Transit Time Devices Research Articles

Prospects of graphene-based heat sink and its computational thermal analysis in avalanche transit time devices

Prospects of graphene-based heat sink and its computational thermal analysis in avalanche transit time devices

Bias Current Modulation of DC and RF Properties of AlmGa1‐m∼GaN Heterojunction IMPATT at Sub‐Millimeter Wave Frequency

AbstractLarge signal RF properties of double drift region (DDR) Impact Avalanche Transit Time (IMPATT) device based on AlmGa1‐m∼GaN heterojunction (HT) are investigated and their variation with bias currents has been presented here. These RF properties and their modulation will significantly validate the acceptability of the particular HT in the sub‐millimeter wave frequency domain. Avalanche noise plays the prime ground for making the IMPATT diode basically noisy and tunneling decays the performance of the device, as we move towards the higher frequency range like the sub‐millimeter frequency. These setbacks can be done with if the homojunction (HM) is replaced by heterojunction. This authenticates the choice of heterojunction over homojunction. Here, the author takes 500 GHz as the frequency of concern in the sub‐millimeter wave range. Here, two complementary heterojunctions of AlmGa1‐m∼GaN is taken with m = 0.4 or the mole fraction of Al in the alloy being 40% and their RF results are compared with two homojunctions of GaN and AlGaN for double drift region (DDR) IMPATT diode. Outcomes indicate that HT diodes surpass the high‐frequency performance of HM diodes if compared on the basis of dc to RF conversion efficiency and output power due to the influence of large amplitude ac signal. It is also observed that with increasing bias voltage dependent current, which is varied from 20 × 108to 40 × 108Am−2, the efficiency of conversion together with the power output increases. These results will be very important for further research in this domain.

Space charge studies in graphene based avalanche transit time devices

Avalanche multiplication process in avalanche transit time devices results in large number of electrons-hole pairs (EHPs). Therefore, there is considerable temperature rise in the junction. This high temperature than normal operating one, leads to large space charge effects by creating extra space charges. As a result, practical efficiency of transit time devices is below 30%. It is quite important to understand the effects of space charges on device performances. The space charge effect is computationally studied and analysed here for the graphene material based double drift region (DDR) impact ionization and avalanche transit time (GIMPATT) diode. Also, the effect in GIMPATT is compared with IMPATT based on other material like Si, Ge, GaAs, WzGaN, InP, 4H–SiC. • There is considerable temperature rise in the junction of avalanche transit time devices. • This high temperature than normal operating one, leads to large space charge effects by creating extra space charges. • Practical efficiency of transit time devices is below 30%. It is important to understand the effects of space charges. • The space charge effect is computationally studied and analysed here for the GIMPATT diode . • The effect in GIMPATT is compared with IMPATT based on other material like Si, Ge, GaAs, WzGaN, InP, 4H–SiC.

Strained Si/Si1−yCy superlattice based quasi-read avalanche transit-time devices for terahertz ultrafast switches

Effects of selective carbon (C) incorporation in silicon (Si) quasi-read-avalanche-transit-time (QRATT) devices are studied through indigenously developed non-linear Strain-corrected-mixed-quantum-tunneling-drift–diffusion-model (SMQTDDM). A superlattice with alternate thin films of strained-Si and comparatively thick layers of Si0.99C0.01 stressors constitutes the active region. Out-of-plane mobility enhancement occurs due to the in-plane biaxial strain at Si/Si0.99C0.01 interfaces. Band offset between Si/Si0.99C0.01results in high injection velocity. Combined effect of strain-engineering and band offset amounts to the application of periodic accelerating pulse along the active region. This subsequently reduces carrier transit-time and results in THz oscillation in Si-ATT-diode. Remarkable RF performance (RF-power ~ $$23.2\times {10}^{8}$$ W/m2 at 0.73 THz) of exotic Si-QRATT-devices is reported for the first time. The simulation incorporates quantum-effects, process-induced-strain, parasitic-resistance, thermal-model and inter-sub-band-tunneling in the dispersion relation of the multiple-quantum-wells through a combined solution of Schrodinger–Poisson equations. The theoretical analysis is verified with experimental observations for in-house-fabricated Si-ATT-diodes. QRATT-device-based THz series-shunt switches are further explored.

Millimetre‐wave high–low IMPATT source development: First on‐chip experimental verification

This paper reports design and development of high–low type Si/SiC-based Impact Ionisation Avalanche Transit Time device and its on-chip characterisation. The design has been performed with indigenously developed strain engineered non-linear self-consistent large-signal simulator. On-chip hetero-structure SiC Impact Ionisation Avalanche Transit Time at 94 GHz has been successfully fabricated and on-chip DC testing (forward and reverse) results are reported for the first time. The device breaks down at 185 V (simulated result: 188 V) and breakdown current is ∼12.5 mA. If the diode chip is mounted properly with W-band waveguide, it is expected to generate ∼2 W of Radio Frequency (RF) power at W-band window frequency (∼94 GHz). The experimental verification of newly developed in-house strain-corrected mixed quantum tunnelling drift diffusion simulator is done successfully.

Analyzing 24-Hour Blood Pressure Measurements with a Novel Cuffless Pulse Transit Time Device in Clinical Practice-Does the Software for Heartbeat Detection Matter?

Background: The Somnotouch-Non-Invasive-Blood-Pressure (NIBP) device delivers raw data consisting of electrocardiography and photoplethysmography for estimating blood pressure (BP) over 24 h using pulse-transit-time. The study’s aim was to analyze the impact on 24-hour BP results when processing raw data by two different software solutions delivered with the device. Methods: We used data from 234 participants. The Somnotouch-NIBP measurements were analyzed using the Domino-light and Schiller software and compared. BP values differing >5 mmHg were regarded as relevant and explored for their impact on BP classification (normotension vs. hypertension). Results: Mean (±standard deviation) absolute systolic/diastolic differences for 24-hour mean BP were 1.5 (±1.7)/1.1 (±1.3) mm Hg. Besides awake systolic BP (p = 0.022), there were no statistically significant differences in systolic/diastolic 24-hour mean, awake, and asleep BP. Twenty four-hour mean BP agreement (number (%)) between the software solutions within 5, 10, and 15 mmHg were 222 (94.8%), 231 (98.7%), 234 (100%) for systolic and 228 (97.4%), 232 (99.1%), 233 (99.5%) for diastolic measurements, respectively. A BP difference of >5 mmHg was present in 24 (10.3%) participants leading to discordant classification in 4–17%. Conclusion: By comparing the two software solutions, differences in BP are negligible at the population level. However, at the individual level there are, in a minority of cases, differences that lead to different BP classifications, which can influence the therapeutic decision.

Design and development of an AlGaN/GaN heterostructure nano‐ATT oscillator: experimental feasibility studies in THz domain

Numerical analysis followed by experimental investigations are done with heterostructure GaN/AlGaN single drift MIxed Tunnelling and Avalanche Transit Time (MITATT) devices grown on Si〈111〉 substrate, at around 1 THz. A generalised self-consistent, nonlinear Mixed Quantum Drift-Diffusion (MQDD) simulator is developed and used for designing p++-n−-n-n++ type room temperature solid-state source within 0.75–1.1 THz regime. The epitaxial heterojunction layers are grown by III–V nitride molecular beam epitaxy which has enabled the realisation of GaN/AlGaN periodic structure on Si〈111〉 substrate with AlN buffer layer. For the first time, the realisation of heterostructure III–V Nitride MITATT oscillator at sub-mm wave/terahertz (THz) frequencies is done. The device is observed to oscillate at 1 THz with ∼4%. A comparison of MQDD model with experimental (On wafer DC testing) results for heterojunction GaN/AlGaN MITATT diodes shows generalised agreement in terms of breakdown voltage (∼27 V) and efficiency (∼4%). First experimental results of GaN/AlGaN heterostructure MITATT diode are reported and compared with the theoretical predictions obtained through MQDD model.

Noise performance of avalanche transit-time devices in the presence of acoustic phonons

Through this paper, the effects of acoustic phonons on the noise performance of avalanche transit-time devices have been investigated and reported. For this study, a double-drift-region silicon-based impact avalanche transit-time diode has been considered at operating frequencies of 94 GHz, 140 GHz and 220 GHz. To analyze the acoustic phonon effects on noise performance, the interactions of charge carriers with acoustic deformation potential and piezoelectric acoustic phonons have been considered in addition to all possible types of scattering events. These effects have been analyzed through a numerical expression for the ionization rate of charge carriers and incorporated in the noise analysis. The noise performance is evaluated in terms of noise spectral density (NSD) and noise measure (NM). The results show that due to acoustic phonons, values of NSD and NM significantly increase.

Millimetre-wave and terahertz IMPATT sources: influence of inter-carrier interactions

A major amount of energy of mobile electrons and holes in a semiconductor under electric field are lost due to inter-carrier collisions prior to ionising collision. This fact causes a decrease in the ionisation probability which leads to the deterioration in ionisation rates especially when the doping density is high. The effects of this phenomenon on the high frequency and noise properties of highly doped impact avalanche transit time (IMPATT) devices based on different wide bandgap (WBG) semiconductors, like 4H-SiC, Wurtzite-GaN (Wz-GaN) and type-IIb diamond (C) have been studied in this paper. Significant deteriorations in the diodes' high frequency and noise performance have been observed in the simulation results. The simulation results have been compared with the experimental data in order to validate those.

Effect of magnetic field on the RF performance of millimeter-wave IMPATT source

In this paper, a two-dimensional (2-D) large-signal model has been presented to study the effect of steady magnetic field on the RF performance of millimeter-wave (mm-wave) double-drift region impact avalanche transit time device. Magnetic field sensitivities of various static and large-signal parameters of the device designed to operate at W-band have been calculated and discussed in detail. Results show that the frequency tuning of the source of around 3.64 GHz is achievable with maximum sensitivity of about $$-$$-1.59 GHz $$\hbox {T}^{-1}$$T-1 for the application of transverse magnetic field varying from 4.0 to 5.0 T. The nature of both frequency and power tuning of the device due to application of transverse magnetic field is in good agreement with the experimental results carried out earlier.

High gain hybrid graphene-organic semiconductor phototransistors.

Hybrid phototransistors of graphene and the organic semiconductor poly(3-hexylthiophene-2,5-diyl) (P3HT) are presented. Two types of phototransistors are demonstrated with a charge carrier transit time that differs by more than 6 orders of magnitude. High transit time devices are fabricated using a photoresist-free recipe to create large-area graphene transistors made out of graphene grown by chemical vapor deposition. Low transit time devices are fabricated out of mechanically exfoliated graphene on top of mechanically exfoliated hexagonal boron nitride using standard e-beam lithography. Responsivities exceeding 10(5) A/W are obtained for the low transit time devices.

Quantum corrected drift-diffusion model for terahertz IMPATTs based on different semiconductors

The authors have developed a quantum corrected drift-diffusion model for impact avalanche transit time (IMPATT) devices by coupling the density gradient model with the classical drift-diffusion model. A large-signal simulation technique has been developed by incorporating the quantum potentials in the current density equations for the analysis of double-drift region IMPATT devices based on different semiconductors such as Wurtzite---GaN, InP, type-IIb diamond (C), 4H---SiC and Si deigned to operate at different millimeter-wave (mm-wave) and terahertz (THz) frequencies. It is observed that, the RF power output and DC to RF conversion efficiency of the devices operating at higher mm-wave ($$>$$>140 GHz) and THz frequencies reduce due to the incorporation of quantum corrections in the model; but the effect of quantum corrections are negligible for the devices operating at lower mm-wave frequencies ($$\le $$≤140 GHz).

Large-signal characterizations of DDR IMPATT devices based on group III–V semiconductors at millimeter-wave and terahertz frequencies

Large-signal (L-S) characterizations of double-drift region (DDR) impact avalanche transit time (IMPATT) devices based on group III–V semiconductors such as wurtzite (Wz) GaN, GaAs and InP have been carried out at both millimeter-wave (mm-wave) and terahertz (THz) frequency bands. A L-S simulation technique based on a non-sinusoidal voltage excitation (NSVE) model developed by the authors has been used to obtain the high frequency properties of the above mentioned devices. The effect of band-to-band tunneling on the L-S properties of the device at different mm-wave and THz frequencies are also investigated. Similar studies are also carried out for DDR IMPATTs based on the most popular semiconductor material, i.e. Si, for the sake of comparison. A comparative study of the devices based on conventional semiconductor materials (i.e. GaAs, InP and Si) with those based on Wz-GaN shows significantly better performance capabilities of the latter at both mm-wave and THz frequencies.

Quantum drift-diffusion model for IMPATT devices

Quantum correction is necessary on the classical drift-diffusion (CLDD) model to predict the accurate behavior of high frequency performance of ATT devices at frequencies greater than 200 GHz when the active layer of the device shrinks in the range of 150---350 nm. In the present work, a quantum drift-diffusion model for impact avalanche transit time (IMPATT) devices has been developed by incorporating appropriate quantum mechanical corrections based on density-gradient theory which macroscopically takes into account important quantum mechanical effects such as quantum confinement, quantum tunneling, etc. into the CLDD model. Quantum potentials (synonymous as Bohm potentials) have been incorporated in the current density equations as necessary quantum mechanical corrections for the analysis of millimeter-wave (mm-wave) and Terahertz (THz) IMPATT devices. It is observed that the large-signal (L-S) performance of the device is degraded due to the incorporation of quantum corrections into the model when the frequency of operation increases above 200 GHz; while the effect of quantum corrections are negligible for the devices operating at lower mm-wave frequencies.

IMPATT devices based on group III–V compound semiconductors: prospects as potential terahertz radiators

The potentialities of double-drift region (DDR) impact avalanche transit time (IMPATT) devices based on group III–V semiconductor material Wurtzite-gallium nitride (Wz-GaN) as possible millimetre-wave (mm-wave) and terahertz (THz) sources have been explored in this paper. A large-signal (L-S) simulation technique based on a non-sinusoidal voltage excitation model (NVSE) developed by the authors has been used to study the high-frequency properties of DDR Wz-GaN IMPATTs at both mm-wave and THz frequencies. Similar studies have also been carried out for DDR IMPATTs based on some other conventional semiconductor materials such as GaAs (group III–V), InP (group III–V) and Si (group IV). Results show that DDR Wz-GaN IMPATTs excel all other devices under consideration as regards radio-frequency performance at both mm-wave and THz frequencies.

Potentiality of semiconducting diamond as the base material of millimeter-wave and terahertz IMPATT devices

An attempt is made in this paper to explore the potentiality of semiconducting type-IIb diamond as the base material of double-drift region (DDR) impact avalanche transit time (IMPATT) devices operating at both millimetre-wave (mm-wave) and terahertz (THz) frequencies. A rigorous large-signal (L-S) simulation based on the non-sinusoidal voltage excitation (NSVE) model developed earlier by the authors is used in this study. At first, a simulation study based on avalanche response time reveals that the upper cut-off frequency for DDR diamond IMPATTs is 1.5 THz, while the same for conventional DDR Si IMPATTs is much smaller, i.e. 0.5 THz. The L-S simulation results show that the DDR diamond IMPATT device delivers a peak RF power of 7.79 W with an 18.17% conversion efficiency at 94 GHz; while at 1.5 THz, the peak power output and conversion efficiency decrease to 6.19 mW and 8.17% respectively, taking 50% voltage modulation. A comparative study of DDR IMPATTs based on diamond and Si shows that the former excels over the later as regards high frequency and high power performance at both mm-wave and THz frequency bands. The effect of band to band tunneling on the L-S properties of DDR diamond and Si IMPATTs has also been studied at different mm-wave and THz frequencies.

Optical control of large-signal properties of millimeter-wave and sub-millimeter-wave DDR Si IMPATTs

The authors have proposed a complete large-signal (L-S) model to investigate the optical modulation of high frequency properties of double-drift region (DDR) impact avalanche transit time (IMPATT) devices operating at different millimeter-wave and sub-millimeter-wave frequencies. Simulation is carried out based on the proposed model to study the effect of photo-irradiation of different values of incident optical power of different wavelengths from 600---1000 nm on the DC and L-S characteristics of DDR IMPATTs based on Si designed to operate at 94, 140, 220, 300 and 500 GHz. Two different optical illumination configurations such as top mount and flip chip are taken into account for the present study. Results show that the maximum optical tuning of L-S parameters of the device can be achieved for optical illumination of wavelength 700 nm, i.e. near the wavelength corresponding to the responsivity peak of Si in both top mount and flip chip configurations. Further, better photo-sensitivity of top mount structure is observed as compared to its flip chip counterpart. The simulation results are found to be in good agreement with the experimental results reported earlier. Suitable tunable light source information is also suggested to carry out the optical control experiment within the wavelength range under consideration.

Large-signal characterization of DDR silicon IMPATTs operating in millimeter-wave and terahertz regime

The authors have carried out the large-signal characterization of silicon-based double-drift region (DDR) impact avalanche transit time (IMPATT) devices designed to operate up to 0.5 THz using a large-signal simulation method developed by the authors based on non-sinusoidal voltage excitation. The effect of band-to-band tunneling as well as parasitic series resistance on the large-signal properties of DDR Si IMPATTs have also been studied at different mm-wave and THz frequencies. Large-signal simulation results show that DDR Si IMPATT is capable of delivering peak RF power of 633.69 mW with 7.95% conversion efficiency at 94 GHz for 50% voltage modulation, whereas peak RF power output and efficiency fall to 81.08 mW and 2.01% respectively at 0.5 THz for same voltage modulation. The simulation results are compared with the experimental results and are found to be in close agreement.

Studies on Anisotype Si/Si1-xGex Heterojunction DDR IMPATTs: Efficient Millimeter-wave Sources at 94 GHz Window

In this paper, an attempt is made to investigate the millimeter-wave performance of anisotype Si/Si1-xGex heterojunction Double-drift region (DDR) Impact Avalanche Transit Time (IMPATT) devices operating at 94 GHz window frequency by using a generalized double-iterative computer method based on drift-diffusion model developed by the authors. Simulation study on both anisotype Np and nP type Si/Si1-xGex heterojunction DDR IMPATTs for Ge mole fractions, x = 0.1 and x = 0.3, is carried out and their mm-wave properties are compared with a Si homojunction DDR IMPATT operating at the same frequency. Results show that significant improvements in DC to RF conversion efficiency and RF power output can be achieved in Si/Si1-xGex heterojunction DDR IMPATTs as compared to their Si homojunction counterpart. It is observed that the mm-wave performance of nP Si0.7Ge0.3/Si heterojunction DDR as regards to RF power output and conversion efficiency is best among all the devices under consideration. DC to RF conversion efficiency and continuous wave (CW) RF power output of nP Si0 7Ge0 3/Si DDR are obtained as 20.04% and 731.7 mW, respectively, near 94 GHz; whereas for Si DDR, the same parameters are obtained as 10.40% and 609.3 mW, respectively.

Large-signal characterization of DDR silicon IMPATTs operating up to 0.5 THz

Large-signal (L-S) characterization of double-drift region (DDR) impact avalanche transit time (IMPATT) devices based on silicon designed to operate at different millimeter-wave (mm-wave) and terahertz (THz) frequencies up to 0.5 THz is carried out in this paper using an L-S simulation method developed by the authors based on non-sinusoidal voltage excitation (NSVE) model. L-S simulation results show that the device is capable of delivering peak RF power of 657.64 mW with 8.25% conversion efficiency at 94 GHz for 50% voltage modulation; whereas RF power output and efficiency reduce to 89.61 mW and 2.22% respectively at 0.5 THz for same voltage modulation. Effect of parasitic series resistance on the L-S properties of DDR Si IMPATTs is also investigated, which shows that the decrease in RF power output and conversion efficiency of the device due to series resistance is more pronounced at higher frequencies especially at the THz regime. The NSVE L-S simulation results are compared with well established double-iterative field maximum (DEFM) small-signal (S-S) simulation results and finally both are compared with the experimental results. The comparative study shows that the proposed NSVE L-S simulation results are in closer agreement with experimental results as compared to those of DEFM S-S simulation.