Wide Band Gap Semiconductors

Wide bandgap semiconductors are extremely attractive for the gamut of power electronics applications from power conditioning to microwave transmitters for communications and radar. Of the various materials and device technologies, the AlGaN/GaN high-electron mobility transistor seems the most promising. This paper attempts to present the status of the technology and the market with a view of highlighting both the progress and the remaining problems.

UV response of ZnO nanowire nanosensor has been studied under ambient condition. By utilizing Schottky contact instead of Ohmic contact in device fabrication, the UV sensitivity of the nanosensor has been improved by four orders of magnitude, and the reset time has been drastically reduced from ϳ417 to ϳ0.8 s. By further surface functionalization with function polymers, the reset time has been reduced to ϳ20 ms even without correcting the electronic response of the measurement system. These results demonstrate an effective approach for building high response and fast reset UV detectors.

We report a novel approach in fabricating high-performance enhancement mode (E-mode) AlGaN/GaN HEMTs. The fabrication technique is based on fluoride-based plasma treatment of the gate region in AlGaN/GaN HEMTs and post-gate rapid thermal annealing with an annealing temperature lower than 500 C. Starting with a conventional depletion-mode HEMT sample, we found that fluoride-based plasma treatment can effectively shift the threshold voltage from 4 0 to 0.9 V. Most importantly, a zero transconductance ( ) was obtained at gs = 0 V, demonstrating for the first time true E-mode operation in an AlGaN/GaN HEMT. At gs = 0 V, the off-state drain leakage current is 28 A mm at a drain-source bias of 6 V. The fabricated E-mode AlGaN/GaN HEMTs with 1 m-long gate exhibit a maximum drain current density of 310 mA/mm, a peak of 148 mS/mm, a current gain cutoff frequency of 10.1 GHz and a maximum oscillation frequency max of 34.3 GHz.

This paper presents a method with an accurate control of threshold voltages (V th ) of AlGaN/GaN high-electron mobility transistors (HEMTs) using a fluoride-based plasma treatment. Using this method, the V th of AlGaN/GaN HEMTs can be continuously shifted from −4 V in a conventional depletion-mode (D-mode) AlGaN/GaN HEMT to 0.9 V in an enhancement-mode AlGaN/GaN HEMT. It was found that the plasma-induced damages result in a mobility degradation of two-dimensional electron gas. The damages can be repaired and the mobility can be recovered by a post-gate annealing step at 400 • C. At the same time, the shift in V th shows a good thermal stability and is not affected by the post-gate annealing.

The effects of aluminum, indium and tin dopants on the microstructure and electrical properties of ZnO thin films prepared on silica glass substrates by the sol–gel method were investigated. As a starting material, zinc acetate dihydrate was used. 2-methoxyethanol and monoethanolamine were used as the solvent and stabilizer, respectively. The dopant sources were aluminum chloride, indium chloride and tin chloride. For each dopant, films doped with 1 at.% aluminum, 1 at.% indium and 2 at.% tin concentrations exhibited a stronger c-axis orientation perpendicular to the substrate and had larger grain, a high smooth surface morphology as well as high conductivity and transmittance than the others. In addition, the electrical resistivity value of ZnO thin films reduced by applying the second heat-treatment in nitrogen with 5% hydrogen. When the aluminum doping concentration was 1 at.%, the film had a columnar structure, a resistivity value of 1.1×10−2 Ω cm and a transmittance higher than 90% in the visible spectra region.

Silicon-based power semiconductor devices, ranging from diodes, thyristors, gate turn-off thyristors, metal-oxide-semiconductor field-effect transistors, and, more recently, insulated-gate bipolar transistors, integrated gate-commutated thyristors, and metal-oxide-semiconductor turn-off thyristors, are the workhorse of power electronic systems and circuits. Silicon offers multiple advantages to power circuit designers, but at the same time suffers from limitations that are inherent to silicon material properties, such as low bandgap energy, low thermal conductivity, and switching frequency limitations. Wide bandgap semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN), provide larger bandgaps, higher breakdown electric field, and higher thermal conductivity. Power semiconductor devices made with SiC and GaN are capable of higher blocking voltages, higher switching frequencies, and higher junction temperatures than silicon devices. SiC is by far the most advanced material and, hence, is the subject of attention from power electronics and systems designers. This paper looks at the benefits of using SiC in power electronics applications, reviews the current state of the art, and shows how SiC can be a strong and viable candidate for future power electronics and systems applications.

SiC electronic device technology has made rapid progress during the past decade. In this paper, we review the evolution of SiC power MOSFETs between 1992 and the present, discuss the current status of device development, identify the critical fabrication issues, and assess the prospects for continued progress and eventual commercialization.

We investigate the breakdown (Vbr) enhancement potential of the field plate (FP) technique in the context of AlGaN/GaN power HEMTs. A comprehensive account of the critical geometrical and material variables controlling the field distribution under the FP is provided. A systematic procedure is given for designing a FP device, using two-dimensional (2-D) simulation, to obtain the maximum Vbr , with minimum degradation in on-resistance and frequency response. It is found that significantly higher Vbr can be achieved by raising the dielectric constant (εi) of the insulator beneath the FP. Simulation gave the following estimates. The FP can improve the Vbr by a factor of 2.8-5.1, depending on the 2-DEG concentration (ns) and εi. For n s=1×1013/cm2, the Vbr can be raised from 123 V to 630 V, using a 2.2 μm FP on a 0.8 μm silicon nitride, and 4.7 μm gate-drain separation. The methodology of this paper can be extended to the design of FP structures in other lateral FETs, such as MESFETs and LD-MOSFETs

Silicon carbide (SiC) offers significant advantages for power-switching devices because the critical field for avalanche breakdown is about ten times higher than in silicon. SiC power devices have made remarkable progress in the past five years, demonstrating currents in excess of 100 A and blocking voltages in excess of 19 000 V. In this paper, we describe the latest progress in three classes of SiC devices: diodes (p-in and Schottky), transistors (junction field-effect transistor, metal-oxide-semiconductor field-effect transistor, bipolar junction transistor), and thyristors (gate turn-off).

This paper evaluates the capability of SiC power semiconductor devices, in particular JFET and Schottky barrier diodes (SBD) for application in high-temperature power electronics. SiC JFETs and SBDs were packaged in high temperature packages to measure the dc characteristics of these SiC devices at ambient temperatures ranging from 25 C (room temperature) up to 450 C. The results show that both devices can operate at 450 C, which is impossible for conventional Si devices, at the expense of significant derating. The current capability of the SiC SBD does not change with temperature, but as expected the JFET current decreases with rising temperatures. A 100V, 25W dc-dc converter is used as an example of a high-temperature power-electronics circuit because of circuit simplicity. The converter is designed and built in accordance with the static characteristics of the SiC devices measured under extremely high ambient temperatures, and then tested up to an ambient temperature of 400 C. The conduction loss of the SiC JFET increases slightly with increasing temperatures, as predicted from its dc characteristics, but its switching characteristics hardly change. Thus, SiC devices are well suited for operation in harsh temperature environments like aerospace and automotive applications.

uring recent years, silicon carbide (SiC) power electronics has gone from being a promising future technology to being a potent alternative to state-of-the-art silicon (Si) technology in high-efficiency, highfrequency, and high-temperature applications. The reasons for this are that SiC power electronics may have higher voltage ratings, lower voltage drops, higher maximum temperatures, and higher thermal conductivities. It is now a fact that several manufacturers are capable of developing and processing high-quality transistors at cost that permit introduction of new products in application areas where the benefits of the SiC technology can provide significant system advantages. The additional cost for the SiC transistors in comparison

GaN films have been grown on a variety of substrates by electron cyclotron resonance microwave plasma-assisted molecular beam epitaxy (ECR-MBE). The films were grown in two steps. First, a GaN-buffer was grown at low temperature and then the rest of the film was grown at higher temperatures. We found that this method of growth leads to a relatively small two-dimensional nucleation rate (-20 nuclei/pm' h) and high lateral growth rate (100 times faster than the vertical growth rate). This type of quasi-layer-by-layer growth results in a smooth surface morphology to within 100 A. Growth on Si(l0 0) leads to single-crystalline GaN films having the zinc-blende structure. Growth on Si( 1 1 1) leads to GaN films having the wurtzitic structure with a large concentration of stacking faults. The crystallographic orientation and the surface morphology of GaN films on sapphire depends on the orientation of sapphire. To this date, the best films were grown on the basal plane of sapphire.

We have observed significant instability in the threshold voltage of 4H-SiC metal-oxide-semiconductor field-effect transistors due to gate-bias stressing. This effect has a strong measurement time dependence. For example, a 20-mus-long gate ramp used to measure the I-V characteristic and extract a threshold voltage was found to result in a instability three to four times greater than that measured with a

Using a GaN nanorod template in a hydride vapor phase epitaxy (HVPE) system can manufacture a freestanding GaN (FS-GaN) substrate with threading dislocation densities down to ~ 107 cm-2. In this letter, we report InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) grown on this FS-GaN substrate. The defect densities in the homoepitaxially grown LEDs were substantially reduced, leading to improved light emission efficiency. Compared with the LED grown on sapphire, we obtained a lower forward voltage, smaller diode ideality factor, and higher light-output power in the same structure grown on FS-GaN. The external quantum efficiency (EQE) of LEDs grown on FS-GaN were improved especially at high injection current, which brought the efficiency droop phenomenon greatly reduced at high current density.

Interference effects on indium tin oxide enhanced Raman scattering J. Appl. Phys. 111, 033110 (2012) Optical properties of a-plane (Al, Ga)N/GaN multiple quantum wells grown on strain engineered Zn1−xMgxO layers by molecular beam epitaxy Appl. Phys. Lett. 99, 261910 (2011) Spectrally and temporarily resolved luminescence study of short-range order in nanostructured amorphous ZrO2 J. Appl. Phys. 110, 103521 Photovoltaic effect of CdS/Si nanoheterojunction array J. Appl. Phys. 110, 094316 Electrical control of photoluminescence wavelength from semiconductor quantum dots in a ferroelectric polymer matrix Appl. Phys. Lett. 99, 153112 (2011) Additional information on J. Appl. Phys.

Abstract—Strain-compensated InGaN–AlGaN quantum wells (QW) are investigated as improved active regions for lasers and light emitting diodes. The strain-compensated QW structure consists of thin tensile-strained AlGaN barriers surrounding the InGaN QW. The band structure ...

We report on the calculation of electrical characteristics of AlGaN/GaN heterojunction field effect transistors (HFETs). The model is based on the self-consistent solution of the Schrodinger and Poisson equations coupled to a quasi-2D model for the current flow. Both single and double heterojunction devices are analyzed for [0001] or [000-1] growth directions. The onset of a parasitic p-channel for particular growth directions and alloy concentrations is also shown

An extremely low operating electric field has been achieved on zinc oxide (ZnO) nanowire field emitters grown on carbon cloth. Thermal vaporization and condensation was used to grow the nanowires from a mixture source of ZnO and graphite powders in a tube furnace. An emission current density of 1 mA/ cm 2 was obtained at an operating electric field of 0.7 V / m. Such low field results from an extremely high field enhancement factor of 4.11ϫ 10 4 due to a combined effect of the high intrinsic aspect ratio of ZnO nanowires and the woven geometry of carbon cloth.

ZnS:Mn was produced in nanocrystalline form by a chemical method using polyvinylpyroledone as a chemical capping agent. Mn was stoichiometrically substituted for Zn in ZnS. The manganese ͑Mn͒ concentration was varied over its whole solid solution limit in ZnS, i.e., from 0 to 40%. In the high concentration regime this material formed may be thus written as nanocrystalline ͑Zn, Mn͒S. The material formed is thus a wide gap diluted magnetic semiconductor. The characterized material was in powder form. X-ray diffraction was used to estimate the crystallite size and to confirm formation of the material in single phase. The average crystallite size obtained was about 2 nm. The material remained cubic over the whole Mn solid solution range. The room temperature photoluminescence ͑PL͒ when deconvoluted using a Gaussian fit showed two extra peaks in nanocrystalline ZnS:Mn when compared to pure nanocrystalline ZnS, which had only two peaks. Mn incorporation significantly enhanced the PL intensity in nanocrystalline ZnS:Mn ͑400-850 nm range͒ thereby suggesting Mn 2ϩ induced PL. The red shift of the two new peaks with increase in Mn 2ϩ concentration can be attributed to the change in band structure due to the formation of ZnS:Mn alloy. These extra peaks were due to ͑a͒ various Mn 2ϩ transitions in the ZnS host, ͑b͒ related to S as the nearest neighbor of Mn 2ϩ ion in the nanocrystallite ͑due to the high concentration of Mn 2ϩ), or ͑c͒ Mn-Mn interactions at high Mn concentrations. However, our prepared pure MnS samples did not show any photoluminescence at room temperature. So it is concluded that the observed PL is Mn 2ϩ induced in the nanocrystalline ZnS host.

We present temperature-dependent pulsed X-ray data on the decay time spectra, wavelengths, and intensities of fast (ns) radiative recombination in five direct, wide-bandgap semiconductors: CuI, HgI 2 , PbI 2 , and n-doped ZnO:Ga and CdS:In. At 12 K the luminosity of powder samples is 0.30, 1.6, 0.40, 2.0, and 0.15, respectively, relative to that of BGO powder at room temperature. Increasing the temperature of CuI to 346 K decreases the luminosity by a factor of 300 while decreasing the fwhm of the decay time spectra from 0.20 to 0.11 ns. Increasing the temperature of HgI 2 to 102 K decreases the luminosity by a factor of 53 while decreasing the fwhm from 1.6 to 0.5 ns. Increasing the temperature of PbI 2 to 165 K decreases the luminosity by a factor of 27 while decreasing the fwhm from 0.52 to 0.15 ns. Increasing the temperature of ZnO:Ga to 365 K decreases the luminosity by a factor of 33 while decreasing the fwhm from 0.41 to 0.21 ns. Increasing the temperature of CdS:In to 295 K decreases the luminosity by a factor of 30 while decreasing the fwhm from 0.20 to 0.17 ns. All emission wavelengths are near the band edge. The luminosities decrease much faster than the radiative lifetimes, therefore, the reduction in luminosity is not primarily due to thermal quenching of the excited states, but mostly due to thermally activated trapping of charge carriers on nonradiative recombination centers. Since the radiative and nonradiative processes occur on different centers, increasing the ratio of radiative to nonradiative centers could result in a class of inorganic scintillators whose decay time and radiative efficiency would approach fundamental limits (i.e. o1 ns and 100 000 photons/MeV).

We model the carrier recombination mechanisms in GaInN/GaN light-emitting diodes as R = An + Bn 2 + Cn 3 + f͑n͒, where f͑n͒ represents carrier leakage out of the active region. The term f͑n͒ is expanded into a power series and shown to have higher-than-third-order contributions to the recombination. The total third-order nonradiative coefficient ͑which may include an f͑n͒ leakage contribution and an Auger contribution͒ is found to be 8 ϫ 10 −29 cm 6 s −1 . Comparison of the theoretical ABC + f͑n͒ model with experimental data shows that a good fit requires the inclusion of the f͑n͒ term.

We have fabricated Ni Schottky rectifiers on 2 7 10 16 cm 3 n-type 6H-SiC epilayer using an effective edge termination based on an oxide ramp profile around the Schottky contact. Several anneals of the Schottky contacts were experimented. In particular, the diodes annealed at 900 C showed excellent reverse characteristics with a nearly ideal breakdown at about 800 V. Forward characteristics follow the thermionic emission theory with the ideality factor close to one at low biases. An accurate analytical model and complete parameter extraction of the forward characteristics of the Ni/6H-SiC Schottky barrier diodes (SBDs) for low and high-level current densities are presented. The model takes into account the high-level injection effects and the current dependence of the series resistance. Direct extraction of the SBD parameters is carried out. A very good agreement between the simulated forward curves using extracted parameters and measured data up to 500 A/cm 2 is obtained.

The breakdown voltages in unpassivated nonfieldplated AlGaN/GaN HFETs on sapphire substrates were studied. These studies reveal that the breakdown is limited by the surface flashover rather than by the AlGaN/GaN channel. After elimination of the surface flashover in air, the breakdown voltage scaled linearly with the gate-drain spacing reaching 1.6 kV at 20 µm. The corresponding static ON-resistance was as low as 3.4 mΩ · cm 2 . This translates to a power device figure-of-merit V 2 BR /R ON = 7.5 × 10 8 V 2 · Ω −1 cm −2 , which, to date, is among the best reported values for an AlGaN/GaN HFET.

In this paper, a simple and reliable method to estimate the channel temperature of GaN high-electron mobility transistors (HEMTs) is proposed. The technique is based on electrical measurements of performance-related figures of merit (I D max and R ON ) with a synchronized pulsed I-V setup. As our technique involves only electrical measurement, no special design in device geometry is required, and packaged devices can be measured. We apply this technique to different device structures and validate its sensitivity and robustness.

Abstract—Improvement of light extraction efficiency of InGaN LEDs using colloidal-based SiO2 /polystyrene (PS) mi-crolens arrays was demonstrated. The size effect of the SiO2 mi-crospheres and the thickness effect of the PS layer on the light ex-traction efficiency of III-nitride LEDs ...

We demonstrate that remote plasma hydrogenation can increase electron concentrations in ZnO single crystals by more than an order of magnitude. We investigated the effects of this treatment on Hall concentration and mobility as well as on the bound exciton emission peak I 4 for a variety of ZnO single crystals-bulk air annealed, Li doped, and epitaxially grown on sapphire. Hydrogen increases I 4 intensity in conducting samples annealed at 500 and 600°C and partially restores emission in the I 4 range for Li-diffused ZnO. Hydrogenation increases carrier concentration significantly for the semi-insulating Li doped and epitaxial thin film samples. These results indicate a strong link between the incorporation of hydrogen, increased donor-bound exciton PL emission, and increased n-type conductivity.

We demonstrate the surface plasmon (SP) enhanced green light-emitting diodes (LEDs). The Au nanoparticles were embedded in the p-GaN of LEDs. The photoluminescence and electroluminescence measurements showed improved optical properties of LEDs with Au nanoparticles due to an increase in the spontaneous emission rate by resonance coupling between the excitons in multiple quantum wells and localized surface plasmons in Au nanoparticles. The optical output power of SP-enhanced green LEDs with Au nanoparticles was increased by 86% without showing degradation of the electrical characteristics of LEDs compared to LEDs without Au nanoparticles.

The performance of AlGaN/GaN high-electronmobility transistors (HEMTs) on diamond and SiC substrates is examined. We demonstrate GaN-on-diamond transistors with periphery W G = 250 µm, exhibiting f t = 27.4 GHz and yielding a power density of 2.79 W/mm at 10 GHz. Additionally, the temperature rise in similar devices on diamond and SiC substrates is reported. To the best of our knowledge, these represent the highest frequency of operation and first-reported thermal and X -band power measurements of GaN-on-diamond HEMTs.

We report recent measurements of the drift velocity of electrons parallel to the basal plane in 6H and 4H silicon carbide (SiC) as a function of applied electric field. The dependence of the low field mobility and saturated drift velocity on temperature are also reported. The saturated drift velocities at room temperature are approximately 1 9 10 7 cm/s in 6H-SiC and 2 2 10 7 cm/s in 4H-SiC.

In this work, we investigated the dye-sensitized solar cells (DSSCs) using photoanode, which were fabricated directly from electrospinning of TiO2 nanofibres onto the substrate. The electrochemical impedance spectroscopy (EIS) and current–voltage diagram were used to analyse electron transport in electrospun nanofibres and determine their applicability in DSSCs. In order to improve the short-circuit photocurrent of fabricated cells, we treated the electrospun TiO2 electrode with the TiCl4 aqueous solution. The modification of the photoelectrode significantly improves the power conversion efficiency (over 26%) of the solar cells attributed to the higher electron lifetime (τ) and reduction in recombination processes as indicated by the EIS of the solar cells.

The effect of the asymmetry in carrier concentration and mobility is studied in GaInN pn-junction light-emitting diodes (LEDs). We propose and present experimental evidence that the asymmetry in carrier concentration and mobility, and associated high-level injection phenomena, cause efficiency droop in GaInN LEDs. Low temperatures exacerbate the degree of asymmetry of the junction by reducing acceptor ionization, and shift high-injection-phenomena to lower currents. Accordingly, at temperatures near 80 K, we measure a greater droop compared to room temperature. The analysis of temperature-dependent I-V curves shows an excellent correlation between the onset of high-level injection and the onset of droop. V

Previously developed methods used to grow Ge1-ySny alloys on Si are extended to Sn concentrations in the 1019-1020 cm-3 range. These concentrations are shown to be sufficient to engineer large increases in the responsivity of detectors operating at 1550 nm. The dopant levels of Sn are incorporated at temperatures in the 370–390 °C range, yielding atomically smooth layers devoid of threading defects at high growth rates of 15–30 nm/min. These conditions are far more compatible with complementary metal-oxide semiconductor processing than the high growth and processing temperatures required to achieve the same responsivity via tensile strain in pure Ge on Si. A detailed study of a detector based on a Sn-doped Ge layer with 0.25% (1.1 × 1020 cm-3) Sn range demonstrates the responsivity enhancement and shows much better I-V characteristics than previously fabricated detectors based on Ge1-ySny alloys with y = 0.02.

| Light-emitting diodes (LEDs) have become quite a high-performance device of late and are revolutionizing the display and illumination sectors of our economy. Due to demands for better performance and reduced energy consumption there is a constant race towards converting every single electron hole pair in the device to photons and extracting them as well while using only the minimum required voltage.

Efficient deep‐blue organic light‐emitting diodes were demonstrated using 4,4'‐bis(9‐ethyl‐3‐carbazovinylene)‐1,1'‐biphenyl doped in double‐emission layers (D‐EMLs). The D‐EML system, which consists of 2‐methyl‐9,10‐di(2‐naphthyl)anthracene and 1,4‐(dinaphthalen‐2‐yl)‐naphthalene as blue hosts, was employed to broaden the recombination zone and to ensure the good confinement of the holes and electrons. The optimized device showed a peak current efficiency of 4.47 cd/A, a peak external quantum efficiency of 4.09%, and Commission Internationale de L'Eclairage coordinates of (0.16, 0.10).

An ultrasonic rangefinder has a working range of 30 mm to 450 mm and operates at a 375 Hz maximum sampling rate. The random noise increases with distance and equals 1.3 mm at the maximum range. The range measurement principle is based on pulse-echo time-of-flight measurement using a single transducer for transmit and receive. The transducer consists of a piezoelectric AlN membrane with 400 µm diameter, which was fabricated using a low-temperature process compatible with processed CMOS wafers. The performance of the system exceeds the performance of other micromechanical rangefinders.

We review the features of GaN-based FETs and describe their expected development direction. GaN has a high breakdown field, but this does not necessarily mean it is suitable for highvoltage and high-power applications. The main advantage is that it enables scaling down beyond the silicon MOSFET miniaturization limitation from the Maxwell-Boltzmann distribution. Thus, fine gate patterns together with a high carrier velocity make GaNbased FETs be suited for millimeter and near millimeter wavelength high-power applications. In addition, by using large-area sapphire substrates, high-performance and low-cost MMICs can be produced on GaN. We expect that such devices will be the key to future advanced wireless communication systems.

Threshold voltage and drain current instabilities in state-of-the-art 4H-SiC MOSFETs with thermal as-grown SiO2 and NO-annealed gate oxides have been studied using fast I-V measurements. These measurements reveal the full extent of the instability underestimated by dc measurements. Furthermore, fast measurements allow the separation of negative and positive bias stress effects. Postoxidation annealing in NO was found to passivate the

W4H-SiC Schottky photodiodes have been fabricated with the device areas up to 1cm2. The I-Vcharacteristics and photo-response spectra have been measured and analyzed. For a 5mmx5mm area device leakage current of ~X~O -'~A at zero bias and 1 . 2~1 0 "~A at -lV have been established. The quantum efficiency is over 30% from 240nrn. to 320nm. The specific detectivity, D*, has been calculated from the directly measured leakage current and quantum efficiency data and are shown to be higher than 10"~mHz''~/W from 210nm to 350nm with a peak D* of 3 . 6~1 0 '~ cmHz'/2/W at 300nm.

Wide bandgap, high saturation velocity, and high thermal stability are some of the properties of GaN, which make it an excellent material for high-power, high frequency, and high temperature applications. Given the predicted widespread use, reliable models are needed for simulationbased optimization. As several application areas require the devices to operate at elevated temperatures, a proper modeling of the temperature dependences of the band structure and transport parameters is highly important. We present two-dimensional hydrodynamic simulations of AlGaN/GaN high electron mobility transistors (HEMTs) supported by measured data at high temperatures. The temperature dependence of the low-field mobility at low and high carrier concentrations is modeled by using power laws [1]. Fig. 1 shows our model for the electron mobility in GaN as a function of temperature in the two-dimensional electron gas in comparison to experimental values from various groups. The model parameters are calibrated against own Monte Carlo (MC) simulation results and consider high-quality GaN material. A decrease of the maximum mobility with temperature (~T í1.5), in agreement with the power term of the acoustic phonon mobility expression [2] is assumed. Our MC simulation results and recent experiments from [3] confirmed that the latter is the dominant scattering mechanism at high temperatures. A weak temperature dependence (~T í0.2) of the electron mobility at high concentrations is adopted. A two-valley hydrodynamic mobility model describes the high-field electron transport. The model is used to simulate HEMT T-gate structures with the two-dimensional device simulator Minimos-NT [4]. Both the l g =0.25 ȝm and l g =0.5 ȝm devices share the same layer specification and gate width w g =2x50 ȝm (taken as 1x100 ȝm in the simulation). The structures consist of GaN buffer, 22 nm thick Al 0.22 Ga 0.78 N barrier layer, 3 nm thick GaN cap layer, and SiN passivation. The l g =0.25 ȝm device is used for calibration. Self-heating effects are accounted for by using substrate thermal contact resistance. The densities of the polarization charges at the channel/barrier interface and at the barrier/cap interface are determined by calibration against the experimental data to be 9.5×10 12 cm í2 and í2.5×10 12 cm í2 , respectively [5]. Thus, an excellent agreement is achieved both for the transfer (Fig. 2) and the output characteristics at all three ambient temperatures. As an example Fig. 3 shows the output characteristics at 425 K. Using the calibrated model set, the predicted results for the l g =0.5 ȝm device match nicely the measured transfer characteristics at 300 K, 365 K, and 425 K (Fig. 4), and the output characteristics. The RF device performance is studied by small-signal AC analysis. Fig. 5 shows the current gain |h 21 | for the 0.25 ȝm device for the three temperatures. The gain decrease with higher temperature in the simulation agrees well with the measurements, and consequently, the calculated cutoff frequency f t (Fig. 6). The slight overestimation of f t can be contributed either to the hydrodynamic model used, or to parasitics between the gate fingers of the real structure.

In this paper, an accurate table-based large-signal model for AlGaN/GaN HEMTs accounting for trapping-and self-heating-induced current dispersion is presented. The B-spline-approximation technique is used for the model-element construction, which improves the intermodulation-distortion (IMD) simulation. The dynamic behavior of the trapping and self-heating processes is taken into account in the implementation of the model. The model validity is verified by comparing the simulated and measured outputs of the device tested under pulsed and continuous large-signal excitations for devices of 1-mm gate width. Single-and two-tone simulation results show that the model can efficiently predict the output power and its harmonics and the associated IMD under different input-power and bias conditions.

This letter presents the high-temperature performance of AlGaN/GaN HEMT direct-coupled FET logic (DCFL) integrated circuits. At 375 • C, enhancement-mode (E-mode) AlGaN/GaN HEMTs which are used as drivers in DCFL circuits exhibit proper E-mode operation with a threshold voltage (V TH ) of 0.24 V and a peak current density of 56 mA/mm. The monolithically integrated E/D-mode AlGaN/GaN HEMTs DCFL circuits deliver stable operations at 375 • C: An E/D-HEMT inverter with a drive/load ratio of 10 exhibits 0.1 V for logic-low noise margin (NM L ) and 0.3 V for logic-high-noise margin (NM H ) at a supply voltage (V DD ) of 3.0 V; a 17-stage ring oscillator exhibits a maximum oscillation frequency of 66 MHz, corresponding to a minimum propagation delay (τ pd ) of 446 ps/stage at V DD of 3.0 V. Index Terms-AlGaN/GaN, CF 4 plasma treatment, directcoupled FET logic (DCFL), enhancement mode (E-mode), high electron mobility transistor (HEMT), high-temperature electronics, IC, threshold voltage.

We have analyzed the time-temperature dependence of positive bias stress ͑PBS͒ and recovery in amorphous indium-gallium-zinc-oxide ͑a-IGZO͒ Thin-film-transistors ͑TFTs͒ incorporating SiO 2 back channel passivation. The data are fitted to stretched exponentials, yielding the time constant and stretch parameter ␤ as fitting parameters. As-fabricated samples and samples annealed in vacuum at 250°C 200 h are compared. The time constant for room temperature stress increases fivefold with the 200 h anneal to the value = 1.3ϫ 10 6 s. The dependence of from stress temperature is well described by an Arrhenius plot, with activation E = 0.95 eV. Stress and recovery show very similar activation energies, supporting the defect formation in the bulk or at the gate insulator/a-IGZO interface as the mechanism responsible for PBS.