Reverse blocking characteristics and mechanisms in Schottky-drain AlGaN/GaN HEMT with a drain field plate and floating field plates

In this paper, a novel Al Ga N/Ga N HEMT with a Schottky drain and a compound field plate(SD-CFP HEMT) is presented for the purpose of better reverse blocking capability. The compound field plate(CFP) consists of a drain field plate(DFP) and several floating field plates(FFPs). The physical mechanisms of the CFP to improve the reverse breakdown voltage and to modulate the distributions of channel electric field and potential are investigated by two-dimensional numerical simulations with Silvaco-ATLAS. Compared with the HEMT with a Schottky drain(SD HEMT) and the HEMT with a Schottky drain and a DFP(SD-FP HEMT), the superiorities of SD-CFP HEMT lie in the continuous improvement of the reverse breakdown voltage by increasing the number of FFPs and in the same fabrication procedure as the SD-FP HEMT.Two useful optimization laws for the SD-CFP HEMTs are found and extracted from simulation results. The relationship between the number of the FFPs and the reverse breakdown voltage as well as the FP efficiency in SD-CFP HEMTs are discussed. The results in this paper demonstrate a great potential of CFP for enhancing the reverse blocking ability in Al Ga N/Ga N HEMT and may be of great value and significance in the design and actual manufacture of SD-CFP HEMTs.     

Mechanism of improving forward and reverse blocking voltages in AlGaN/GaN HEMTs by using Schottky drain

In this paper, we demonstrate that a Schottky drain can improve the forward and reverse blocking voltages(BVs)simultaneously in AlGaN/GaN high-electron mobility transistors(HEMTs). The mechanism of improving the two BVs is investigated by analysing the leakage current components and by software simulation. The forward BV increases from72 V to 149 V due to the good Schottky contact morphology. During the reverse bias, the buffer leakage in the Ohmicdrain HEMT increases significantly with the increase of the negative drain bias. For the Schottky-drain HEMT, the buffer leakage is suppressed effectively by the formation of the depletion region at the drain terminal. As a result, the reverse BV is enhanced from-5 V to-49 V by using a Schottky drain. Experiments and the simulation indicate that a Schottky drain is desirable for power electronic applications.     

Influence of a drain field plate on the forward blocking characteristics of an AlGaN/GaN high electron mobility transistor

In this paper,the influence of a drain field plate(FP)on the forward blocking characteristics of an AlGaN/GaN high electron mobility transistor(HEMT)is investigated.The HEMT with only a gate FP is optimized,and breakdown voltage VBRis saturated at 1085 V for gate–drain spacing LGD≥8μm.On the basis of the HEMT with a gate FP,a drain FP is added with LGD=10μm.For the length of the drain FP LDF≤2μm,VBRis almost kept at 1085 V,showing no degradation.When LDFexceeds 2μm,VBRdecreases obviously as LDFincreases.Moreover,the larger the LDF,the larger the decrease of VBR.It is concluded that the distance between the gate edge and the drain FP edge should be larger than a certain value to prevent the drain FP from affecting the forward blocking voltage and the value should be equal to the LGDat which VBR begins to saturate in the first structure.The electric field and potential distribution are simulated and analyzed to account for the decrease of VBR.     

Influence of drain bias on the electron mobility in AlGaN/AlN/GaN heterostructure field-effect transistors

Using measured capacitance-voltage curves and current-voltage characteristics for the AlGaN/AlN/GaN heterostructure field-effect transistors with different gate lengths and drain-to-source distances,the influence of drain bias on the electron mobility is investigated.It is found that below the knee voltage the longitudinal optical(LO) phonon scattering and interface roughness scattering are dominant for the sample with a large ratio of gate length to drain-to-source distance(here 4/5),and the polarization Coulomb field scattering is dominant for the sample with a small ratio(here 1/5).However,the above polarization Coulomb field scattering is weakened in the sample with a small drain-to-source distance(here 20 μm) compared with the one with a large distance(here 100 μm).This is due to the induced strain in the AlGaN layer caused by the drain bias.     

AlGaN/GaN高电子迁移率晶体管肖特基高温退火机理研究

在不同应力条件下,研究了AlGaN/GaN高电子迁移率晶体管高温退火前后的电流崩塌、栅泄漏电流以及击穿电压的变化.结果表明,AlGaN/GaN高电子迁移率晶体管通过肖特基高温退火以后,器件的特性得到很大的改善.利用电镜扫描(SEM)和X射线光电子能谱(XPS)对高温退火前、后的肖特基接触界面进行深入分析,发现器件经过高温退火后,Ni和AlGaN层之间介质的去除,并且AlGaN材料表面附近的陷阱减少,使得肖特基有效势垒提高,从而提高器件的电学特性. 关键词： AlGaN/GaN高电子迁移率晶体管 肖特基接触 界面陷阱     

Influence of Schottky drain contacts on the strained AlGaN barrier layer of AlGaN/AlN/GaN heterostructure field-effect transistors

Rectangular Schottky drain AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) with different gate contact areas and conventional AlGaN/AlN/GaN HFETs as control were both fabricated with same size. It was found there is a significant difference between Schottky drain AlGaN/AlN/GaN HFETs and the control group both in drain series resistance and in two-dimensional electron gas (2DEG) electron mobility in the gate-drain channel. We attribute this to the different influence of Ohmic drain contacts and Schottky drain contacts on the strained AlGaN barrier layer. For conventional AlGaN/AlN/GaN HFETs, annealing drain Ohmic contacts gives rise to a strain variation in the AlGaN barrier layer between the gate contacts and the drain contacts, and results in strong polarization Coulomb field scattering in this region. In Schottky drain AlGaN/AlN/GaN HFETs, the strain in the AlGaN barrier layer is distributed more regularly.     

High-electric-field-stress-induced degradation of SiN passivated AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated by employing SiN passivation, this paper investigates the degradation due to the high-electric-field stress. After the stress, a recoverable degradation has been found, consisting of the decrease of saturation drain current I_Dsat, maximal transconductance g_m, and the positive shift of threshold voltage V_TH at high drain-source voltage V_DS. The high-electric-field stress degrades the electric characteristics of AlGaN/GaN HEMTs because the high field increases the electron trapping at the surface and in AlGaN barrier layer. The SiN passivation of AlGaN/GaN HEMTs decreases the surface trapping and 2DEG depletion a little during the high-electric-field stress. After the hot carrier stress with V_DS=20 V and V_GS=0 V applied to the device for 10⁴ sec, the SiN passivation decreases the stress-induced degradation of I_Dsat from 36% to 30%. Both on-state and pulse-state stresses produce comparative decrease of I_Dsat, which shows that although the passivation is effective in suppressing electron trapping in surface states, it does not protect the device from high-electric-field degradation in nature. So passivation in conjunction with other technological solutions like cap layer, prepassivation surface treatments, or field-plate gate to weaken high-electric-field degradation should be adopted.     

开态应力下电压和电流对AlGaN/GaN高电子迁移率晶体管的退化作用研究

通过采集等功率的两种不同开态直流应力作用下AlGaN/GaN高电子迁移率晶体管(HEMTs)漏源电流输出特性、源区和漏区大信号寄生电阻、转移特性、阈值电压随应力时间的变化, 并使用光发射显微镜观察器件漏电流情况, 研究了开态应力下电压和电流对AlGaN/GaN高电子迁移率晶体管的退化作用. 结果表明, 低电压大电流应力下器件退化很少, 高电压大电流下器件退化较明显. 高电压是HEMTs退化的主要因素, 栅漏之间高电场引起的逆压电效应对参数的永久性退化起决定性作用. 除此之外, 器件表面损坏部位的显微图像表明低电压大电流下器件失效是由于局部电流密度过高, 出现热斑导致器件损伤引起的.     

Kink effect in AlGaN/GaN high electron mobility transistors by electrical stress

The kink effect is studied in an AlGaN/GaN high electron mobility transistor by measuring DC performance during fresh, short-term stress and recovery cycle with negligible degradation. Vdg plays an assistant role in detrapping electrons and short-term stress results in no creation of new category traps but an increase in number of active traps. A possible mechanism is proposed that electrical stress supplies traps with the electric field for activation and when device is under test field-assisted hot-electrons result in electrons detrapping from traps, thus deteriorating the kink effect. In addition, experiments show that the impact ionization is at a relatively low level, which is not the dominant mechanism compared with trapping effect. We analyse the complicated link between the kink effect and stress bias through groups of electrical stress states: Vds = 0-state, off-state, on-state (on-state with low voltage, high-power state, high field state). Finlly, a conclusion is drawn that electric field brings about more severe kink effect than hot electrons. With the assistance of electric field, hot electrons tend to be possible to modulate the charges in deep-level trap.     

Recessed-gate quasi-enhancement-mode AlGaN/GaN high electron mobility transistors with oxygen plasma treatment

In this paper,the enhancement-mode AlGaN/GaN HEMT combined with the low damage recessed-gate etching and the optimized oxygen plasma treatment was fabricated.Scanning electron microscope/energy dispersive spectrometer(SEM/EDS) method and x-ray photoelectron spectroscopy(XPS) method were used to confirm the formation of oxides.Based on the experimental results,the obtained enhancement-mode HEMT exhibited a threshold voltage of 0.5 V,a high peak transconductance of 210 mS/mm,and a maximum drain current of 610 mA/mm at the gate bias of 4 V.Meanwhile,the on/off current ratio of enhancement-mode HEMT was as high as 10~8,drain induced barrier lowering(DIBL) was as low as 5 raV/V,and subthreshold swing(SS) of 80 mV/decade was obtained.Compared with the conventional HEMT,the Schottky reverse current of enhancement-mode HEMT was three orders of magnitude lower,and the off-state breakdown voltage of which was higher.In addition,a power gain cutoff frequency(/max) of the enhancement-mode HEMT was larger than that of the conventional one.     

Enhancement-mode AlGaN/GaN high electronic mobility transistors with thin barrier

An enhancement-mode (E-mode) AlGaN/GaN high electron mobility transistor (HEMTs) was fabricated with 15-nm AlGaN barrier layer. E-mode operation was achieved by using fluorine plasma treatment and post-gate rapid thermal annealing. The thin barrier depletion-HEMTs with a threshold voltage typically around --1.7 V, which is higher than that of the 22-nm barrier depletion-mode HEMTs (--3.5 V). Therefore, the thin barrier is emerging as an excellent candidate to realize the enhancement-mode operation. With 0.6-μ m gate length, the devices treated by fluorine plasma for 150-W RF power at 150 s exhibited a threshold voltage of 1.3 V. The maximum drain current and maximum transconductance are 300 mA/mm, and 177 mS/mm, respectively. Compared with the 22-nm barrier E-mode devices, V_T of the thin barrier HEMTs is much more stable under the gate step-stress.     

Electric field driven plasmon dispersion in AlGaN/GaN high electron mobility transistors

We present a theoretical study on the electric field driven plasmon dispersion of the two-dimensional electron gas(2DEG)in AlGaN/GaN high electron mobility transistors(HEMTs).By introducing a drifted Fermi–Dirac distribution,we calculate the transport properties of the 2DEG in the AlGaN/GaN interface by employing the balance-equation approach based on the Boltzmann equation.Then,the nonequilibrium Fermi–Dirac function is obtained by applying the calculated electron drift velocity and electron temperature.Under random phase approximation(RPA),the electric field driven plasmon dispersion is investigated.The calculated results indicate that the plasmon frequency is dominated by both the electric field and the angle between wavevector and electric field.Importantly,the plasmon frequency could be tuned by the applied source–drain bias voltage besides the gate voltage(change of the electron density).     

Neutron irradiation effects on AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) were exposed to 1 MeV neutron irradiation at a neutron fluence of 1 × 10¹⁵ cm^-2. The dc characteristics of the devices, such as the drain saturation current and the maximum transconductance, decreased after neutron irradiation. The gate leakage currents increased obviously after neutron irradiation. However, the rf characteristics, such as the cut-off frequency and the maximum frequency, were hardly affected by neutron irradiation. The AlGaN/GaN heterojunctions have been employed for the better understanding of the degradation mechanism. It is shown in the Hall measurements and capacitance-voltage tests that the mobility and concentration of two-dimensional electron gas (2DEG) decreased after neutron irradiation. There was no evidence of the full-width at half-maximum of X-ray diffraction (XRD) rocking curve changing after irradiation, so the dislocation was not influenced by neutron irradiation. It is concluded that the point defects induced in AlGaN and GaN by neutron irradiation are the dominant mechanisms responsible for performance degradations of AlGaN/GaN HEMT devices.     

High temperature characteristics of AlGaN/GaN high electron mobility transistors

Direct current (DC) and pulsed measurements are performed to determine the degradation mechanisms of AlGaN/GaN high electron mobility transistors (HEMTs) under high temperature. The degradation of the DC characteristics is mainly attributed to the reduction in the density and the mobility of the two-dimensional electron gas (2DEG). The pulsed measurements indicate that the trap assisted tunneling is the dominant gate leakage mechanism in the temperature range of interest. The traps in the barrier layer become active as the temperature increases, which is conducive to the electron tunneling between the gate and the channel. The enhancement of the tunneling results in the weakening of the current collapse effects, as the electrons trapped by the barrier traps can escape more easily at the higher temperature.     

AlGaN/GaN 高电子迁移率晶体管漏电流退化机理研究

本文首先制备了与AlGaN/GaN高电子迁移率晶体管 (HEMT) 结构与特性等效的AlGaN/GaN异质结肖特基二极管, 采用步进应力测试比较了不同栅压下器件漏电流的变化情况, 然后基于电流-电压和电容-电压测试验证了退化前后漏电流的传输机理, 并使用失效分析技术光发射显微镜 (EMMI) 观测器件表面的光发射, 研究了漏电流的时间依赖退化机理. 实验结果表明: 在栅压高于某临界值后, 器件漏电流随时间开始增加, 同时伴有较大的噪声. 将极化电场引入电流与电场的依赖关系后, 器件退化前后的 log(I_FT/E)与√E 都遵循良好的线性关系, 表明漏电流均由电子Frenkel-Poole (FP) 发射主导. 退化后 log(I_FT/E)与√E 曲线斜率的减小, 以及利用EMMI在栅边缘直接观察到了与缺陷存在对应关系的“热点”, 证明了漏电流退化的机理是: 高电场在AlGaN层中诱发了新的缺陷, 而缺陷密度的增加导致了FP发射电流I_FT的增加. 关键词： AlGaN/GaN 高电子迁移率晶体管 漏电流 退化机理     

An AlGaN/GaN HEMT with enhanced breakdown and near-zero breakdown voltage temperature coefficient

An AlGaN/GaN high-electron mobility transistor (HEMT) with a novel source-connected air-bridge field plate (AFP) is experimentally verified. The device features a metal field plate that jumps from the source over the gate region and lands between the gate and drain. When compared to a similar size HEMT device with conventional field plate (CFP) structure, the AFP not only minimizes the parasitic gate to source capacitance, but also exhibits higher OFF-state breakdown voltage and one order of magnitude lower drain leakage current. In a device with a gate to drain distance of 6 μm and a gate length of 0.8 μm, three times higher forward blocking voltage of 375 V was obtained at V_GS=-5 V. In contrast, a similar sized HEMT with CFP can only achieve a breakdown voltage no higher than 125 V using this process, regardless of device dimensions. Moreover, a temperature coefficient of 0 V/K for the breakdown voltage is observed. However, devices without field plate (no FP) and with optimized conventional field plate (CFP) exhibit breakdown voltage temperature coefficients of -0.113 V/K and -0.065 V/K, respectively.     

Degradation and recovery properties of AlGaN/GaN high-electron mobility transistors under direct current reverse step voltage stress

Direct current (DC) reverse step voltage stress is applied on the gate of AlGaN/GaN high-electron mobility transistor (HEMT). Experiments show that parameters degenerate under stress. Large-signal parasitic source/drain resistance (R_S/R_D) and gate-source forward I-V characteristics are recoverable after breakdown of the device under test (DUT). Electrons trapped by both the AlGaN barrier trap and the surface state under stress lead to this phenomenon, and surface state recovery is the major reason for the recovery of device parameters.     

Channel temperature determination of multifinger AlGaN/GaN high electron mobility transistor using micro-Raman technique

Self-heating in multifinger AlGaN/GaN high electron mobility transistor (HEMT) is investigated by micro-Raman spectroscopy. The device temperature is probed on the die as a function of applied bias. The operating temperature of AlGaN/GaN HEMT is estimated from the calibration curve of passively heated AlGaN/GaN structure. A linear increase of junction temperature is observed when direct current dissipated power is increased. When the power dissipation is 12.75 W at a drain voltage of 15 V, a peak temperature of 69.1 ℃ is observed at the gate edge on the drain side of the central finger. The position of the highest temperature corresponds to the high-field region at the gate edge.     

Transport mechanism of reverse surface leakage current in AlGaN/GaN high-electron mobility transistor with SiN passivation

The transport mechanism of reverse surface leakage current in the AlGaN/GaN high-electron mobility transistor(HEMT) becomes one of the most important reliability issues with the downscaling of feature size.In this paper,the research results show that the reverse surface leakage current in AlGaN/GaN HEMT with SiN passivation increases with the enhancement of temperature in the range from 298 K to 423 K.Three possible transport mechanisms are proposed and examined to explain the generation of reverse surface leakage current.By comparing the experimental data with the numerical transport models,it is found that neither Fowler-Nordheim tunneling nor Frenkel-Poole emission can describe the transport of reverse surface leakage current.However,good agreement is found between the experimental data and the two-dimensional variable range hopping(2D-VRH) model.Therefore,it is concluded that the reverse surface leakage current is dominated by the electron hopping through the surface states at the barrier layer.Moreover,the activation energy of surface leakage current is extracted,which is around 0.083 eV.Finally,the SiN passivated HEMT with a high Al composition and a thin AlGaN barrier layer is also studied.It is observed that 2D-VRH still dominates the reverse surface leakage current and the activation energy is around 0.10 eV,which demonstrates that the alteration of the AlGaN barrier layer does not affect the transport mechanism of reverse surface leakage current in this paper.     

Time-dependent degradation of threshold voltage in AlGaN/GaN high electron mobility transistors

This paper gives a detailed analysis of the time-dependent degradation of the threshold voltage in AlGaN/GaN high electron mobility transistors(HEMTs) submitted to off-state stress. The threshold voltage shows a positive shift in the early stress, then turns to a negative shift. The negative shift of the threshold voltage seems to have a long recovery time. A model related with the balance of electron trapping and detrapping induced by shallow donors and deep acceptors is proposed to explain this degradation mode.