Transit Time Devices Research Articles

Potentiality of IMPATT Devices as Terahertz Source: An Avalanche Response Time-based Approach to Determine the Upper Cut-off Frequency Limits

Potentiality of Impact Avalanche Transit Time (IMPATT) devices based on different semiconductor materials such as InP, 4H-SiC, and Wurtzite-GaN (Wz-GaN) has been explored for operation at terahertz (THz) frequencies. Drift-diffusion model is used to design double-drift region (DDR) IMPATTs based on above mentioned materials at different millimeter-wave (mm-wave) and THz frequencies and the upper cut-off frequency limits of those devices are obtained from the avalanche response times at those mm-wave and THz frequencies. Results show that the upper cut-off frequency limits of both InP and 4H-SiC DDR IMPATTs are 1.0 THz; whereas, the same is 5.0 THz in Wz-GaN DDR IMPATTs. The Wz-GaN DDR IMPATTs emerge as the most suitable devices for generation of THz frequencies due to their small avalanche response time, high DC to RF conversion ratio, and sufficiently high RF power output at THz frequencies. But, it is observed that up to 1.0 THz, 4H-SiC DDR IMPATTs excel Wz-GaN DDR IMPATTs due to their higher output power densities. Thus, the wide bandgap semiconductors such as Wz-GaN and 4H-SiC are highly suitable materials for DDR IMPATTs at both mm-wave and THz frequency ranges.

Effect of junction temperature on the large-signal properties of a 94 GHz silicon based double-drift region impact avalanche transit time device

The authors have developed a large-signal simulation technique extending an in-house small-signal simulation code for analyzing a 94 GHz double-drift region impact avalanche transit time device based on silicon with a non-sinusoidal voltage excitation and studied the effect of junction temperature between 300 and 550 K on the large-signal characteristics of the device for both continuous wave (CW) and pulsed modes of operation. Results show that the large-signal RF power output of the device in both CW and pulsed modes increases with the increase of voltage modulation factor up to 60%, but decreases sharply with further increase of voltage modulation factor for a particular junction temperature; while the same parameter increases with the increase of junction temperature for a particular voltage modulation factor. Heat sinks made of copper and type-IIA diamond are designed to carry out the steady-state and transient thermal analysis of the device operating in CW and pulsed modes respectively. Authors have adopted Olson's method to carry out the transient analysis of the device, which clearly establishes the superiority of type-IIA diamond over copper as the heat sink material of the device from the standpoint of the undesirable effect of frequency chirping due to thermal transients in the pulsed mode.

A proposed simulation technique to study the series resistance and related millimeter-wave properties of Ka-band Si IMPATTs from the electric field snapshots

A large-signal model and a simulation technique based on non-sinusoidal voltage excitation are used to obtain the electric field snapshots from which the series resistance and related high-frequency properties of a 35 GHz Silicon Single-Drift Region (SDR) Impact Avalanche Transit Time (IMPATT) device have been estimated for different bias current densities. A novel method is proposed in this paper to determine the parasitic series resistance of a millimeter-wave IMPATT device from large-signal electric field snapshots at different phase angles of a full cycle of steady-state oscillation. The method is based on the depletion width modulation of the device under a large-signal condition. The series resistance of the device is also obtained from the large-signal admittance characteristics at threshold frequency. The values of series resistance of a 35 GHz SDR IMPATT diode obtained from the proposed method and the large-signal admittance method are compared with experimentally reported values. The results show that the proposed method provides better and closer agreement with the experimental value.

Studies on anisotype Si/Si1-xGexheterojunction DDR IMPATTs: efficient millimeter-wave sources at 94 GHz window

AbstractIn 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 convers...

Prospects of IMPATT devices based on wide bandgap semiconductors as potential terahertz sources

In this paper the potentiality of impact avalanche transit time (IMPATT) devices based on different semiconductor materials such as GaAs, Si, InP, 4H-SiC and Wurtzite-GaN (Wz-GaN) has been explored for operation at terahertz frequencies. Drift–diffusion model is used to design double-drift region (DDR) IMPATTs based on different materials at millimeter-wave (mm-wave) and terahertz (THz) frequencies. The performance limitations of these devices are studied from the avalanche response times at different mm-wave and THz frequencies. Results show that the upper cut-off frequency limits of GaAs and Si DDR IMPATTs are 220 GHz and 0.5 THz, respectively, whereas the same for InP and 4H-SiC DDR IMPATTs is 1.0 THz. Wz-GaN DDR IMPATTs are found to be excellent candidate for generation of RF power at THz frequencies of the order of 5.0 THz with appreciable DC to RF conversion efficiency. Further, it is observed that up to 1.0 THz, 4H-SiC DDR IMPATTs excel Wz-GaN DDR IMPATTs as regards their RF power outputs. Thus, the wide bandgap semiconductors such as Wz-GaN and 4H-SiC are highly suitable materials for DDR IMPATTs at both mm-wave and THz frequency ranges.

GaN based transfer electron and avalanche transit time devices

A new model is developed to study the microwave/mm wave characteristics of two-terminal GaN-based transfer electron devices (TEDs), namely a Gunn diode and an impact avalanche transit time (IMPATT) device. Microwave characteristics such as device efficiency and the microwave power generated are computed and compared at D-band (140 GHz center frequency) to see the potentiality of each device under the same operating conditions. It is seen that GaN-based IMPATT devices surpass the Gunn diode in the said frequency region.

Dependence of Noise Properties on Photon Flux Incident on Silicon MITATT Device at Millimeter-Wave Window Frequencies

The effect of photon flux density on the noise properties of optically illuminated Double Drift Region (DDR) Mixed Tunneling Avalanche Transit Time (MITATT) device is investigated in this paper. An analytical model is proposed to obtain a relationship between the normalized difference of electron and hole current density of DDR device and the incident photon flux on it. Simulation is carried out to study the small signal noise spectral density and noise measure of the device under optical illumination. Two illumination configurations i.e., Flip Chip (FC) and Top Mounted (TM) structures are considered in the simulation. The device is designed to operate at mm-wave window frequency at W-band. The results show that avalanche noise measures of FC and TM structures of photo irradiated device are 37.1 dB and 40.2dB respectively for an incident photon flux density of 1024 m-2 sec-1 at 1000nm wavelength near band gap absorption of Silicon. The noise measure of the un-illuminated device is found to be 35dB. The results indicate that the increase of avalanche noise due to the incident photon flux can be reduced if FC structure is taken instead of TM structure.

Potential of asymmetrical Si/Ge and Ge/Si based hetero-junction transit time devices over homo-junction counterparts for generation of high power

Static and dynamic properties of both complementary n-Ge/p-Si and p-Ge/n-Si hetero-junction Double-Drift IMPATT diodes have been investigated by an advanced and realistic computer simulation technique, developed by the authors, for operation in the Ka-, V- and W-band frequencies. The results are further compared with corresponding Si and Ge homo-junction devices. The study shows high values of device efficiency, such as 23%, 22% and 21.5%, for n-Ge/p-Si IMPATTs at the Ka, V and W bands, respectively. The peak device negative conductances for n-Si/p-Ge and n-Ge/p-Si hetero-junction devices found to be 50.7 × 106 S/m2 and 71.3 × 106 S/m2, which are ∼3–4 times better than their Si and Ge counterparts at the V-band. The computed values of RF power-density for n-Ge/p-Si hetero-junction IMPATTs are 1.0 × 109, 1.1 × 109 and 1.4 × 109 W/m2, respectively, for Ka-, V- and W-band operation, which can be observed to be the highest when compared with Si, Ge and n-Si/p-Ge devices. Both of the hetero-junctions, especially the n-Ge/p-Si hetero-junction diode, can thus become a superior RF-power generator over a wide range of frequencies. The present study will help the device engineers to choose a suitable material pair for the development of high-power MM-wave IMPATT for applications in the civil and defense-related arena.

α-SiC nanoscale transit-time diodes: performance of the photo-irradiated terahertz sources at elevated temperature

The effects of elevated junction temperature on the terahertz (THz) frequency characteristics of α-(hexagonal, 4H and 6H) silicon carbide (SiC) based double-drift region (DDR, p++ p n n++ type) impact ionization avalanche transit-time (IMPATT) devices are studied and compared for the first time through simulation experiments. This study reveals that at 300 K < T < 600 K, a 4H-SiC IMPATT diode may yield 3.5 W of output power (efficiency (η) ∼ 8.6%) at 1.3 THz, while its 6H-SiC counterpart can deliver 3 W of output power (η = 6.3%) at 1.2 THz. It is interesting to observe that at elevated temperature, the performance of a 6H-SiC IMPATT diode degrades more in comparison with its 4H-SiC counterpart. These comparative analyses reveal the superiority of 4H-SiC diodes over their 6H-SiC counterparts, and thus establish the potential of the former as a high-power THz IMPATT oscillator even in harsh environments. Mobile space charge effects and the effect of positive series resistance on the high-temperature performance of the THz devices are also simulated, and it is found that series resistance reduces the output power level of the diodes by at least 15.0%. Moreover, the effects of increased junction temperature on the photo-sensitivity of top mounted (TM) and flip chip (FC) α-SiC IMPATTs are also investigated using a modified simulation technique. The device operating frequencies, under TM illumination configuration, shifts upward by at least 40.0 GHz, whereas the operating frequency shifts upward by at least 100.0 GHz under FC illumination configuration. The simulation results and the proposed experimental methodology presented here may be used for realizing optically controlled α-SiC transit-time devices for application in THz communication.

PROPERTIES OF ELECTROMAGNETIC FIELDS AND EFFECTIVE PERMITTIVITY EXCITED BY DRIFTING PLASMA WAVES IN SEMICONDUCTOR-INSULATOR INTERFACE STRUCTURE AND EQUIVALENT TRANSMISSION LINE TECHNIQUE FOR MULTI-LAYERED STRUCTURE

Strong interests are recently emerging for development of solid-state devices operating in the so-called \terahertz region for possible application in radio astronomy, industry and defense. To flll the THz gap by using conventional electron approach or transit time devices seems to be very di-cult due to the limitation that comes from the carrier transit time where extremely small feature sizes are required. One way to overcome this limitation is to employ the traveling wave type approach in semiconductors like classical traveling wave tubes (TWTs) where no transit time limitation is imposed. In this paper, the analysis method to analyze the properties of drifting plasma waves in semiconductor-insulator structure based on the transverse magnetic (TM) mode analysis is presented. Two waves components (quasi-lamellar wave and quasi-solenoidal wave), electromagnetic flelds (Ey, Ez and Hx) and !- and k-dependent efiective permittivity are derived where these parameters are the main parameters to explain the interaction between propagating electromagnetic waves and drifting carrier plasma waves in semiconductor. A method to determine the surface impedances in semiconductor-insulator multi-layered structure using equivalent transmission line representation method is also presented since multi- layered structure is also a promising structure for fabricating such a so-called plasma wave device.

State of the art and future of electronic sources at terahertz frequencies

A review is presented of the state of the art of electronic sources based on semiconductor devices that have either demonstrated substantial amounts of output power or have a strong potential of producing significant output powers at frequencies above 300 GHz. Both fundamental sources and harmonic power generation using varactor or varistor diodes are discussed. The key devices are Schottky diodes, heterojunction barrier varactors, heterojunction bipolar transistors, high-electron mobility transistors, resonant tunnelling diodes, tunnel-injection transit-time devices, Gunn devices, and superlattice electron devices.

Negative terahertz dynamic conductivity in electrically induced lateral p–i–n junction in graphene

We analyze a graphene tunneling transit-time device based on a heterostructure with a lateral p–i–n junction electrically induced in the graphene layer and calculate its ac characteristics. Using the developed device model, it is shown that the ballistic transit of electrons and holes generated due to interband tunneling in the i-section results in the negative ac conductance in the terahertz frequency range. The device can serve as an active element of terahertz oscillators

Comparative study on the high-bandgap material (GaN and SiC)-based impact avalanche transit time device

The dynamic characteristics of GaN and SiC-based impact avalanche transit times (IMPATTs) are reported at D-band. The device properties are compared at the same operating conditions and frequency of operations. A noise analysis model is employed to assess the performance studies. The noise of SiC-based IMPATTs is found to be higher than that of GaN-based IMPATTs. It is shown that the high-bandgap semiconductor material-based IMPATTs are potential candidates for replacing traditional IMPATTs at high frequency of operation. The computed power densities are found to be, respectively, 2.48times10 7 , 2.78times10 7 and 0.07times10 7 W/cm 2 for SiC, GaN and GaAs-based IMPATTs at the same operating conditions.

Photosensitivity Analysis of Gallium Nitride and Silicon Carbide Terahertz IMPATT Oscillators: Comparison of Theoretical Reliability and Study on Experimental Feasibility

Reliability of terahertz-frequency (~1.0 THz) characteristics of wide-bandgap (WBG) wurtzite (Wz)-GaN- and 4H-SiC-based p++nn++-type single-drift-region (SDR) impact avalanche transit time (IMPATT) devices (normal and photoilluminated) is compared through a simulation scheme. The simulation experiment reveals that an RF power density of 3.37 times 1011 W middotm-2 (efficiency of 18.2%) at around 1.126 THz may be realized from the optimized unilluminated GaN IMPATT device, whereas the unilluminated 4H-SiC IMPATT device is expected to generate an RF power density of 1.35 times 1011 W middotm-2 (efficiency of 9%) at 1.05 THz. However, the parasitic series resistance reduces the maximum exploitable power density from the terahertz devices. Under optical illumination, additional photogenerated carriers are created in the devices, and these carriers change the admittance and negative resistance properties of the terahertz IMPATT diodes. The performance modulation of the terahertz devices is simulated, and the results are compared in this paper. Under external radiation, the operating frequencies of the GaN- and SiC-based diodes are found to shift upward by 6.0 and 40.0 GHz, respectively, with degradation of maximum output-power density level and device negative resistance. The extensive simulation experiments establish that, although the photosensitivity of the 4H-SiC-based IMPATT device is better than its GaN counterpart, the overall terahertz performance of the unilluminated GaN IMPATT device is far better than the 4H-SiC-based device, particularly in terms of output power and efficiency. The simulation results and the proposed experimental methodology presented here can be used for realizing optically tuned WBG IMPATT oscillators for terahertz communication.

Analysis and design of Si terahertz transit-time diodes

This paper presents a numerical simulation of a Si MITATT diode working in the submillimeter-wave and lower terahertz frequency range. A full band Monte Carlo simulation that included the microscopic details of carrier transport and a simpler drift-diffusion based model were used to investigate the diode DC, small-signal and large-signal properties of MITATT diodes operating between 200 and 300GHz. Although the full band Monte Carlo simulation shows the effects of transient velocities and dead zones required by energy conservation during ionization, there is still a reasonable agreement between the two simulations. The results show that silicon based transit-time devices can produce significant power up to high submillimeter-wave frequencies. The paper will describe the simulation of transit-time devices at very high frequencies, compare results from a detailed physical Monte Carlo model with a numerically simpler drift–diffusion approach and describe device performance between 200 and 300GHz.

Silicon carbide TUNNETT diodes

The theoretical analysis of microwave characteristics of the n +p + νn + TUNNETT diodes made of silicon carbide is carried out. The expressions for the impedance and its active and reactive components as well as expressions for the frequency range, where the negative differential resistance takes place, and the maximal frequency of oscillations are obtained for the TUNNETT diodes made of SiC. After numerical calculations, the conclusion is made that the performance potential of TUNNETTs is better than that of the other transit time devices in very high frequency range. The use of silicon carbide for the manufacture of TUNNETTs can promise better microwave parameters over the range of frequencies 100–500 GHz.

Edge instability elimination of GaAs/ALGaAs MQW avalanche transit time oscillators by p+ substrate

AbstractThe p+ substrate plays an important role on the edge stability of p+n multiquantum well avalanche transit time devices. The p+n multiquantum well avalanche transit time devices on n+ substrate easily burn out along the device edge at low breakdown current. The same structure on p+ substrate can have the desired band diagram on device edge to eliminate edge burn‐out and CW operation is thus achieved at 100.3 GHz. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 40: 196–197, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.11326

Theory and small signal analysis for a new bipolar injection transit time device (BIPOLITT)

We have proposed and analyzed a new millimeter and submillimeter wave transit time device, the bipolar injection and transit time diode (BIPOLITT). The theory and small signal analysis for a two-terminal type rf operation is presented here. The device structure is similar to a heterojunction bipolar transistor (HBT) with an rf floating base. The relatively longer base is utilized to achieve an injection-phase delay close to 180/spl deg/. The collector depletion region is used as drift region. As an example, a BIPOLITT diode for 300-400 GHz operation is designed, and its small signal specific negative resistance is calculated to be about -3/spl times/10/sup -7/ /spl Omega//spl middot/cm/sup 2/.

New low-noise heterojunction transit-timedevice for millimetre waves

A new two-terminal millimetre wave transit-time device is presented than can be used for power generation. Theoretical calculations predict good noise properties. DC measurements are in agreement with a theoretical model of current transport across the employed GaAs/GaAlAs heterojunction. A first realisation of the structure leads to several milliwatts of power output at 60 GHz under pulsed conditions.

Temperature distribution in cylinder symmetric MM-wave devices

A general solution of the nonlinear heat equation for temperature dependent thermal conductivity has been developed in cylinder coordinates. The solution is helpful in the design and analysis of high frequency power devices and circuits. The application to diamond heatsinks leads to a design rule height of diamond/spl ap/0.5/spl times/radius of diamond. Coplanar circuits with an active device modeled as a focal plane can be analyzed. Due to the temperature dependent thermal conductivity the thermal resistance becomes dependent on the input power. Especially for small radii of the focal plane and large input powers significant deviations from the approximation "constant thermal conductivity" result. The results are used to calculate the maximum current density of monolithically integrated transit time devices. The comparison with experimental results yields a rather good agreement.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>