Computational Nanoelectronics Laboratory

Victor I. Ryzhii Professor	Irina I. Khmyrova Associate Professor	Maxim V. Ryzhii Associate Professor	Fedir Vasko Visiting Researcher

The research activity of the Computational Nano-Electronics Laboratory (former Computer Solid State Physics Laboratory) is focused on theoretical studies and computer modeling of physical processes in semiconductor micro- and nanostructures and in novel electronic and optoelectronic classical and quantum devices based on such structures. Since the laboratory establishment in 1993, its members have published more than 180 journal articles and made more than 190 presentations at international conferences. One of the members of the laboratory is a Fellow of the Institute of Electrical and Electronics Engineers (IEEE), Fellow of the American Physical Society, and Corresponding member of the Russian (national) Academy of Sciences. Two professors are the IEEE Senior Members. The research efforts of the laboratory members are now centered on theory and computer modeling of: The theory and computer modeling of graphene-based devices - the basis of future Carbon electronics. Semiconductor classical and quantum heterostructure devices operating in the terahertz range of frequencies (terahertz detectors, mixers, generators, and photomixers). Quantum-well and quantum-dot infrared photodetectors (QWIPs and QDIPs), QWIP- and QWIP-based functional devices. Original research results obtained in this academic year have been published in refereed journal papers (published in Physical Review B, Journal of Applied Physics, Applied Physics Letters, Japanese Journal of Applied Physics, and others) and presented at different international conferences. The Computational Nano-Electronics Laboratory conducts research in cooperation with leading research groups at: Tohoku University (Sendai, Japan), Rensselaer Polytechnic Institute (Troy, USA), Institute for Physics of Microstructures RAS (Nizhny Novgorod, Russia), University at Buffalo (Buffalo, USA), Moscow Institute of Physics and Technology, Ioffe Physical-Technical Institute (St. Petersburg, Russia), Gothenburg University (Gothenburg, Sweden), and others.

Refereed Journal Papers

[khmyrova-01:2008]	T. Nishimura, N. Magome, I. Khmyrova, T. Suemitsu, W. Knap, and T. Otsuji. Analysis of fringing effect on resonant plasma frequency in plasma wave devices. Jpn.J.Appl.Phys., 48:04C096 (4), 2009.
	We investigate influence of fringing effects due to nonideality of the gate-2DEG channel capacitance on fundamental frequency of plasma oscillations excited in the high-electron mobility transistor (HEMT) channel. Spatial distribution of sheet electron density in the fringed region of the 2DEG channel is calculated. In the equivalent circuit model developed gated and fringed regions of the 2DEG channel have been represented by cascaded distributed circuit. Estimate of fundamental plasma frequency and results of IsSpice simulation based on cascaded distributed circuit model show that reduction of the fundamental frequency of plasma oscillations can be attributed to the impact of fringing effects. The results obtained are in rather good agreement with experimental data.
[m-ryzhii-01:2008, v-ryzhii-01:2008]	V. Ryzhii and M. Ryzhii. Graphene bilayer fieldeffect phototransistor for terahertz and infrared detection. Phys. Rev. B, 79:245311(8), 2009.
	A graphene bilayer phototransistor (GBL-PT) is proposed and analyzed. The GBLPT under consideration has the structure of a field-effect transistor with a GBL as the channel and the back and top gates. The positive bias of the back gate results in the formation of conducting source and drain sections in the channel, while the negatively biased top gate provides the potential barrier which is controlled by the charge of the photogenerated holes. The features of the GBL-PT operation are associated with the variations of both the potential distribution and the energy gap in different sections of the channel when the gate voltages change. Using the developed GBL-PT device model, the spectral characteristics, dark current, responsivity and detectivity are calculated as functions of the applied voltages, energy of incident photons, intensity of electron and hole scattering, and geometrical parameters. It is shown that the GBL-PT spectral characteristics are voltage tuned. The GBL-PT performance as photodetector in the terahertz and infrared photodetectors can markedly exceed the performance of other photodetectors.
[m-ryzhii-01:2008, v-ryzhii-01:2008]	V. Ryzhii and M. Ryzhii. Graphene bilayer fieldeffect phototransistor for terahertz and infrared detection. Phys. Rev. B, 79:245311(8), 2009.
	A graphene bilayer phototransistor (GBL-PT) is proposed and analyzed. The GBLPT under consideration has the structure of a field-effect transistor with a GBL as the channel and the back and top gates. The positive bias of the back gate results in the formation of conducting source and drain sections in the channel, while the negatively biased top gate provides the potential barrier which is controlled by the charge of the photogenerated holes. The features of the GBL-PT operation are associated with the variations of both the potential distribution and the energy gap in different sections of the channel when the gate voltages change. Using the developed GBL-PT device model, the spectral characteristics, dark current, responsivity and detectivity are calculated as functions of the applied voltages, energy of incident photons, intensity of electron and hole scattering, and geometrical parameters. It is shown that the GBL-PT spectral characteristics are voltage tuned. The GBL-PT performance as photodetector in the terahertz and infrared photodetectors can markedly exceed the performance of other photodetectors.
[m-ryzhii-02:2008, v-ryzhii-02:2008]	V. Ryzhii, M. Ryzhii, A. Satou, T. Otsuji, and N. Kirova. Device model for graphene bilayer field-effect transistor. J. Appl. Phys., 105:104510 (9), 2009.
	We present an analytical device model for a graphene bilayer field-effect transistor (GBL-FET) with a graphene bilayer as a channel and with back and top gates. The model accounts for the dependences of the electron and hole Fermi energies as well as energy gap in different sections of the channel on the bias back-gate and top-gate voltages. Using this model, we calculate the dc and ac source-drain currents and the transconductance of GBL-FETs with both ballistic and collision dominated electron transport as functions of structural parameters, the bias back-gate and top-gate voltages, and the signal frequency. It is shown that there are two threshold voltages, Vth,1 and Vth,2, so that the dc current versus the top-gate voltage relation markedly changes depending on whether the section of the channel beneath the top gate gated section is filled with electrons, depleted, or filled with holes. The electron scattering leads to a decrease in the dc and ac currents and transconductances, whereas it weakly affects the threshold frequency. As demonstrated, the transient recharging of the gated section by holes can pronouncedly influence the ac transconductance resulting in its nonmonotonic frequency dependence with a maximum at fairly high frequencies.
[m-ryzhii-03:2008, v-ryzhii-03:2008]	V. Ryzhii, M. Ryzhii, V. Mitin, and M.S. Shur. Graphene tunneling transit-time terahertz oscillator based on electrically induced p-i-n junction. Appl. Phys. Express, 2:034503 (3), 2009.
	We propose and analyze a graphene tunneling transit time device based on a heterostructure with a lateral p-i-n junction electrically induced in the graphene layer by the applied gate voltages of different polarity. The depleted i-section of the graphene layer (between the gates) serves as both the tunneling injector and the transit region. Using the developed device model, we demonstrate 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, so that the device can serve as a terahertz oscillator.
[m-ryzhii-04:2008, v-ryzhii-04:2008]	V. Ryzhii, M. Ryzhii, N. Ryabova, V. Mitin, and T. Otsuji. Graphene nanoribbon phototransistor: proposal and analysis. Jpn. J. Appl. Phys., 48:04C144 (5), 2009.
	We consider a concept of a graphene nanoribbon phototransistor (GNR-PT) based on an array of GNRs operating as a photodetector of far-infrared (FIR) and terahertz (THz) radiation. The photodetector has the structure of a GNR field effect transistor with the back and relatively short top gates. To calculate the GNR-PT characteristics, we develop an analytical model of the device. This model generalizes the model we proposed previously by accounting for the possibility of not only the thermionic regime but also the tunneling regime of the GNR-PT operation. Using the developed model, we derive analytical formulas for the source-drain current as a function of the intensity and frequency of the incident radiation and bias voltages, and estimate the detector responsivity. The obtained formulas can be used for detector optimization by varying the dark current, photoelectric current gain, and voltage control of the spectral properties. The dependences of the absorption edge on GNR width and bias voltages can be utilized for the development of multicolor voltage tunable FIR/THz photodetectors.
[m-ryzhii-05:2008, v-ryzhii-05:2008]	M. Ryzhii, A. Satou, V. Ryzhii, and T. Otsuji. Highfrequency properties of graphene nanoribbon field-effect transistor. J. AppL. Phys., 104(11):114505 (6), 2008.
	We propose an analytical device model for a graphene nanoribbon field-effect transistor (GNR-FET). The GNR-FET under consideration is based on a heterostructure which consists of an array of nanoribbons clad between the highly conducting substrate (the back gate) and the top gate controlling the dc and ac source-drain currents. Using the model developed, we derive explicit analytical formulas for the GNR-FET transconductance as a function of the signal frequency, collision frequency of electrons, and the top gate length. The transition from the ballistic and to strongly collisional electron transport is considered.
[m-ryzhii-06:2008, v-ryzhii-06:2008]	V. Ryzhii, A. Satou, M. Ryzhii, T. Otsuji, and M.S. Shur. Mechanism of self-excitation of terahertz plasma oscillations in periodically double-gated electron channels. J. Phys.: Condensed Matter, 20:384207 (6), 2008.
	We develop a device model for a heterostructure device with an electron channel and with a periodic system of interdigitated gates. Using this model, we find the conditions of the self-excitation of plasma oscillations in portions of the channel. It is shown that the self-excitation of plasma oscillations in these devices and the terahertz emission observed in the experiments might be attributed to the electrontransit- time effect in the barrier regions.
[m-ryzhii-07:2008, v-ryzhii-07:2008]	V. Ryzhii, V. Mitin, M. Ryzhii, N. Ryabova, and T. Otsuji. Device model for graphene nanoribbon phototransistor,. Appl. Phys. Express, 1:063002 (3), 2008.
	An analytical device model for a graphene nanoribbon phototransistor (GNR-PT) is presented. GNR-PT is based on an array of graphene nanoribbons with the side source and drain contacts, which is sandwiched between the highly conducting substrate and the top gate. Using the developed model, we derive the explicit analytical relationships for the source-drain current as a function of the intensity and frequency of the incident radiation and find the detector responsivity. It is shown that GNR-PTs can be rather effective photodetectors in infrared and terahertz ranges of spectrum.
[m-ryzhii-08:2008, v-ryzhii-08:2008]	V.G. Leiman, M. Ryzhii, A. Satou, N. Ryabova, V. Ryzhii, T. Otsuji, and M.S. Shur. Analysis of resonant detection of terahertz radiation in high-electron mobility transistor with a nanostring/carbon nanotube as mechanically floating gate. J. Appl. Phys., 104:024514 (7), 2008.
	We develop a device model for a resonant detector of electromagnetic radiation with a frequency in the terahertz (THz) range modulated by a Gigahertz (GHz) signal based on a micromachined high-electron mobility transistor (HEMT) with a metallized nanostring (NS) or metallic carbon nanotube (CNT) as mechanically floating gate and analyze the detector operation. The device model describes both the NS/CNT mechanical motion and plasma effects in the HEMT two-dimensional electron channel. Using this model, we calculate the output gate ac current and the detector responsivity as functions of the carrier (in the THz range) and modulation (in the GHz range) frequencies. It is shown that the THz detector responsivity exhibits sharp and high maxima under the conditions of both mechanical and plasma resonances.
[m-ryzhii-09:2008, v-ryzhii-09:2008]	V. Ryzhii, M. Ryzhii, and T. Otsuji. Thermionic and tunneling transport mechanisms in graphene field-effect transistors. Physica Status Solidi (a), 205:1527–1533, 2008.
	We present an analytical device model for a graphene field-effect transistor (GFET) on a highly conducting substrate, playing the role of the back gate, with relatively short top gate which controls the source-drain current. The equations of the GFET device model include the Poisson equation in the weak nonlocality approximation. Using this model, we find explicit analytical formulae for the spatial distributions of the electric potential along the channel and for the voltage dependences of the thermionic and tunneling currents.
[m-ryzhii-10:2008, v-ryzhii-10:2008]	V. Ryzhii, M. Ryzhii, A. Satou, and T. Otsuji. Current-voltage characteristics of a graphene nanoribbon field-effect transistor. J. Appl. Phys., 103:094510 (8), 2008.
	We calculate the high-frequency response of a graphene nanoribbon field-effect transistor (GNR-FET) using the developed analytical model. The GNR-FET under consideration is based on a heterostructure which consists of an array of nanoribbons clad between the highly conducting substrate (the back gate) and the top gate controlling the dc and ac source-drain currents. We derive explicit analytical formulas for the GNR-FET transconductance as a function of the signal frequency, collision frequency of electrons, and the top gate length. The transition from the ballistic and to strongly collisional electron transport is considered.
[v-ryzhii-11:2008]	V.Ya. Aleshkin, A.A. Dubinov, and V. Ryzhii. Terahertz laser based on optically pumped graphene: model and feasibility of realization. JETP Lett., 89(2):63–67, 2009.
	We consider a terahertz laser based on optically pumped graphene layer and bilayer as the active media and suggested waveguide structure. Using the developed model, we calculate the spectral dependences the dynamic conductivity of the optically pumped graphene layer and bilayer associated with the interband and intraband transitions, estimate the pumping optical power required for lasing, and demonstrate the feasibility of realization of such a laser.
[v-ryzhii-12:2008]	A.A. Ivanov, V.I. Ryzhii, E.A. Vostrikova, V.M. Chechetkin, and A.M. Oparin. Model of terahertz radiation source on the base of field-effect transistor. Radioengineering and Electronics, 53:1, 2008.
	/in Russian/
[v-ryzhii-013:2008]	O.G. Balev, .F.T. Vasko, and V. Ryzhii. Carrier heating in intrinsic graphene by a strong dc electric field. Phys. Rev. B, 79:165432 (8), 2009.
	We consider the heating of carriers in an intrinsic graphene in a strong dc electric field. The intraband energy relaxation due to acoustic phonon scattering and the interband generation-recombination transitions due to thermal radiation are taken into account. The distributions of nonequilibrium carriers are obtained for the cases when the carrier-carrier scattering is unessential and when the intercarrier Coulomb scattering effectively establishes the quasiequilibrium distribution with the temperature and the density of carriers determined by the balance equations. Due to an interplay between weak energy relaxation and generation-recombination processes, the nonlinear response is characterized by a very low threshold electric field. The nonlinear current-voltage characteristics as well as the field-dependent carrier concentration are calculated for the case of the momentum relaxation associated with the elastic scattering. The obtained current-voltage characteristics exhibit a low threshold of nonlinearity and an appearance of the second ohmic region, for strong fields.
[v-ryzhii-14:2008]	A. Satou, F.T. Vasko, and V. Ryzhii. Nonequilibrium carriers in intrinsic graphene under interband photoexcitation. Phys. Rev. B, 78:115431 (6), 2008.
	We study nonequilibrium carriers (electrons and holes) in intrinsic graphene at low temperatures under far-infrared (IR) and midinfrared radiation in a wide range of its intensities. The energy distributions of carriers are calculated using a quasiclassical kinetic equation which accounts for the energy relaxation due to acoustic phonons and the radiative generation-recombination processes associated with thermal radiation and the carrier photoexcitation by incident radiation. It is found that the nonequilibrium distributions are determined by an interplay between weak energy relaxation on acoustic phonons and generation-recombination processes as well as by the effect of pumping saturation. Due to the effect of saturation, the carrier distribution functions can exhibit plateaus around the pumping region at elevated intensities. As shown, at sufficiently strong mid-IR pumping, the population inversion can occur below the pumping energy. The graphene dc conductivity as a function of the pumping intensity exhibits a pronounced nonlinearity with a sublinear region at fairly low intensities and a saturation at a strong pumping. However, an increase in the pumping intensity in very wide range leads only to a modest increase in the carrier concentration and, particularly, the dc conductivity. The graphene conductivity at mid-IR irradiation exhibit strong sensitivity to mechanisms of carrier momentum relaxation.
[v-ryzhii-15:2008]	V. Vyurkov and V. Ryzhii. Effect of Coulomb scattering on graphene conductivity. JEPT Lett., 88(5):322–325, 2008.
	The effect of the Coulomb scattering on graphene conductivity in field-effect transistor structures is discussed. Interparticle scattering (electron-electron, hole-hole, and electron-hole) and scattering on charged defects are taken into account in a wide range of gate voltages. It is shown that an intrinsic conductivity of graphene (purely ambipolar system, where both electron and hole densities exactly coincide) is defined by a strong electron-hole scattering. It has a universal value independent of the temperature. We give an explicit derivation based on the scaling theory. When there is even a small discrepancy in the electron and hole densities caused by the applied gate voltage, the conductivity is determined by both a strong electron-hole scattering and a weak external scattering: on the defects or phonons. We suggest that the density of the charged defects (occupancy of defects) depends on the Fermi energy to explain the sublinear dependence of conductivity on a fairly high gate voltage observed in the experiments. We also eliminate the contradictions between the experimental data obtained in the deposited and suspended graphene structures regarding the graphene conductivity.
[v-ryzhii-16:2008]	F. Vasko and V. Ryzhii. Photoconductivity of an intrinsic graphene. Phys. Rev. B, 77:195433 (8), 2008.
	We examine the photoconductivity of an intrinsic graphene associated with farand midinfrared irradiation at low temperatures. The model under consideration accounts for the excitation of the electron-hole pairs by incident radiation, the interband generation-recombination transitions due to thermal radiation, and the intraband energy relaxation due to acoustic phonon scattering. The momentum relax- ation is assumed to be caused by elastic scattering. The pertinent collision integrals are adapted for the case of the massless energy spectrum of carriers that interact with the longitudinal acoustic mode and the thermal radiation. It is found that the photoconductivity is determined by interplay between weak energy relaxation and generation-recombination processes. Due to this, the threshold of nonlinear response is fairly low.

Unrefereed Papers

[khmyrova-02:2008]

N. Magome, T. Nishimura, I. Khmyrova, T. Suemitsu, W. Knap, and T.Otsuji. Effect of parasitic gate fringing capacitance on the terahertz plasma resonance in HEMT’s. IEICE Techn. Report, 108(369):ED2008–195, pp.53–58, 2008.

Influence of fringing electric field due to nonideality of the gate-two-dimensional electron gas (2DEG) channel capacitance on fundamental resonant frequency of plasma waves in HEMT channel was investigated. Cascaded transmission line (TL) equivalent circuit model was developed to represent the gated and ungatedfringed regions of the 2DEG channel. Results of calculation and IsSpice simulation based on cascaded TL model show that plasma frequency reduction can be caused by the fringing effects.

Refereed Proceeding Papers

[khmyrova-03:2008]	T. Nishimura, N. Magome, I. Khmyrova, T. Suemitsu, W. Knap, and T. Otsuji. Effect of nonideality of the gate-2DEG channel capacitance on the frequency of plasma oscillations in the plasma wave devices. In Ext. Abstr. 2008 Int. Conf. on Solid State Devices and Materials (SSDM08), Tsukuba, Japan, Sept. 2008.
	In this paper we focus on the contribution of the nonideality of the gate–2DEG channel capacitance and demonstrate that its neglection results in the overestimate of the fundamental frequency of plasma oscillations excited in the 2DEG channel.
[khmyrova-04:2008]	I. Khmyrova, T. Nishimura, N. Magome, T. Suemitsu, and T. Otsuji. Frequency performance of plasma wave devices for THz applications and the role of fringing effects. In Proc. 2008 IEEE 25th Convention of Electrical and Electronics Engineers in Israel (IEEEI 2008, Eilat, Israel, Dec. 2008.
	Paper is focused on the model in which reduction of resonant frequency of plasma oscillations in the HEMT channel experimentally observed is associated with the gate contact fringing effects. Sheet electron density distribution in the fringed ungated channel region and expression for the resonant plasma frequency are obtained. Cascaded transmission line equivalent circuit model accounting for both gated and fringed ungated channel regions has been developed and used for simulation of HEMT frequency performance with IsSpice software.
[khmyrova-05:2008]	I. Khmyrova. Model and simulation of resonant MEMS based on high- electron-mobility transistor. In 3rd Int. Conf. Smart materials, structures and systems, Acireale, Sicily, Italy, Jun. 2008.
	Model of resonant MEMS based on high-electron mobility transistor (HEMT) with suspended cantilever serving as a gate is developed and used for characterisation implementing SPICE simulation. Resonant MEMS under consideration can be used as a sensor for detection of small forces, masses. Developed model accounts for transduction of mechanical oscillations of the cantilever driven by bias voltage into variation of electric field and electron density in the HEMT channel directly beneath the cantilever which, in turn, results in the variation of source-drain current flowing along the channel. Simulated output characteristics reveal peak at a frequency corresponding to that of cantilever mechanical resonance.
[m-ryzhii-11:2008, v-ryzhii-17:2008]	V. Ryzhii, M. Ryzhii, A. Satou, N. Ryabova, V. Mitin, and T. Otsuji. Some concepts of graphene-based electron and optoelectron devices. In 13th Advanced Heterostructures and Nanostructures Workshop, page 7, Hawaii, USA, Dec. 2008.
	We overview and analyze several proposals and concepts of graphene-based electronic and optoelectronic devices.
[m-ryzhii-12:2008, v-ryzhii-18:2008]	V. Ryzhii, M. Ryzhii, V. Mitin, and M.S. Shur. Negative dynamic conductivity in graphene tunneling transit-time device. In 14th Int. Conf. on Narrow Gap Semiconductors and Systems, (accepted for presentation), Sendai, Japan, July 2009.
	In this paper, we analyze the proposed graphene tunneling transit-time (GTUNNETT) device. This device is based on a graphene layer with the source and drain contacts and two gates. The bias voltages of different polarity applied to the gates result in the formation of a lateral p-i-n junction. The depleted i-section serves as the interband tunneling injector of electron-hole pairs as well as the transit region. As demonstrated using the developed device model, owing to a strong sensitivity of the tunneling rate to the electric field and relatively short electron and hole transit times across the i-section, the real part of the G-TUNNETT source-drain dynamic conductivity can be negative in the terahertz range of frequencies.
[m-ryzhii-13:2008, v-ryzhii-19:2008]	V. Mitin, M. Ryzhii, T. Otsuji, N. Kirova, and V. Ryzhii. Device models for graphene nanoribbon and graphene bilayer phototransistors. In Int. Symp. ’Graphene: Technology, Physics and Modeling’, pages 46–47, Aizu-Wakamatsu, Japan, Nov. 2008.
	We propose device model for a graphene nanoribbon (GNR) phototransistor with the structure similar to that of a GNR field-effect transistor and evaluate its characteristics as a FIR/THz photodetector using this model. Using the developed model, we calculate the dependences of the detector responsivity as a function of the structural parameters.
[m-ryzhii-14:2008, v-ryzhii-20:2008]	V. Ryzhii, M. Ryzhii, N. Ryabova, V. Mitin, and T. Otsuji. Graphene nanoribbon phototransistor: proposal and analysis. In Int. Conf. on Solid State Device and Materials (SSDM2008), page 190, Tsukuba, Japan, Sep. 2008.
	We consider a concept of a graphene nanoribbon phototransistor (GNR-PT) based on an array of GNRs operating as a photodetector of far-infrared (FIR) and terahertz (THz) radiation. The photodetector has the structure of a GNR field effect transistor with the back and relatively short top gates. To calculate the GNR-PT characteristics, we develop an analytical model of the device. This model generalizes the model we proposed previously by accounting for the possibility of not only the thermionic regime but also the tunneling regime of the GNR-PT operation. Using the developed model, we derive analytical formulas for the source-drain current as a function of the intensity and frequency of the incident radiation and bias voltages, and estimate the detector responsivity. The obtained formulas can be used for detector optimization by varying the dark current, photoelectric current gain, and voltage control of the spectral properties. The dependences of the absorption edge on GNR width and bias voltages can be utilized for the development of multicolor voltage tunable FIR/THz photodetectors.
[m-ryzhii-15:2008,v-ryzhii-21:2008]	M. Ryzhii, A. Satou, T. Otsuji, and V. Ryzhii. Dynamic characteristics of graphene nanoribbon field-effect transistors. In Int. Conf. on Solid State Device and Materials (SSDM2008), page 598, Tsukuba, Japan, Sep. 2008.
	We calculate and analyze the dynamic characteristics of a graphene nanoribbon field-effect transistor (GNR-FET), which consists of an array of nanoribbons clad between the highly conducting substrate (the back gate) and the top gate controlling the dc and ac source-drain currents, using the proposed analytical device model. The model is based on the semiclassic Boltzman kinetic equation with the collision term adopted for the specifics of the one-dimensional electron transport along GNRs. Using the model, we derive explicit analytical formulas for the GNRFET transconductance as a function of the signal frequency, collision frequency of electrons, and the top gate length for different electron transport conditions (from the ballistic and to strongly collisional electron transport).
[m-ryzhii-16:2008,v-ryzhii-22:2008]	V. Ryzhii, M. Ryzhii, N. Ryabova, V. Mitin, and T. Otsuji. Far infrared and terahertz devices based on graphene heterostructures. In Int. Symp. on Nanostructures: Physics and Technology, pages 1–4, Vladivostok, Russia, July 2008.
	We demonstrate that the effect of the negative dynamic conductivity can occur in optically pumped gated graphene heterostructures and the heterostructures with patterned graphene layer (array of graphene nanoribbons). We also consider principle of operation of graphene nanoribbon phototransistor and calculate its responsivity as a function of energy of incident photons.
[m-ryzhii-17:2008]	E. Ryzhii, M. Ryzhii, and D. Wei. Computer simulation study of gender differences in cardiac repolarization: normal case and the Brugada syndrome. In 13th Int. Congress of the Int. Society for Holter and Noninvasive Electrocardiology (ISHNE 2009), (accepted for presentation), Yokohama, Japan, June 2009.
	We included gender differences in the ten Tusscher human ventricular cell model adjusting densities of the L-type Ca current (ICal), transient outward K (Ito), and rapid delayed rectifier K current (IKr) according to experimental data. The simulated action potentials for male and female cases were inputted into a state-ofthe- art 3D whole-heart model and corresponding ECG waveforms were calculated.
[m-ryzhii-18:2008, v-ryzhii-23:2008]	V. Vyurkov, I. Semenikhin, M. Ryzhii, T. Otsuji, and V. Ryzhii. Hydrodynamic model of graphene field-effect transistor. In Int. Symp. ’Graphene: Technology, Physics and Modeling’, pages 32–33, Aizu-Wakamatsu, Japan, Nov. 2008.
	We have developed a hydrodynamic model of a graphene field-effect transistor (GFET) using equations derived via a regular procedure from kinetic equations and calculated the transistor’s current-voltage characteristics.
[m-ryzhii-19:2008, v-ryzhii-24:2008]	V. Ryzhii, M. Ryzhii, A. Satou, and T. Otsuji. Graphene nanoribbon field-effect transistor: device model and characteristics. In 2nd Nanotechnology Materials and Devices Conf., page 62, Kyoto, Japan, Oct. 2008. IEEE.
	In this communication, we report on the device model of a graphene nanoribbons field-effect transistor (GNR-FET). The model accounts for the self-consistent electric potential and the electron transport along GNRs. It is assumed that the scattering of electrons in the essential portion of the channel is associated with their interaction with the GNR imperfections and acoustic phonons. The model provides an opportunity to follow the electron transport phenomena from ballistic to strongly collisional regimes. Using the developed model, we calculated the GNRFET dc current as a function of the back and top gate voltages, the source-drain voltage, as well as the device structural parameters, in particular, the top gate length.
[m-ryzhii-20:2008, v-ryzhii-25:2008]	M. Ryzhii, V. Ryzhii, T. Otsuji, and V. Mitin. Graphene nanoribbon and graphene bilayer photodetectors: models and characteristics. In 14th Int. Conf. on Narrow Gap Semiconductors and Systems, (accepted for presentation), Sendai, Japan, July 2009.
	Using the proposed device models for graphene nanoribbon photodetectors (GNRPD) and graphene bilayer photodetectors (GBL-PD), we calculate the dark current, responsivity, and dark current limited detectivity. The similarity and distinctions of the characteristics of GNR-PDs and GBL-PDs are discussed.
[m-ryzhii-21:2008]	E. Ryzhii, M. Ryzhii, and D. Wei. Extension of whole-heart model by coupling with human ventricular cell model. In Progress in Electromagnetics Research Symp. (PIERS 2009), (accepted for presentation), Moscow, Russia, Aug. 2009.
	In this work, we extend the 3D Wei-Harumi whole-heart model by coupling with ion channel human ventricular cell model developed by ten Tusscher et al. We adopted the model of the action potential of human ventricular cells to simulate sex differences in accordance to experimental data for male and female hearts of different species. Using the extended model the surface 12-lead electro-cardiogram (ECG) waveforms were simulated.
[m-ryzhii-22:2008, v-ryzhii-26:2008]	V. Ryzhii, M. Ryzhii, M.S. Shur, and V. Mitin. Negative terahertz dynamic conductivity in electrically induced lateral p-i-n junction in graphene. In 18th Int. Conf. Electronic Properties of Two- Dimensional Systems (EP2DS), (accepted for presentation), Kobe, Japan, July 2009.
	We propose a device based on a gated graphene structure with electrically induced lateral p-i-n junction graphene tunneling transit-time diode (GT-diode) and study its high-frequency characteristics.
[m-ryzhii-23:2008, v-ryzhii-27:2008]	V. Ryzhii, M. Ryzhii, M.S. Shur, and V. Mitin. Graphene tunneling transit-time diode for terahertz generation: proposal, model, and analysis. In Int. Meeting for Future of Electron Devices Kansai (IMFEDK 2009), (accepted for presentation), Osaka, Japan, May 2009.
	Using developed model, we calculated the graphene tunneling transit-time diode (G-TUNNETT) ac conductance (admittance) as a function of the signal frequency and applied bias voltages. It is demonstrated that the ballistic transit of electrons and holes generated owing to interband tunneling leads to the negative ac conductivity in the terahertz frequency range.
[m-ryzhii-24:2008, v-ryzhii-28:2008]	M. Ryzhii, A. Satou, V. Ryzhii, V. Mitin, F.T. Vasko, V.Ya. Aleshkin, A.A. Dubinov, and T. Otsuji. Graphene-based terahertz lasers and detectors: Physics and feasibility of realization. In ESF-FWF Int. Conf. ’Graphene Week 2009’, Obergurgl, Austria, March 2009. European Physical Society.
	We consider and analyze terahertz lasers based on graphene and graphene bilayer heterostructures with optical pumping. We show that the optical pumping followed by emission of a cascade of optical phonons can result in the population inversion between the low energy states. The latter implies the negative contribution of the interband transitions to the graphene (or graphene bilayer) dynamic conductivity in the THz frequency range. Analytical estimates and numerical simulations demonstrate that despite the intraband (Drude) absorption and the loses in the device waveguide structure, this can result in the negative net dynamic conductivity and, hence, provide an opportunity of creating graphene-based THz /FIR lasers. We also present concepts of THz/FIR detectors with the structure of a field-effect transistor (THz/FIR phototransistors) made of graphene nanoribbon arrays as well as of a graphene bilayer and analyze their characteristics (spectral characteristics, responsivity, and detectivity) as functions of the structural parameters and applied voltages.
[m-ryzhii-25:2008, v-ryzhii-29:2008]	A. Satou, M. Ryzhii, F.T. Vasko, T. Otsuji, and V. Ryzhii. Graphene under optical pumping: nonequilibrium distributions, population inversion, and terahertz lasing. In Proceedings of SPIE Photonics West (PW09), page 721119, San Jose, USA, Jan. 2009. SPIE.
	We study nonequilibrium carriers (electrons and holes) in an intrinsic graphene at low temperatures under farand mid-infrared optical pumping in a wide range of its power densities. The energy distributions of carriers are calculated using a quasiclassic kinetic equation which accounts for the energy relaxation due to acoustic phonons and the radiative generation-recombination processes associated with thermal radiation and the carrier photoexcitation by incident radiation. It is found that the nonequilibrium distributions are determined by interplay between weak energy relaxation on acoustic phonons and generation-recombination processes as well as by the effect of pumping saturation. Due to the effect of pumping saturation, the carrier distribution functions can exhibit plateaus whose width increases with increasing pumping power density. The graphene steady-state conductivity as a function of the pumping power density exhibits a pronounced nonlinearity with a sub-linear region at fairly low power densities. As shown, at certain pumping power density the population inversion as well as the dynamic negative conductivity can take place in terahertz and far-infrared frequencies, suggesting the possibility of utilization of graphene under optical pumping for optoelectronic applications, in particular, lasing at such frequencies.
[v-ryzhii-30:2008]	V. Ryzhii. Graphene-based infrared and terahertz devices: concepts and characteristics,. In 2009 Advanced Research Workshop ’Future Trends in Microelectronics: Unmapped Roads’, page (invited), Sardinia, Italy, June 2009.
	In this paper, the features of the physical processes responsible for the device operation (specifics of the energy spectra of electrons and holes, photogeneration and tunneling injection, energy relaxation and recombination, plasma properties), the device characteristics (threshold of lasing, spectral dependences, detector responsivity and detectivity, and voltage tuning) limiting performance, and the feasibility of their realization are considered and discussed.
[v-ryzhii-31:2008]	V.Ya. Aleshkin, A.A. Dubinov, and V. Ryzhii. Graphene terahertz laser. In 17th Int. Symp. ’Nanostructures: Physics and Modeling’, (accepted for presentation), Minsk, Belarus, June 2009.
	We consider a terahertz laser based on optically pumped graphene layer and bilayer as the active media and suggested waveguide structure. Using the developed model, we calculate the spectral dependences the dynamic conductivity of the optically pumped graphene layer and bilayer associated with the interband and intraband transitions, estimate the pumping optical power required for lasing, and demonstrate the feasibility of realization of such a laser.
[v-ryzhii-32:2008]	H. Karasawa, T. Komori, T. Watanabe, M. Suemitsu, V. Ryzhii, and T. Otsuji. Observation of carrier relaxation and recombination dynamics in optically pumped epitaxial graphene heterostructures using terahertz emission spectroscopy. In Photonics Europe 2009, (accepted for presentation), Brussels, Belgium, May 2009.
[v-ryzhii-33:2008]	A. Satou, F.T. Vasko, T. Otsuji, and V. Ryzhii. Population inversion and negative dynamic conductivity in optically pumped graphene. In 2009 Int. Conf. on Simulation of Semiconductor Processes and Devices, page (accepted for presentation), San Diego, USA, Sep. 2009.
[v-ryzhii-34:2008]	T. Otsuji, Y. Tsuda, H. Karasawa, T. Suemitsu, M. Suemitsu, E. Sano, and V. Ryzhii. Emission of terahertz radiation from two-dimensional electron systems in semiconductor nano-heterostructures. In 17th Int. Symp. ’Nanostructures: Physics and Modeling’, page (invited), Minsk, Belarus, June 2009.
	This paper reviews recent advances in emission of terahertz radiation from twodimensional (2D) electron systems in semiconductor nano-heterostructures. 2D plasmon resonance is first presented to demonstrate intense broadband terahertz emission from InGaP/InGaAs/GaAs material systems. The device structure is based on a high-electron mobility transistor and incorporates authors 弛riginal doubly interdigitated grating gates. Second topic focuses on graphene, a monolayer carbon-atomic honeycomb lattice crystal, having attracted attention due to its peculiar carrier transport and optical properties holding massless and gapless energy spectrum. Stimulated terahertz emission from femtosecond laser pumped epitaxial graphene is experimentally observed, reflecting the occurrence of population inversion.

Books

[m-ryzhii-26:2008,v-ryzhii-35:2008]

M. Ryzhii and V. Ryzhii. Physics and modeling of tera- and nano-devices. Number 47 in Selected Topics In Electronics And Systems. World Scientific Publishing Co Pte Ltd., Singapore, 2008.

Grants

[v-ryzhii-36:2008]	V. Ryzhii. Grant-in-Aid (S) from the Japan Society for Promotion of Science (JSPS): Terahertz plasma wave nanoelectronic devices, 2006-2010.
[v-ryzhii-37:2008]	V. Ryzhii. Grant from the Japan Science and Technology Agency, CREST: Development of Graphene on Silicon (GOS) device modeling technology, 2007-2012.
[v-ryzhii-38:2008]	V. Ryzhii. Nation-wide Research Grant from RIEC, 2006-2008.

Academic Activities

[khmyrova-06:2008]	Irina Khmyrova, 2008. Senior Member, IEEE
[khmyrova-07:2008]	M. Ryzhii. Reviewer for International Journal of Bioelectromagnetism
[m-ryzhii-27:2008	M. Ryzhii. Reviewer for International Journal of Bioelectromagnetism
[m-ryzhii-28:2008]	M. Ryzhii. Reviewer for Semiconductor Science and Technology Journal
[m-ryzhii-29:2008]	M. Ryzhii, July 1995-. Member (lifelong), American Physical Society
[m-ryzhii-30:2008]	M. Ryzhii, Nov. 2008. Organizing Committee member of International Symposium on Graphene Devices: Technology, Physics, and Modeling, University of Aizu, Nov. 17-19, 2008.
[m-ryzhii-31:2008]	M. Ryzhii, Dec. 2002-. Senior Member, IEEE
[m-ryzhii-32:2008]	M. Ryzhii. Reviewer for The Journal of Physics D: Applied Physics
[v-ryzhii-39:2008]	V. Ryzhii. Fellow (1994-present), IEEE
[v-ryzhii-40:2008]	V. Ryzhii. Corresponding Member (1987, lifelong), Russian Academy of Sciences (Branch of Nanotechnology and Information Technologies)
[v-ryzhii-41:2008]	V. Ryzhii. Reviewer for Journal of Applied Physics
[v-ryzhii-42:2008]	V. Ryzhii. Reviewer for Journal of Physics Condensed Matter
[v-ryzhii-43:2008]	V. Ryzhii. Reviewer for Physical Review B Journal
[v-ryzhii-44:2008]	V. Ryzhii. Fellow (1995, lifelong), American Physical Society
[v-ryzhii-45:2008]	V. Ryzhii Reviewer for Applied Physics Letters Journal
[v-ryzhii-46:2008]	V. Ryzhii Reviewer for Physical Review Letters Journal
[v-ryzhii-47:2008]	V. Ryzhii Reviewer for Semiconductor Science and Technology Journal
[v-ryzhii-48:2008]	V. Ryzhii. Reviewer for Nanotechnology Journal
[v-ryzhii-49:2008]	V. Ryzhii. Reviewer for Solid-State Electronics Journal
[v-ryzhii-50:2008]	V. Ryzhii, Jan. 2009. Member of the Editorial Board of Journal of Applied Physics
[v-ryzhii-51:2008]	V. Ryzhii, Jan. 2009. Member of the Editorial Board of Applied Physics Letters
[v-ryzhii-52:2008]	V. Ryzhii, 2006–2009. Member of the Int. Advisory Committee and the Editorial Board of Optoelectronics Review Int. Journal
[v-ryzhii-53:2008]	V. Ryzhii, Nov. 2008. Organizer of International Symposium on Graphene Devices: Technology, Physics, and Modeling, University of Aizu, Nov. 17-19, 2008.
[v-ryzhii-54:2008]	V. Ryzhii, June 2008. Program Committee member of SPIE International Conference ’Defense, Security, Sensing’, Orlando, USA, Apr. 13-17, 2009.

Ph.D, Master and Bachelor Theses

[khmyrova-08:2008]	Yusuke Fukuda. Graduation Thesis: Models and IsSPICE simulation of HEMT-based plasma wave devices in the presence of fringing effects, University of Aizu, 2008.
	Thesis Advisor: I. Khmyrova
[m-ryzhii-33:2008]	Koji Yamada. Graduation Thesis: Developing mobile phone software for physics education, University of Aizu, 2009.
	Thesis Advisor: Ryzhii, M.
[m-ryzhii-34:2008]	Atsushi Suzuki. Graduation Thesis: Monte Carlo Simulation of Metal-Semiconductor-Metal Photodetectors, University of Aizu, 2009.
	Thesis Advisor: Ryzhii, M.
[m-ryzhii-35:2008]	Yuu Fukushima. Graduation Thesis: Development of educational Venturi effect simulator for mobile phones, University of Aizu, 2009.
	Thesis Advisor: Ryzhii, M.

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