A semi empirical approach for submicron GaN MESFET using an accurate velocity field relationship for high power applications

A semi empirical model has been proposed for sub-micron GaN MESFET's to calculate the I-V characteristics using an accurate velocity-field relationship obtained by fitting it with the Monte Carlo (MC) simulation. The results so obtained are compared with the experimental results to validate our model and are also compared with the results obtained from the simple saturation model to present the influence of electron drift velocity modeling on the device parameters. The model has been extended to predict the microwave parameters such as transconductance and output conductance of the device.     

Unpassivated GaN/AlGaN/GaN power high electron mobility transistors with dispersion controlled by epitaxial layer design

In this paper, a novel GaN/AlGaN/GaN high electron mobility transistor (HEMT) is discussed. The device uses a thick GaN-cap layer (∼250 nm) to reduce the effect of surface potential fluctuations on device performance. Devices without Si₃N₄ passivation showed no dispersion with 200-ns-pulse-width gate-lag measurements. Saturated output-power density of 3.4 W/mm and peak power-added efficiency (PAE) of 32% at 10 GHz (V_DS=+15 V) were achieved from unpassivated devices on sapphire substrates. Large gate-leakage current and low breakdown voltage prevented higher drain-bias operation and are currently under investigation.     

The effect of built-in electric field in GaN/AlGaN quantum wells with high AIN mole fraction

Spontaneous and piezoelectric polarization in hexagonal GaN/AlGaN heterostructures give rise to large built-in electric fields. The effect of the builtin electric field in GaN/Al_xGa_1−xN quantum wells was investigated for x=0.2 to 0.8 by photoluminescence studies. The quantum well structures were grown by molecular beam epitaxy on (0001) sapphire substrates. Cross-sectional transmission electron microscopy performed on the samples revealed abrupt interfaces and uniform layer thicknesses. The low temperature (4 K) photoluminescence peaks were progressively red-shifted due to the quantum confined Stark effect depending on the AlN mole fraction in the barriers and the thickness of the GaN quantum well. Our results verify the existence of very large built-in electric fields of up to 5 MV/cm in GaN/Al_0.8Ga_0.2N quantum wells.     

X波段AlGaN/GaN HEMTs厂栅场板结构研究

基于SiC衬底成功研制X波段0.25um栅长带有г栅场板结构的AlGaN/GaN HEMT,对比T型栅结构器件,研究了г栅场板引入对器件直流、小信号及微波功率特性的影响.结果表明,г栅场板结构减小器件截止频率及振荡频率,但明显改善器件膝点电压和输出功率密度.针对场板长度分别为0.4、0.7、0.9、1.1 um,得出一定范围内增加场板长度,器件输出功率大幅度提高,并结合器件小信号模型提参结果分析原因.在频率8 GHz下,总栅宽1 mm,场板长度0.9um的器件,连续波输出功率密度7.11 W/mm,功率附加效率(PAE)35.31%,相应线性增益10.25 dB.     

Simulation and fabrication of high voltage AlGaN/GaN based Schottky diodes with field plate edge termination

In this paper, we report the breakdown voltage (BV) of AlGaN/GaN based Schottky diodes with field plate edge termination. Simulation and fabrication of AlGaN/GaN Schottky diodes were carried out. The simulations were performed using the commercial 2-D device simulator DESSIS. From the simulations, it is found that for a given gate-Ohmic distance (L_gd) of 10 μm, 2DEG of 1 × 10¹³ cm⁻² and field plate length (L_FP) of 2.5 μm, highest BV can be obtained for a silicon nitride thickness of 8000 Å and this BV value is more than 5 times that for a Schottky diode without field plate. The breakdown voltages were also simulated for different field plate lengths. The BV values obtained on the fabricated Schottky diodes are compared with the simulation data and the experimental results follow the trend obtained from the simulation. Simulations were also carried out on a Schottky diode with field plate placed over a stepped insulator with L_gd = 10 μm, L_FP = 5 μm and 2DEG = 1 × 10¹³ cm⁻² and the obtained BV values are about 7 times that without field plate.     

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.     

增强型GaN绝缘栅HEMT线性电荷控制解析模型

基于MIS理论和含极化的泊松方程,应用费米能级与二维电子气密度线性近似,并考虑计入了绝缘体/AlGaN界面的陷阱或离子电荷,导出建立了适用于增强型且兼容耗尽型AlGaN/GaN绝缘栅HEMT的线性电荷控制解析模型。研究表明,Insulator/AlGaN界面陷阱密度在1013cm-2数量级;基于该模型的器件转移特性的理论结果与器件的实测转移特性数据比较符合。该模型可望用于器件性能评估和设计优化。     

Enhancement of the Schottky Barrier Height using a Nitrogen-Rich Tungsten Nitride Thin Film for the Schottky Contacts on AlGaN/GaN Heterostructures

Tungsten, stoichiometric W₂N, and nitrogen-rich W₂N films were used as Schottky contacts on AlGaN/GaN heterostructures. The nitrogen content in the film was controlled by varying the nitrogen-to-argon gas flow ratio during the reactive sputter deposition. The diode with the nitrogen-rich film exhibited a higher Schottky barrier height and the leakage current was comparable to that of the Ni/Au Schottky contact. Analysis suggested that this was due to the increase of the tungsten nitride work function as the result of higher nitrogen incorporation. Furthermore, after 600°C thermal annealing, the diode was stable and showed no change in the leakage current.     

用于功率变换的高击穿增强型AlGaN/GaN HEMTs

首次采用CF4等离子体技术实现可用于功率变换的增强性AlGaN/GaN功率器件。实验结果表明,当AlGaN/GaN器件经功率150W和时间150s等离子体轰击后,器件阈值电压从-4V被调制约为0.5V,表现为增强型。当漂移区LGD从5μm增加到15μm,器件的击穿电压从50V迅速增大到400V,电压增幅达350V。采用长度为3μm源场板结构将器件击穿电压明显地提高,击穿电压增加约为475V,且有着比硅基器件更低的比导通电阻,约为2.9mΩ.cm2。器件模拟结果表明,因源场板在远离栅边缘的漂移区中引入另一个电场强度为1.5MV/cm的电场,从而有效地释放了存在栅边缘的电场,将高达3MV/cm的电场减小至1MV/cm。微波测试结果表明,器件的特征频率fT和最大震荡频率fMAX随Vgs改变,正常工作时两参数均在千兆量级。栅宽为1mm的增强型功率管有较好的交直流和瞬态特性,正向电流约为90mA。故增强型AlGaN/GaN器件适合高压高频大功率变换的应用。     

Performance assessment and sub-threshold analysis of gate material engineered AlGaN/GaN HEMT for enhanced carrier transport efficiency

The present work explores the features of gate material engineered (GME) AlGaN/GaN high electron mobility transistor (HEMT) for enhanced carrier transport efficiency (CTE) and suppressed short channel effects (SCEs) using 2-D sub-threshold analysis and device simulation. The model accurately predicts the channel potential, electric field and sub-threshold current for the conventional and GME HEMT, taking into account the effect of work function difference of the two metal gates. This is verified by comparing the model results with the ATLAS simulation results. Further, simulation study has been extended to reflect the wide range of benefits exhibited by GME HEMT for its on-state and analog performance. The simulation results demonstrate that the GME HEMT exhibits much higher on current, lower conductance and higher transconductance as compared to the conventional HEMT due to improved CTE and reduced SCEs. This in turn has a direct bearing on the device figure of merits (FOMs) such as intrinsic gain, device efficiency and early voltage. Tuning of GME HEMT in terms of the relative lengths of the two metal gates, their work function difference and barrier layer thickness has further been carried out to enhance the drive current, transconductance and the device FOMs illustrating the superior performance of GME HEMT for future high-performance high-speed switching, digital and analog applications.     

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.     

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.     

An analytical parasitic resistance dependent Id–Vd model for planar doped InAlAs/InGaAs/InP HEMT using non-linear charge control analysis

An analytical parasitic resistance dependent model for the current voltage characteristics for InAlAs/InGaAs/InP HEMT is proposed. The model uses a new polynomial dependence of sheet carrier concentration on gate voltage to calculate I_d–V_d characteristics and has been extended to obtain transconductance, output conductance and cut-off frequency of the device. A maximum cut-off frequency of 83 and 175 GHz was obtained for channel length of 0.25 and 0.1 μm, respectively. Close agreement with published results confirms the validity of our approach.