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.     

一种氮化镓高迁移率晶体管的栅电流噪声模型

This work presents a theoretical and experimental study on the gate current 1/f noise in Al Ga N/Ga N HEMTs. Based on the carrier number fluctuation in the two-dimensional electron gas channel of Al Ga N/Ga N HEMTs, a gate current 1/f noise model containing a trap-assisted tunneling current and a space charge limited current is built. The simulation results are in good agreement with the experiment. Experiments show that, if Vg Vx, gate current 1/f noise comes from not only the trap-assisted tunneling RTS, but also the space charge limited current RTS. This indicates that the gate current 1/f noise of the Ga N-based HEMTs device is sensitive to the interaction of defects and the piezoelectric relaxation. It provides a useful characterization tool for deeper information about the defects and their evolution in Al Ga N/Ga N HEMTs.     

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.     

Comparative study of AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with Al2O3 and HfO2 gate oxide

AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs) with Al₂O₃ gate oxide which was deposited by atomic layer deposition (ALD) were fabricated and their performance was then compared with that of AlGaN/GaN MOSHFETs with HfO₂ gate oxide. The capacitance (C)-voltage (V) curve of the Al₂O₃/GaN MOS diodes showed a lower hysteresis and lower interface state density than the C-V curve of the HfO₂/GaN diodes, indicating better quality of the Al₂O₃/GaN interface. The saturation of drain current in the I_D-V_GS relation of the Al₂O₃ AlGaN/GaN MOSHFETs was not as pronounced as that of the HfO₂ AlGaN/GaN MOSHFETs. The gate leakage current of the Al₂O₃ MOSHFET was five to eight orders of magnitude smaller than that of the HfO₂ MOSHFETs.     

14.2 W/mm internally-matched AlGaN/GaN HEMT for X-band applications

High-performance X-band AlGaN/GaN high electron mobility transistor (HEMT) has been achieved by Γ-gate process in combination with source-connected field plate. Both its Schottky breakdown voltage and pinch-off breakdown voltage are higher than 100 V. Beside, excellent superimposition of direct current (DC) I-V characteristics in different V_ds sweep range indicates that our GaN HEMT device is almost current collapse free. As a result, both outstanding breakdown characteristics and reduction of current collapse effect guarantee high microwave power performances. Based upon it, we have developed an internally-matched GaN HEMT amplifier with single chip of 2.5 mm gate periphery, which exhibits power density of 14.2 W/mm with 45.5 dBm (35.5 W) output power and a power added efficiency (PAE) of 48% under V_ds = 48 V pulse operating condition at 8 GHz. To the best of our knowledge, it is the highest power density at this power level.     

基于再生长欧姆接触工艺的220 GHz InAlN/GaN 场效应晶体管

本文在蓝宝石衬底上研制了具有高电流增益截止频率(f_T)的InAlN/GaN异质结场效应晶体管 (HFETs)。基于MOCVD外延n -GaN欧姆接触工艺实现了器件尺寸的缩小,有效源漏间距(L_sd)缩小至600 nm。此外,采用自对准工艺制备了50 nm直栅。由于器件尺寸的缩小,V_gs= 1 V下器件最大饱和电流(I_ds)达到2.11 A/mm,峰值跨导达到609 mS/mm。小信号测试表明,器件f_T达到220 GHz、最大振荡频率(f_max)达到48 GHz。据我们所知,该f_T值是目前国内InAlN/GaN HFETs器件报道的最高结果。     

SiC衬底上栅长70 nm fT / fmax >160 GHz的InAlN/GaN HEMTs器件

在SiC衬底上制备了InAlN/GaN 高电子迁移率晶体管（HEMTs）,并进行了表征。为提高器件性能,综合采用了多种技术,包括高电子浓度,70 nm T型栅,小的欧姆接触电阻和小源漏间距。制备的InAlN/GaN器件在栅偏压为1 V时得到的最大饱和漏电流密度为1.65 A/mm,最大峰值跨导为382 mS/mm。70 nm栅长器件的电流增益截止频率fT和最大振荡频率fmax分别为162 GHz和176 GHz。     

The effects of gate length variation and trapping effects on the transient response of AlGaN/GaN HEMT’s on SiC substrates

In AlGaN/GaN heterostructure field-effect transistors, the surface defects and dislocations may serve as trapping centers and affect the device performance via leakage current. In this paper we report results of our investigation of the trapping characteristics of Al_0.25Ga_0.75N/GaN HEMT using the Conductance Deep Level Transient Spectroscopy (CDLTS). Two deep level electronic defects were observed labeled E₁ and HL₁, with activation energies E_a1 = 1.36 eV and E_a2 = 0.63 eV. The hole-trap HL₁ is characterized for the first time in our studies. We identified the characteristics of the traps at the AlGaN/GaN interface adjoining the channel and the surface along the ungated region between the gate and the drain, as well as the effects of the surface traps.     

半开态直流应力作用下AlGaN/GaN高电子迁移率晶体管的失效机制分析

本文研究了半开态直流应力条件下,AlGaN/GaN高电子迁移率晶体管的退化机制。应力实验后,器件的阈值电压电压正漂,栅漏串联电阻增大。利用数据拟合发现,沟道电流的退化量与阈值电压及栅漏串联电阻的变化量之间有密切的关系。分析表明,阈值电压的退化是引起饱和区沟道电流下降的主要因素,对于线性区电流,在应力开始的初始阶段,栅漏串联电阻的增大导致线性区电流的退化,随后沟道电流退化主要由阈值电压的退化引起。分析表明,在半开态应力作用下,栅泄露电流及热电子效应使得电子进入AlGaN层,被缺陷俘获,进而导致沟道电流退化。其中反向栅泄露电流中的电子被栅电极下AlGaN层内的缺陷俘获,导致阈值电压正漂;而热电子效应则使得栅漏串联区电阻增大。     

AlGaN/GaN metal-oxide semiconductor heterostructure field-effect transistor with photo-chemical-vapor deposition SiO<Subscript>2</Subscript> gate oxide

High-quality SiO₂ was successfully deposited onto GaN by photo-chemicalvapor deposition (photo-CVD) using a D₂ lamp as the excitation source. The AlGaN/GaN metal-oxide semiconductor, heterostructure field-effect transistors (MOSHFETs) were also fabricated with photo-CVD oxide as the insulating layer. Compared with AlGaN/GaN metal-semiconductor HFETs (MESHFETs) with similar structure, we found that we could reduce the gate-leakage current by more than four orders of magnitude by inserting the photo-CVD oxide layer in between the AlGaN/GaN and the gate metal. With a 2-μm gate, it was found that the saturated I_ds, maximum g_m, and gate-voltage swing (GVS) of the fabricated nitride-based MOSHFET were 512 mA/mm, 90.7 mS/mm, and 6 V, respectively.     

Static and low-frequency noise characterization in submicron MOSFETs for memories cells applications

In this paper, we present the extraction of oxide traps properties of n-metal-oxide-semiconductor (N-MOS) field effect transistors with W×L=0.5×0.1 μm² using low-frequency (LF and random telegraph signal) noise and static I(V) characterizations. The impact of oxide thickness, on static and noise parameters is analyzed. Static measurements on N-MOS devices with different tunnel oxide thickness show anomalies (a significant increase in V_t values for low temperature and kink effect) attributed to traps located in the oxide. From LF noise analysis we find that 1/f noise stems from carrier number fluctuations. The slow oxide trap concentration deduced from the noise data is about 10¹⁵ eV/cm³ in agreement with the state-of-the-art gate oxides. Finally, drain current RTS amplitude as large as 10% have been observed.     

Source/drain extension region engineering in nanoscale double gate SOI MOSFETs: Novel design methodology for low-voltage analog applications

The present paper proposes for the first time, a novel design methodology based on the optimization of source/drain extension (SDE) regions to significantly improve the trade-off between intrinsic voltage gain (A_VO) and cut-off frequency (f_T) in nanoscale double gate (DG) devices. Our results show that an optimally designed 25 nm gate length SDE region engineered DG MOSFET operating at drain current of 10 μA/μm, exhibits up to 65% improvement in intrinsic voltage gain and 85% in cut-off frequency over devices designed with abrupt SDE regions. The influence of spacer width, lateral source/drain doping gradient and symmetric as well as asymmetrically designed SDE regions on key analog figures of merit (FOM) such as transconductance (g_m), transconductance-to-current ratio (g_m/I_ds), Early voltage (V_EA), output conductance (g_ds) and gate capacitances are examined in detail. The present work provides new opportunities for realizing future low-voltage/low-power analog circuits with nanoscale SDE engineered DG MOSFETs.     

Nonlinear HEMT model formulated from the second-order derivative of the I-V and Q-V characteristics

In this paper, an empirical nonlinear model of high electron mobility transistors (HEMTs) suitable for a wide bias range is presented. Unlike the conventional large-signal models whose fitting parameters are coupled to the measured drain current and gate capacitance characteristics, the derived modeling equations are direct formulated from the second-order derivative of drain current (I-V) and gate charge (Q-V) with respect to gate voltage. As a consequence, the proposed nonlinear model is kept continuously differentiable and accurate enough to the higher-order I-V and Q-V derivatives. Besides, the thermal and trapping effects have been implemented in the large-signal model along with its dependence on temperature and quiescent-bias state. The composite nonlinear model is shown to accurately predict the S-parameters, large-signal power performances as well as the two-tone intermodulation distortion products for various types of GaAs and GaN HEMTs.     

The effect of external stress on the electrical characteristics of AlGaN/GaN HEMTs

The electrical characteristics of AlGaN/GaN high electron mobility transistors under the application of uniform in-plane tensile and compressive stress were measured. The results demonstrate the change of the drain–source I_ds–V_ds characteristics as a function of the external stress. The output current at V_ds = 10 V increases linearly with the stress with the slope about 3 × 10⁻⁶ A MPa⁻¹. It is associated with the piezoelectric effect and kink effect. Moreover, the magnitude of the kink effect is found to be affected by the stress. It displays a linear changing trend with the slope of 3.3 × 10⁻⁴ mS MPa⁻¹ within the stress level. The energy band structure is suggested to be responsible for the dependence of the kink effect on the stress.     

Hot electron induced degradation of undoped AlGaN/GaN HFETs

This work presents the effects of hot electron stress on the degradation of undoped Al_0.3GaN_0.7/GaN power HFET’s with SiN passivation. Typical degradation characteristics consist of a decrease in the drain current and maximum transconductance, an increase in the drain series resistance, gate leakage and a subthreshold current. Degradation mechanism has been investigated by means of gate lag measurements (pulsed I-V) and current-mode deep level transient spectroscopy (DLTS). Stressed devices suffered from aggravated drain current slump (DC to RF dispersion) which indicates possible changes in surface charge profiles occurred during hot electron stress test. The DLTS was used to identify the trap creation by hot electron stress. The DLTS spectra of stressed device revealed the evidence of trap creation due to hot electron stress.     

Identification of an RF degradation mechanism in GaN based HEMTs triggered by midgap traps

Quantitative defect spectroscopy was performed on low gate leakage operational S-band GaN HEMTs before and after RF accelerated life testing (ALT) to investigate and quantify potential connections between the evolution of observed traps and RF output power loss in these HEMTs after stressing. Constant drain current deep level transient spectroscopy and deep level optical spectroscopy (CI_D-DLTS and CI_D-DLOS, respectively) were used to interrogate thermally-emitting traps (CI_D-DLTS) and deeper optically-stimulated traps (CI_D-DLOS) so that the entire bandgap can be probed systematically before and after ALT. Using drain-controlled CI_D-DLTS/DLOS, with which traps in the drain access region are resolved, it is found that an increase in the concentration of a broad range of deep states between E_C–1.6 to 3.0 eV, detected by CI_D-DLOS, causes a persistent increase in on-resistance of ~ 0.22 Ω-mm, which is a likely source for the 1.2 dB reduction in RF output power that was observed after stressing. In contrast, the combined effect of the upper bandgap states at E_C–0.57 and E_C–0.72 eV, observed by CI_D-DLTS, is responsible for only ~ 10% of the on-resistance increase. These results demonstrate the importance of discriminating between traps throughout the entire bandgap with regard to the relative roles of individual traps on degradation of GaN HEMTs after ALT.     

Degradation of AlGaN/GaN HEMTs under elevated temperature lifetesting

Elevated temperature lifetesting was performed on 0.25 μm AlGaN/GaN HEMTs grown by MOCVD on 2-in. SiC substrates. A temperature step stress (starting at T_a of 150 °C with a step of 15 °C; ending at T_a of 240 °C; 48 h for each temperature cycle) was employed for the quick reliability evaluation of AlGaN/GaN HEMTs. It was found that the degradation of AlGaN/GaN HEMTs was initiated at ambient temperature of 195 °C. The degradation characteristics consist of a decrease of drain current and transconductance, and an increase of channel-on-resistance. However, there is no noticeable degradation of the gate diode (ideality factor, barrier height, and reverse gate leakage current). The FIB/STEM technique was used to examine the degraded devices. There is no detectable ohmic metal or gate metal interdiffusion into the epitaxial materials. Accordingly, the degradation mechanism of AlGaN/GaN HEMTs under elevated temperature lifetesting differs from that observed in GaAs and/or InP HEMTs. The reliability performance was also compared between two vendors of AlGaN/GaN epilayers. The results indicate that the reliability performance of AlGaN/GaN HEMTs could strongly depend on the material quality of AlGaN/GaN epitaxial layers on SiC substrates.     

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.     

Enhanced gate induced drain leakage current in HfO2 MOSFETs

The substrate current of high-κ dielectric MOSFETs has been studied using dc sweep and transient (down to 100 μs per I-V curve) electrical measurements. These measurements reveal trap-assisted substrate current components in addition to the traditional bell-shaped impact ionization current. By separating the transversal and lateral electric field contributions, the gate induced drain leakage (GIDL) is shown to dominate the substrate current at low gate biases. At high gate biases, tunneling of valence band electrons from the bulk to the gate dominates. The results show that the GIDL current is the result of band-to-band tunneling assisted by traps located at the HfO₂/SiO₂ interface and transition layer, and not the result of oxide charging.     

Comparative DC Characteristic Analysis of AlGaN/GaN HEMTs Grown on Si(111) and Sapphire Substrates by MBE

A comparative assessment of AlGaN/GaN high-electron-mobility transistors (HEMTs) grown by molecular beam epitaxy on silicon and sapphire substrates has been carried out. Large-area power GaN HEMTs with identical device dimensions were fabricated on both substrates. A thicker AlN buffer layer was used for the GaN HEMT on silicon to achieve similar quality and uniformity of GaN epitaxy for rational comparison with that grown on sapphire. Direct-current analysis and physical characterization were carried out to understand the performance of the devices. Mathematical measurement of the instability of the current–voltage (I–V) characteristic at high applied drain bias was carried out to evaluate the performance of both devices. An improved two-dimensional (2D) analysis of the I–V characteristic was performed from a thermal perspective including appropriate scattering effects on the 2D electron gas mobility. The experimental and analytical studies were correlated to reveal the effects of temperature-sensitive scattering phenomena on the mobility as well as on the I–V characteristic at high drain bias in terms of lattice thermal heating. It is observed that the HEMT on Si has improved stability compared with sapphire due to its weaker scattering phenomena at high drain bias, associated with its thermal conductivity. Simulation of 2D thermal mapping was also carried out to distinguish the hot-spot regions of the devices. The comparable electrical performance of these devices illustrates the viability of AlGaN/GaN HEMTs on Si(111) to achieve low-cost stable devices with better thermal power handling for high-voltage applications.