Browsing by Author "Tang, CS"

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A study of the threshold voltage variations for ultrathin body double gate SOI MOSFETs
(2004)
Tang, CS
;
Lo, SC
;
Lee, JW
;
Tsai, JH
;
Li, YM
Silicon on insulator (SOI) devices have been of great interest in these years. In this paper, simulation with density-gradient transport model is performed to examine the variation of threshold voltage (VTH) for double gate SOI MOSFETs. Different thickness of silicon (Si) film, oxide thickness, channel length and doping concentration are considered in this work. According to the numerical study, both drift-difftision (DD) and density gradient (DG) models demonstrate that the thickness of Si film greatly affects the threshold voltage (5 similar to 15% variation). It is found that the thickness of Si film decreases, VTH variation increases; and the dependence relation is nonlinear. Therefore, this effect must be taken into account for the realization of double gate SOI ULSI circuit.
Coherent quantum transport in narrow constrictions in the presence of a finite-range longitudinally polarized time-dependent field
(1999-07-15)
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
We have studied the quantum transport in a narrow constriction acted upon by a finite-range longitudinally polarized time-dependent electric field. The electric field induces coherent inelastic scatterings that involve both intrasubband and intersideband transitions. Subsequently, the dc conductance G is found to exhibit suppressed features. These features are recognized as the quasi-bound-state (QBS) features that are associated with electrons making transitions to the vicinity of a subband bottom, of which the density of states is singular. Having valleylike instead of diplike structures, these QBS features are different from the G characteristics for constrictions acted upon by a finite-range time-modulated potential. In addition, the subband bottoms in the time-dependent electric-field region are shifted upward by an energy proportional to the square of the electric field and inversely proportional to the square of the frequency. This effective potential barrier is originated from the square of the vector potential, and it leads to the interesting field-sensitive QBS features. An experimental setup is proposed for the observation of these features. [S0163-1829(99)01827-5].
Coherent quantum transport in the presence of a finite-range transversely polarized time-dependent field
(2000-10-01)
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
This work investigates the quantum transport in a narrow constriction acted upon by a finite-range transversely polarized time-dependent field. A generalized scattering-matrix method is developed that has incorporated a time-dependent mode-matching scheme. The transverse field induces coherent inelastic scatterings that include both intersubband and intersideband transitions. These scatterings give rise to the DC conductance G a general suppressed feature that escalates with the chemical potential. In addition, particular suppressed features - the dip structures - are found in G. These features are recognized as the quasibound state (QBS) features that arise from electrons making intersubband transitions to the vicinity of a subband bottom. For the case of larger held intensities, the QBS features that involve more photons are more evident. These QBS features are closely associated with the singular density of states at the subband threshold. An experimental setup is proposed for the observation of these features. (C) 2000 Elsevier Science B.V. All rights reserved.
Connecting wave functions at a three-leg junction of one-dimensional channels
(2006-01-01)
Voo, KK
;
Chen, SC
;
Tang, CS
;
Chu, CS
;
電子物理學系
We propose a scheme to connect the wave functions on different one-dimensional branches of a three-leg junction (Y junction). Our scheme differs from that due to Griffith [Trans. Faraday Soc. 49, 345 (1953)] in the respect that ours can model the difference in the widths of the quasi-one-dimensional channels in different systems. We test our scheme by comparing results from a doubly connected one-dimensional system and a related quasi-one-dimensional system, and we find a good agreement. Therefore our scheme may be useful in the construction of one-dimensional effective models out of (multiply connected) quasi-one-dimensional systems.
dc spin current generation in a Rashba-type quantum channel
(2006-02-01)
Wang, LY
;
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
We propose and demonstrate theoretically that resonant inelastic scattering (RIS) can play an important role in dc spin current generation. The RIS makes it possible to generate dc spin current via a simple gate configuration: a single finger gate that locates atop and orients transversely to a quantum channel in the presence of Rashba spin-orbit interaction. The ac-biased finger gate gives rise to a time variation in the Rashba coupling parameter, which causes spin-resolved RIS and, subsequently, contributes to the dc spin current. The spin current depends on both the static and the dynamic parts in the Rashba coupling parameter, alpha(0) and alpha(1), respectively, and is proportional to alpha(0)alpha(2)(1). The proposed gate configuration has the added advantage that no dc charge current is generated. Our study also shows that the spin current generation can be enhanced significantly in a double finger-gate configuration.
Differential conductance of a saddle-point constriction with a time-modulated gate-voltage
(1998-11-01)
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
This work investigates how a time-modulated gate-voltage influences the differential conductance G of a saddle-point constriction. The constriction is modeled by a symmetric saddle-point potential and the time-modulated gate-voltage is represented by a potential of the form V(0)Theta(a/2 - x - x(c)) cos(omega t). For h omega less than half of the transverse subband energy level spacing, gate-voltage-assisted (suppressed) feature occurs when the chemical potential mu is less (greater) than but close to the threshold energy of a subband. Our results indicate that as mu increases, G exhibits, alternatively, the assisted and the suppressed feature. For a larger h omega, these two features may overlap. In addition, dip structures are found in the suppressed regime, and mini-steps are found in the assisted regime only when the gate-voltage covers a region sufficiently distant from the center of the constriction. (C) 1998 Elsevier Science B.V. All rights reserved.
Differential conductance of a saddle-point constriction with a time-modulated gate-voltage
(1998-11-01)
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
This work investigates how a time-modulated gate-voltage influences the differential conductance G of a saddle-point constriction. The constriction is modeled by a symmetric saddle-point potential and the time-modulated gate-voltage is represented by a potential of the form V(0)Theta(a/2 - \x - x(c)\) cos(omega t). For h omega less than half of the transverse subband energy level spacing, gate-voltage-assisted (suppressed) feature occurs when the chemical potential mu is less (greater) than but close to the threshold energy of a subband. Our results indicate that as mu increases, G exhibits, alternatively, the assisted and the suppressed feature. For a larger h omega, these two features may overlap. In addition, dip structures are found in the suppressed regime, and mini-steps are found in the assisted regime only when the gate-voltage covers a region sufficiently distant from the center of the constriction. (C) 1998 Elsevier Science B.V. All rights reserved.
Effects of a time-dependent transverse electric field on the quantum transport in narrow channels
(1996-01-01)
Chu, CS
;
Tang, CS
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
The quantum transport in a narrow channel (NC) is studied in the presence of a time-dependent delta-profile electric field. Thr electric field is taken to be tranversely polarized, with frequency omega, causing inter-subband and inter-sideband transitions. Suppression in the de conductance G is found, which escalates with the chemical potential. There are structures in G which are related to the quasi-bound state (QBS) features. Major dip, and dip-and-peak, structures occur when an incident electron makes transition to a subband edge by absorbing or emitting one, and two, h omega, respectively. Structures associated with 3h omega processes are recognized. The QBS are closely associated with the singular density of states (DOS) at subband bottoms. Our results demonstrate that, due to this singular features of the DOS, the interaction of the electron with the electric field has to be treated beyond finite order perturbation.
Effects of a time-dependent transverse electric field on the quantum transport in narrow channels
(1996-01-01)
Chu, CS
;
Tang, CS
;
電子物理學系
;
Department of Electrophysics
The quantum transport in a narrow channel (NC) is studied in the presence of a time-dependent delta-profile electric field. Thr electric field is taken to be tranversely polarized, with frequency omega, causing inter-subband and inter-sideband transitions. Suppression in the de conductance G is found, which escalates with the chemical potential. There are structures in G which are related to the quasi-bound state (QBS) features. Major dip, and dip-and-peak, structures occur when an incident electron makes transition to a subband edge by absorbing or emitting one, and two, h omega, respectively. Structures associated with 3h omega processes are recognized. The QBS are closely associated with the singular density of states (DOS) at subband bottoms. Our results demonstrate that, due to this singular features of the DOS, the interaction of the electron with the electric field has to be treated beyond finite order perturbation.
Effects of a time-dependent transverse electric field on the quantum transport in narrow channels
(1996-01-01)
Chu, CS
;
Tang, CS
;
交大名義發表
;
電子物理學系
;
National Chiao Tung University
The quantum transport in a narrow channel (NC) is studied in the presence of a time-dependent delta-profile electric field. The electric field is taken to be transversely polarized, with frequency omega, causing inter-subband and inter-sideband transitions Suppression in the de conductance G is found, which escalates with the chemical potential. There are structures in G which are related to the quasi-bound state (QBS) features. Major dip, and dip-and-peak, structures occur when an incident electron makes transition to a subband edge by absorbing or emitting one, and two, h omega, respectively. Structures associated with 3h omega processes are recognized. The QBS are closely associated with the singular density of states (DOS) at subband bottoms. Our results demonstrate that, due to this singular features of the DOS, the interaction of the electron with the electric field has to be treated beyond finite order perturbation.
Effects of an elastic scatterer on the DC spin current generation in a Rashba-type quantum channel
(2006-05-01)
Wang, LY
;
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
In this work, we consider a Rashba-type quantum channel (RQC) consisting of one AC-biased finger-gates (FG) that orient perpendicularly and located above the RQC. Such an AC-biased FG gives rise to a local time-modulation in the Rashba coupling parameter, and is shown recently to generate a DC spin current [L.Y. Wang, C.S. Tang, C.S. Chu, Cond-mat/0409291, 2004]. No charge current, however, is generated in this configuration. We explore the robustness of such DC spin current generation against elastic scattering in the RQC. The effect of backscattering is studied by introducing a static barrier that is uniform in the transverse dimension. The effects of both backscattering and subband mixing is studied by introducing a static partial-barrier that is spatially localized and non-uniform in the transverse dimension. In addition, we compare the cases of attractive and repulsive partial-barriers. It is found that attractive partial-barrier gives rise to additional DC spin current structures due to resonant inter-subband and inter-sideband transition to quasi-bound states formed just beneath subband thresholds. (c) 2006 Elsevier B.V. All rights reserved.
Energy levels of a parabolically confined quantum dot in the presence of spin-orbit interaction
(2004-06-01)
Kuan, WH
;
Tang, CS
;
Xu, W
;
電子物理學系
;
Department of Electrophysics
We present a theoretical study of the energy levels in a parabolically confined quantum dot in the presence of the Rashba spin-orbit interaction (SOI). The features of some low-lying states in various strengths of the SOI are examined at zero and nonzero magnetic fields. It is shown that the spin-polarized electronic states can be more easily achieved in a weakly confined dot when SOI is greater than a critical value of the confinement strength. The presence of a magnetic field enhances the possibility of the spin polarization and the SOI leads to different energy dependence on magnetic fields applied. Furthermore, in high magnetic fields, the spectra of low-lying states show basic features of Fock-Darwin levels as well as Landau levels. (C) 2004 American Institute of Physics.
Finger-gate array quantum pumps: Pumping characteristics and mechanisms
(2004-08-01)
Chung, SW
;
Tang, CS
;
Chu, CS
;
Chang, CY
;
電子物理學系
We study the pumping effects, in both the adiabatic and nonadiabatic regimes, of a pair of finite finger-gate array (FGA) on a narrow channel. Connection between the pumping characteristics and associated mechanisms is established. The pumping potential is generated by ac biasing the FGA pair. For a single pair (N=1) of finger gates (FG's), the pumping mechanism is due to the coherent inelastic scattering of the traversing electron to its subband threshold. For a pair of FGA with pair number N>2, the dominant pumping mechanism becomes that of the time-dependent Bragg reflection. The contribution of the time-dependent Bragg reflection to the pumping is enabled by breaking the symmetry in the electron transmission when the pumping potential is of a predominant propagating type. This propagating wave condition can be achieved both by an appropriate choice of the FGA pair configuration and by the monitoring of a phase difference phi between the ac biases in the FGA pair. The robustness of such a pumping mechanism is demonstrated by considering a FGA pair with only pair number N=4.
Nonadiabatic quantum pumping in mesoscopic nanostructures
(2001)
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
We consider a nonadiabatic quantum pumping phenomenon in a ballistic narrow constriction. The pumping is induced by a potential that has both spatial and temporal periodicity characterized by K and Omega. In the zero frequency (Omega = 0) limit, the transmission through narrow constriction exhibits valley structures due to the opening up of energy gaps in the pumping region - a consequence of the K periodicity. These valley structures remain robust in the regime of finite Omega, while their energies of occurrence are shifted by about h Omega /2. The direction of these energy shifts depend on the directions of both the phase-velocity of the pumping potential and the transmitting electrons. This frequency dependent feature of the valley structures gives rise to both the asymmetry in the transmission coefficients and tho pumping current. An experimental setup is suggested for a possible observation of our nonadiabatic quantum pumping finding. (C) 2001 Published by Elsevier Science Ltd.
Quantum transport in the presence of a finite-range time-modulated potential
(1996-02-15)
Tang, CS
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
Quantum transport in a narrow constriction and in the presence of a finite-range time-modulated potential is studied. The potential takes the form V(x,t) = V-0 theta(x)theta(a-x)cos(omega t), with a the range of the potential and x the transmission direction. Intrasubband transitions for the electrons and for arbitrary omega are made possible by the finiteness in the potential range. Our results show that, as the chemical potential mu increases, the dc conductance G exhibits dip structures when mu is at nn omega above the threshold energy of a subband. These structures in G are found in both the small a (a similar to lambda) and the large a (a much greater than lambda) regime. These dips are associated with the formation of quasi-bound-states. Our results can be reduced to the limiting case of a delta-profile oscillating potential when both a much less than lambda and V(0)a much less than 1 are satisfied. The assumed form of the time-modulated potential is expected to be realized in a gate-induced potential configuration.
Simulation of electrical characteristics of surrounding- and omega-shaped-gate nanowire FinFETs
(2004)
Tang, CS
;
Yu, SM
;
Chou, HM
;
Lee, JW
;
Li, YM
In this paper, electrical characteristics of sub-10 not nanowire FinFETs are investigated using a three-dimensional (3D) quantum correction simulation. Two different nanowire FinFETs, surrounding-gate FinFET and omega-shaped-gate one, are simulated with density-gradient-based model and compared in terms of on/off current, turn-on resistance, gate capacitance. It is found that the characteristic difference between surrounding-gate FinFET and omega-shaped-gate FinFET with a 70% coverage is insignificant. However, the former possesses better DIBL effect than that latter with different coverage ratios. Our examination presented here is useful in the fabrication of omega-shaped nanowire FinFETs.
Spin-current generation and detection in the presence of an ac gate
(2003-12-01)
Mal'shukov, AG
;
Tang, CS
;
Chu, CS
;
Chao, KA
;
電子物理學系
We predict that in a narrow gap III-V semiconductor quantum well or quantum wire, an observable electron spin current can be generated with a time-dependent gate to modify the Rashba spin-orbit coupling constant. Methods to rectify the so generated ac current are discussed. An all-electric method of spin-current detection is suggested, which measures the voltage on the gate in the vicinity of a two-dimensional electron gas carrying a time-dependent spin current. Both the generation and detection do not involve any optical or magnetic mediator.
Strain-induced coupling of spin current to nanomechanical oscillations
(2005-09-02)
Mal'shukov, AG
;
Tang, CS
;
Chu, CS
;
Chao, KA
;
電子物理學系
We propose a setup which allows us to couple the electron spin degree of freedom to the mechanical motions of a nanomechanical system not involving any of the ferromagnetic components. The proposed method employs the strain-induced spin-orbit interaction of electrons in narrow gap semiconductors. We have shown how this method can be used for detection and manipulation of the spin flow through a suspended rod in a nanomechanical device.
Transport spectroscopy in a time-modulated open quantum dot
(2003-05-15)
Tang, CS
;
Tan, YH
;
Chu, CS
;
電子物理學系
;
Department of Electrophysics
We have investigated the time-modulated coherent quantum transport phenomenon in a ballistic open quantum dot. Conductance G and the electron dwell time in the dots are calculated by a time-dependent mode-matching method. Under high-frequency modulation, the traversing electrons are found to exhibit three types of resonant scatterings. They are intersideband scatterings into quasibound states in the dots, into true bound states in the dots, and into quasibound states just beneath the subband threshold in the leads. Dip structures or fano structures in G are their signatures. Our results show structures due to 2 (h) over bar omega intersideband processes. At the above scattering resonances, we have estimated, according to our dwell time calculation, the number of round-trip scatterings which the traversing electrons undertake between the two dot openings.
Transport through a quantum ring, dot, and barrier embedded in a nanowire in magnetic field
(2005-06-01)
Gudmundsson, V
;
Lin, YY
;
Tang, CS
;
Moldoveanu, V
;
Bardarson, JH
We investigate the transport through a quantum ring, a dot, and a barrier embedded in a nanowire in a homogeneous perpendicular magnetic field. To be able to treat scattering potentials of finite extent in a magnetic field we use a mixed momentum-coordinate representation to obtain an integral equation for the multiband scattering matrix. For a large embedded quantum ring we are able to obtain Aharonov-Bohm type of oscillations with superimposed narrow resonances caused by interaction with quasibound states in the ring. We also employ the scattering matrix approach to calculate the conductance through a semiextended barrier or well in the wire. The numerical implementations we resort to in order to describe the cases of weak and intermediate magnetic field allow us to produce high resolution maps of the "near field" scattering wave functions, which are used to shed light on the underlying scattering processes.