Pulsed Bias Conditions Research Articles

2D study of AlGaN/AlN/GaN/AlGaN HEMTs’ response to traps

In this work, the effects of GaN channel traps and temperature on the performance of AlGaN/AlN/GaN/AlGaN high electron mobility transistors (HEMTs) on Si (111) substrate, were investigated. 2D simulations carried out using the Silvaco TCAD simulator tool for different drain and gate voltages showed that acceptor-like traps in the channel have a significant influence on the DC and RF characteristics. It was found that deeper acceptors below the conduction band with larger concentration have a more pronounced effect on the transistor performance. Meanwhile, the donor-like traps show no influence. Pulsing the device with different pulse widths and bias conditions, as well as increasing temperature, showed that the traps are more ionized when the pulse is wider or the temperature is higher, which can degrade the drain current and thus the DC characteristics of the transistor. Passivation of the transistor has also a beneficial effect on performance.

A review of pulsed NBTI in P-channel power VDMOSFETs

Negative bias temperature instability (NBTI) is a phenomenon commonly observed in p-channel metal-oxide-semiconductor (MOS) devices simultaneously exposed to negative gate voltage and elevated temperature. In this study we provide overview of threshold voltage instabilities observed in commercial p-channel power vertical double-diffused MOS field-effect transistors (VDMOSFET) IRF9520 under the pulsed NBT stress conditions. These instabilities are caused by the NBT stress induced changes in oxide trapped charge and interface trap densities, and are more significant at higher voltages and/or temperatures. NBT stress induced degradation under the pulsed gate bias conditions is generally lower as compared to static stress. Less significant degradation of threshold voltage found after pulsed bias stressing is ascribed to the recovery effects. Partial recovery occurs during the low level of pulsed gate voltage as a consequence of the removal of recoverable component of degradation and is associated with passivation/neutralization of shallow oxide traps that have not been transformed into the deeper traps (permanent component). A specific approach has been applied in this study in order to assess the recoverable and permanent components of degradation in commercial p-channel power VDMOSFETs subjected to NBT stressing. Experimental data have been analysed in terms of the mechanisms responsible for changes in the densities of gate oxide charge and interface traps.

Characterization of AlGaN/GaN HEMTs Using Gate Resistance Thermometry

In this paper, gate resistance thermometry (GRT) was used to determine the channel temperature of AlGaN/GaN high electron-mobility transistors. Raman thermometry has been used to verify GRT by comparing the channel temperatures measured by both techniques under various bias conditions. To further validate this technique, a thermal finite-element model has been developed to model the heat dissipation throughout the devices. Comparisons show that the GRT method averages the temperature over the gate width, yielding a slightly lower peak temperature than Raman thermography. Overall, this method provides a fast and simple technique to determine the average temperature under both steady-state and pulsed bias conditions.

Effects of pulsed negative bias temperature stressing in p-channel power VDMOSFETs

Our recent research of the effects of pulsed bias NBT stressing in p-channel power VDMOSFETs is reviewed in this paper. The reduced degradation normally observed under the pulsed stress bias conditions is discussed in terms of the dynamic recovery effects, which are further assessed by varying the duty cycle ratio and frequency of the pulsed stress voltage. The results are analyzed in terms of the effects on device lifetime as well. A tendency of stress induced degradation to decrease with lowering the duty cycle and/or increasing the frequency of the pulsed stress voltage, which leads to the increase in device lifetime, is explained in terms of enhanced dynamic recovery effects.

Near-Infrared Detection Using Pulsed Tunneling Junction in Silicon Devices

We exploit the potential of the pulsed tunneling bias condition in silicon junctions (the Zener junction and the drain surface junction in nMOSFETs) to detect near-infrared (NIR) photon signals. A combination of the Franz–Keldysh effect and avalanche multiplication with minimization of the thermal effect through a pulsed tunneling bias is found to provide infrared radiation sensitivities as high as 1.18 and 0.89 A/W at the wavelengths of 1310 and 1550 nm, respectively, in a Zener diode. The potential of the drain surface junction in MOSFETs to function as an NIR sensor is also investigated using a similar bias scheme.

Negative bias temperature instability in p-channel power VDMOSFETs: recoverable versus permanent degradation

In this study, which is aimed at assessing a possible relationship between the recoverable and permanent components of negative bias temperature instability (NBTI) degradation, we investigate NBTI in commercial IRF9520 p-channel VDMOSFETs (vertical double diffused MOSFETs) stressed under particular pulsed bias conditions by varying the pulse on-time while keeping the off-time constant and vice versa. The stress-induced threshold voltage shifts are found to be practically independent of duty cycle when the pulse on-time is kept short or the off-time is kept long, and are found to start increasing with duty cycle only when the on-time is increased or the off-time is decreased beyond specific values. These results, which are discussed in terms of dynamic recovery effects and the mechanisms leading to NBTI degradation, point to the existence of an important correlation between the recoverable and permanent components of degradation.

Synthesis and tribological properties of WSex films prepared by magnetron sputtering

WSex films with variable Se/W ratio were deposited by non-reactive r.f. magnetron sputtering from WSe2 target changing the applied d.c. pulsed bias conditions and substrate temperature. The structural and chemical properties were measured by cross-sectional scanning electron microscopy (X-SEM), energy dispersive analysis (EDX), X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS). The tribological properties were measured in ambient air (RH = 30–40%) and dry nitrogen by means of a reciprocating ball-on-disk tribometer. A clear correlation was found between the Se/W ratio and the measured friction coefficient displaying values below 0.1 (in ambient air) and 0.03 (in dry N2) for ratios Se/W ≥ 0.6 as determined by electron probe microanalysis (EPMA). The results demonstrated that notable tribological results could be obtained even in ambient air (friction ≤ 0.07 and wear rate ≈10−7 mm3 Nm−1) by controlling the film microstructure and chemical composition. By incorporating carbon, wear and chemical resistance can be gained by formation of non-stoichiometric carbides and/or alloying into the defective WSex hexagonal structure. The existence of a WSe2 rich interfacial layer (either on the ball scar or embedded in the film track) was evidenced by Raman in low friction conditions. The improvement in tribological performance is therefore obtained by means of layered WSex, the formation of gradient composition from metallic W (hard) to WSe2 (lubricant) and carbon incorporation.

Dependence of Transient Current and Capacitance of Organic Light-Emitting Diodes on Pulsed Bias Conditions

We have employed a simple circuit combining pulse generator and capacitance meter to investigate transient current (TC) and capacitance of organic light-emitting diodes (OLEDs). The dependence of TC on OLEDs structure with the change in thickness of hole transport layer (HTL), and pulse regimes with the change in voltage and frequency have been studied. Results revealed that two different evolution pattern of TC at low and high electric fields, and the change in HTL affected TC much greatly at high electric fields. The capacitance-voltage curve with different frequency was observed and showed trapped- charge state and interfacial charge transport process near the electrode.

Development of diamond-based X-ray detection for high-flux beamline diagnostics

High-quality single-crystal and polycrystalline chemical-vapor-deposition diamond detectors with platinum contacts have been tested at the white-beam X28C beamline at the National Synchrotron Light Source under high-flux conditions. The voltage dependence of these devices has been measured under both DC and pulsed-bias conditions, establishing the presence or absence of photoconductive gain in each device. Linear response consistent with the theoretically determined ionization energy has been achieved over eleven orders of magnitude when combined with previous low-flux studies. Temporal measurements with single-crystal diamond detectors have resolved the nanosecond-scale pulse structures of both the NSLS and the APS. Prototype single-crystal quadrant detectors have provided the ability to simultaneously resolve the X-ray beam position and obtain a quantitative measurement of theflux.

Development of Diamond-Based X-ray Detection for High Flux Beamline Diagnostics

AbstractHigh quality single crystal and polycrystalline CVD diamond detectors with platinum contacts have been tested at the white beam X28C beamline at the National Synchrotron Light Source under high-flux conditions. The voltage dependence of these devices has been measured under DC and pulsed-bias conditions, establishing the presence or absence of photoconductive gain in each device. Linear response has been achieved over eleven orders of magnitude when combined with previous low flux studies. Temporal measurements with single crystal diamond detectors have resolved the ns scale pulse structure of the NSLS.

Technology aspects of GaN-based diodes for high-field operation

Abstract This work reports important aspects of technology development and characterization for GaN based diodes operating at high electric fields. The considered operation conditions result, in comparison to III–V semiconductor devices, from the higher values of threshold field for intervalley transfer of electrons. This lies above 150 kV/cm and requires correspondingly higher biasing voltages and currents through semiconducting layers of transferred electron devices, switches or NDR (negative differential resistance) diodes. Mesa-based vertical and lateral devices using GaN layers on sapphire substrate were considered for current–voltage characteristics under very high electric field conditions. A systematic investigation of MOCVD-grown diode structures with regular, tapered mesa designs and variable dimensions was carried out under pulsed-bias condition. The current–voltage characteristics showed threshold voltages for saturation corresponding to electric fields well above the critical value of 150 kV/cm in the active layer. Self-heating and electromigration effects have been addressed in relation with biasing and metallization conditions.

Amorphous carbon films PACVD in CH 4–CO 2 under pulsed and continuous substrate bias conditions

This work is a comparative study of the different effects on the growth of a-C:H films obtained with the application of a pulsed bipolar (±100 V) rather than of a continuous, only negative (−100 V), substrate bias. The depositions were performed from a CH 4–CO 2 gas mixture in a RF-PACVD system, in which an external bias was applied to the substrates to produce an energetic ion bombardment of the growing film. Pulsing the bias turned out to be much more effective in the densification and hardening of the material than the application of a continuous negative voltage and in assuring, at the same time, better adhesion to the substrate.

Ultraviolet AlGaN multiple-quantum-well laser diodes

We demonstrate ultraviolet emission from current-injection AlGaN multiple-quantum-well laser diodes grown on sapphire substrates by metalorganic chemical vapor deposition. Lasing was obtained in gain-guided laser diode test structures with uncoated facets and cavity length ranging from 400 to 1500 μm. Under pulsed bias conditions, threshold current densities as low as 23 kA/cm2 have been achieved for laser diodes with emission wavelengths between 359.7 and 361.6 nm. The maximum output power was 45 mW per facet with differential quantum efficiencies of 1.3%.

Continuous-wave operation of ultraviolet InGaN/InAlGaN multiple-quantum-well laser diodes

We demonstrate ultraviolet InGaN/InAlGaN multiple-quantum-well laser diodes operating under continuous-wave (cw) conditions. The laser diodes were grown on sapphire substrates by metalorganic chemical vapor deposition. Under pulsed bias conditions, we have achieved threshold current densities as low as 5 kA/cm2 for laser diodes with emission wavelengths between 368 nm and 378 nm and have demonstrated lasing at 363.2 nm at room temperature, the shortest wavelength yet reported for a semiconductor laser diode. The cw operation up to a heat sink temperature of 40 °C was demonstrated on a series of narrow ridge-waveguide devices processed with chemically assisted ion beam etched mirrors and high reflective coating on both facets. The shortest wavelength emission under cw operation conditions was 373.5 nm with output powers of more than 1 mW.

Microwave power limits of AlGaN/GaN HEMTs under pulsed-bias conditions

Dynamic loadline analysis illustrating the microwave performance limits of state-of-the-art AlGaN/GaN high electron-mobility transistors (HEMTs) under pulsed RF biasing and drive conditions are presented. Calculation of dynamic loadlines concurrent with load-pull measurements show the increase of the device RF knee voltage with increasing drain voltage as the cause of reduced output power-added efficiency (PAE) at high drain biases. In this study, an 8-GHz saturated output power of 14.1 W (9.4 W/mm) is achieved on a 1500 /spl times/ 0.25 /spl times/ /spl mu/m/sup 2/ AlGaN/GaN HEMT at a pulsed drain bias of V/sub D/=40 V. The pulsed-bias conditions considered here preclude device self-heating as a mechanism responsible for the lower than expected output power and decrease in PAE with increasing drain bias. These data suggest electron trapping associated with the surface of the device between the gate and drain as the mechanism that limits the ultimate power, PAE, and linearity of AlGaN/GaN HEMTs.

Multilayered carbon films for tribological applications

Recent work has shown that for nano-layered structures consisting of two materials, an effect may be shown in which the elastic modulus and fracture toughness depend on the period of the structure. In this paper we create carbon multilayer structures by two methods using cathodic arc deposition with pulsed bias applied to the substrate. The multilayers are created in one class of structures simply by interrupting the deposition to form relaxed layers. In the other class of structures, the pulse bias conditions are varied periodically. The in-service performance of the structures is assessed by impact-enhanced pin-on-disc wear testing and indentation hardness. The structures are examined using bright field and dark field transmission electron microscopy. A 10 nm non-convergent electron probe was used for the micro-diffraction studies. A model for the structure of the interfaces between the relaxed layers and the as-deposited material, including evidence of preferred orientation is presented.

Time-dependent breakdown of ultra-thin SiO2 gate dielectrics under pulsed biased stress

Ultra-thin SiO/sub 2/ films (t/sub ox//spl sim/2.0 nm) were stressed under DC, unipolar, and bipolar pulsed bias conditions up to a pulse repetition frequency of 50 kHz. The time-to-breakdown (t/sub BD/), the number of defects at breakdown (N/sub BD/), and the number of defects generated inside the oxide as a function of stress time were monitored during each stress condition. Oxide lifetime under unipolar pulsed bias is similar to that under DC conditions; however lifetime under bipolar pulsed bias is significantly improved and exhibits a dependence on pulse repetition frequency. The observation of a lifetime increase under bipolar pulsed bias for the oxide thickness and voltage range used in this study suggests that a different physical mechanism may be responsible for the lifetime increase from that assumed in earlier studies for thicker films.

Study of self-heating effects, temperature-dependent modeling, and pulsed load-pull measurements on GaN HEMTs

On-wafer RF and IV characterizations are performed for the first time on power GaN high electron-mobility transistors (HEMTs) under pulse and continuous conditions at different temperatures. These measurements give an in-depth understanding of self-heating effects and allow one to investigate the possibility of improving heat-dissipation mechanisms. A pulsed load-pull system that measures the power gain of the device-under-test (DUT) under pulsed RF and bias condition has been developed. To the best of our knowledge, this is the first time that the reflected power at the DUT is measured under the pulse mode of operation. Additionally, an improved small-signal model for power GaN HEMTs that incorporates the geometry of the device is developed at various temperatures. This is the basis for empirical large-signal modeling.

Long Time-Constant Trap Effects in Nitride Heterostructure Field Effect Transistors

ABSTRACTCurrent collapse effects in an Al0.25Ga0.75N/GaN HFET have been investigated under pulsed bias conditions, and a detailed investigation of current responses to changes in drain or gate bias voltage (drain-lag and gate-lag, respectively) has been performed. Three components of transient current response to changes in drain and gate bias voltages are distinguished. Surface treatment using KOH etching and the influence of pulsed bias conditions on threshold voltage are investigated to explore the origins of traps associated with each current transient component.

Self-organized In0.4Ga0.6As quantum-dot lasers grown on Si substrates

We report growth of self-organized In0.4Ga0.6As quantum dots on Si substrates by molecular-beam epitaxy. Low-temperature (17 K) photoluminescence spectra show that the optical properties of In0.4Ga0.6As quantum dots grown on Si are comparable to quantum dots grown on GaAs substrates. We also present preliminary characteristics of In0.4Ga0.6As quantum-dot lasers grown on Si substrates. Light versus current measurements at 80 K under pulsed bias conditions show that Ith=3.85 kA/cm2. The lasing spectral output has a peak emission wavelength of 1.013 μm and a linewidth (full width at half maximum) of ∼4 Å at the threshold.