Electron Saturation Velocity Research Articles

Thermodynamic Studies of β-Ga2O3 Nanomembrane Field-Effect Transistors on a Sapphire Substrate.

The self-heating effect is a severe issue for high-power semiconductor devices, which degrades the electron mobility and saturation velocity, and also affects the device reliability. On applying an ultrafast and high-resolution thermoreflectance imaging technique, the direct self-heating effect and surface temperature increase phenomenon are observed on novel top-gate β-Ga2O3 on insulator field-effect transistors. Here, we demonstrate that by utilizing a higher thermal conductivity sapphire substrate rather than a SiO2/Si substrate, the temperature rise above room temperature of β-Ga2O3 on the insulator field-effect transistor can be reduced by a factor of 3 and thereby the self-heating effect is significantly reduced. Both thermoreflectance characterization and simulation verify that the thermal resistance on the sapphire substrate is less than 1/3 of that on the SiO2/Si substrate. Therefore, maximum drain current density of 535 mA/mm is achieved on the sapphire substrate, which is 70% higher than that on the SiO2/Si substrate due to reduced self-heating. Integration of β-Ga2O3 channel on a higher thermal conductivity substrate opens a new route to address the low thermal conductivity issue of β-Ga2O3 for power electronics applications.

High-Velocity Saturation in Graphene Encapsulated by Hexagonal Boron Nitride.

We measure drift velocity in monolayer graphene encapsulated by hexagonal boron nitride (hBN), probing its dependence on carrier density and temperature. Due to the high mobility (>5 × 104 cm2/V/s) of our samples, the drift velocity begins to saturate at low electric fields (∼0.1 V/μm) at room temperature. Comparing results to a canonical drift velocity model, we extract room-temperature electron saturation velocities ranging from 6 × 107 cm/s at a low carrier density of 8 × 1011 cm-2 to 2.7 × 107 cm/s at a higher density of 4.4 × 1012 cm-2. Such drift velocities are much higher than those in silicon (∼107 cm/s) and in graphene on SiO2, likely due to reduced carrier scattering with surface optical phonons whose energy in hBN (>100 meV) is higher than that in other substrates.

Review of GaN-based devices for terahertz operation

GaN provides the highest electron saturation velocity, breakdown voltage, operation temperature, and thus the highest combined frequency-power performance among commonly used semiconductors. The industrial need for compact, economical, high-resolution, and high-power terahertz (THz) imaging and spectroscopy systems are promoting the utilization of GaN for implementing the next generation of THz systems. As it is reviewed, the mentioned characteristics of GaN together with its capabilities of providing high two-dimensional election densities and large longitudinal optical phonon of ∼90 meV make it one of the most promising semiconductor materials for the future of the THz emitters, detectors, mixers, and frequency multiplicators. GaN-based devices have shown capabilities of operation in the upper THz frequency band of 5 to 12 THz with relatively high photon densities in room temperature. As a result, THz imaging and spectroscopy systems with high resolution and deep depth of penetration can be realized through utilizing GaN-based devices. A comprehensive review of the history and the state of the art of GaN-based electronic devices, including plasma heterostructure field-effect transistors, negative differential resistances, hetero-dimensional Schottky diodes, impact avalanche transit times, quantum-cascade lasers, high electron mobility transistors, Gunn diodes, and tera field-effect transistors together with their impact on the future of THz imaging and spectroscopy systems is provided.

RHEED intensities from two-dimensional heteroepitaxial nanoscale systems of GaN on a 3C-SiC(111) surface

This paper presents a version of simulation program, which facilitates the calculation of changes to the intensity of RHEED oscillations in the function of the glancing angle of incidence of the electron beam, employing various models of scattering crystal potential for heteroepitaxial structure of hexagonal (0001) GaN film nucleated on a 3C-SiC surface, including the possible existence of various diffuse scattering models through the layer parallel to the surface. New version program summaryTitle of program: RHEED_DIFF_ZProgram Files doi:http://dx.doi.org/10.17632/52nxmwzkxx.1Licensing provisions: GNU General Public License 3Programming language used: C++Journal reference of previous version: Computer Physics Communications 207 (2016) 536–538Does the new version supersede the previous version?: No. It is a supplement to the previous version.Reasons for new version:Responding to users’ feedback we present a practical procedure of construction of simulation program, which facilitates the calculation of changes to the intensity of RHEED rocking curves, employing various models of scattering crystal potential for heteroepitaxial structure of hexagonal (0001) GaN film nucleated on a 3C-SiC(111) surface.Nature of problem:Growth of GaN layers on SiC is of importance for potential electronic device applications.Silicon carbide crystallizes in numerous different modifications, so-called polytypes [1]. Among them, only hexagonal structure 4H-SiC and 6H-SiC have been thoroughly investigated while the cubic 3C-SiC polytype is lagging behind in technological developments. The 3C-SiC has a Zincblende crystal structure and is characterized by an identical orientation of all bilayers in the crystal, where the atomic geometry is repeated every three bilayers along the c-axis this crystal structure (Fig. 1). For this reason, 3C-SiC has the high electron mobility and saturation velocity by reducing phonon scattering resulting from the high symmetry. The lattice mismatch between GaN(0001) and 3C-SiC(111) is about 3.3%, and is smaller than the mismatch between GaN(0001) and Si(111) [2]. Therefore progress in investigations on crystal growth of GaN on 3C-SiC is a key issue for device developments related to this heterostructure. Researchers and technologists manufacturing two-dimensional heterostructures frequently use RHEED rocking curves to control growth of samples at the atomistic level of accuracy. The fundamental scientific problem in such research is to specify both interface type and growth mechanism for subsequent layers.Method of solution:RHEED intensities are calculated within the general framework described in Refs. [2] and [3].Summary of revisions:The presented version of the program implements an original algorithm of self-consistent calculations for scattering potentials GaN(0001)/3C-SiC(111) and solving a time-independent Schrödinger equation for high-energy electrons.During the numerical calculations of the changes in the intensity of the specular beam in the function of the glancing angle, it was assumed that the azimuth of the incident beam direction corresponds to the one-beam condition, the electron energy equals 10 keV, the glancing angle was increased from 0.5∘ to 6.5∘, and the value of αparameter=0.1 and β=0.5 for the MODEL3 of the scattering potential [3] with the model of ideal SiC/GaN interface [4].

GaN Lateral High Voltage Photoconductive Semiconductor Switches

Wide bandgap GaN photoconductive semiconductor switches (PCSSs) have gained recent attention due to high critical electric field strength and high electron saturation velocity to provide high power ultrafast devices. To reduce leakage currents, PCSSs have previously been demonstrated on semi-insulating GaN achieved by Fe compensation doping. However, Fe is known to have strong memory effects, which can redistribute Fe into subsequent films, as well as a narrow window of acceptable doping levels. PCSSs with fast response time are required for applications requiring high repetition rates. C-doped GaN is used to reduce leakage current and improve the photocurrent of lateral PCSSs. The C-doping is performed by simply tuning the growth parameters, such as temperature, pressure and III/V ratio and creates shallow accepter levels, compensating any unintentional n-type doping. In this work, we demonstrate high voltage GaN:C PCSSs with fast response times measured using a picosecond pulsed laser and high breakdown voltages, over 4000 V, measured in a vacuum probe station.

Investigation of electron mobility and saturation velocity limits in gallium nitride using uniaxial dielectric continuum model

Here we introduce a uniaxial dielectric continuum model with temperature-dependent phonon mode frequencies to study temperature- and orientation-dependent polar-optical-phonon limited electron mobility and saturation velocity in uniaxial semiconductors. The formalism for calculating electron scattering rates, momentum relaxation rates, and rate of energy change as a function of the electron kinetic energy and incident electron angle with respect to the c-axis are presented and evaluated numerically. Electron–longitudinal-optical-phonon interactions are shown to depend weakly on the electron incident angle, whereas the electron–transverse-optical-phonon interactions around the emission threshold energy are observed to depend strongest on the electron incident angle when varied from π/4 to π/2 (with respect to the c-axis). We provide electron mobility and saturation velocity limits in different GaN crystal orientations as a function of temperature and electron concentration. At room temperature and for an electron density of 5 × 1018 cm−3, electron mobility limit of ∼3200 cm2/V s and electron saturation velocity limit of 3.15 × 107 cm/s are calculated. Both GaN electron mobility and saturation velocity are observed to be governed by the longitudinal-optical-phonon interaction, and their directional anisotropy is shown to vary less than 5% as the electron incident angle with respect to the c-axis is varied from 0 to π/2. Overall, we develop a theoretical formalism for calculating anisotropic properties of uniaxial wurtzite semiconductors.

Hot electron induced non-saturation current behavior at high electric field in InAlN/GaN heterostructures with ultrathin barrier

The high-field transport characteristics of nearly lattice-matched InAlN/GaN heterostructures with different barrier thickness were investigated. It is found that the current in the InAlN/GaN heterostructures with ultrathin barrier shows unsaturated behaviors (or secondary rising) at high voltage, which is different from that of AlGaN/GaN heterostructures. This phenomenon is more obvious if the barrier thickness is thinner and the channel width is narrower. The experimental results demonstrate that it is the increasing carrier density excited from the more defect states by the hot electrons with larger electron saturation velocity that results in the unsaturated current behaviors in InAlN/GaN heterostructures. Our results pave a way for further optimizing InAlN barrier design and improving the reliability of InAlN/GaN HEMTs.

Electron velocity saturation and intervalley transfer in monolayer MoS2

Monolayer MoS2 is a material with a rich history and that is being suggested for many applications in electronics, including novel electron devices. Recent experiments has shown that it has a saturation velocity at high electric fields well below other electronic materials such as Si. This is a very important property that is crucial to high performance electron devices. Here, we study this property with ensemble Monte Carlo simulations of the electron transport. We find that the velocity at high electric fields is larger than the experiments, and does not show a saturation up to 100 kV cm−1. In addition, the transport at high fields is dominated by inter-valley transfer to the T valleys.

Deep Trap Levels Responsible for Current Collapse in AlGaN/GaN MISHFET

GaN-based heterostructure field-effect transistors (HFETs) have been attracted considerably as candidates for next-generation devices for both high-power and high frequency applications, because of its excellent material properties such as high breakdown field, high polarization-induced two dimensional electron gas (2DEG) and high saturated electron velocity [1-3]. Despite the advantages, current collapse has been concerned for reliability of AlGaN/GaN HFETs. In this work, we investigated origins of the current collapse in AlGaN/GaN MISHFETs after negative gate bias by observing the spectral photo-responsive drain current and gate-drain spacing dependent effect. Schematic structure of the fabricated MISHFETs is shown in Fig. 1(a), in which the epitaxial layers were grown on the sapphire (0001) substrate by MOCVD. After mesa isolation by TCP-RIE, a 5 nm-thick Al2O3 layer was deposited using PAALD. Si/Ti/Al/Ni/Au (1/25/10/40/100 nm) were deposited for ohmic metal stacks by e-beam evaporator and rapid thermal annealing (RTA) was performed at 850 ˚C for 30 s in N2flow. Subsequently, Ni/Au (40/50 nm) was deposited as the schottky gate contacts. The defined gate length and width of the fabricated devices were 3 μm and 50 μm, respectively. Fig 1(b) shows typical output characteristics of the fabricated MISHFET. Fig. 2 shows the drain currents measured before and after the bias stress when the monochrome lights of different energy were illuminated. Under the illumination condition, it showed only slight collapses of drain current for 500 ~ 600 nm light after the negative bias stress, while showed serious collapse for the other wavelengths. It means that the photons with energy of 2.25 eV was effective to excite the electrons trapped by the deep level which was attributable to the current collapse due to the negative gate bias. This trap energy might be related to the Ga vacancy in GaN bulk layer as widely known as a culprit in previous researches in AlGaN/GaN HFET [4]. Fig. 3 shows a schematic band diagram for the current collapse caused by a negative bias stress. When a high negative bias stress is applied to the gate, the energy band near AlGaN/GaN junction will be bended and the electrons be released from crystal interface which have been trapped in acceptor like states such as Ga vacancy levels as shown in Fig. 3 (a). Fig. 4 shows the drain current suppression at a bias condition of Vgs=1 V and Vds=7V after negative bias stress of -20 V, in which the current decreases as the gate-drain spacing increases. For wide gate-drain spacing, even the electric field between gate and drain decreases, more states seemed to be exposed and contributed to the current collapse at the AlGaN or GaN layer. In conclusion, the current collapse in AlGaN/GaN MISHFET was significant for the wide gate-drain spacing. The photons with energy of 2.25 eV was effective to excite the electrons trapped by the deep levels which were attributed to the current collapse induced by the negative gate bias. This trap energy might be related to the Ga vacancy in GaN bulk layer. Reference [1] P. Waltereit, W. Bronner, R. Quay, M. Dammann, R. Kiefer, S. Müller, M. Musser, J. Kühn, F. van Raay, M. Seelmann, M. Mikulla, O. Ambacher, F. van Rijs, T. Rödle, and K. Riepe, “GaN HEMT and MMIC development at Fraunhofer IAF: performance and reliability,” phys. Status Solidi A, vol. 206, pp. 1215-1220, 2009. [2] D. S. Kim, K. S. Im, K. W. Kim, H. S. Kang, D. K. Kim, S. J. Chang, Y. Bae, S. H. Hahm, S. Cristoloveanu, and J. H. Lee, “Normally-off GaN MOSFETs on insulating substrate,” Solid-State Electron., vol. 90, pp. 79-85, 2013. [3] S. T. Sheppard, K. Doverspike, W. L. Probble, S. T. Allen, J. W. Palmour, L. T. Kehias, and T. J. Jenkins, “High-power microwave GaN/AlGaN HEMTs on semi-insulating silicon carbide substrates,” IEEE Electron Device Lett., vol. 20, pp. 161-163, 1999. [4] J. Neugebauer and C. G. Van de Walle, “Gallium vacancies and the yellow luminescence in GaN,” Appl. Phys. Lett., vol. 69, pp. 503-505, 1996. Figure 1

Kinetic Investigation on the Photoetching Reaction of n-Type GaAs by Scanning Electrochemical Microscopy

The physicochemical principle of photocorrosion and photoetching is the internal photoelectric effect of semiconductors. However, the kinetic investigation of interfacial charge transfer induced by this phenomenon has been seldom reported due to its microdomain bipolarity. GaAs is a direct band gap semiconductor with high saturated electron velocity and high electron mobility. Once the photogenerated electrons on a n-type GaAs surface are removed by Fe3+ in the solution, it will dissolve due to the residual positive holes; i.e., the photoetching process will occur. By employing scanning electrochemical microscopy (SECM), the photoetching rates of n-type GaAs are obtained about ∼10–4 mol·m–2·s–1 with Fe3+ cation as electron acceptor. The rate-determining step (rds) is proved as the charge separation process at low illuminating intensity, and the mass transfer of Fe3+ in the solution at high illuminating intensity. Moreover, the photoetching process is developed as a controllable micromachining method for s...

(Electronics and Photonics Division Award) High Power Nitride Based Field Effect Transistors

Nitride semiconductors have a great potential for high power applications because of a high breakdown field, a very large density of polarization induced two-dimensional electrons, high electron saturation and overshoot velocities, a relatively high mobility, excellent thermal properties, chemical inertness, and radiation hardness. GaN power transistors grown on silicon substrates are already being commercialized. However, while their performance characteristics are competitive or even surpass the best silicon based power transistors, the full potential of the nitride materials for power applications is still to be achieved and will require dramatic changes in the device design that account for the materials properties of wide band gap materials. Such new features include the use of diamond substrates for a better heat removal, adding a low conducting passivation in the high field region between the gate and drain, changing the gate geometry to the perforated channel with perforations extending beyond the gate, using AlInN for lattice matching design to improve stability and reach a high temperature operation and employing lateral-vertical designs and cascade solutions. This presentation will review of the state-of-the –art power GaN based transistors and project the potential improvements in their performance

The Integration of Functional Oxide Thin Film with GaAs By Laser Molecular Beam Epitaxy

Integration of crystalline oxide thin films with semiconductors has attracted considerable attention in recent years. Compared to Si, GaAs have a higher saturated electron velocity and electron mobility, transistors based on GaAs can function at a much higher frequencies. In this talk, SrTiO3 films were grown on GaAs (100) substrates as an intermediate buffer layer for the epitaxial growth of oxide thin films including ferroelectric BaTiO3 and semiconductor ZnO by laser molecular beam epitaxy (LMBE). The properties of the multilayers in terms of growth modes, strain and interface structures were characterized by the insitu reflective high energy electron diffraction (RHEED) pattern. The crystalline quality and surface morphology of the oxide/GaAs heterostructure were investigated respectively by a combination of Xray diffraction (XRD) and atomic force microscopy (AFM). These heterostructure exhibit good functional properties such as ferrelectrisity and resistive switching. Furthermore, the fundamental interface properties of these heterostructure with an emphasis on the strain effect were discussed in relation with the electrical properties. The results provide an understanding of the integration of oxide/GaAs heterostructure with full control over the interface structure at the atomic scale.

Analytical Study of Electron Mobility in Hemts Algan/Gan

The hetero junctions GaN based offer an excellent potential for power applications at high frequency. This is due to the important energy of the bandgap and high saturation velocity of electrons. The high mobility transistors (HEMT - High Electron Mobility Transistor) are based on the heterojunction AlGaN/GaN. Our work is the subject of an analytical study of the carrier mobility HEMTs AlGaN/GaN calculating Ionized impurities scattering, Residual impurities scattering, Interface roughness scattering, Alloy disorder scattering, dislocations scattering, Phonons and Dipoles taking into account the impact of technological parameters (doping, aluminium content) and geometric (thickness barrier, interface roughness). The results allowed us to take account of the variation of carrier density in the wells of 2D electronic gas.

Investigation of plasma density gratings in current carrying plasmas by particle in cell method

Plasma density gratings induced in current carrying plasmas are investigated through the particle in cell method. Simulation results show that the electron phase space holes can be formed at the saturation time which confirms the counter-streaming in these plasmas. It is found that the generation of grating-like pattern strongly depends on the saturation time and velocity of electrons and positrons. Moreover, in the presence of delta distribution function the grating-like pattern is obvious, compared to the Maxwellian distribution. Finally, using the fluid model, it is demonstrated that increasing the velocity of electrons and positrons causes to increase the maximum amount of growth rate of instability which is in good agreement with those obtained using the simulation model.

Impact of Contact on the Operation and Performance of Back-Gated Monolayer MoS2 Field-Effect-Transistors

Metal contacts to atomically thin two-dimensional (2D) crystal based FETs play a decisive role in determining their operation and performance. However, the effects of contacts on the switching behavior, field-effect mobility, and current saturation of monolayer MoS2 FETs have not been well explored and, hence, is the focus of this work. The dependence of contact resistance on the drain current is revealed by four-terminal-measurements. Without high-κ dielectric boosting, an electron mobility of 44 cm(2)/(V·s) has been achieved in a monolayer MoS2 FET on SiO2 substrate at room temperature. Velocity saturation is identified as the main mechanism responsible for the current saturation in back-gated monolayer MoS2 FETs at relatively higher carrier densities. Furthermore, for the first time, electron saturation velocity of monolayer MoS2 is extracted at high-field condition.

(Invited) Ion Implantation into GaN and Implanted GaN Power Transistors

GaN is a promising material for high-power, high-frequency and high-efficiency electron devices owing to their high saturation electron velocity and large break-down electric field (Eb ). The heavily doped layer with a low sheet resistance is strongly desired to reduce the power loss of GaN device and to increase the transconductance and maximum saturation current through the decrease of series resistances in devices. To reduce the series resistances in a device, a heavily doped epitaxial layer is often grown on the active layer, which would be removed by a dry etching technique to fabricate the recess structure [1]. On the other hand ion implantation is also one of very common technology to make lightly and highly selective doping area in Si [2] and SiC [3,4] devices and it is crucial for making high performance devices. We have previously demonstrated Si ion implantation for GaN HEMTs [5] and MESFETs [6] to reduce the contact resistance and fabricate a lightly-doped channel region. In addition, we have recently demonstrated normally-off GaN MISFETs with Si ion implantation into p-type GaN [7] and N ion implantation for the device isolation [8]. Si implanted MISFETs have achieved the high threshold voltage (Vth ) of +3.4 V [9]. Moreover, we have succeeded to fabricate selective p-GaN regions with Mg ion implantation and this result made it possible to control the Vth of GaN MISFETs from -5 V to +1.5 V. These results indicate a definite availability for power switching device applications. As known well, it is very difficult to form a p-type conductive layer with ion implantation on GaN. The GaN-based devices are usually fabricated on either a sapphire substrate or a Si substrate with a large diameter and at low cost. However these substrates cause many defects in GaN due to large lattice mismatch between epitaxial layers and the substrate. Such defects of high threading dislocation density over 109 /cm2 cause substantial current leakage and such layers would be deteriorated by the high temperature annealing for activation of the implanted Mg and create donor-like defects which compensates the Mg acceptors. P-type conversion by Mg ion implantation was attempted using high quality n--GaN epitaxial layer grown on a free-standing GaN substrate. Low temperature PL spectra showed Mg-acceptor related emissions. Vertical diodes showed clear rectifying I-V characteristics. UV and blue-green light emission was observed at forward biased condition. Positive Hall coefficients were observed by van der Pauw measurements. These results indicate the p-type conversion was possible using high quality GaN layers grown on free-standing GaN substrates, which can lead new device structures and processing for future GaN power devices. [1] B.R. Park, et al., Semisonductor Science and technology, Vol. 28, No. 12, (2013) [2] E. Suekawa, et al., Proceeding of ISPSD & ICs, 11.12, 249 – 252 (1998) [3] S.H. Ryu, et al., IEEE Electron Device Letters, Vol. 22, No. 3, 124 – 126, March (2001) [4] K.W. Chu, et al., IEEE Electron Device Letters, Vol. 34, No. 2, 286 – 288, February (2013) [5] K.Nomoto, T. Nakamura, et al., IEEE Electron Device Letters, Vol. 28, No. 11, 939 – 941, November (2007) [6] K.Nomoto, T. Nakamura, et al., Proceeding of MRS Fall Meeting, Vol. 892 (2005) [7] S. Gu, K. Nomoto, T. Nakamura, et al., physica status solidi (c), Vol. 10, Issue. 5, 820 – 823, May (2013) [8] H. Kasai, T. Nakamura, et al., physica status solidi (c), Vol. 11, Issue. 3-4, 914 – 917, April (2014) [9] S. Taguchi, K. Nomoto, T. Nakamura, et al., physica status solidi (c), Vol. 9, Issue. 3-4, 858 – 860, March (2012)

Ultimate response time of high electron mobility transistors

We present theoretical studies of the response time of the two-dimensional gated electron gas to femtosecond pulses. Our hydrodynamic simulations show that the device response to a short pulse or a step-function signal is either smooth or oscillating time-decay at low and high mobility, μ, values, respectively. At small gate voltage swings, U0 = Ug − Uth, where Ug is the gate voltage and Uth is the threshold voltage, such that μU0/L < vs, where L is the channel length and vs is the effective electron saturation velocity, the decay time in the low mobility samples is on the order of L2/(μU0), in agreement with the analytical drift model. However, the decay is preceded by a delay time on the order of L/s, where s is the plasma wave velocity. This delay is the ballistic transport signature in collision-dominated devices, which becomes important during very short time periods. In the high mobility devices, the period of the decaying oscillations is on the order of the plasma wave velocity transit time. Our analysis shows that short channel field effect transistors operating in the plasmonic regime can meet the requirements for applications as terahertz detectors, mixers, delay lines, and phase shifters in ultra high-speed wireless communication circuits.

(Invited) The Stability of High Voltage AlGaN/GaN HEMTs

Si-based semiconductor devices have long dominated the power device market and are approaching their performance limit because of the inherent material limitations. On the other hand, wide bandgap semiconductors such as GaN and SiC promise significant performance boost coming from the superior material properties, such as a high critical electric field (3x106 V/cm), a high peak electron saturation velocity (2.5x107 cm/s), and a wide bandgap (3.4 eV) in GaN case [1]. In the III-nitride material system, a popular structure for power switching devices is the AlGaN/GaN HEMTs, because of the excellent two-dimensional electric gas (2DEG) in the hetrojunction interface, which greatly reduces the on-resistance of the device. Combined with the enhanced breakdown voltage from a large critical electric field, the figure of merit BV2/Ron,sp of reported AlGaN HEMTs have far surpassed the Si limit. The enhancement mode (E-mode) operation is strongly preferred for low power consumption, driver circuit simplicity, and fail-safe requirements for power electronic circuits. For AlGaN/GaN HEMTs realized utilizing epitaxial layers on the gallium face, the piezoelectric polarization, together with the spontaneous polarization, greatly enhances the 2DEG concentration, which is beneficial for reducing the on-resistance. But this makes it difficult to achieve the E-mode operation since the 2DEG under the gate region must vanish at zero gate bias. Several approaches have been proposed, including the recess gate structure, fluoride plasma treatment, the nano-ribbon channel, the p-GaN gate, and others. Although AlGaN/GaN HEMTs have demonstrated excellent performance with E-mode operation, their stability and reliability remains to be inspected before they can be commercialized. With different approaches to realize E-mode, the gate related stability problems arise differently depending on the gate design. In this work, we systematically investigate the stability of different device structures under different gate bias conditions, including Schottky gate HEMTs, p-GaN gate HEMTs, recessed MIS gate HEMTs, and multi-layer fluorinated MIS gate HEMTs. Figure 1 shows the pulsed Id-Vg characteristics of a recessed MIS gate AlGaN/GaN HEMT on a silicon substrate. The gate insulator is a 30 nm ALD deposited Al2O3 layer. The gate length is 3 μm and the gate-to-drain spacing is 20 μm for >1kV breakdown voltage. The pulse width is 5 ms and the period is 100 ms for all cases. The base voltage was varied from -15 V to 15 V and the drain current was measured during the pulse duration as shown in the inset. The threshold voltage Vth, which is defined when Id is 1mA/mm, increases with the base voltage, implying that negative charges are trapped in the gate insulator during the base duration. Figure 2 summarizes the extracted Vth for different gate structures using pulsed Id-Vg measurement with different gate base voltages. The Schottky gate HEMT shows a very stable Vth and Id-Vd characteristics against different pulse conditions but the negative Vth makes them less favorable. The p-GaN gate HEMT exhibits a positive but unstable Vth. The Vth shift can be as large as 1 V. Further annealing at 550oC in an O2ambient improves the p-type contact and hence the stability of the Vth but still limits the maximum gate voltage to 3 V. The decrease in Vth with increased gate base voltages implies trapping of holes in the p-GaN gate, which differs from the MIS gate device [2]. The MIS gate HEMT shows a positive threshold voltage about 6 V and a relatively large gate voltage swing. But the variation in the threshold voltage even at low gate base voltages indicates the occurrences of trapped charges in the gate insulator, which requires further examination and process improvement. More detailed discussion and measurement results will be shown. Acknowledgement This project was funded in part by financial support from Ministry of Economic Affairs 103A0002J9. Reference [1] T. P. Chow, R. Tyagi, IEEE Trans. Electron Devices 41(1994) 1481. [2] T.-F. Chang et al. to appear in IEEE Trans. Electron Devices. Figure 1

GaN HEMT器件结构的研究进展

Ga N high electron mobility transistor( HEMT) has been widely acknowledged for use in high-frequency,high-power,and high-temperature applications because of their features such as its wide band gap,high electron saturation velocity,high 2-DEG density at the hetero-interface,high breakdown voltage( BV),and high thermal conductivity. The issues that limit the gallium nitride high electron mobility transistor device performance improvement and some solutions are introduced firstly. And then,the latest research progress on the high-frequency,high-power area of gallium nitride high electron mobility transistor is reviewed in detail with focus on the material structural design and the device structural design. Finally,the direction for the development of the device is discussed briefly.

High‐frequency performance of AlGaN channel HEMTs with high breakdown voltage

The high-frequency characteristics of high electron mobility transistors applying AlGaN for a channel layer (AlGaN channel HEMTs) with greater impacts of alloy disorder scattering than those in conventional HEMTs with a GaN channel layer (GaN channel HEMTs) are investigated. The obtained electron saturation velocity was 4.7 × 106 cm/s and the cutoff frequency was 7 GHz in the AlGaN channel HEMTs with a gate length of 1 μm. These results are promising for several gigahertz band operations of AlGaN channel HEMTs.