effect transistors (HFETs) formed on qualified epitaxial layers on Si substrates was successfully suppressed using graded field-plate (FP) structures. To improve the reproducibility of the FP structure manufacturing process, a simple process for linearly graded SiO2 profile formation was developed. An HFET with a graded FP structure exhibited a significant decrease in an on-resistance increase ratio of 1.16 even after application of a drain bias of 600 V.
Eiji MIYAZAKI Shigeru KISHIMOTO Takashi MIZUTANI
We performed the (NH4)2S surface treatments before Al2O3 deposition to improve the Al2O3/III-Nitride interface quality in Al2O3/AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs). Interface state density at the Al2O3/GaN interface was decreased by the (NH4)2S treatment. The hysteresis width in ID-VGS and gm-VGS characteristics of the Al2O3/AlGaN MOSHFETs with the (NH4)2S treatment was smaller than that without the (NH4)2S treatment. In addition, transconductance (gm) decrease at a large gate voltage was relaxed by the (NH4)2S treatment. We also performed ultraviolet (UV) illumination during the (NH4)2S treatment for further improvement of the Al2O3/III-Nitride interface quality. Interface state density of the Al2O3/GaN MOS diodes with the UV illumination was smaller than that without the UV illumination.
Noboru NEGORO Masayuki KURODA Tomohiro MURATA Masaaki NISHIJIMA Yoshiharu ANDA Hiroyuki SAKAI Tetsuzo UEDA Tsuyoshi TANAKA
High output power AlGaN/GaN metal-insulator-semiconductor (MIS) hetero-junction field effect transistor (HFET) on Si substrate for millimeter-wave application has developed. High temperature chemical vapor deposition (HT-CVD) grown SiN as a gate insulator improves the breakdown characteristics which enables the operation at high drain voltage of 55 V. The device exhibits high drain current of 1.1 A/mm free from the current collapse and high RF gain of 10.4 dB. The amplifier module developed AlGaN/GaN MIS-HFET with the gate width of 5.4 mm exhibits an output power of 10.7 W and a linear gain of 4 dB at 26.5 GHz. The resultant high output power is very promising for long-distance communication at millimeter-wave in the future which would enable high speed and high density data transmission.
Masayuki ABE Noriaki KOGUSHI Kian Siong ANG René HOFSTETTER Kumar MANOJ Louis Nicholas RETNAM Hong WANG Geok Ing NG Chon JIN Dimitris PAVLIDIS
Novel thermopiles based on modulation doped AlGaAs/InGaAs and AlGaN/GaN heterostructures are proposed and developed for the first time, for uncooled infrared FPA (Focal Plane Array) image sensor application. The high responsivity with the high speed response time are designed to 4,900 V/W with 110 µs for AlGaAs/InGaAs, and to 460 V/W with 9 µs for AlGaN/GaN thermopiles, respectively. Based on integrated HEMT-MEMS technology, the AlGaAs/InGaAs 3232 matrix FPAs are fabricated to demonstrate its enhanced performances by black body measurement. The technology presented here demonstrates the potential of this approach for low-cost uncooled infrared FPA image sensor application.
Yusuke IKAWA Yorihide YUASA Cheng-Yu HU Jin-Ping AO Yasuo OHNO
Drain collapse in AlGaN/GaN HFET is analyzed using a two-dimensional device simulator. Two-step saturation is obtained, assuming hole-trap type surface states on the AlGaN surface and a short negative-charge-injected region at the drain side of the gate. Due to the surface electric potential pinning by the surface traps, the negative charge injected region forms a constant potential like in a metal gate region and it acts as an FET with a virtual gate. The electron concentration profile reveals that the first saturation occurs by pinch-off in the virtual gate region and the second saturation occurs by the pinch-off in the metal gate region. Due to the short-channel effect of the virtual gate FET, the saturation current increases until it finally reaches the saturation current of the intrinsic metal gate FET. Current collapses with current degradation at the knee voltage in the I-V characteristics can be explained by the formation of the virtual gate.
Cheng-Yu HU Katsutoshi NAKATANI Hiroji KAWAI Jin-Ping AO Yasuo OHNO
To improve the high voltage performance of AlGaN/GaN heterojunction field effect transistors (HFETs), we have fabricated AlGaN/GaN HFETs with p-GaN epi-layer on sapphire substrate with an ohmic contact to the p-GaN (p-sub HFET). Substrate bias dependent threshold voltage variation (VT-VSUB) was used to directly determine the doping concentration profile in the buffer layer. This VT-VSUB method was developed from Si MOSFET. For HFETs, the insulator is formed by epitaxially grown and heterogeneous semiconductor layer while for Si MOSFETs the insulator is amorphous SiO2. Except that HFETs have higher channel mobility due to the epitaxial insulator/semiconductor interface, HFETs and Si MOSFETs are basically the same in the respect of device physics. Based on these considerations, the feasibility of this VT-VSUB method for AlGaN/GaN HFETs was discussed. In the end, the buffer layer doping concentration was measured to be 21017 cm-3, p-type, which is well consistent with the Mg concentration obtained from secondary ion mass spectroscopy (SIMS) measurement.
Novel thermopiles based on modulation doped AlGaAs/InGaAs, AlGaN/GaN, and ZnMgO/ZnO heterostructures are proposed and designed for the first time, for uncooled infrared image sensor application. These devices are expected to offer high performances due to both the superior Seebeck coefficient and the excellently high mobility of 2DEG and 2DHG due to high purity channel layers at the heterojunction interface. The AlGaAs/InGaAs thermopile has the figure-of-merit Z of as large as 1.110-2/K (ZT = 3.3 over unity at T = 300 K), and can be realized with a high responsivity R of 15,200 V/W and a high detectivity D* of 1.8109 cmHz1/2/W with uncooled low-cost potentiality. The AlGaN/GaN and the ZnMgO/ZnO thermopiles have the advantages of high sheet carrier concentration due to their large polarization charge effects (spontaneous and piezo polarization charges) as well as of a high Seebeck coefficient due to their strong phonon-drag effect. The high speed response time τ of 0.9 ms with AlGaN/GaN, and also the lower cost with ZnMgO/ZnO thermopiles can be realized. The modulation-doped heterostructure thermopiles presented here are expected to be used for uncooled infrared image sensor applications, and for monolithic integrations with other photon detectors such as InGaAs, GaN, and ZnO PiN photodiodes, as well as HEMT functional integrated circuit devices.
Hideyuki OKITA Toshiharu MARUI Shinichi HOSHI Masanori ITOH Fumihiko TODA Yoshiaki MORINO Isao TAMAI Yoshiaki SANO Shohei SEKI
Current collapse phenomenon is a well known obstacle in the AlGaN/GaN HEMTs. In order to improve the surface stability of HEMTs, we have investigated the SiN passivation film deposited by T-CVD, and we found that it improves both gate leakage current and current collapse phenomenon [1]. Moreover, we compared the T-CVD and PE-CVD passivation films, on high electric field DC and RF characteristics. We found that T-CVD SiN passivation film improves BVds-off by 30% because of the reduction of gate leakage current. It also improved ηd in the output power characteristics by load-pull measurement, which indicates the decrease of the current collapse phenomenon. Also we fabricated a multi-fingered 50 W-class AlGaN/GaN HEMT with T-CVD SiN passivation film and achieved 61.2% of high drain efficiency at frequency of 2.14 GHz, which was 3.6 points higher than that with PE-CVD SiN passivation film.
Masafumi ITO Shigeru KISHIMOTO Fumihiko NAKAMURA Takashi MIZUTANI
We have fabricated AlGaN/GaN HEMTs with a thin InGaN cap layer to implement normally-off HEMTs with a small extrinsic source resistance. The key idea is to employ the polarization-induced field in the InGaN cap layer, by which the conduction band is raised leading to the normally-off operation. Fabricated HEMT with an In0.2Ga0.8N cap layer with a thickness of 5 nm showed normally-off operation with a threshold voltage of 0.4 V and a maximum transconductance of 85 mS/mm for the device with a 1.9-µm-long gate. By etching-off the In0.2Ga0.8N cap layer at the region except under the gate using gate and ohmic electrodes as etching masks, the sheet resistance has decreased from 2.7 to 0.75 kΩ/, and the maximum transconductance has increased from 85 to 130 mS/mm due to a reduction of the extrinsic source resistance. The transconductance was increased from 130 to 145 mS/mm by annealing the devices at 250 for 20 minutes in a N2 atmosphere.
Toshiharu MARUI Shinich HOSHI Masanori ITOH Isao TAMAI Fumihiko TODA Hideyuki OKITA Yoshiaki SANO Shohei SEKI
In AlGaN/GaN high electron mobility transistors (HEMTs), drain current reduction by current collapse phenomenon is a big obstacle for a high efficient operation of power amplifier application. In this study, we investigated the effects of SiN passivation film quality on the electrical characteristics of AlGaN/GaN HEMTs. First, we conducted some experiments to investigate the relationship between electrical characteristics of AlGaN/GaN HEMTs and various conditions of SiN passivation film by plasma enhanced chemical vapor deposition (PE-CVD). We found that both gate current leakage and current collapse were improved simultaneously by SiN passivation film deposited by optimized condition of NH3 and SiH4 gas flow. It is found that the critical parameter in the optimization is a IN-H/ ISi-H ratio measured by Fourier transforms infrared spectroscopy (FT-IR) spectra. Next, a thermal CVD SiN was applied to the passivation film to be investigated from the same point of view, because a thermal CVD SiN is well known to have good quality with low hydrogen content and high IN-H/ISi-H ratio. We confirmed that the thermal CVD SiN passivation could improve much further both of the gate leakage current and the current collapse in AlGaN/GaN-HEMTs. Furthermore, we tried to apply the thermal CVD SiN to the gate insulator in MIS (Metal Insulator Semiconductor) structure of AlGaN/GaN HEMTs. The thermal CVD SiN passivation was more suitable for the gate insulator than PE-CVD SiN passivation in a view of reducing current collapse phenomena. It could be believed that the thermal CVD SiN film is superior to the PE-CVD SiN film to achieve good passivation and gate insulator film for AlGaN/GaN HEMTs due to the low hydrogen content and the high IN-H/ISi-H ratio.
Keita MATSUDA Takeshi KAWASAKI Ken NAKATA Takeshi IGARASHI Seiji YAEGASSI
To reduce the gate leakage current of AlGaN/GaN HEMTs, we selected ITO/Ni/Au for Schottky electrodes and Schottky characteristics were compared with those of Ni/Au electrodes. ITO/Ni/Au and Ni/Au electrodes were deposited by vacuum evaporation and annealed at 350 in nitrogen atmosphere. From the I-V evaluation results of ITO/Ni/Au electrodes, forward and reverse leakage currents were reduced. Schottky characteristics of ITO/Ni/Au electrodes were also improved compared to these of Ni/Au electrodes. In addition, substantial decrease of leakage currents was confirmed after the annealing of HEMTs with ITO/Ni/Au electrodes. This may be explained that ITO/AlGaN interface state became lower by the annealing. By the temperature dependence of I-V curves, clear dependence was confirmed for the gates with ITO/Ni/Au electrodes. On the other hand, small dependence was observed for those with Ni/Au electrodes. From these results, tunnel leakage currents were dominant for the gates with Ni/Au electrode. Thermal emission current was dominant for the gates with ITO/Ni/Au electrode. The larger temperature dependence was caused that ITO/AlGaN interface states were smaller than those for Ni/Au electrode. It was suggested that suppressed AlGaN Schottky barrier thinning was caused by the surface defect donors, then tunneling leakage currents were decreased. We evaluated HEMT characteristics with ITO/Ni/Au electrode and Ni/Au electrode. Id max and Gm max were similar characteristics, but Vth with ITO/Ni/Au electrode was shifted +0.4 V than that with Ni/Au electrode due to the higher Schottky barrier. It was confirmed to have a good pinch-off currents and low gate leakage currents by ITO/Ni/Au electrodes.
Jin-Ping AO Yuya YAMAOKA Masaya OKADA Cheng-Yu HU Yasuo OHNO
The mechanism of current collapse of AlGaN/GaN heterojunction field-effect transistors (HFETs) was investigated by gate bias stress with and without illumination. It is clarified that there are two positions where negative charges accumulate, at the gate edge and in the bulk epi-layer. In the gate-edge mode, the charge comes either through the passivation film or the AlGaN layer, depending on the resistance of the films. Reduction of leakage current in the passivation film will be important to suppress the surface-related collapse.
Shinichi HOSHI Toshiharu MARUI Masanori ITOH Yoshiaki SANO Shouhei SEKI
In AlGaN/GaN high electron mobility transistors (HEMTs), Si3N4 passivation film brings effective improvements in the current collapse phenomenon, however, the suppression of this phenomenon in a high voltage operation can not be achieved in only the Si3N4 deposition process. In order to solve this problem, we have demonstrated an NH3-plasma surface pretreatment in the chamber of plasma enhanced chemical vapor deposition (PE-CVD) just before Si3N4 deposition process. We found that the optimized NH3-plasma pretreatment could improve the current collapse as compared with only the Si3N4 deposition and an excessive pretreatment made it worse adversely in AlGaN/GaN-HEMTs. It was confirmed by Auger electron spectroscopy (AES) analysis that the optimized NH3-plasma pretreatment decreased the carbon contamination such as hydrocarbon on the AlGaN surface and the excessive pretreatment degraded the stoicheiometric composition of AlGaN surface.
Masaya OKADA Ryohei TAKAKI Daigo KIKUTA Jin-Ping AO Yasuo OHNO
This investigation of the temperature and illumination effects on the AlGaN/GaN HFET threshold voltage shows that it shifts about -1 V under incandescent lamp or blue LED illumination, while almost no shift takes place under red LED illumination. The temperature coefficient for the threshold voltage shift is +3.44 mV/deg under the illuminations and +0.28 mV/deg in darkness. The threshold voltage variation can be attributed to a virtual back-gate effect caused by light-generated buffer layer potential variations. The expressions for the potential variation are derived using Shockley-Read-Hall (SRH) statistics and the Maxwell-Boltzmann distribution for the carriers and deep traps in the buffer layer. The expressions indicate that large photoresponses will occur when the electron concentration in the buffer layer is extremely small, that is, highly resistive. In semi-insulating substrates, the substrate potential varies so as to keep the trap occupation function constant. The sign and the magnitude of the threshold voltage variation are explained by the shift of the pinning energy calculated from the Fermi-Dirac distribution function.
Yong CAI Yugang ZHOU Kei May LAU Kevin J. CHEN
Based on fluoride-based plasma treatment of the gate region in AlGaN/GaN HEMTs and post-gate rapid thermal annealing (RTA), enhancement mode (E-mode) AlGaN/GaN HEMTs with low on-resistance and low knee-voltage were fabricated. The fabricated E-mode AlGaN/GaN HEMT with 1 µm-long gate exhibits a threshold voltage of 0.9 V, a knee-voltage of 2.2 V, a maximum drain current density of 310 mA/mm, a peak gm of 148 mS/mm, a current gain cutoff frequency fT of 10.1 GHz and a maximum oscillation frequency fmax of 34.3 GHz. In addition, the fluoride-based plasma treatment was also found to be effective in lowering the gate leakage current, in both forward and reverse bias. Two orders of magnitude reducation in gate leakage current was observed in the fabricated E-mode HEMTs compared to the conventional D-mode HEMTs without fluoride-based plasma treatment.
Yoshikazu HIROSE Akira HONSHIO Takeshi KAWASHIMA Motoaki IWAYA Satoshi KAMIYAMA Michinobu TSUDA Hiroshi AMANO Isamu AKASAKI
The correlation between ohmic contact resistivity (ρc) and transconductance (gm) in AlGaN/GaN high-electron-mobility transistors (HEMTs) was investigated. To characterize ρc precisely, we fabricated a circular transmission line model (c-TLM) pattern adjoined to a field-effect transistor (FET) pattern on an HEMT. By measuring ohmic contact resistance and sheet resistance using the adjoined c-TLM, intrinsic transconductance (gm0), which is not influenced by the source resistance, can be estimated. The gm0 thus obtained is between 179 and 206 mS/mm. Then, it became possible to calculate the correlation between gm and (ρc. We found that ρc should be below 10-5 Ωcm2 for the improvement of gm in AlGaN/GaN HEMT when Rsh 400 Ω/.