AlGaN/GaN/InGaN/GaN DH-HEMTs with an InGaN notch for enhanced carrier confinement

We report an AlGaN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) with high-mobility two-dimensional electron gas (2-DEG) and reduced buffer leakage. The device features a 3-nm thin In/sub x/Ga/sub 1-x/N(x=0.1) layer inserted into the conventional AlGaN/GaN HEMT structure. Assisted by the InGaN layers polarization field that is opposite to that in the AlGaN layer, an additional potential barrier is introduced between the 2-DEG channel and buffer, leading to enhanced carrier confinement and improved buffer isolation. For a sample grown on sapphire substrate with MOCVD-grown GaN buffer, a 2-DEG mobility of around 1300 cm/sup 2//V/spl middot/s and a sheet resistance of 420 /spl Omega//sq were obtained on this new DH-HEMT structure at room temperature. A peak transconductance of 230 mS/mm, a peak current gain cutoff frequency (f/sub T/) of 14.5 GHz, and a peak power gain cutoff frequency (f/sub max/) of 45.4 GHz were achieved on a 1/spl times/100 /spl mu/m device. The off-state source-drain leakage current is as low as /spl sim/5 /spl mu/ A/mm at V/sub DS/=10 V. For the devices on sapphire substrate, maximum power density of 3.4 W/mm and PAE of 41% were obtained at 2 GHz.     

Influence of AlGaN deep level defects on AlGaN/GaN 2-DEG carrierconfinement

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures and to correlate their effect on two-dimensional electron gas (2-DEG) confinement. We investigated AlGaN/GaN heterostructures with different electrical properties using incident electron beam energies of 0.5 to 15 keV to probe electronic state transitions within each of the heterostructure layers. AlGaN heterostructures of 25 nm thickness and nominal 30% Al concentration grown on GaN buffer layers on sapphire substrates by plasma-assisted molecular beam epitaxy exhibited a range of polarization-induced electron densities and room temperature mobilities. In general, the spectra exhibit AlGaN band edge emission at ~3.8 eV or ~4.0 eV, GaN band edge emission at ~3.4 eV, yellow luminescence (YL) features at 2.18 eV and 2.34 eV, and a large emission in the infrared (<1.6 eV) from the GaN cap layer used to passivate the AlGaN outer surface. These heterostructures also show high strain in the 2 nm-thick GaN layer with evidence for a Franz-Keldysh red shift due to piezoelectric charging. The LEEN depth profiles reveal differences between the structures with and without 2-DEG confinement and highlight the importance of AlGaN defects in the near 2-DEG region     

Influence of annealed ohmic contact metals on electron mobility of strained AlGaN/GaN heterostructures

The influence of annealed ohmic contact metals on the electron mobility of a two dimensional electron gas (2DEG) is investigated on ungated AlGaN/GaN heterostructures and AlGaN/GaN heterostructure field effect transistors (AlGaN/GaN HFETs). Current-voltage (I-V) characteristics for ungated AlGaN/GaN heterostructures and capacitance-voltage (C-V) characteristics for AlGaN/GaN HFETs are obtained, and the electron mobility for the ungated AlGaN/GaN heterostructure is calculated. It is found that the electron mobility of the 2DEG for the ungated AlGaN/GaN heterostructure is decreased by more than 50% compared with the electron mobility of Hall measurements. We propose that defects are introduced into the AlGaN barrier layer and the strain of the AlGaN barrier layer is changed during the annealing process of the source and drain, causing the decrease in the electron mobility.     

Mobility spectrum analysis of anisotropic electron transport in N-polar GaN/AlGaN heterostructures on vicinal sapphire substrates

In this work, we present results of a study of anisotropic two-dimensional electron gas (2DEG) transport in N-polar GaN/AlGaN heterostructures grown on slightly mis-oriented sapphire substrates. High-resolution mobility spectrum analysis of magnetic-field dependent Hall-effect and resistivity indicate an isotropic 2DEG sheet carrier density, yet significant anisotropy was observed in carrier mobility. A single electron species with a narrow mobility distribution was found to be responsible for conduction parallel to the multi-atomic steps resulting from growth on the vicinal substrates; whilst in the perpendicular direction two distinct electrons peaks are evident at T ? 150 K, which merge near room temperature. The linewidth of the mobility distributions for transport in the perpendicular direction was found to be significantly broader than that of the single electron in the parallel direction. The broader mobility distribution and the lower average mobility for the 2DEG in the perpendicular direction are attributable to interface roughness scattering associated with the GaN/AlGaN interfacial steps.     

Characterization of Electrical Properties of AlGaN/GaN Interface Using Coupled Schr?dinger and Poisson’s Equations

The electrical characterization of AlGaN/GaN interface is reported. The dependence of two-dimensional electron gas (2-DEG) density at the interface on the Al mole fraction and thickness of AlGaN layer as well as on the thickness of GaN cap layer is presented. This information can be used to design and fabricate AlGaN/GaN based MODFET (modulation doped field effect transistor) for optimum DC and RF characteristics.     

Low- and high-frequency capacitance of aluminum gallium nitride/gallium nitride heterostructures with interface traps

We have studied frequency dependence of capacitance properties of aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructures with interface traps at the AlGaN/GaN interface. We have shown that ability of charge in interface traps to respond to external measuring signal is responsible for the frequency dispersion of capacitance curves. The difference between capacitance curves measured at low and high frequencies in experimental structures is similar to the difference between simulated capacitances of the heterostructures with interface traps measured at low and high frequencies. At a high frequency when the charge in interface traps does not follow the measuring signal, the capacitance curves are only shifted in voltage compared to the curve of the structure without interface traps. But for low frequency a capacitance peak is observed. Interface traps hence contribute to experimentally observed capacitance dispersion.     

Effect of gate impurity concentration on inversion-layer mobility in MOSFETs with ultrathin gate oxide layer

In this paper, the influence of poly-Si-gate impurity concentration, N/sub poly/, on inversion-layer electron mobility is experimentally investigated in MOSFETs with ultrathin gate oxide layer. The split capacitance-voltage C-V method is modified to directly measure an effective mobility, paying attention to both 1) accurate current-voltage I-V and capacitance-voltage (C-V) measurements with high gate leakage current and 2) correct surface carrier density, N/sub s/, estimation at a finite drain bias. It is demonstrated that the mobility in ultrathin gate oxides becomes low significantly for highly doped gate, strongly suggesting the contribution of remote Coulomb scattering due to the gate impurities, which is quantitatively discriminated from that of Coulomb scattering due to substrate impurities and interface states. It is also found that the mobility lowering becomes significant rapidly at T/sub ox/ of 1.5 nm or less. The mobility-lowering component is weakly dependent on N/sub s/, irrespective of N/sub poly/, which cannot be fully explained by the existing theoretical models of remote impurity scattering.     

AlGaN/GaN异质结构中二维电子气的高温输运性质

采用高温Hall测量仪对一个全应变和一个部分应变弛豫的AlGaN/GaN异质结构中2DEG的高温输运特性进行了研究,温度变化范围从室温到680K.研究结果表明:在高温段2DEG的迁移率主要受LO声子散射限制; 在室温,异质界面处的非均匀压电极化场对2DEG迁移率的散射也是一个主要的散射机制.同时,计算结果显示,随着温度升高,更多的电子跃迁到更高的子带,在更高的子带,其波函数逐渐扩展到AlGaN层内部以及GaN体内更深的位置,导致LO声子散射的屏蔽效应减弱且来自AlGaN层内的合金无序散射增强.     

On the resolution of the mechanism for reverse gate leakage in AlGaN/GaN HEMTs

We provide following important clues for resolving the reverse gate leakage mechanism in AlGaN/GaN high-electron mobility transistors (HEMTs), based on two-dimensional (2-D) simulation and analysis. First, measurement of the gate current-voltage, I/sub G/-V/sub G/, characteristics on devices having different gate structures, passivation layers and interface charges, can reveal the field sensitivity of this mechanism. Second, of the different mechanisms proposed so far, namely - direct tunneling (DT), direct tunneling through a thin surface barrier (DTTSB) and trap-assisted tunneling (TT), DT/DTTSB is sensitive to the 2-D field, while the TT is not. Finally, the DT/DTTSB mechanism appears unlikely, since its 2-D calculations fit the measured I/sub G/-V/sub G/ shape, only if we assume a physically unrealistic voltage-variable charge at the interface and/or the TSB layer.     

Influence of high channel doping on the inversion layer electron mobility in strained silicon n-MOSFETs

In this letter, we investigate the dependence of electron inversion layer mobility on high-channel doping required for sub-50-nm MOSFETs in strained silicon (Si), and we compare it to co-processed unstrained Si. For high vertical effective electric field E/sub eff/, the electron mobility in strained Si displays universal behavior and shows enhancement of 1.5-1.7/spl times/ compared to unstrained Si. For low E/sub eff/, the mobility for strained Si devices decreases toward the unstrained Si data due to Coulomb scattering by channel dopants.     

Thermal effects in AlGaN/GaN/Si high electron mobility transistors

Group III-nitride compounds are of increasing interest for designing high power and high temperature transistors. A considerable progress in the growth and process technology of these devices has been achieved. However, there are still limitations concerning particularly the lack of native substrates. Comparison of the AlGaN/GaN high electron mobility transistors investigated favours the SiC substrate. Recently, encouraging results have been reported for AlGaN/GaN/Si. The crucial problem found in AlGaN/GaN transistors operating at high biases is the self-heating induced by high power dissipation in the active zone. The present work reports on a study of the self-heating in AlGaN/GaN HEMTs grown on Si(1 1 1). The electron-band parameters of the heterostructures have been calculated self-consistently by taking into account the piezoelectric and spontaneous polarizations. As an experiment support, direct-current characteristics of AlGaN/GaN/Si HEMTs have been used to derive the drain voltage-dependent temperature rise in the conductive channel. As has been found, the self-heating is relatively weak. An improvement in the electron transport is achieved by optimizing the epilayers and adjusting the electrode sizes at output of the transistors investigated.     

Analysis of Interface Scattering in AlGaN/GaN/InGaN/GaN Double-Heterojunction High-Electron-Mobility Transistors

This paper presents detailed investigations on the direct-current (DC) characteristics of an AlGaN/GaN/InGaN/GaN double-heterojunction high-electron-mobility transistor (DH-HEMT) using two-dimensional numerical analysis. In this work, the hot-electron effect is taken into account and implemented in the hydrodynamic model. The results indicate that carrier transport in this kind of device exhibits properties significantly different from that in a conventional AlGaN/GaN HEMT. Due to imperfections at the GaN/InGaN interface, scattering caused by the interface roughness, phonons, etc. inhibit the negative differential conductance in high electric field. In addition, the velocity increment of electrons around the gate edge is dominated by the overshoot effect rather than the phonon effect. The energy exchange between phonons and electrons, as presented in this paper, illustrates that the dissipated power is just a small portion of the exchanged energy. For further performance improvement, more lattice-matched material with strong polarization for the barrier layer is proposed.     

Physically based modeling of low field electron mobility in ultrathin single- and double-gate SOI n-MOSFETs

In this paper, we have extensively investigated the silicon thickness dependence of the low field electron mobility in ultrathin silicon-on-insulator (UT-SOI) MOSFETs operated both in single- and in double-gate mode. A physically based model including all the scattering mechanisms that are known to be most relevant in bulk MOSFETs has been extended and applied to SOI structures. A systematic comparison with the measurements shows that the experimental mobility dependence on the silicon thickness (T/sub SI/) cannot be quantitatively explained within the transport picture that seems adequate for bulk transistors. In an attempt to improve the agreement with the experiments, we have critically rediscussed our model for the phonon scattering and developed a model for the scattering induced by the T/sub SI/ fluctuations. Our results suggest that the importance of the surface optical (SO) phonons could be significantly enhanced in UT-SOI MOSFETs with respect to bulk transistors. Furthermore, both the SO phonon and the T/sub SI/ fluctuation scattering are remarkably enhanced with reducing T/sub SI/, so that they could help explain the experimental mobility behavior.     

Elevated-Temperature Annealing Effects on AlGaN/GaN Heterostructures

The effect of high-temperature annealing of undoped AlGaN/GaN heterostructures on different substrates was systematically studied between 1100°C and 1230°C. An AlN spacer layer was found to add stability to structures on sapphire substrates. AlGaN/GaN heterostructures on SiC substrates demonstrated excellent robustness for the temperature range studied, maintaining their mobility, sheet resistance, and sheet concentration values, even after annealing. A silicon nitride, SiN _x , capping layer was found to assist in minimizing surface roughness during annealing and maintaining the electrical characteristics of the heterostructures. AlGaN/GaN heterostructures on SiC substrates showed a 20% decrease in mobility for uncapped samples compared with SiN _x -capped samples.     

A surface-potential-based model for AlGaN/AlN/GaN HEMT

A new surface-potential-based model for AlGaN/AlN/GaN high electron mobility transistor(HEMT) is proposed in this paper. Since the high polarization effects caused by AlN interlayer favorably influence the two dimensional electron gas(2DEG) and scattering mechanisms, we first add spontaneous and piezoelectric charge terms to the source equation of surface-potential, and a mobility model for AlGaN/AlN/GaN HEMT is rewritten. Compared with TCAD simulations, the DC characteristics of AlGaN/AlN/GaN HEMT are faithfully reproduced by the new model.     

垂直电场作用下的钎锌矿相GaN电子输运特性的Monte Carlo研究

在GaN NMOSFET中,沟道电子由于受垂直于其运动方向电场的作用而产生界面散射,从而影响MOSFET特性.研究采用Monte Carlo体模拟方法计算钎锌矿相GaN材料在界面散射下的电子输运特性.模拟中在电子漂移方向加一个水平电场,同时在与其垂直的方向加另外一个电场,在垂直电场作用下,电子发生界面散射.采用基于指数...     

高阈值电压低界面态增强型Al2O3/GaN MIS-HEMT

采用高温热氧化栅极凹槽刻蚀工艺并结合高温氮气氛围退火技术,制备出了高阈值电压的硅基GaN增强型Al_2O_3/GaN金属-绝缘体-半导体高电子迁移率晶体管(MIS-HEMT)。采用高温热氧化栅极凹槽刻蚀工艺刻蚀AlGaN层,并在AlGaN/GaN界面处自动终止刻蚀,可有效控制刻蚀的精度并降低栅槽表面的粗糙度。同时,利用高温氮气退火技术能够修复Al_2O_3/GaN界面的界面陷阱,并降低Al_2O_3栅介质体缺陷,因此能够减少Al_2O_3/GaN界面的界面态密度并提升栅极击穿电压。采用这两项技术制备的硅基GaN增强型Al_2O_3/GaN MIS-HEMT具有较低的栅槽表面平均粗糙度(0.24 nm)、较高的阈值电压(4.9 V)和栅极击穿电压(14.5 V)以及较低的界面态密度(8.49×10¹¹ cm^-2)。     

Influence of the dynamic access resistance in the g/sub m/ and f/sub T/ linearity of AlGaN/GaN HEMTs

The decrease of transconductance g/sub m/ and current gain cutoff frequency f/sub T/ at high drain current levels in AlGaN/GaN high-electron mobility transistors (HEMTs) severely limits the linearity and power performance of these devices at high frequencies. In this paper, the increase of the differential source access resistance r/sub s/, with drain current is shown to play an important role in the fall of g/sub m/ and f/sub T/. The increase of r/sub s/ occurs due to the quasi-saturation of the electron velocity in the source region of the channel at electric fields higher than 10 kV/cm. This has been confirmed by both experimental measurements and two-dimensional drift-diffusion simulations. Through simulations, we have identified HEMT structures with source implanted regions (or n/sup ++/ cap layers) as good candidates in order to increase the linearity of the g/sub m/ and f/sub T/ versus current profile.     

MBE-grown nanoheterostructures with increased electron mobility

Multilayered nitride heterostructures AlN/AlGaN/GaN/AlGaN are grown using a Nanofab-100 multichamber domestic complex involving a STE3N3 specialized installation of molecular beam epitaxy. The obtained heterostructures have high structural quality, smooth morphology, and a low dislocation density, which demonstrates the high potential of the nanotechnological complex.     

InAlN/GaN HEMTs: a first insight into technological optimization

High-electron mobility transistors (HEMTs) were fabricated from heterostructures consisting of undoped In/sub 0.2/Al/sub 0.8/N barrier and GaN channel layers grown by metal-organic vapor phase epitaxy on (0001) sapphire substrates. The polarization-induced two-dimensional electron gas (2DEG) density and mobility at the In/sub 0.2/Al/sub 0.8/N/GaN heterojunction were 2/spl times/10/sup 13/ cm/sup -2/ and 260 cm/sup 2/V/sup -1/s/sup -1/, respectively. A tradeoff was determined for the annealing temperature of Ti/Al/Ni/Au ohmic contacts in order to achieve a low contact resistance (/spl rho//sub C/=2.4/spl times/10/sup -5/ /spl Omega//spl middot/cm/sup 2/) without degradation of the channels sheet resistance. Schottky barrier heights were 0.63 and 0.84 eV for Ni- and Pt-based contacts, respectively. The obtained dc parameters of 1-/spl mu/m gate-length HEMT were 0.64 A/mm drain current at V/sub GS/=3 V and 122 mS/mm transconductance, respectively. An HEMT analytical model was used to identify the effects of various material and device parameters on the InAlN/GaN HEMT performance. It is concluded that the increase in the channel mobility is urgently needed in order to benefit from the high 2DEG density.