Increase In Cutoff Frequency Research Articles

Operational Characteristics of AlGaN/GaN High-Electron-Mobility Transistors with Various Dielectric Passivation Structures for High-Power and High-Frequency Operations: A Simulation Study.

This study investigates the operational characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) by employing various passivation materials with different dielectric constants and passivation structures. To ensure the simulation reliability, the parameters were calibrated based on the measured data from the fabricated basic Si3N4 passivation structure of the HEMT. The Si3N4 passivation material was replaced with high-k materials, such as Al2O3 and HfO2, to improve the breakdown voltage. The Al2O3 and HfO2 passivation structures achieved breakdown voltage improvements of 6.62% and 17.45%, respectively, compared to the basic Si3N4 passivation structure. However, the increased parasitic capacitances reduced the cut-off frequency. To mitigate this reduction, the operational characteristics of hybrid and partial passivation structures were analyzed. Compared with the HfO2 passivation structure, the HfO2 partial passivation structure exhibited a 7.6% reduction in breakdown voltage but a substantial 82.76% increase in cut-off frequency. In addition, the HfO2 partial passivation structure exhibited the highest Johnson's figure of merit. Consequently, considering the trade-off relationship between breakdown voltage and frequency characteristics, the HfO2 partial passivation structure emerged as a promising candidate for high-power and high-frequency AlGaN/GaN HEMT applications.

High harmonic generation in graphene quantum dots

We study theoretically the generation of high harmonics in disk graphene quantum dots placed in linearly polarized short pulse. The quantum dots (QD) are described within an effective model of the Dirac type and the length gauge was used to describe the interaction of quantum dots with an optical pulse. The generated radiation spectra of graphene quantum dots can be controlled by varying the quantum dot size, i.e. its radius. With increasing the quantum dot radius, the intensities of low harmonics mainly decrease, while the cutoff frequency increases. The sensitivity of the cutoff frequency to the QD size increases with the intensity of the pulse.

High order harmonic spectra of CO under external electrostatic field

In this work, we use Lewenstein’s theory to calculate the high order harmonic spectra of CO molecule in a linearly polarized laser field combined with an external electrostatic field. The results show that the characteristics of the high order harmonic spectra of CO molecule depend strongly on the direction of the external static electric field relative to the orientation of CO. Especially, when the direction of the external static electric field points from O to C, the plateau of the harmonic spectrum becomes wider and the cutoff frequency reaches a larger value than the scenario without external static electric field. When the direction of the external static electric field points from C to O, the harmonic spectrum shows a double-plateau structure. Using Lewenstein theory, these phenomena can be understood from the viewpoint that the harmonic generation comes from a coherent superposition of the contributions of two kinds of channels characterized by C-end ionization and O-end ionization. The C(O)-end ionization channel means that the electron is ionized from C(O) end, then accelerated by the driving electric field, finally recombines with its parent molecular ion at either C or O end, emitting the harmonics. For the same harmonic order, the contribution of the C-end ionization channel is greater than that of the the O-end ionization channel. The two kinds of channels emit harmonics in adjacent half period of laser, where the external static electric field causes the cutoff frequency of the harmonic spectrum to increase and decrease in the adjacent half period of the laser field. Especially, when the direction of external static electric field is from the C to O, the cutoff frequency of the harmonic spectrum of the C-end ionization channel decreases, resulting in a higher first plateau in the spectrum. While, the increase of cut-off frequency of the O-end ionization channel will result in a lower second plateau. When the direction of external static electric field is reversed, the cutoff frequency of the harmonics of the C-end channel increases, leading the plateau of CO harmonic spectrum to become wider than that without the external static electric field. The cut-off frequency of the O-end ionization channel decreases. Because the contribution of the O-end ionization channel can be ignored compared with that of C-end ionization channel, the C-end channel dominates the contribution to harmonic generation and hence there is only one plateau in the harmonic spectrum. This work provides a clear physical picture for the formation of a double-plateau structure of CO harmonic spectra under the action of an external static electric field.

Microscopic Simulation of the RF Performance of SiGe HBTs With Additional Uniaxial Mechanical Stress

A very detailed investigation of high-speed silicon–germanium (SiGe) heterojunction bipolar transistors (HBTs) showed an underestimation of the measured peak cutoff frequency by simulations with a hydrodynamic (HD) model. It was speculated that this might be due to a breakdown of the HD approximation or unknown additional mechanical stress. We repeated those simulations with the more fundamental Boltzmann transport equation (BTE) based on the same device model (doping profiles, 2-D geometry, parasitics, and so on) and obtained almost similar results showing that this failure was not due to a breakdown of the HD approximation. Since additional uniaxial stress along the direction of the lateral base has been shown to increase the cutoff frequency, we investigated this effect. We found by 2-D device simulations that the increase in the peak cutoff frequency is rather small, and at high stress levels, it even decreases, if the uniaxial stress is applied homogenously. This is due to the reduction of the conductivity of the highly doped collector layer by the stress. If the stress is limited to the intrinsic transistor, the increase in the cutoff frequency is monotonic with growing stress. On the other hand, the collector current for a given base–emitter voltage also increases with stress leading to an overestimation of the collector current compared with the measurements. If this increase is corrected by a slight decrease in the germanium profile, the gain in the peak cutoff frequency is lost. Thus, the underestimation of the peak cutoff frequency cannot be explained by an additional homogeneous uniaxial stress in the intrinsic transistor.

Speech Perception Performance of Native Speakers of Marathi: Effect of Filtered Speech Stimulus and Degree of Hearing Impairment

The study investigated the effect of filtered speech stimulus on speech perception performance of native speakers of Marathi as a function of degree of hearing impairment. Speech identification score (SIS) testing was performed to measure speech perception on three groups (Group I, Group II, and Group III consisted of participants with moderate, moderately-severe, and severe sensorineural hearing impairment respectively). Speech stimuli comprised eight word-lists with each list consisting of 25 words in Marathi. The first seven word-lists (first to seven) were filtered at 500 Hz, 1000 Hz, 1500 Hz, 2000 Hz, 2500 Hz, 3000 Hz, and 3500 Hz cut-off frequencies, respectively, while word list 8 was left unfiltered. Although, the SIS improved with increase in cut-off frequency, the improvement in SIS with increase in cut-off frequency of speech stimulus was noticed up to 3000 Hz, 2500 Hz, and 2000 Hz for participants of Group I, Group II, and Group III, respectively. In addition, the improvement in speech perception performance did not correspond to what would be anticipated with an increase in the cut-off frequency of speech stimulus for participants of Group II and Group III compared to participants of Group I. Although, there was a significant reduction in SIS as a function of the degree of hearing impairment for speech stimulus filtered at 1500 Hz, 2000 Hz, 2500 Hz, and 3000 Hz cut-off frequencies, there was no significant effect of degree of hearing impairment on SIS for speech stimulus filtered at 500 Hz and 1000 Hz cut-off frequencies.

Impact of reconstruction parameters on spatial resolution and its comparison between cadmium-zinc-telluride SPECT/CT and conventional SPECT/CT.

To evaluate the impact of reconstruction parameters on the spatial resolution of the tomographic image in single photon emission computed tomography (SPECT)/computed tomography (CT), and compare spatial resolution between a new polyvalent whole-body Cadmium-Zinc-Telluride camera (CZT-SPECT/CT) and a conventional dual-head Anger camera (conventional SPECT/CT). Spatial resolution was evaluated with four-line sources filled with 99mTc in tomographic images reconstructed by varying reconstruction parameters. Ordered-subset expectation maximization (OSEM) algorithm was performed with varying iterations (1-20), the number of subsets was fixed at 10. Butterworth filter, Gauss filter and no-filter were selected, respectively. Computed tomography-based attenuation correction (CTAC), scatter correction, resolution recovery and no correction (NC) were adopted for image correction. Filtered back projection (FBP) with Butterworth filter and CTAC was performed in image reconstruction. Spatial resolution was expressed by the full width at half-maximum (FWHM) value. The impact of reconstruction parameters on the spatial resolution was identical in both cameras: FWHM values decreased with the increase of iterations and converged uniformly when the number of iterations was over 4. FWHM values decreased with the increase of cutoff frequency of the Butterworth filter and increased with the increase of the Gauss filter. scatter correction and resolution recovery improved spatial resolution, whereas CTAC had a negligible effect on spatial resolution when reconstructed by OSEM. FWHM was generally lower with OSEM reconstruction than FBP reconstruction. On the whole, under the same reconstruction conditions, CZT-SPECT/CT had a lower FWHM value than conventional SPECT/CT. The spatial resolution was improved with the increase of iterations. Increasing the cutoff frequency of the Butterworth filter and decreasing the Gauss filter enhanced spatial resolution. The spatial resolution was better reconstructed by OSEM associated with attenuation correction, scatter correction and resolution recovery than FBP. CZT-SPECT/CT had better spatial resolution than conventional SPECT/CT.

Effect of Spectrally Modified Speech on Speech Perception of Native Speakers of Marathi

Objective: We investigated the effect of spectrally modified speech on speech perception of native speakers of Marathi. Methods: A total of eight word-lists with each list consisting of 25 words in Marathi were used as speech stimuli. The speech stimuli in List 1, List 2, List 3, List 4, List 5, List 6, and List 7 were spectrally modified by filtering them at 500 Hz, 1000 Hz, 1500 Hz, 2000 Hz, 2500 Hz, 3000 Hz, and 3500 Hz cutoff frequencies, respectively. The speech stimuli in List 8 were unfiltered. The speech identification score (SIS) testing was performed as a measure of speech perception using eight word-lists on a total of 125 normal hearing native speakers of Marathi in the age range between 18 and 25 years. Results: The results revealed that there was a statistically significant effect of spectral modification on speech perception. An increase in cutoff frequency of speech stimuli was analogous to increase in speech perception performance. The normal range of SIS (90%–100%) was achieved for speech stimuli filtered at 2500 Hz cutoff frequency. Conclusion: Although speech is a broadband signal, the findings of this study suggest that the spectral information till 2500 Hz is sufficient for optimum perception of speech of Marathi.

Evaluation method for moisture content of oil‐paper insulation based on segmented frequency domain spectroscopy: From curve fitting to machine learning

In recent years, frequency domain spectroscopy (FDS) is often used to evaluate oil paper insulation state in power transformer bushing. But it is still very difficult to evaluate the moisture content accurately and quickly. In order to solve this problem, this paper proposes an intelligent algorithm based on random forest regression (RFR) to construct an efficient evaluation method through segmented FDS curves. Furthermore, the characteristics of FDS curves were studied and the intelligent method was compared with support vector regression (SVR) and deep neural networks (DNN). The results show that the dielectric loss, the real part and imaginary part of complex capacitance all move upward with the moisture increasing, so they can be used as the input feature of the evaluation model; The moisture content evaluation accuracy of the RFR model in the whole frequency band is higher than that of SVR and DNN models; With the increase of lower cut off frequency (FDS test stop frequency), the FDS test time is greatly shortened, and the accuracy of the RFR model can still meet the evaluation requirements. Therefore, the data in a compromise frequency band can be used to evaluate the moisture content of oil paper insulation accurately and quickly.

Investigation of New Multi-Channel HEMT Based on InAs0.3P0.7/InP for Future Power Terahertz HEMT’s

In this paper, we perform the increase of cut-off frequency for a HEMT based on the InP substrate by the use of a new concept which can be presented by the use of a Multi-Channel (M-Ch) based on InAs[Formula: see text]P[Formula: see text]/InP. This concept leads to formation of two confining interfaces and also an added barrier in the channel level which in turn leads to enhance the confining conditions of charge carriers. The obtained cut-off frequency for the proposed HEMT based on InAs[Formula: see text]P[Formula: see text]/InP has reached the Tera-hertz range which is the best of all existing power technologies. This morphology of structure is the first power structure that has reached this frequency range due to the use of InAs[Formula: see text]P[Formula: see text] as the main channel, it has better transport proprieties compared to that of InP. The same task is for the drain current in which the proposed structure has shown a high value compared to the structure based on a single-channel (S-Ch) formed by InP due to increasing confining interface number where the booth interface InAlAs/InAsP and InAsP/InP are being used.

Comparison of Low-Pass Filters for SPECT Imaging.

In single photon emission computed tomography (SPECT) imaging, the choice of a suitable filter and its parameters for noise reduction purposes is a big challenge. Adverse effects on image quality arise if an improper filter is selected. Filtered back projection (FBP) is the most popular technique for image reconstruction in SPECT. With this technique, different types of reconstruction filters are used, such as the Butterworth and the Hamming. In this study, the effects on the quality of reconstructed images of the Butterworth filter were compared with the ones of the Hamming filter. A Philips ADAC forte gamma camera was used. A low-energy, high-resolution collimator was installed on the gamma camera. SPECT data were acquired by scanning a phantom with an insert composed of hot and cold regions. A Technetium-99m radioactive solution was homogenously mixed into the phantom. Furthermore, a symmetrical energy window (20%) centered at 140 keV was adjusted. Images were reconstructed by the FBP method. Various cutoff frequency values, namely, 0.35, 0.40, 0.45, and 0.50 cycles/cm, were selected for both filters, whereas for the Butterworth filter, the order was set at 7. Images of hot and cold regions were analyzed in terms of detectability, contrast, and signal-to-noise ratio (SNR). The findings of our study indicate that the Butterworth filter was able to expose more hot and cold regions in reconstructed images. In addition, higher contrast values were recorded, as compared to the Hamming filter. However, with the Butterworth filter, the decrease in SNR for both types of regions with the increase in cutoff frequency as compared to the Hamming filter was obtained. Overall, the Butterworth filter under investigation provided superior results than the Hamming filter. Effects of both filters on the quality of hot and cold region images varied with the change in cutoff frequency.

Synthesis, Microstruture and Electromagnetic Performance of NixZn1-xFe2O4 Ferrites with Different Ni/Zn Ratios Prepared by a Novel Molten Salt Method

NixZn1-xFe2O4 ferrites (x=0.0∼1.0 with a step of 0.1) were prepared by a novel molten salt method. The effects of Ni/Zn ratio on the microstructure and electromagnetic performance of the acquired ferrites were investigated. All the samples are composed of a pure ferrite with cubic spinel structure, and their microstructures are quite homogeneous. With the increase of Ni content, the lattice constant of the samples decreases, the grain size fluctuates in a very narrow range around 2.0 μm, and the saturation magnetization firstly increases and then decreases, presenting the maximum value with x=0.6, while the coercivity basically displays an inverse trend to the saturation magnetization. In the frequency range from 1 M to 1 GHz, the initial permeability of the acquired NixZn1-xFe2O4 ferrites decreases, the cut-off frequency increases, and the magnetic loss decreases with increasing Ni content.

A novel noise model based on balanced detection for an ultrafast line-scan imaging system

A novel noise model is proposed for a balanced-detection ultrafast imaging system to comprehensively analyze the noise in the system. The whole noise model is obtained by solving for erbium-doped fiber amplifier (EDFA) noise, intersymbol interference (ISI) noise and low-pass filter (LPF) noise successively with stochastic process theory. Then, the nonmonotonic relationship between the cut-off frequency and the signal-to-noise ratio (SNR) for certain experimental parameters are derived quantitatively and qualitatively: the SNR first increases and then decreases when the cut-off frequency increases, while the unique extreme point is located at 6.3 GHz. In addition, imaging results are obtained accordingly. The model and the results can provide a reference for system design and parameter selection.

Effect of Temperature on Reliability Issues of Ferroelectric Dopant Segregated Schottky Barrier Tunnel Field Effect Transistor (Fe DS-SBTFET)

This paper addresses reliability issues associated with temperature of Ferroelectric Dopant Segregated Schottky Barrier Tunnel Field Effect Transistor (Fe DS-SBTFET). The simulated results are compared with Dopant Segregated Schottky Barrier TFET (DS-SBTFET). This is achieved by varying the operating temperature from 300 to 500 K. DC parameters such as ION/IOFF ratio, drain current characteristics and subthreshold swing (SS) for a range of temperature have been highlighted. Moreover, the influence of temperature on various RF figure of merits such as gate capacitance (CGG), intrinsic delay, cutoff frequency (fT) etc. have been investigated. The device linearity has been analyzed by considering the effect of temperature variation on linearity parameters like gm2, gm3, 1-dB compression point, VIP2, VIP3 and IIP3. The device characteristics get upgraded by the increase in cut-off frequency and reduction in intrinsic delay at elevated temperature.

Drain-Engineered TFET With Fully Suppressed Ambipolarity for High-Frequency Application

In this paper, we propose and simulate a novel drain-engineered structure of a quadruple-gate tunnel field-effect transistor (TFET). The proposed device employs a lateral dual source with a vertical drain extension on top of T-shaped channel region. This enables the modification of screening length ( $\lambda $ ) by varying the silicon film ( ${t}_{\textsf {Si},\textsf {v}}$ ) and the oxide ( ${t}_{\textsf {Si},\textsf {v}}$ ) thicknesses at the channel–drain junction to overcome the limitations of high ambipolar leakage in the conventional double-gate TFET (DG-TFET). A 2-D calibrated simulation study has revealed the doubling of on-current ( ${I}_{ \mathrm{\scriptscriptstyle ON}}$ ) and significantly suppressed ambipolar leakage ( ${I}_{\textsf {AMB}}$ ) in the proposed device as compared to the conventional DG-TFET. Furthermore, a five orders of magnitude improvement in ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ , 73% increase in transconductance ( ${g}_{\textsf {m}}$ ), 62% increase in cutoff frequency ( ${f}_{\textsf {T}}$ ), 72% increase in gain-bandwidth product, and 54% improvement in fall propagation delay ( ${t}_{\textsf {pHL}}$ ) are achieved in the proposed device.

Temperature Associated Reliability Issues of Heterogeneous Gate Dielectric—Gate All Around—Tunnel FET

In this paper, the temperature associated reliability issues of heterogeneous gate dielectric-gate all around-tunnel FET (HD GAA TFET) has been addressed, and the results are simultaneously compared with gate all around tunnel FET (GAA TFET). This is done by investigating the effect of interface trap charges such as donor (positive interface charges) and acceptor (negative interface charges) at various operating temperatures on the device analog parameters and RF figure of merits. It is observed that, at high gate bias, TFET exhibits weak temperature dependence owing to the weak dependence of band to band tunneling phenomenon on the temperature in comparison to the large temperature variation for lower gate bias due to the temperature dependence of Shockley-Read-Hall (SRH) phenomenon. Results reveal that extremely high off current at elevated temperatures degarades the device performance, making the device less reliable for high-temperature applications. Moreover, at elevated temperature, the decrease in threshold voltage and intrnsic delay, and increase in cut off frequency is found, thereby upgrading the device characteristics. All the simulations have been done on ATLAS device simulator.

Operational frequency degradation induced trapping in scaled GaN HEMTs

Cut-off frequency increase from 12.1GHz to 26.4GHz, 52.1GHz and 91.4GHz is observed when the 1μm gate length GaN HEMT is laterally scaled down to LG=0.5μm, LG=0.25μm and LG=0.125μm, respectively. The study is based on accurately calibrated transfer characteristics (ID-VGS) of the 1μm gate length device using Silvaco TCAD. If the scaling is also performed horizontally, proportionally to the lateral (full scaling), the maximum drain current is reduced by 38.2% when the gate-to-channel separation scales from 33nm to 8.25nm. Degradation of the RF performance of a GaN HEMT due to the electric field induced acceptor traps experienced under a high electrical stress is found to be about 8% for 1μm gate length device. The degradation of scaled HEMTs reduces to 3.5% and 7.3% for the 0.25μm and 0.125 gate length devices, respectively. The traps at energy level of ET=EV+0.9eV (carbon) with concentrations of NIT=5×1016cm−3, NIT=5×1017cm−3 and NIT=5×1018cm−3 are located in the drain access region where highest electrical field is expected. The effect of traps on the cut-off frequency is reduced for devices with shorter gate lengths down to 0.125μm.

SOI Dual-Gate Trench LDMOSFET for RF Integrated Power Amplifiers

ABSTRACTIn this paper, we propose a dual-gate trench laterally diffused MOSFET (DGT-LDMOS) on silicon-on-insulator by utilizing trenches in the drift region. The proposed device has one horizontal-gate placed on the surface and another vertical-gate located in a trench. The dual-gate structure creates two channels in p-base which carry drain current in parallel. Another trench filled with oxide is placed in the drift region to enhance the reduced-surface-field effect. Based on two-dimensional simulations, it is demonstrated that an 80 V DGT-LDMOS can achieve 2.3 times higher drain current, 6.5 times reduction in specific on-resistance, 86% improvement in peak transconductance, 3 times increase in cut-off frequency, and 2.3 times improvement in maximum oscillation frequency with 3 times reduction in cell pitch when compared with 80 V conventional LDMOS.

Sidewall spacer layer engineering for improvement of analog/RF performance of nanoscale double-gate junctionless transistors

We investigate the impact of sidewall spacers on the analog/RF performance of double gate junctionless transistors at channel length of 30 nm using extensive numerical device simulation. Furthermore, we report the performance of a common source amplifier built employing such devices. The simulation deck is calibrated with experimental results published elsewhere. The influence of spacer layers is analyzed in terms of the dielectric constant and spacer layer thickness. Our findings reveal that peak transconductance and peak intrinsic gain increase by 5.2 and 71.3 % for spacer dielectric constant k = 30 as compared to the respective value for k = 3.9 while peak unity gain cut-off frequency and maximum frequency of oscillations increase by 37 and 83.3 % for k = 3.9 compared with the value for k = 30. It is evident from our studies that peak transconductance, peak transconductance generation factor, peak gain and peak cut-off frequency increase by 13, 10, 27 and 20 %, respectively, for spacer length of 5 nm compared with the corresponding value for spacer length of 15 nm. Additionally, our analysis shows that the peak gain of a common source amplifier increases with higher value of load resistance and spacer dielectric constant while it reduces with a larger spacer length. Moreover, we present a comparison of various device parameters of JLTs associated with analog/RF performance and gain of the common source amplifier with the corresponding parameter obtained with undoped inversion mode transistors.

Graphene FETs with Low-Resistance Hybrid Contacts for Improved High Frequency Performance.

This work proposes a novel geometry field effect transistor with graphene as a channel—graphene field-effect transistor (GFET), having a hybrid contact that consists of an ohmic source/drain and its extended part towards the gate, which is capacitively coupled to the channel. The ohmic contacts are used for direct current (DC) biasing, whereas their capacitive extension reduces access region length and provides the radio frequency (RF) signal a low impedance path. Minimization of the access region length, along with the paralleling of ohmic contact’s resistance and resistive part of capacitively coupled contact’s impedance, lower the overall source/drain resistance, which results in an increase in current gain cut-off frequency, fT. The DC and high-frequency characteristics of the two chosen conventional baseline GFETs, and their modified versions with proposed hybrid contacts, have been extensively studied, compared, and analyzed using numerical and analytical techniques.

A Multi-Channel Trench-Gate Radio Frequency LDMOS on Silicon-on-Insulator

ABSTRACTA new radio frequency (RF) laterally diffused semiconductor field-effect transistor (MOSFET) (LDMOS) structure called multi-channel trench-gate (MCTG-LDMOS) is proposed on silicon-on-insulator. The MCTG-LDMOS structure consists of identical trenches in the drift region in which gate-electrodes are placed vertically. An n+-Si layer over buried-oxide serves as drain region which is connected to drain metal placed in a trench. Each gate-electrode creates two channels vertically in p-base on either side of its trench. In this study, four gate-trenches are built in a cell pitch of 5 µm resulting in formation of eight channels which carry current in parallel from drain to source electrodes. Based on two-dimesional simulations results, a 50 V MCTG-LDMOS is demonstrated to achieve 6.1 times higher drive current, 2.2 times reduction in specific on-resistance, 6.9 times improvement in transconductance, 2.1 times increase in cut-off frequency, and 3.2 times higher maximum oscillation frequency when compared with a 50 V conventional LDMOS for same cell pitch.