High-Temperature Microwave Performance of Submicron AlGaN/GaN HEMTs on SiC

The effect of temperature on the small-signal radio-frequency (RF) performance of submicron AlGaN/GaN high-electron-mobility transistors on SiC has been studied from room temperature (RT) up to 600 K. A relation between ambient and channel temperatures has been established by means of finite-element simulations. The thermal behavior of the intrinsic parameters $C_{rm gs}$, $C_{rm gd}$, $g_{m, {rm int}}$, and $g_{rm ds}$ has been extracted accurately from RF measurements by means of the small-signal equivalent circuit. Main dc parameters $(I_{D}, g_{m, {rm ext}})$ show reductions close to 50% between RT and 600 K, mainly due to the decrease in the electron mobility and drift velocity. In the same range, $f_{T}$ and $f_{max}$ suffer a 60% decrease due to the reduction in $g_{m, {rm ext}}$ and a slight increase of $C_{rm gs}$ and $C_{rm gd}$. An anomalous thermal evolution of $C_{rm gd}$ at low $I_{D}$ has been identified, which is indicative of the presence of traps.      

High-performance E-mode AlGaN/GaN HEMTs

Enhancement-mode AlGaN/GaN high electron-mobility transistors have been fabricated with a gate length of 160 nm. The use of gate recess combined with a fluorine-based surface treatment under the gate produced devices with a threshold voltage of +0.1 V. The combination of very high transconductance (> 400 mS/mm) and low gate leakage allows unprecedented output current levels in excess of 1.2 A/mm. The small signal performance of these enhancement-mode devices shows a record current cutoff frequency (f/sub T/) of 85 GHz and a power gain cutoff frequency (f/sub max/) of 150 GHz.     

Temperature-dependent nonlinearities in GaN/AlGaN HEMTs

Temperature-dependent nonlinearities of GaN/AlGaN HEMTs are reported. The large-signal device model of the transistor is obtained by using a physics-based analysis. The model parameters are obtained as functions of bias voltages and temperature. The analysis of the device has been carried out using a time-domain technique. f_max for a 0.23 μm×100 μm Al_0.13Ga_0.87N/GaN FET is calculated as 69 GHz at 300 K, while at 500 K, f_max decreases to 30 GHz, which are in agreement with the experimental data within 7% error. f_max as obtained from calculated unilateral gain, decreases monotonically with increasing temperature. For shorter gate lengths irrespective of the operating temperature f_max is less sensitive to bias voltage scaling. For longer gate length devices, f_max becomes less sensitive to the bias voltage scaling at elevated temperatures. 1-dB compression point (P_1-dB ) at 4 GHz for a 1 μm×500 μm Al_0.15Ga_0.85N/GaN FET is 13 dBm at 300 K. At 500 K, P_1-dB decreases to 2.5 dBm for the same operating frequency. Similar results for output referred third intercept point (OIP3) are reported for different gate length devices     

High-frequency measurements of AlGaN/GaN HEMTs at high temperatures

High-frequency measurements of the 1.3-μm-long gate AlGaN-GaN HEMTs have been performed at temperatures ranging from 23 to 187°C. The cutoff frequency f_T decreased with increasing temperature. It was 13.7 and 8.7 GHz at 23 and 187°C, respectively. The effective electron velocities υ_eff in the channel evaluated from the total delay time versus I_D-inverse relation were 1.2 and 0.8×10⁷ cm/s at 23 and 187°C, respectively     

High-power polarization-engineered GaN/AlGaN/GaN HEMTs without surface passivation

In this paper, a high-power GaN/AlGaN/GaN high electron mobility transistor (HEMT) has been demonstrated. A thick cap layer has been used to screen surface states and reduce dispersion. A deep gate recess was used to achieve the desired transconductance. A thin SiO/sub 2/ layer was deposited on the drain side of the gate recess in order to reduce gate leakage current and improve breakdown voltage. No surface passivation layer was used. A breakdown voltage of 90 V was achieved. A record output power density of 12 W/mm with an associated power-added efficiency (PAE) of 40.5% was measured at 10 GHz. These results demonstrate the potential of the technique as a controllable and repeatable solution to decrease dispersion and produce power from GaN-based HEMTs without surface passivation.     

增强型AlGaN/GaN HEMT器件工艺的研究进展

随着高压开关和高速射频电路的发展,增强型GaN基高电子迁移率晶体管(HEMT)成为该领域内的研究热点。增强型GaN基HEMT只有在加正栅压才有工作电流,可以大大拓展该器件在低功耗数字电路中的应用。近年来,国内外对增强型GaN基HEMT阈值电压的研究主要集中以下两个方面:在材料生长方面,通过生长薄势垒、降低Al组分、生长无极化电荷的AlGaN/GaN异质材料、生长InGaN或p-GaN盖帽层,来控制二维电子气浓度;在器件工艺方面,采用高功函数金属、MIS结构、刻蚀凹栅、F基等离子体处理,来控制表面电势,影响二维电子气浓度。从影响器件阈值电压的相关因素出发,探讨了实现和优化增强型GaN基HEMT的各种工艺方法和发展方向。     

Structure optimization of field-plate AlGaN/GaN HEMTs

AlGaN/GaN high electron mobility transistors (HEMTs) on 6H-SiC with varying field-plate length and gate-drain spacing were fabricated and analyzed. The classical small signal FET model and the well-known ColdFET method were used to extract the small signal parameters of the devices. Though the devices with field plates exhibited lower better f_T characteristic, they did demonstrate better f_max, MSG and power density performances than the conventional devices without field plate. Besides, no independence of DC characteristic on field-plate length was observed. With the increase of the field-plate length and the gate-drain spacing, the characteristic of f_T and f_max degraded due to the large parasitic effects. Loadpull method was used to measure the microwave power performance of the devices. Under the condition of continuous wave at 5.4 GHz, an output power density of 4.69 W/mm was obtained for device with field-plate length of 0.5 μm and gate-drain length of 2 μm.     

Enhancement-mode AlGaN/GaN HEMTs on silicon substrate

High-performance enhancement-mode AlGaN/GaN HEMTs (E-HEMTs) were demonstrated with samples grown on a low-cost silicon substrate for the first time. The fabrication process is based on a fluoride-based plasma treatment of the gate region and postgate annealing at 450 /spl deg/C. The fabricated E-HEMTs have nearly the same peak transconductance (G/sub m/) and cutoff frequencies as the conventional depletion-mode HEMTs fabricated on the same wafer, suggesting little mobility degradation caused by the plasma treatment.     

High-power AlGaN/GaN HEMTs for Ka-band applications

We report on the fabrication and high-frequency characterization of AlGaN/GaN high-electron mobility transistors (HEMTs) grown by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). In devices with a gate length of 160 nm, a record power density of 10.5 W/mm with 34% power added efficiency (PAE) has been measured at 40 GHz in MOCVD-grown HEMTs biased at V/sub DS/=30 V. Under similar bias conditions, more than 8.6 W/mm, with 32% PAE, were obtained on the MBE-grown sample. The dependence of output power, gain, and PAE on gate and drain voltages, and frequency have also been analyzed.     

Punchthrough-Voltage Enhancement of AlGaN/GaN HEMTs Using AlGaN Double-Heterojunction Confinement

In this paper, we present an enhancement of punchthrough voltage in AlGaN/GaN high-electron-mobility-transistor devices by increasing the electron confinement in the transistor channel using an AlGaN buffer-layer structure. An optimized electron confinement results in a scaling of punchthrough voltage with device geometry and a significantly reduced subthreshold drain leakage current. These beneficial properties are pronounced even further if gate-recess technology is applied for device fabrication. Physical-based device simulations give insight in the respective electronic mechanisms.      

Electron transport in passivated AlGaN/GaN/Si HEMTs

AlGaN/GaN/Si high electron mobility transistors (HEMTs) grown by molecular beam epitaxy are investigated using direct-current and radio-frequency measurements. As has been found, the maximum of drain current achieves 881 mA/mm with an extrinsic current gain cutoff frequency of 37 GHz for a 0.25 µm gate length. Pulsed characteristics also showed a reduction of trapping centers that improves the quality of the epilayers.     

Deep levels and nonlinear characterization of AlGaN/GaN HEMTs on silicon carbide substrate

Deep levels in AlGaN/GaN high electron mobility transistors (HEMTs) on SiC substrate are known to be responsible for trapping processes like: threshold voltage shift, leakage current, degradation current, kink effect and hysteresis effect. The related deep levels are directly characterized by conductance deep level transient spectroscopy (CDLTS) method. Hereby, we have detected five carrier traps with activation energy ranging from 0.84 to 0.07 eV. In this study, we have revealed the presence of two hole-like traps (HL1 and HL2) observed for the first time by CDLTS with activations energy of 0.40 and 0.84 eV. The localisation and the identification of these traps are presented. Finally, the correlation between the anomalies observed on output and defects is discussed.     

Study of Impact of Access Resistance on High-Frequency Performance of AlGaN/GaN HEMTs by Measurements at Low Temperatures

This letter studies the effect of access resistance on the high-frequency performance of AlGaN/GaN high-electron-mobility transistors. To systematically reduce the sheet access resistance, the transistors were measured at different temperatures. The increase of mobility at lower temperatures allowed more than four-fold reduction in the sheet access resistances. Both the current- and power-gain cutoff frequencies are observed to increase at low temperatures. Also, the intrinsic effective velocity has been estimated in these devices, as well as the parasitic delays involved in the final performance. Channel charging delay, which was expected to be most sensitive to parasitics, is observed to decrease at low temperatures. However, the drain delay, intrinsic delay, and effective electron velocity remain unaffected by temperature     

Deep-Submicrometer AlGaN/GaN HEMTs With Slant Field Plates

Deep-submicrometer AlGaN/GaN HEMTs with integrated slant field plates have been fabricated. Simulation showed this technology had the ability to minimize both the dc-RF dispersion and parasitic capacitance. Prototype device results demonstrated an excellent millimeter-wave power density of 4.9 W/mm with a power-added efficiency of 45% at 30 GHz at a drain bias of 30 V.      

Ultraviolet photoresponse of ZnO nanostructured AlGaN/GaN HEMTs

We present the first active visible blind ultraviolet (UV) photodetector based on zinc oxide (ZnO) nanostructured AlGaN/GaN high electron mobility transistors (HEMTs). The ZnO nanorods (NRs) are selectively grown on the gate area by using hydrothermal method. It is shown that ZnO nanorod (NR)-gated UV detectors exhibit much superior performance in terms of response speed and recovery time to those of seed-layer-gated detectors. It is also found that the best response speed (~10 and~190 ms) and responsivity (~1.1×10⁵ A/W) were observed from detectors of the shortest gate length of 2 µm among our NR-gated devices of three different gate dimensions, and this responsivity is about one order higher than the best performance of ZnO NR-based UV detectors reported to date.     

Effect of fluorine interface redistribution on performance of AlGaN/GaN HEMTs

The effect of fluorine interface redistribution on dc and microwave performances of SF₆ plasma-treated AlGaN/GaN high-electron mobility transistors (HEMTs) was investigated. Selective SF₆ plasma treatment of the AlGaN/GaN HEMT gate interface yielded increases in the current gain cut-off frequency (f_T) and maximum frequency of oscillation (f_max) of almost 60%. Annealing induced fluorine interface redistribution showed a low impact on the electron drift mobility and a negligible impact on the peak transconductance of the HEMTs. A large impact of the fluorine interface redistribution was observed for the threshold voltage and sheet carrier concentration of two-dimensional electron gas (2DEG). Consequently, it led to a decrease in the f_T and f_max values, but the values were still higher than those of conventional reference HEMTs.     

Intrinsic noise equivalent-circuit parameters for AlGaN/GaN HEMTs

Intrinsic noise sources and their correlation in gallium-nitride high electron-mobility transistors (HEMTs) are extracted and studied. Microwave noise measurements have been performed over the frequency range of 0.8-5.8 GHz. Using measured noise and scattering parameter data, the gate and drain noise sources and their correlation are determined using an equivalent-circuit representation. This model correctly predicts the frequency-dependent noise for two devices having different gate length. Three noise mechanisms are identified in these devices, namely, those due to velocity fluctuation, gate leakage, and traps.     

Undoped AlGaN/GaN HEMTs for microwave power amplification

Undoped AlGaN/GaN structures are used to fabricate high electron mobility transistors (HEMTs). Using the strong spontaneous and piezoelectric polarization inherent in this crystal structure a two-dimensional electron gas (2DEG) is induced. Three-dimensional (3-D) nonlinear thermal simulations are made to determine the temperature rise from heat dissipation in various geometries. Epitaxial growth by MBE and OMVPE are described, reaching electron mobilities of 1500 and 1700 cm ²/Ns, respectively, For electron sheet density near 1×10¹³/cm², Device fabrication is described, including surface passivation used to sharply reduce the problematic current slump (dc to rf dispersion) in these HEMTs. The frequency response, reaching an intrinsic f_t of 106 GHz for 0.15 μm gates, and drain-source breakdown voltage dependence on gate length are presented. Small periphery devices on sapphire substrates have normalized microwave output power of ~4 W/mm, while large periphery devices have ~2 W/mm, both thermally limited. Performance, without and with Si₃N₄ passivation are presented. On SiC substrates, large periphery devices have electrical limits of 4 W/mm, due in part to the limited development of the substrates     

Effect of Gate Leakage in the Subthreshold Characteristics of AlGaN/GaN HEMTs

This letter studies the effect of gate leakage on the subthreshold slope and ON/OFF current ratio of AlGaN/GaN high-electron mobility transistors (HEMTs). We found a strong correlation between the gate leakage current and the transistor subthreshold characteristics: the lower the gate leakage, the higher the ON/OFF ratio and the steeper the subthreshold slope. To improve the subthreshold characteristics in GaN HEMTs, the gate leakage current was reduced with an $hbox{O}_{2}$ plasma treatment prior to the gate metallization. The $hbox{O}_{2}$ plasma treatment effectively reduces the gate leakage current by more than four orders of magnitude, it increases the ON/OFF ratio to more than seven orders of magnitude and the improved AlGaN/GaN HEMT shows a nearly ideal subthreshold slope of 64 mV/dec.      

Investigation of High-Electric-Field Degradation Effects in AlGaN/GaN HEMTs

High-electric-field degradation phenomena are investigated in GaN-capped AlGaN/GaN HEMTs by comparing experimental data with numerical device simulations. Under power- and OFF-state conditions, 150-h DC stresses were carried out. Degradation effects characterizing both stress experiments were as follows: a drop in the dc drain current, the amplification of gate-lag effects, and a decrease in the reverse gate leakage current. Numerical simulations indicate that the simultaneous generation of surface (and/or barrier) and buffer traps can account for all of the aforementioned degradation modes. Experiments also showed that the power-state stress induced a drop in the transconductance at high gate-source voltages only, whereas the OFF-state stress led to a uniform transconductance drop over the entire gate-source-voltage range. This behavior can be reproduced by simulations provided that, under the power-state stress, traps are assumed to accumulate over a wide region extending laterally from the gate edge toward the drain contact, whereas, under the OFF-state stress, trap generation is supposed to take place in a narrower portion of the drain-access region close to the gate edge and to be accompanied by a significant degradation of the channel transport parameters.