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.     

Post-process thermal treatment for microwave power improvement of AlGaN/GaN HEMTs

The effects of post-process rapid thermal annealing (RTA) treatment after device fabrication on direct current, microwave and power performances of AlGaN/GaN high electron mobility transistors (HEMTs) with a gate-length of 0.2 μm were fully investigated. By 3 min post-process RTA treatment at 350 °C under N₂ atmosphere, the direct current (DC), radio frequency (RF) small signal and power performances of AlGaN/GaN HEMTs have been much improved. The output power, power gain and power added efficiency (PAE) of GaN HEMT device with gate wide of 1 mm increase from 37.09 dBm, 6.09 dB and 42.79% to 38.22 dBm, 7.22 dB and 67.3%. The post-process RTA after device fabrication has two merits. On the one hand, it improves passivation effect of SiN_x dielectric layer on AlGaN/GaN HEMT surface, suppressing RF current dispersion. On the other hand, it helps recover dry-etch damage at the Schottky metal/AlGaN interface, leading to reduction of reverse Schottky leakage current.     

Role of nanoscale AlN and InN for the microwave characteristics of AlGaN/(Al,In)N/GaN-based HEMT

A new AlGaN/GaN-based high electron mobility transistor (HEMT) is proposed and its micro-wave characteristics are discussed by introducing a nanoscale AlN or InN layer to study the potential improvement in their high frequency performance. The 2DEG transport mechanism including various sub-band calculations for both (Al,In) N-based HEMTs are also discussed in the paper. Apart from direct current characteristics of the proposed HEMT, various microwave parameters such as transconductance, unit current gain (h ₂₁ = 1) cut-off frequency (f _t ), high power-gain frequency (f _max). Masons available/stable gain and masons unilateral gain are also discussed for both devices to understand its suitable deployment in microwave frequency range.     

Thermal stability investigations of AlGaN/GaN HEMTs with various high work function gate metal designs

The operation of high power RF transistor generates a huge amount of heat and thermal effect is a major consideration for improving the efficiency of power transistors. AlGaN/GaN high electron mobility transistors (HEMTs) on silicon substrates have been studied extensively because of their high thermal conductivity. This study comprehensively investigates the DC, low frequency noise, microwave and RF power performance of Al_0.27Ga_0.73N/GaN HEMTs on silicon substrates at temperatures from room temperature to 100 °C using high work function metals such as palladium (Pd) and iridium (Ir) gate metals. Although the conventional Ni gate exhibited a good metal work function with AlGaN, which is beneficial for increasing the Schottky barrier height of HEMTs, the diffusion of Ni metal toward the AlGaN and GaN layers influences the DC and RF stability of the device at high temperatures or over long-term operation. Pd and Ir exhibited less diffusion at high temperature than Ni, resulting in less degradation of device characteristics after high temperature operation.     

MOCVD-Grown AlGaN Buffer GaN HEMTs With V-Gates for Microwave Power Applications

We report the performance of AlGaN buffer GaN high-electron mobility transistors (HEMTs) grown by metal–organic chemical vapor deposition. GaN HEMTs on high-quality AlGaN buffer were grown on SiC substrates. The incorporation of an AlGaN buffer into the GaN HEMT significantly improves channel confinement and suppresses the short-channel effect. Advanced deep-recess V-gate structures were employed to optimize the device for better microwave power performance. With a 10-nm GaN channel layer sandwiched between the AlGaN barrier and buffer, excellent power performance was achieved. The output power density is 13.1 W/mm, and the associated power-added efficiency is 72% at 4-GHz frequency and 48-V drain bias. This power performance is comparable to the state-of-the-art GaN HEMTs grown on GaN buffers, indicating that the AlGaN buffer in our optimized device structure does not introduce any noticeable trapping.      

Evidence of surface states for AlGaN/GaN/SiC HEMTs passivated Si<Subscript>3</Subscript>N<Subscript>4</Subscript> by CDLTS

In AlGaN/GaN heterostructure field-effect transistors (HEMTs) structures, the surface defects and dislocations may serve as trapping centers and affect the device performance via leakage current and low frequency noise. This work demonstrates the effect of surface passivation on the current-voltage characteristics and we report results of our investigation of the trapping characteristics of Si₃N₄-passivated AlGaN/GaN HEMTs on SiC substrates using the conductance deep levels transient spectroscopy (CDLTS) technique. From the measured of CDLTS we identified one electron trap had an activation energy of 0.31 eV it has been located in the AlGaN layer and two hole-likes traps H ₁, H ₂. It has been pointed out that the two hole-likes traps signals did not originate from changes in hole trap population in the channel, but reflected the changes in the electron population in the surface states of the HEMT access regions.     

Reliability and degradation mechanism of AlGaN/GaN HEMTs for next generation mobile communication systems

Excellent reliability performance of AlGaN/GaN HEMTs on SiC substrates for next generation mobile communication systems has been demonstrated using DC and RF stress tests on 8 × 60 μm wide and 0.5 μm long AlGaN/GaN HEMTs at a drain voltage of V_d = 50 V. Drain current recovery measurements after stress indicate that the degradation is partly caused by slow traps generated in the SiN passivation or in the HEMT epitaxial layers. The traps in the SiN passivation layer were characterized using high and low frequency capacitance-voltage (CV) measurements of MIS test structures on thick lightly doped GaN layers.     

一种氮化镓高迁移率晶体管的栅电流噪声模型

This work presents a theoretical and experimental study on the gate current 1/f noise in Al Ga N/Ga N HEMTs. Based on the carrier number fluctuation in the two-dimensional electron gas channel of Al Ga N/Ga N HEMTs, a gate current 1/f noise model containing a trap-assisted tunneling current and a space charge limited current is built. The simulation results are in good agreement with the experiment. Experiments show that, if Vg Vx, gate current 1/f noise comes from not only the trap-assisted tunneling RTS, but also the space charge limited current RTS. This indicates that the gate current 1/f noise of the Ga N-based HEMTs device is sensitive to the interaction of defects and the piezoelectric relaxation. It provides a useful characterization tool for deeper information about the defects and their evolution in Al Ga N/Ga N HEMTs.     

Degradation of AlGaN/GaN HEMTs under elevated temperature lifetesting

Elevated temperature lifetesting was performed on 0.25 μm AlGaN/GaN HEMTs grown by MOCVD on 2-in. SiC substrates. A temperature step stress (starting at T_a of 150 °C with a step of 15 °C; ending at T_a of 240 °C; 48 h for each temperature cycle) was employed for the quick reliability evaluation of AlGaN/GaN HEMTs. It was found that the degradation of AlGaN/GaN HEMTs was initiated at ambient temperature of 195 °C. The degradation characteristics consist of a decrease of drain current and transconductance, and an increase of channel-on-resistance. However, there is no noticeable degradation of the gate diode (ideality factor, barrier height, and reverse gate leakage current). The FIB/STEM technique was used to examine the degraded devices. There is no detectable ohmic metal or gate metal interdiffusion into the epitaxial materials. Accordingly, the degradation mechanism of AlGaN/GaN HEMTs under elevated temperature lifetesting differs from that observed in GaAs and/or InP HEMTs. The reliability performance was also compared between two vendors of AlGaN/GaN epilayers. The results indicate that the reliability performance of AlGaN/GaN HEMTs could strongly depend on the material quality of AlGaN/GaN epitaxial layers on SiC substrates.     

SiN钝化前的NF3等离子体处理对AlGaN/GaN HEMT性能的影响

研究了SiN钝化前利用感应耦合等离子体（ICP）对AlGaN/GaN HEMT表面进行NF3等离子体处理对器件性能的影响. 结果表明,运用低能量的NF3等离子体处理钝化前的AlGaN/GaN HEMT表面能有效抑制器件电流崩塌,而器件直流及微波小信号特性则未受影响. 微波功率测试表明,经过6min NF3等离子体处理的AlGaN/GaN HEMT在2GHz, 30V工作电压下达到6.15W/mm的输出功率密度,而未经过处理的器件只达到1.82W/mm的输出功率密度.     

14W X波段AlGaN/GaN HEMT功率MMIC

报道了研制的SiC衬底AIGaN/GaN HEMT微带结构微波功率MMIC,芯片工艺采用凹槽栅场板结构提高AlGaN/GaNHEMTs的微波功率特性.S参数测试结果表明AlGaN/GaN HEMTs的频率特性随器件的工作电压变化显著.研制的该2级功率MMIC在9～11GHz带内30V工作,输出功率大于10W,功率增益大于12dB,带内峰值输出功率达到14.7W,功率增益为13.7dB,功率附加效率为23%,该芯片尺寸仅为2.0mm×1.1mm.与已发表的X波段AlGaN/GaN HEMT功率MMIC研制结果相比,本项工作在单位毫米栅宽输出功率和芯片单位面积输出功率方面具有优势.     

SiC衬底上栅长70 nm fT / fmax >160 GHz的InAlN/GaN HEMTs器件

在SiC衬底上制备了InAlN/GaN 高电子迁移率晶体管（HEMTs）,并进行了表征。为提高器件性能,综合采用了多种技术,包括高电子浓度,70 nm T型栅,小的欧姆接触电阻和小源漏间距。制备的InAlN/GaN器件在栅偏压为1 V时得到的最大饱和漏电流密度为1.65 A/mm,最大峰值跨导为382 mS/mm。70 nm栅长器件的电流增益截止频率fT和最大振荡频率fmax分别为162 GHz和176 GHz。     

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.     

AlGaN/GaN高电子迁移率晶体管小信号等效电路的参数提取

本文在高电子迁移率晶体管(HEMT)小信号等效电路模型的基础上,考虑了AlGaN/GaN HEMT的结构特性,具体分析了寄生参数和本征参数的提取方法.采用这些方法,实际测量了5～10 GHz频率下HEMT器件的小信号S参数并提取了它的电学参数,S参数的计算值与实际测量值进行了比较.实验结果表明此方法简单易行,较为精确.     

AlGaN/GaN self-aligned MODFET with metal oxide gate for millimeter wave application

As promising candidates for future microwave power devices, GaN-based high-electron mobility transistors (HEMTs) have attracted much research interest. An investigation of the operation of AlGaN/GaN n type self-aligned MOSFET with modulation doped GaN channels is presented. Liquid phase deposited (LPD) SiO₂ is used as the insulating material. An analytical model based on modified charge control equations is developed. The investigated critical parameters of the proposed device are the maximum drain current (I_Dmax), the threshold voltage (V_th), the peak DC trans-conductance (g_m), break down voltage (V_br) and unity current gain cut-off frequency (f_T). The typical DC characteristics for a gate length of 1 μm with 100 μm gate width are following: I_max=800 mA/mm, V_break-down=50 V, g_{m_extrinsic}=200 mS/mm, V_pinchoff=−10 V. The analysis and simulation results on the transport characteristics of the MOS gate MODFET structure is compared with the previously measured experimental data. The calculated values of f_T (20-130 GHz) suggest that the operation of the proposed device effectively, has sufficiently high current gain cutoff frequencies over a wide range of drain voltage, which is essential for high-power performance at microwave frequencies. The proposed device offers lower on-state resistance. The results so obtained are in close agreement with the experimental data.     

A new two-dimensional C-V model for prediction of maximum frequency of oscillation (fmax) of deep submicron AlGaN/GaN HEMT for microwave and millimeter wave applications

An analytical two-dimensional capacitance-voltage model for AlGaN/GaN high electron mobility transistor (HEMTs) is developed, which is valid from a linear to saturation region. The gate source and gate drain capacitances are calculated for 120 nm gate length including the effects of fringing field capacitances. We obtain a cut-off frequency (f_T) of 120 GHz and maximum frequency of oscillations (f_max) of 160 GHz. The model is very useful for microwave circuit design and analysis. Additionally, these devices allow a high operating voltage V_DS, which is demonstrated in the present analysis. These results show an excellent agreement when compared with the experimental data.     

RF and microwave characteristics of a 10 nm thick InGaN-channel gate recessed HEMT

A new depletion-mode gate recessed AlGaN/InGaN/GaN-high electron mobility transistor(HEMT)with 10 nm thickness of InGaN-channel is proposed.A growth of AlGaN over GaN leads to the formation of twodimensional electron gas(2DEG)at the heterointerface.High 2DEG density(ns)is achieved at the heterointerface due to a strain induced piezoelectric effect between AlGaN and GaN layers.The electrons are confined in the InGaN-channel without spilling over into the buffer layer,which also reduces the buffer leakage current.From the input transfer characteristics the threshold voltage is obtained as 4:5 V and the device conducts a current of 2 A/mm at a drain voltage of 10 V.The device also shows a maximum output current density of 1.8 A/mm at Vds of 3 V.The microwave characteristics like transconductance,cut-off frequency,max frequency of oscillation and Mason’s Unilateral Gain of the device are studied by AC small-signal analysis using a two-port network.The stability and power performance of the device are analyzed by the Smith chart and polar plots respectively.To our knowledge this proposed InGaN-channel HEMT structure is the first of its kind.     

AlGaN/GaN HEMT器件工艺的研究进展

首先论述了Al GaN/GaN高电子迁移率晶体管(HEMT)在微波大功率领域的应用优势和潜力;其次,介绍并分析了影响Al GaN/GaN HEMT性能的主要参数,分析表明要提高Al-GaN/GaN HEMT的频率和功率性能,需改善寄生电阻、电容、栅长和击穿电压等参数。然后,着重从材料结构和器件工艺的角度阐述了近年来Al GaN/GaN HEMT的研究进展,详细归纳了目前主要的材料生长和器件制作工艺,可以看出基本的工艺思路是尽量提高材料二维电子气的浓度和材料对二维电子气的限制能力的同时减小器件的寄生电容和电阻,增强栅极对沟道的控制能力。另外,根据具体情况调节栅长及沟道电场。最后,简要探讨了Al GaN/GaN HEMT还存在的问题以及面临的挑战。     

Effect of Temperature Variation on the Characteristics of Microwave Power AlGaN/GaN MODFET

The prime motivation for developing the proposed model of AlGaN/GaN microwave power device is to demonstrate its inherent ability to operate at much higher temperature. An investigation of temperature model of a 1 μm gate AlGaN/GaN enhancement mode n-type modulation-doped field effect transistor (MODFET) is presented. An analytical temperature model based on modified charge control equations is developed. The proposed model handles higher voltages and show stable operation at higher temperatures. The investigated temperature range is from 100 °K–600 °K. The critical parameters of the proposed device are the maximum drain current (I_Dmax), the threshold voltage (V_th), the peak dc trans-conductance (g_m), and unity current gain cut-off frequency (f_T). The calculated values of f_T (10–70 GHz) at elevated temperature suggest that the operation of the proposed device has sufficiently high current handling capacity. The temperature effect on saturation current, cutoff frequency, and trans-conductance behavior predict the device behavior at elevated temperatures. The analysis and simulation results on the transport characteristics of the MODFET structure is compared with the previously measured experimental data at room temperature. The calculated critical parameters suggest that the proposed device could survive in extreme environments.     

基于氟等离子体表面处理技术的AlGaN/GaN HEMT器件栅正向泄漏电流研究

The gate forward leakage current in AlGaN/GaN high electron mobility transistors(HEMTs) is investigated. It is shown that the current which originated from the forward biased Schottky-gate contributed to the gate forward leakage current.Therefore,a fluorine-plasma surface treatment is presented to induce the negative ions into the AlGaN layer which results in a higher metal-semiconductor barrier.Consequently,the gate forward leakage current shrinks.Experimental results confirm that the gate forward leakage current is decreased by one order magnitude lower than that of HEMT device without plasma treatment.In addition,the DC characteristics of the HEMT device with plasma treatment have been studied.