Effect of gate engineering in submicron GaAs MESFET for microwave frequency applications

We present an approach of GaAs MESFET incorporating the gate engineering effect to improve immunity against the short channel effects in order to enhance the scaling capability and the device performance for microwave frequency applications. In this context, a physics-based model for I–V characteristics and various microwave characteristics such as transconductance, cut-off frequency and maximum frequency of oscillation of submicron triple material gate(TM) GaAs MESFET are developed. The reduced short channel effects have also been discussed in combined designs i.e. TM, DM and SM in order to show the impact of our approach on the GaAs MESFETs-based device design. The proposed analytical models have been verified by their good agreement with 2D numerical simulations. The models developed in this paper will be useful for submicron and microwave analysis for circuit design.     

Thermally stimulated current spectroscopy of high-purity semi-insulating 4H-SiC substrates

High-purity semi-insulating (HPSI) 4H-SiC has been widely used as a substrate material for AlGaN/GaN high electron-mobility transistors because of its fairly good lattice match with GaN and its high thermal conductivity. To control material quality, it is important to understand the nature of the deep traps. For this purpose, we have successfully applied thermally stimulated current (TSC) spectroscopy to investigate deep traps in two HPSI 4H-SiC samples grown by physical vapor transport (PVT) and high-temperature chemical vapor deposition (HTCVD), respectively. Fundamentals of TSC spectroscopy, typical TSC spectra obtained on the two samples, and theoretical fittings of a boron-related trap (peaked at ∼ 150 K) will be presented. Based on literature data for deep traps in conductive 4H-SiC, the impurity and point-defect nature of several commonly observed TSC traps, peaked at ∼105 K (0.22 eV), ∼150 K (0.29 eV), ∼175 K (∼0.33 eV), ∼260 K (∼0.53 eV), ∼305 K (∼0.63 eV), and ∼360 K (0.91 eV), in the HPSI 4H-SiC will be discussed.     

A new analytical model for photo-dependent capacitances of GaAs MESFET’s with emphasis on the substrate related effects

A new analytical model for optical and bias dependent nonlinear capacitances of GaAs MESFET which is valid for both linear and saturation regions has been proposed in this paper. The novelty lies in modeling of internal and external photovoltaic effects that includes deep level traps in the substrate and surface recombination at metal–semiconductor interface of the gate. The effect of high field domain formation at the drain end in the saturation region has also been included to improve the accuracy of the present model. The model presents backgating effects on gate–source and gate–drain capacitances of GaAs MESFET for the first time in literature. Finally, the proposed model has been compared to the reported results to show the validity. The proposed model may be very useful for the designing of photonic MMIC’s and optical receivers using GaAs MESFET’s.     

Identifying the spatial position and properties of traps in GaN HEMTs using current transient spectroscopy

Time constant spectra are extracted from current transients based on the Bayesian deconvolution and used to characterize traps in GaN high-electron mobility transistors. Two kinds of traps with different time constants in an actual device were identified in the AlGaN barrier layer and the GaN layer, respectively. In particular, the trapping process in the AlGaN barrier layer was identified at the region near the drain side under gate contact. Trapping mechanisms of both two traps are discussed. Additionally, we observe that the trap in the AlGaN barrier layer requires sufficient electric field to activate the trapping process and a high drain voltage (V_ds) accelerates the trapping processes both in the AlGaN barrier layer and the GaN layer. In addition, detrapping experiments with different filling conditions were performed to confirm their spatial positions. The influence of self-heating is excluded during the experiment by keeping the power density at a very low level, and the trapping effect is the sole factor accounting for the current transients.     

Two-dimensional analysis of the surface state effects in 4H-SiC MESFETs

Two-dimensional small-signal ac and transient analysis of surface trap effects in 4H-SiC MESFETs have been performed in this paper. The mechanism by which acceptor-type traps effect the transconductance and drain current changes has been discussed. The simulation results show that transconductance exhibits negative frequency dispersion behavior, which is caused by the charge exchange via the surface states existing between the gate-source and gate-drain terminals. The current degradation behavior is also observed due to acceptor-type traps, acting as electron traps, in MESFET devices. A detailed study involving the density, ionization and energy level of traps reveals conclusive results in the devices analyzed.     

Modeling and simulation of enhancement mode p-GaN Gate AlGaN/GaN HEMT for RF circuit switch applications

An enhancement mode p-GaN gate AlGaN/GaN HEMT is proposed and a physics based virtual source charge model with Landauer approach for electron transport has been developed using Verilog-A and simulated using Cadence Spectre, in order to predict device characteristics such as threshold voltage, drain current and gate capacitance. The drain current model incorporates important physical effects such as velocity saturation, short channel effects like DIBL (drain induced barrier lowering), channel length modulation (CLM), and mobility degradation due to self-heating. The predicted I_d-V_ds, I_d-V_gs, and C-V characteristics show an excellent agreement with the experimental data for both drain current and capacitance which validate the model. The developed model was then utilized to design and simulate a single-pole single-throw (SPST) RF switch.     

一种改进的4H-SiC MESFET大信号解析模型

提出了一种基于器件物理和结构参数并可直接应用于射频电路CAD工具的4H-SiC MESFET大信号解析模型.大信号模型基于4H-SiC MESFET的物理工作机理,源漏电流模型采用Materka的改进模型,沟道长度调制系数和饱和电压系数采用了栅源电压的一次函数建模;大信号电容模型采用电荷-电压(Q-V)的电容积分形式.该大信号模型已经应用在CAD工具中.模拟结果与实验结果符合良好,模型的有效性得到验证.     

4H-SiC MESFET的特性研究

对4H-SiC MESFET的特性研究发现,在室温下4H-SiC MESFET饱和漏电流的值为0.75A/mm,随着温度的上升,器件的饱和漏电流和跨导一直下降;栅长越短,沟道层掺杂浓度越高,饱和漏电流就越大.300K时器件的击穿电压为209V,计算出来的最大功率密度可达19.22W/mm.这些结果显示了4H-SiC在高温、高压、大功率器件应用中的优势.     

Trap effects in p-channel GaAs MESFET's

After processing of conventional n-channel GaAs MESFETs, traps in the channel and channel interface regions cause several deleterious parasitic device effects. It is known that a p-well GaAs MESFET structure eliminates all of the undesirable parasitic effects in n-channel devices; moreover, complementary p-channel MESFETs are realizable with the same p-well technology. The hole capture and emission processes of deep-level traps associated with p-channel GaAs MESFETs are characterized here using temperature-dependent drain current transient measurements. The transient behavior is dominated by trapping in the channel-substrate interface region analogous to an n-channel MESFET. By employing a one-level model to extract the activation energy and capture cross section, the traps in the channel-substrate region of the p-channel MESFET are attributed to an EL2 antisite defect (As_Ga )     

High breakdown voltage 4H-SiC MESFETs with floating metal strips

A high breakdown voltage 4H-SiC MESFET with floating metal strips (FMS) was proposed. The maximum electrical field of the MESFET gate is clamped after surface depletion layer punch through to FMS. The optimized results showed that the breakdown voltage of the 4H-SiC MESFET with two strips and one strip are 180% and 95% larger than that of the conventional one without FMS and meanwhile maintain almost same saturation drain current. The maximum theoretical output power density of the 4H-SiC MESFET with two strips and one strip are 14.5 W and 10.0 W compared to 4.8 W of the conventional structure. The cut-off frequency (f_T) of 14.7 GHz and 15.6 GHz and the maximum oscillation frequency (f_max) of 44.8 GHz and 48.7 GHz for the 4H-SiC MESFET with two strips and one strip are obtained respectively, which is just a little bit lower than that of the conventional structure.     

Mechanism of drain current droop in GaAs MESFETs

The mechanisms responsible for the drain current droop in GaAs MESFETs are discussed and their relative contributions evaluated. Contrary to a common belief that the cause is mainly self-heating, it is shown on the example of a power MESFET that deep level effects (surface states and bulk traps) have a higher contribution     

Record gain at 3.1 GHz of 4H-SiC high power RF MESFET

In this paper, a very high gain 4H-SiC power MESFET with incorporation of L-gate and source field plate (LSFP-MESFET) structures for high power and RF applications is proposed. The influence of L-gate and source field plate structures on saturation current, breakdown voltage (V_b) and small-signal characteristics of the LSFP-MESFET was studied by numerical device simulation. The optimized results showed that V_b of the LSFP-MESFET is 91% larger than that of the 4H-SiC conventional MESFET (C-MESFET), which meanwhile maintains almost 77% higher saturation drain current characteristics. The maximum output power densities of 21.8 and 5.5 W/mm are obtained for the LSFP-MESFET and C-MESFET, respectively, which means about 4 times larger output power for the proposed device. Also, the cut-off frequency (f_T) of 23.1 GHz and the maximum oscillation frequency (f_max) of 85.3 GHz for the 4H-SiC LSFP-MESFET are obtained compared to 9.4 and 36.2 GHz for that of the C-MESFET structure, respectively. The proposed LSFP-MESFET shows a new record maximum stable gain exceeding 22.7 dB at 3.1 GHz, which is 7.6 dB higher than that of the C-MESFET. To the best of our knowledge, this is 2.5 dB greater than the highest gain yet reported for SiC MESFETs, showing the potential of this device for high power RF applications.     

Fabrication and characterization of 4H-SiC planar MESFETs

The planar 4H-SiC MESFETs were fabricated by employing an ion-implantation process instead of a recess gate etching process, which is commonly adapted in compound semiconductor MESFETs, to eliminate potential damage to the gate region during etching process. Excellent ohmic and Schottky contact properties were achieved by using the modified RCA cleaning of 4H-SiC surface and the sacrificial thermal oxide layer. The fabricated MESFETs was also free from drain current instability, which the most of SiC MESFETs have been reported to suffer for the charge trapping. The drain current recovery characteristics were also improved by passivating the surface with a thermal oxide layer and eliminating the charge trapping at the surface. The performance of fabricated MESFETs was characterized by analyzing the small-signal equivalent circuit parameters extracted from the measured parameters.     

Effects of Electron Irradiation on Deep Centers in High-Purity Semi-Insulating 6H-SiC

Many point-defect–related centers have been investigated in electron-irradiated 6H-SiC by deep-level transient spectroscopy (DLTS). Most of them are believed to be related to vacancies. Our DLTS studies on deep centers produced by electron-irradiation (EI) in conductive epi-6H-SiC are in agreement with the literature data. However, for semi-insulating SiC, DLTS cannot be used for trap studies so we have applied thermally stimulated current (TSC) spectroscopy. At least nine TSC traps have been observed in high-purity/semi-insulating (HPSI) 6H-SiC. To understand the nature of these centers, 1-MeV EI and postirradiation annealing at 600°C were applied to the sample. The TSC spectroscopy and 4.2 K photoluminescence (PL) have been used to study the effects of EI and annealing on the centers in HPSI 6H-SiC. It was found that (1) some of the major EI-induced DLTS centers in conductive 6H-SiC, such as ED1, E₁/E₂, E_i, Z₁/Z₂, and L₉, have TSC counterparts even in as-grown HPSI 6H-SiC; (2) EI-induced TSC centers in HPSI 6H-SiC are due to point defects, which have been confirmed by typical PL lines (such as S, L, and V lines); and (3) the concentration of the 1.1-eV center, which controls material conductivity, can be increased by 1-MeV EI and decreased by 600°C annealing.     

A three-region analytical model for short-channel SiC MESFETs

An improved analytical three-region model is proposed for short-channel SiC metal semiconductor field effect transistors (MESFETs). It takes into account two regions in the channel under the gate, and a third ungated high field region between the gate and the drain. This third region, which has been omitted in SiC MESFETs analytical models reported so far, experiences a large potential drop in short channel devices operating under high drain voltages. Therefore, its inclusion is critical in providing an accurate device modeling. To further improve our model, we have also incorporated parasitic resistances and incomplete ionization of dopants. Using this three-region analytical model, we have simulated the I-V characteristics of SiC MESFET with a 0.70 μm gate length, and obtained excellent agreement when compared with published experimental results.     

Comparison of SiC, GaAs, and Si RF MESFET power densities

The maximum power density of Si, GaAs, and 4H-SiC MESFET's was modeled using material parameters, a planar MESFET cross section, and a piecewise linear MESFET drain characteristic. The maximum power density for the Si, GaAs, and 4H-SiC was calculated to be 0.45 W/mm, 0.78 W/mm, and 17.37 W/mm at drain voltages of 8.4 V, 8.3 V, and 105 V, respectively. Modeling power density as a function of drain voltage showed that, for low voltage applications, the GaAs MESFET has the highest power density because of its high electron mobility and very low channel resistance (R_on). For high voltage applications, the 4H-SiC MESFET has the highest absolute power density because of the higher breakdown voltage of this material. Experiment data agree qualitatively with the modeled results     

带温度补偿的6H-SiC PMOS模拟与分析

提出了一个在较宽温度范围内能精确描述6H-SiC PMOS性能的器件模型。该模型将阈值电压、沟道迁移率、体漏电流、源漏薄层电阻的温度效应等效为相应的补偿电流源,并计入界面态电荷高斯分布模型及体内Poole-Frenkel效应。模拟结果表明,阈值电压是引起高温条件下输出电流变化的主要因素,同时随着温度的升高,由于体内缺陷的存在导致体漏电流所占比例不断增大,逐渐成为Ids的重要组成部分。     

High-purity semi-insulating 4H-SiC for microwave device applications

High-purity, semi-insulating (HPSI) 4H-SiC crystals with diameters up to 75 mm have been grown by the seeded sublimation technique without the intentional introduction of elemental deep-level dopants, such as vanadium. Wafers cut from these crystals exhibit homogeneous activation energies near mid gap and thermally stable semi-insulating (SI) behavior (>10⁹ ohm-cm) throughout device processing. Secondary ion mass spectroscopy, deep-level transient spectroscopy, optical admittance spectroscopy, and electron paramagnetic resonance data suggest that the SI behavior originates from several deep levels associated with intrinsic point defects. Micropipe densities in HPSI substrates have been demonstrated to be as low as 10 cm⁻² in 2-in. substrates, and the room-temperature thermal conductivity of this material is near the theoretical maximum of 5 W/cm·K for 4H-SiC. Devices fabricated on these HPSI wafers do not exhibit any substrate related back-gating effects and have power densities as high as 5.2 W/mm with 63% power added efficiency.     

Model for photo-induced long-term drain current transients in GaAs MESFETs

A simple model is presented for the negative drain current transients observed in GaAs MESFETs when subjected to ionizing radiation. The two dominant mechanisms are proposed to be electron trapping under the Schottky gate and in the neutral semi-insulating substrate. The model is suitable for the design and evaluation of radiation-resistant GaAs MESFET integrated circuits using common electrical simulators such as SPICE3.