A novel double-recessed 4H-SiC MESFET with partly undoped space region

In this paper, a novel double-recessed 4H-SiC metal semiconductor field effect transistor (MESFET) with partly undoped space region (DRUS-MESFET) is introduced. The key idea in this work is to improve the DC and RF characteristics of the device by introducing an undoped space region. Using two-dimensional and two-carrier device simulation, we demonstrate that breakdown voltage (V_BR) increases from 109 V in conventional double recessed MESFET (DR-MESFET) structure to 144.5 V in the DRUS-MESFET structure due to the modified channel electric field distribution of the proposed structure. The maximum output power density of the DRUS-MESFET structure is about 25.4% larger than that of the DR-MESFET structure. Furthermore, lower gate-drain capacitance (C_GD), higher cut-off frequency (f_T), larger maximum available gain (MAG), and higher maximum oscillation frequency (f_max) are achieved for the DRUS-MESFET structure. The results show that the f_max and f_T of the proposed structure improve 95.6% and 13.07% respectively, compared with that of the DR-MESFET structure. Also, the MAG of the DRUS-MESET is 4.5 dB higher than that of the DR-MESFET structure at 40 GHz. The results show that the DRUS-MESFET structure has superior electrical characteristics and performances in comparison with the DR-MESFET structure.     

A novel 4H–SiC MESFET with modified channel depletion region for high power and high frequency applications

In this paper, a novel 4H–SiC metal semiconductor field effect transistor (MESFET) with modified depletion region is introduced. The key idea in this work is modifying the depletion region in the channel for improving the electrical performances. The proposed structure consists of upper and lower gates. Also, the lower gate is divided into a number (N) of smaller step-shaped sections. Therefore, we have called the proposed structure multiple-recessed 4H–SiC MESFET (MR-MESFET). DC and RF characteristics of the MR-MESFET structure with various lower gate segments are analyzed by 2D numerical simulation. The simulated results show that as the number of the lower gate sections increases, the channel depletion region is modified and the drain current (I_D) enhances. Also, by increasing the number of the lower gate sections, the breakdown voltage (V_BR) enhances, too. Improvement of the I_D and V_BR leads to a further increase in the output power density of the device. Also, cut-off frequency (f_T), maximum oscillation frequency (f_max), and maximum available gain (MAG) improvements are achieved for the MR-MESFET structure with further number of the lower gate sections. The results show that the MR-MESFET structure with higher number of the lower gate segments has superior electrical characteristics and performances in comparison with the MR-MESFET structure with fewer number of the lower gate sections.     

Effects of gate-buffer combined with a p-type spacer structure on silicon carbide metal-semiconductor field-effect transistors

An improved structure of silicon carbide metal-semiconductor field-effect transistors (MESFET) is proposed for high power microwave applications. Numerical models for the physical and electrical mechanisms of the device are presented, and the static and dynamic electrical performances are analysed. By comparison with the conventional structure, the proposed structure exhibits a superior frequency response while possessing better DC characteristics. A p-type spacer layer, inserted between the oxide and the channel, is shown to suppress the surface trap effect and improve the distribution of the electric field at the gate edge. Meanwhile, a lightly doped n-type buffer layer under the gate reduces depletion in the channel, resulting in an increase in the output current and a reduction in the gate-capacitance. The structural parameter dependences of the device performance are discussed, and an optimized design is obtained. The results show that the maximum saturation current density of 325 mA/mm is yielded, compared with 182 mA/mm for conventional MESFETs under the condition that the breakdown voltage of the proposed MESFET is larger than that of the conventional MESFET, leading to an increase of 79% in the output power density. In addition, improvements of 27% cut-off frequency and 28% maximum oscillation frequency are achieved compared with a conventional MESFET, respectively.     

Effects of gate-buffer combined with a p-type spacer structure on silicon carbide metalben semiconductor field-effect transistors

An improved structure of silicon carbide metal-semiconductor field-effect transistors (MESFET) is proposed for high power microwave applications. Numerical models for the physical and electrical mechanisms of the device are presented, and the static and dynamic electrical performances are analysed. By comparison with the conventional structure, the proposed structure exhibits a superior frequency response while possessing better DC characteristics. A p-type spacer layer, inserted between the oxide and the channel, is shown to suppress the surface trap effect and improve the distribution of the electric field at the gate edge. Meanwhile, a lightly doped n-type buffer layer under the gate reduces depletion in the channel, resulting in an increase in the output current and a reduction in the gate-capacitance. The structural parameter dependences of the device performance are discussed, and an optimized design is obtained. The results show that the maximum saturation current density of 325 mA/mm is yielded, compared with 182 mA/mm for conventional MESFETs under the condition that the breakdown voltage of the proposed MESFET is larger than that of the conventional MESFET, leading to an increase of 79% in the output power density. In addition, improvements of 27% cut-off frequency and 28% maximum oscillation frequency are achieved compared with a conventional MESFET, respectively.     

Improved 4H–SiC MESFET with double source field plate structures

An improved 4H–SiC power MESFET with double source field plates (DSFP) for high-power applications is proposed (DSFP-MESFET). The DSFP structure significantly modifies the electric field in the drift layer. The influence of the DSFP structure on saturation current, breakdown voltage (V_b), and small-signal characteristics of the DSFP-MESFET were studied by numerical device simulation. The V_b of 359 V is obtained for the DSFP-MESFET compared to 301 V of the conventional source field plate MESFET (LSFP-MESFET). Hence, the maximum output power density of 24.7 and 21.8 W/mm are achieved for the DSFP-MESFET and LSFP-MESFET, respectively, which means 13% improvement for the proposed device. Also, the cut-off frequency (f_T) of 24.5 and the maximum oscillation frequency (f_max) of 89.1 GHz for the 4H–SiC DSFP-MESFET are obtained compared to 23.1 and 85.3 GHz for that of the LSFP-MESFET structure, respectively. The DSFP-MESFET shows a superior maximum stable gain (MSG) exceeding 23.3 dB at 3.1 GHz, which is presenting the potential of the proposed device for high-power operations.     

Improved performance of 4H-SiC metal-semiconductor field-effect transistors with step p-buffer layer

An improved 4H-SiC metal-semiconductor field-effect transistors (MESFETs) with step p-buffer layer is proposed, and the static and dynamic electrical performances are analysed in this paper. A step p-buffer layer has been applied not only to increase the channel current, but also to improve the transconductance. This is due to the fact that the variation in p-buffer layer depth leads to the decrease in parasitic series resistance resulting from the change in the active channel thickness and modulation in the electric field distribution inside the channel. Detailed numerical simulations demonstrate that the saturation drain current and the maximum theoretical output power density of the proposed structure are about 30% and 37% larger than those of the conventional structure. The cut-off frequency and the maximum oscillation frequency of the proposed MESFETs are 14.5 and 62 GHz, respectively, which are higher than that of the conventional structure. Therefore, the 4H-SiC MESFETs with step p-buffer layer have superior direct-current and radio-frequency performances compared to the similar devices based on the conventional structure.     

High-power SiC MESFET using dual p-buffer layer for S-band power amplifier

Silicon carbide (SiC) based metal semiconductor field effect transistor (MESFET) is fabricated by using a standard SiC MESFET structure with the application of a dual p-buffer layer and a multi-recessed gate to the process for S-band power amplifier. The lower doped upper-buffer layer serves to maintain the channel current, while the higher doped lower-buffer layer is used to provide excellent electron confinement in the channel layer. A 20-mm gate periphery SiC MESFET biased at a drain voltage of 85 V demonstrates a pulsed wave saturated output power of 94 W, a linear gain of 11.7 dB, and a maximum power added efficiency of 24.3% at 3.4 GHz. These results are improved compared with those of the conventional single p-buffer MESFET fabricated in this work using the same process. A radio-frequency power output greater than 4.7 W/mm is achieved, showing the potential as a high-voltage operation device for high-power solid-state amplifier applications.     

双沟4H-SiC MESFET优化结构的解析模型及性能

优化双沟4H-SiC MESFET结构,通过求解一维和二维泊松方程,建立优化结构的解析模型,分析这种结构的直流和交流特性.结果表明,饱和电流密度的计算结果与实验一致,结构优化后4H-SiC MESFET的饱和电流密度和击穿电压分别为420μA·μm^-1和155 V,明显高于优化前的275μA·μm^-1和141 V;最高输出功率密度为7.4 W·mm^-1,比优化前提高约64%;截止频率和最高振荡频率比优化前略微提高.双沟结构经优化后其交流小信号特性未退化而功率特性获得明显改善.     

High-power SiC MESFET using a dual p-buffer layer for an S-band power amplifier

A silicon carbide (SiC) based metal semiconductor field effect transistor (MESFET) is fabricated by using a standard SiC MESFET structure with the application of a dual p-buffer layer and a multi-recessed gate to the process for an S-band power amplifier. The lower doped upper-buffer layer serves to maintain the channel current, while the higher doped lower-buffer layer is used to provide excellent electron confinement in the channel layer. A 20-mm gate periphery SiC MESFET biased at a drain voltage of 85 V demonstrates a pulsed wave saturated output power of 94 W, a linear gain of 11.7 dB, and a maximum power added efficiency of 24.3% at 3.4 GHz. These results are improved compared with those of the conventional single p-buffer MESFET fabricated in this work using the same process. A radio-frequency power output greater than 4.7 W/mm is achieved, showing the potential as a high-voltage operation device for high-power solid-state amplifier applications.     

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.     

Top-gate amorphous IGZO thin-film transistors with a SiO buffer layer inserted between active channel layer and gate insulator

In this paper, top-gate thin-film transistors (TFTs) using amorphous In-Ga-Zn-O as the n-channel active layer and SiO₂ as gate insulator were fabricated by radio frequency magnetron sputtering at room temperature. In this device, a SiO layer was used to be a buffer layer between active layer and gate insulator for preventing the damage of the InGaZnO surface by the process of sputtering SiO₂ with relatively high sputtering power. The thickness of buffer layers was studied and optimized for enhancing the TFTs performances. Contrasting to the TFTs without buffer layer, the optimized thickness of 10 nm SiO buffer layer improved the top-gate TFTs performances greatly: mobility increases 30%, reached 1.29 cm²/V s, the I_on/I_off ratio increases 3 orders, and the trap density at the interface of channel/insulator decreases about 1 order, indicated that the improvement of semiconductor/dielectric interface by buffering the sputtering power.     

AlGaN/GaN-based HEMT on SiC substrate for microwave characteristics using different passivation layers

In this paper, a new gate-recessed AlGaN/GaN-based high electron mobility transistor (HEMT) on SiC substrate is proposed and its DC as well as microwave characteristics are discussed for Si₃N₄ and SiO₂ passivation layers using technology computer aided design (TCAD). The two-dimensional electron gas (2DEG) transport properties are discussed by solving Schr?dinger and Poisson equations self-consistently resulting in various subbands having electron eigenvalues. From DC characteristics, the saturation drain currents are measured to be 600?mA/mm and 550?mA/mm for Si₃N₄ and SiO₂ passivation layers respectively. Apart from DC, small-signal AC analysis has been done using two-port network for various microwave parameters. The extrinsic transconductance parameters are measured to be 131.7?mS/mm at a gate voltage of V _gs?= ?0.35?V and 114.6?mS/mm at a gate voltage of V _gs?= ?0.4?V for Si₃N₄ and SiO₂ passivation layers respectively. The current gain cut-off frequencies (f _t) are measured to be 27.1?GHz and 23.97?GHz in unit-gain-point method at a gate voltage of ?0.4?V for Si₃N₄ and SiO₂ passivation layers respectively. Similarly, the power gain cut-off frequencies (f _max) are measured to be 41?GHz and 38.5?GHz in unit-gain-point method at a gate voltage of ?0.1?V for Si₃N₄ and SiO₂ passivation layers respectively. Furthermore, the maximum frequency of oscillation or unit power gain (MUG = 1) cut-off frequencies for Si₃N₄ and SiO₂ passivation layers are measured to be 32?GHz and 28?GHz respectively from MUG curves and the unit current gain, ?O?h ₂₁??O?=?1 cut-off frequencies are measured to be 140?GHz and 75?GHz for Si₃N₄ and SiO₂ passivation layers respectively from the abs ?O?h ₂₁??O curves. HEMT with Si₃N₄ passivation layer gives better results than HEMT with SiO₂ passivation layer.     

4H-SiC同质外延的绿带发光与缺陷的关系

利用室温光致发光谱(PL)对CVD法生长的4H-SiC同质外延特性进行研究,发现有绿带发光(GL)特性.用扫描电子显微镜(SEM) 、二次离子质谱(SIMS)和X射线光电子谱(XPS)技术获得了4H-SiC样品纵截面形貌和元素相对含量分布.结果表明,GL与4H-SiC晶体中碳空位(V_C)及络合体缺陷相关,V_C和缓冲层的扩展缺陷(点缺陷和刃位错等)是GL微观来源.GL的半峰宽(FWHM) 反映了参与复合发光的V_C及其络合缺陷能级分散的程度.室温下获得的样品GL强度和光谱波长度可用于分析4H-SiC外延中缺陷分布和晶体质量. 关键词： 绿带发光 4H-SiC同质外延 晶体缺陷     

(1-x)MgAl2O4-xTiO2 dielectrics for microwave and millimeter wave applications

The MgAl₂O₄ ceramics were prepared by the conventional solid-state ceramic route and the dielectric properties studied in the microwave frequency region (3–13 GHz). The phase purity and crystal structure were identified using the X-ray diffraction technique. The MgAl₂O₄ spinel ceramics show interesting microwave dielectric properties (ε_r=8.75, Q_uxf=68900 GHz (loss tangent = 0.00017 at 12.3 GHz), τ_f=-75 ppm/°C). The MgAl₂O₄ has high negative τ_f, which precludes its immediate use in practical applications. Hence the microwave dielectric properties of MgAl₂O₄ spinels were tailored by adding different mole fractions of TiO₂. The ε_r and Q factor of the mixed phases were increased with the molar addition of TiO₂ into the spinel to form mixtures based on (1-x)MgAl₂O₄-xTiO₂ (x=0.0-1.0). For x=0.25 in (1-x)MgAl₂O₄-xTiO₂, the microwave quality factor reaches a maximum value of Q_uxf=105400 GHz (loss tangent = 0.00007 at 7.5 GHz) where ε_r and τ_f are 11.035 and -12 ppm/°C, respectively. The microwave dielectric properties of the newly developed 0.75MgAl₂O₄-0.25TiO₂ dielectric is superior to several commercially available low loss dielectric substrates. PACS 77.22.-d; 84.40.-x; 81.05.Je     

Design and Analysis of InGaN-GaN Modulation Doped Field-Effect Transistors (MODFETs) for Over 60 GHz Operation

A Modulation-Doped Field-Effect Transistor (MODFET) structure realized in InGaN-GaN material system is presented for the first time. An analytical model predicting the transport characteristics of the proposed MODFET structure is given in detail. Electron energy levels inside and outside the quantum well channel of the MODFET are evaluated. The two-dimensional electron gas (2DEG) density in the channel is calculated by self-consistently solving Schrödinger and Poisson's equations simultaneously. Analytical results of the current-voltage and transconductance characteristics are presented. The unity-current gain cutoff frequency (f _T) of the proposed device is computed as a function of the gate voltage V _G . The results are compared well with experimental f _T value of a GaN/AlGaN HFET device. By scaling the gate length down to 0.25 m the proposed InGaN-GaN MODFET can be operated up to about 80GHz. It is shown in this paper that InGaN-GaN system has small degradation in f _T as the operating temperature is increased from 300°K to 400°K.     

Drain-induced barrier lowering effect for short channel dual material gate 4H silicon carbide metal—semiconductor field-effect transistor

Sub-threshold characteristics of the dual material gate 4H-SiC MESFET (DMGFET) are investigated and the analytical models to describe the drain-induced barrier lowering (DIBL) effect are derived by solving one- and two- dimensional Poisson’s equations. Using these models, we calculate the bottom potential of the channel and the threshold voltage shift, which characterize the drain-induced barrier lowering (DIBL) effect. The calculated results reveal that the dual material gate (DMG) structure alleviates the deterioration of the threshold voltage and thus suppresses the DIBL effect due to the introduced step function, which originates from the work function difference of the two gate materials when compared with the conventional single material gate metal-semiconductor field-effect transistor (SMGFET).     

碳化硅基上3UCVD淀积二氧化硅及其C-V性能测试

利用3UCVD技术在p型4H-SiC上制备了栅氧化层，其正的氧化物电荷密度仅有1.6×10¹¹cm^-2,这一结果优于传统的热氧化工艺.为检验氧化层质量所做的高频C-V测试采用了正面接地的新的测试结构，克服了常规测试结构的缺点. 关键词： 淀积 高频C-V测试 二氧化硅 碳化硅     

Design and Analysis of InGaN-GaN Modulation-Doped Field-Effect Transistors (MODFETs) for 90 GHz Operations

A Modulation-Doped Field-Effect Transistor (MODFET) structure having quantum wire channel realized in InGaN-GaN material system is presented. This paper presents design and analysis of a novel one-dimensional Modulation-Doped Field-Effect transistor (1D MODFET) in InGaN-GaN material system for microwave and millimeter wave applications. An analytical model predicting the transport characteristics of the proposed MODFET device is also presented. Analytical results of the current-voltage and transconductance characteristics are presented. The unity-current gain cutoff frequency (f _T) of the proposed device is computed as a function of the gate voltage V _G. The results are compared with two-dimensional GaN/AlGaN MODFET and HFET devices. The analytical model also predicts that 0.25 m channel length devices will extend the use of InGaN-GaN MODFETs to above 90GHz.     

Experimental and numerical analysis of the multi-recessed gate structure for microwave silicon carbide power MESFETs

This paper reports that multi-recessed gate 4H-SiC MESFETs (metal semiconductor filed effect transistors) with a gate periphery of 5-mm are fabricated and characterized. The multi-recessed region under the gate terminal is applied to improve the gate--drain breakdown voltage and to alleviate the trapping induced instabilities by moving the current path away from the surface of the device. The experimental results demonstrate that microwave output power density, power gain and power-added efficiency for multi-finger 5-mm gate periphery SiC MESFETs with multi-recessed gate structure are about 29%, 1.1dB and 7% higher than those of conventional devices fabricated in this work using the same process.