A new physics-based self-heating effect model for 4H-SiC MESFETs

A new self-heating effect model for 4H-SiC MESFETs is proposed based on a combination of an analytical and a computer aided design (CAD) oriented drain current model. The circuit oriented expressions of 4H-SiC low-field electron mobility and in-complete ionization rate, which are related to temperature, are presented in this model, which are used to estimate the self-heating effect of 4H-SiC MESFETs. The verification of the present model is made, and the good agreement between simulated results and measured data of DC I-V curves with the self-heating effect is obtained.     

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.     

Ohmic contacts of 4H-SiC on ion-implantation layers

The ohmic contacts of 4H-SiC are fabricated on nitrogen ion implanted layers made by performing box-like-profile implantation three and four times. Implantation parameters such as the standard deviation σ and the projection range R_p are calculated by the Monte Carlo simulator TRIM. Ni/Cr ohmic contacts on Si-face 4H-SiC implantation layers are measured by transfer length methods (TLMs). The results show that the values of sheet resistance R_sh are 30~kΩ /□ and 4.9~kΩ/□ and the values of specific contact resistance ρ_c of ohmic contacts are 7.1× 10^-4Ω.cm² and 9.5× 10^-5Ω.cm² for the implanted layers with implantation performed three and four times respectively.     

Parameter analysis for gate metal-oxide-semiconductor structures of ion-implanted 4H silicon carbide metal-semiconductor field-effect transistors

From the theoretical analysis of the thermionic emission model of current-voltage characteristics, this paper extracts the parameters for the gate Schottky contact of two ion-implanted 4H-SiC metal-semiconductor field-effect transistors (sample A and sample B for three and four times multiple ion-implantation channel region respectively) fabricated in the experiment, including the ideality factor, the series resistance, the zero-field barrier height, the interface oxide capacitance, the interface state density distribution, the neutral level of interface states and the fixed space charge density. The methods to improve the interface of the ion-implanted Schottky contact are given at last.     

Characterization of ion-implanted 4H-SiC Schottky barrier diodes

Ion-implantation layers are fabricated by multiple nitrogen ion-implantations (3 times for sample A and 4 times for sample B) into a p-type 4H-SiC epitaxial layer. The implantation depth profiles are calculated by using the Monte Carlo simulator TRIM. The fabrication process and the I--V and C--V characteristics of the lateral Ti/4H-SiC Schottky barrier diodes (SBDs) fabricated on these multiple box-like ion-implantation layers are presented in detail. Measurements of the reverse I--V characteristics demonstrate a low reverse current, which is good enough for many SiC-based devices such as SiC metal--semiconductor field-effect transistors (MESFETs), and SiC static induction transistors (SITs). The parameters of the diodes are extracted from the forward I--V and C--V characteristics. The values of ideality factor n of SBDs for samples A and B are 3.0 and 3.5 respectively, and the values of series resistance R_\rm s are 11.9 and 1.0~kΩ respectively. The values of barrier height φ _\rm B of Ti/4H-SiC are 0.95 and 0.72 eV obtained by the I--V method and 1.14 and 0.93 eV obtained by the C--V method for samples A and B respectively. The activation rates for the implanted nitrogen ions of samples A and B are 2\% and 4\% respectively extracted from C--V testing results.     

SIMPLE TECHNIQUES FOR DETERMINING THE SMALL-SIGNAL EQUIVALENT CIRCUIT OF MESFET’s

Simple techniques for determining the broadband small signal equivalent circuit (SSEC) of MESFETs are presented in this paper. The intrinsic elements are calculated using two-dimensional method from the Y parameters, which are obtained from the Fourier analysis of the device transient response to voltage-step perturbations at the drain and gate electrodes. Whereas, the parasitic external elements are determined by simple approximations used in transmission line modeling. In addition, a new technique is also proposed to determine the source and drain series resistances. A comparison of the SSEC of three different MESFETs technologies shows that the MESFETs on GaN and 4H-SiC are suitable for high power applications. The method used to determine the intrinsic elements is validated with simulated data obtained by Monte Carlo method.     

Theoretical study on the improvement of the doping efficiency of Al in 4H-SiC by co-doping group-IVB elements

The p-type doping efficiency of 4H silicon carbide (4H-SiC) is rather low due to the large ionization energies of p-type dopants. Such an issue impedes the exploration of the full advantage of 4H-SiC for semiconductor devices. In this study, we show that co-doping group-IVB elements effectively decreases the ionization energy of the most widely used p-type dopant, i.e., aluminum (Al), through the defect-level repulsion between the energy levels of group-IVB elements and that of Al in 4H-SiC. Among group-IVB elements Ti has the most prominent effectiveness. Ti decreases the ionization energy of Al by nearly 50%, leading to a value as low as ~0.13 eV. As a result, the ionization rate of Al with Ti co-doping is up to ~5 times larger than that without co-doping at room temperature when the doping concentration is up to 10¹⁸ cm^-3. This work may encourage the experimental co-doping of group-IVB elements such as Ti and Al to significantly improve the p-type doping efficiency of 4H-SiC.     

栅漏间表面外延层对4H-SiC功率MESFET击穿 特性的改善机理与结构优化

本文提出了一种带栅漏间表面p型外延层的新型MESFET结构并整合了能精确描述4H-SiC MESFET工作机理的数值模型,模型综合考虑了高场载流子饱和、雪崩碰撞离化以及电场调制等效应. 利用所建模型分析了表面外延层对器件沟道表面电场分布的改善作用,并采用突变结近似法对p型外延层参数与器件输出电流(I_ds)和击穿电压(V_B)的关系进行了研究.结果表明,通过在常规MESFET漏端处引入新的电场峰来降低栅极边缘的强电场峰并在栅漏之间的沟道表面引入p-n结内建电场进一步降低电场峰值,改善了表面电场沿电流方向的分布.通过与常规结构以及场板结构SiC MESFET的特性对比表明,本文提出的结构可以明显改善SiC MESFET的功率特性.此外,针对文中给定的器件结构,获得了优化的设计方案,选择p型外延层厚度为0.12 μupm,掺杂浓度为5× 10¹⁵ cm^-3,可使器件的V_B提高33%而保持I_ds基本不变.     

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.     

SiC功率金属-半导体场效应管的陷阱效应模型

针对4H-SiC射频大功率MESFET，建立了一个解析的陷阱效应模型，该模型采用简化参数描述方法，并结合自热效应分析，从理论上完善了SiC MESFET大信号直流I-V特性的解析模型，且避免了数值方法模拟陷阱效应的巨大计算量. 关键词： 碳化硅 陷阱效应 金属-半导体场效应晶体管 深能级陷阱 界面态     

4H-SiC pn结型二极管击穿特性中隧穿效应影响的模拟研究

基于4H-SiC材料特性，建立了4H-SiC pn结型二极管的击穿模型.该模型在碳化硅器件中引入 雪崩倍增效应和隧穿效应.利用该模型，分析了隧穿效应对器件击穿特性的影响；解释了不 同的温度和掺杂条件下，器件的击穿机理.该模型较好地反映了实际器件的击穿特性. 关键词： 4H-SiC 二极管 击穿特性 隧穿效应 碰撞离化 模型     

The Effect of Point Defects on the Activation Processes in Semiinsulating GaAs Irradiated by Si28 Ions

The layered concentration, concentration profile, and mobility of electrons in the Si²⁸ ion-implanted layers (IIL) of semiinsulating GaAs are investigated. The specific resistance of the latter is also studied upon radiation annealing (RA) in the temperature range 590–800 °C using electron energies higher than the threshold energy of defect formation. The IIL are shown to form during RA at much lower temperatures. The layers exhibit high electrical activation of Si²⁸ ions, with the electron-concentration profile corresponding to the calculated one, and a low concentration of residual defects limiting the electron mobility. The radiation annealing increases the resistance of semiinsulating GaAs. The calculations show that these effects are due to the Frenkel pairs (FP) generated by radiation. A high degree of ionization of GaAs atoms significantly reduces energies of potential barriers of diffusion, FP recombination, and electrical impurity activation.     

Physically based model for trapping and self-heating effects in 4H-SiC MESFETs

In order to accurately and simply extract the trapping parameters in SiC metal semiconductor field effect transistors (MESFETs) a method is proposed based on device dc and ac small-signal models. By combining modeling techniques, material physics, and measured device characteristics, we are able to estimate the important information about the trap property and heat flow in 4H-SiC material and their influences on performance of MESFET devices, including gate lag, frequency-related dispersion, and the self-heating effect. Simulations indicate that the gate lag is due to the traps located at the channel/buffer interface and the transition frequency is up to ∼ MHz at 600 K. PACS 72.15.Jf; 72.20.Jv; 72.15.Eb     

Electron traps and positive DLTS signals in VPE GaAs MESFETs

Capacitance DLTS measurements have been performed in VPE GaAs MESFETs prepared on Bridgman Cr-doped and LEC undoped semi-insulating substrates. A band of electron traps not intrinsically related to the VPE growth process and accumulating near the metal (gate) — semiconductor interface was detected in all the samples. Deeper regions into the channel were free from any detectable trap. Near pinch-off conditions, a positive capacitance signal was found to dominate the DLTS spectra only in the case of samples prepared on Cr-doped substrates. The hypothesis of this positive transient being related to changes in the occupation of surface states in the ungated surface access regions has been checked by comparing experimental and calculated dependencies of the signal amplitude on reverse gate voltage. Unexplained discrepancies, together with the absence of positive signal in MESFETs prepared on LEC undoped substrates, suggest the possibility of hole emission from hole traps within the bulk of the device.     

Numerical modelling of anisotropy in 4H－SiC MESFET‘s

A numerical model for 4H-SiC MESFET anisotropy is presented in this paper and the device performances, such as breakdown, temperature and transient characteristics, are demonstrated. The simulation results show obvious effects of the anisotropy for 4H-SiC and are in better accordance with the experimental results. The anisotropy for 4H-SiC should be involved in the device design to acquire better performances.     

LPCVD法制备的高纯半绝缘4H-SiC晶体ESR谱特性

利用电子自旋共振波谱（ESR）仪,分析由低压化学气相沉积（LPCVD）法制备的高纯半绝缘4H-SiC材料本征缺陷.结果发现,在暗场条件下获得的缺陷信息具有碳空位（V_C）及其络合物的特征;谱线具有半高宽较大、峰谷明显不对称的特点.分析认为造成ESR谱线半高宽较大及峰谷不对称现象的主要原因是测试温度较高.同时,吸收谱中峰谷不对称现象及较大半高宽现象的出现还与不对称的晶格结构及缺陷浓度的不均匀分布有关.在110 K测试温度下,能级上的电子分布对ESR谱特性影响很小. 关键词： 低压化学气相沉积 高纯半绝缘4H-SiC 电子自旋共振 本征缺陷     

非故意掺杂4H-SiC外延材料本征缺陷的热稳定性

采用电子顺磁共振（ESR）和低温光致发光（PL）技术,研究了退火温度对低压化学气相沉积法（LPCVD）制备的非故意掺杂 4H-SiC材料中本征缺陷稳定性的影响.结果发现,当退火时间为10 min和30 min时,本征缺陷浓度均随着退火温度的升高而增大,当退火温度达到1573 K时材料中本征缺陷浓度达到最大,继续升高退火温度将使材料中本征缺陷浓度迅速降低.退火温度对材料中本征缺陷的影响主要是由于退火中本征缺陷的稳定化过程及本征缺陷之间发生强烈的相互作用引起的. 关键词： 高温退火 本征缺陷 电子顺磁共振谱 光致发光     

N型4H-SiC同质外延生长

利用水平式低压热壁CVD (LP-HW-CVD) 生长系统,台阶控制生长和衬底旋转等优化技术,在偏晶向的4H-SiC Si(0001) 晶面衬底上进行4H-SiC同质外延生长,生长温度和压力分别为1550℃和10⁴ Pa,用高纯N₂作为n型掺杂剂的4H-SiC原位掺杂技术,生长速率控制在5μm/h左右.采用扫描电镜(SEM)、原子力显微镜(AFM),傅里叶变换红外光谱(FTIR)和Hg/4H-SiC肖特基结构对同质外延表面形貌、厚度、掺杂浓度以及均匀性进行了测试.实验结果表明,4H-SiC同质外延在表面无明显缺陷,厚度均匀性1.74%, 1.99% 和1.32%(σ/mean),掺杂浓度均匀性为3.37%,2.39%和2.01%.同种工艺条件下,样品间的厚度和掺杂浓度误差为1.54%和3.63%,有很好的工艺可靠性. 关键词： 4H-SiC 同质外延生长 水平热壁CVD 均匀性     

Assessing the effect of hydrogen on the electronic properties of 4H-SiC

As a common impurity in 4H silicon carbide (4H-SiC), hydrogen (H) may play a role in tuning the electronic properties of 4H-SiC. In this work, we systemically explore the effect of H on the electronic properties of both n-type and p-type 4H-SiC. The passivation of H on intrinsic defects such as carbon vacancies (V_C) and silicon vacancies (V_Si) in 4H-SiC is also evaluated. We find that interstitial H at the bonding center of the Si-C bond (H_i^bc) and interstitial H at the tetrahedral center of Si (H_i^Si-te) dominate the defect configurations of H in p-type and n-type 4H-SiC, respectively. In n-type 4H-SiC, the compensation of H_i^Si-te is found to pin the Fermi energy and hinder the increase of the electron concentration for highly N-doped 4H-SiC. The compensation of H_i^bc is negligible compared to that of V_C on the p-type doping of Al-doped 4H-SiC. We further examine whether H can passivate V_C and improve the carrier lifetime in 4H-SiC. It turns out that nonequilibrium passivation of V_C by H is effective to eliminate the defect states of V_C, which enhances the carrier lifetime of moderately doped 4H-SiC. Regarding the quantum-qubit applications of 4H-SiC, we find that H can readily passivate V_Si during the creation of V_Si centers. Thermal annealing is needed to decompose the resulting V_Si-nH (n=1-4) complexes and promote the uniformity of the photoluminescence of V_Si arrays in 4H-SiC. The current work may inspire the impurity engineering of H in 4H-SiC.     

Theoretical study on the improvement of the doping efficiency of Al in 4H-SiC by co-doping group-ⅣB elements

The p-type doping efficiency of 4H silicon carbide(4H-SiC)is rather low due to the large ionization energies of p-type dopants.Such an issue impedes the explora...