Modeling of electron tunneling times in asymmetric double quantum wells

A scattering process modeled by an imaginary potential V _I in the wide well of an asymmetric double quantum well structure (DQWS) is used to model the electron tunneling from the narrow well. Taking V _I –5 meV, the ground resonant level lifetimes of the narrow well in the DQWS are in quantitative agreement with the experimental resonance and non-resonance tunneling times. The corresponding scattering time 66 fs is much faster than the intersubband scattering time of LO-photon emission.     

Electric field perturbation due to impurities in GaAs through single electron transistor

The present work shows the presence of inevitable impurities in the semi-insulating GaAs domains when one is developing a single electron transistor (SET) and alters the quantization mechanism of single electron tunneling through the island. It is also indicated that these impurities decrease the amount of energy required to change the number of electrons on the island, which leads to a drastic reduction of SET quality. A theoretical model has been presented for elucidating the I–V characteristics of GaAs nano-crystals. It is found that this proposed model fits well the experimental data.     

A Monte Carlo study of the asymmetric clock or chiral Potts model in two dimensions

The phase diagram of the two-dimensional, three-state chiral Potts or asymmetric clock model is studied using Monte Carlo techniques. The phase boundaries are compared to those obtained using the finite-size renormalization group and the free fermion approximation. The incommensurate phase is described in detail and crossover effects near the Lifshitz point are discussed.     

The multiple-image interactions and the mean-barrier approximation in MOM and MVM tunneling junctions

The effect of the multiple image interactions on theI–V characteristics and the reliability of the mean barrier approximation in calculating the current in MOM and MVM tunneling junctions are critically examined. It is demonstrated that the continued use of the uncorrected form of Simmons' original multiple-image force interaction in the analysis of tunneling junctions can lead to serious errors in both the current and the dynamic resistance. An extensive numerical analysis of planar junctions including the image potential suggests that the basic mean barrier approximation formulated by Simmons is essentially a thick barrier approximation. It also is shown that the conventional mean barrier approximation is a relatively poor approximation for the tunneling barriers of interest, and that it is not possible to establish a reliable a priori estimate of its range of validity.This research was supported in part by the NATO Research Grants Program, Grant No. 1902, Scientific Affairs Division, Brussels, Belgium     

Dynamics of resonant and non-resonant tunneling in asymmetric coupled quantum wells

Within the framework of the effective mass approximation, coherent oscillations of a photoexcited electron wave packet in an asymmetric coupled quantum well structure have been studied using a time-dependent Schrödinger equation. In the method of calculation, the continuity of the current across a semiconductor heterojunction is considered. The amplitude and period of the electronic is obtained and in the case of high bias, it is found the existence of electric field-induced tunelling to semiconductor bulk.     

0.18 μm部分耗尽绝缘体上硅互补金属氧化物半导体电路单粒子瞬态特性研究

利用脉冲激光入射技术研究100级0.18 μm部分耗尽绝缘体上硅互补金属氧化物半导体反相器链的单粒子瞬态效应, 分析了激光入射器件类型及入射位置对单粒子瞬态脉冲传输特性的影响. 实验结果表明, 单粒子瞬态脉冲在反相器链中的传输与激光入射位置有关, 当激光入射第100级到第2级的n型金属-氧化物-半导体器件, 得到的脉冲宽度从287.4 ps增加到427.5 ps; 当激光入射第99级到第1级的p型金属-氧化物-半导体器件, 得到的脉冲宽度从150.5 ps增加到295.9 ps. 激光入射点靠近输出则得到的瞬态波形窄; 靠近输入则得到的瞬态波形较宽, 单粒子瞬态脉冲随着反相器链的传输而展宽. 入射器件的类型对单粒子瞬态脉冲展宽无影响. 通过理论分析得到, 部分耗尽绝缘体上硅器件浮体效应导致的阈值电压迟滞是反相器链单粒子瞬态脉冲展宽的主要原因. 而示波器观察到的与预期结果幅值相反的正输出脉冲, 是输出节点电容充放电的结果.     

RF SET 超灵敏静电计

单电子晶体管(SET)作为灵敏静电计的灵敏度受到噪声的限制，其中散粒噪声(shot noise)是本征噪声，决定着单电子晶体管灵敏度的极限．利用射频(radio frequency，RF)单电子晶体管的极高的工作频率，可以消除SET的1／f噪声，从而达到极限灵敏度．利用一级低温低噪声放大器和一级室温放大器放大工作在反射模式的射频单电子晶体管的输出信号，使用LC共振电路，抬高了SET右边的整个微波系统的阻抗，使之与单电子的输出阻抗匹配，从而提高了RFSET静电计的灵敏度．     

Coherent Tunnelling in Coupled Quantum Wells under a Uniform Magnetic Field

Coherent tunnelling is studied in the framework of the effective mass approximation for an asymmetric coupled quantum well. The Hartree potential due to the electron-electron interaction is considered in our calculation. The effects of the longitudinal and transverse magnetic field on coherent tunnelling characteristics are discussed. It has been found that the external field plays an important role in modulating the electron states.     

The dual role of multiple-transistor charge sharing collection in single-event transients

As technologies scale down in size, multiple-transistors being affected by a single ion has become a universal phenomenon, and some new effects are present in single event transients (SETs) due to the charge sharing collection of the adjacent multiple-transistors. In this paper, not only the off-state p-channel metal-oxide semiconductor field-effect transistor (PMOS FET), but also the on-state PMOS is struck by a heavy-ion in the two-transistor inverter chain, due to the charge sharing collection and the electrical interaction. The SET induced by striking the off-state PMOS is efficiently mitigated by the pulse quenching effect, but the SET induced by striking the on-state PMOS becomes dominant. It is indicated in this study that in the advanced technologies, the SET will no longer just be induced by an ion striking the off-state transistor, and the SET sensitive region will no longer just surround the off-state transistor either, as it is in the older technologies. We also discuss this issue in a three-transistor inverter in depth, and the study illustrates that the three-transistor inverter is still a better replacement for spaceborne integrated circuit design in advanced technologies.     

Single electron tunneling at large conductance: The semiclassical approach

We study the linear conductance of single electron devices showing Coulomb blockade phenomena. Our approach is based on a formally exact path integral representation describing electron tunneling nonperturbatively. The electromagnetic environment of the device is treated in terms of the Caldeira-Leggett model. We obtain the linear conductance from the Kubo formula leading to a formally exact expression which is evaluated in the semiclassical limit. Specifically we consider three models. First, the influence of an electromagnetic environment of arbitrary impedance on a single tunnel junction is studied focusing on the limits of large tunneling conductance and high to moderately low temperatures. The predictions are compared with recent experimental data. Second, the conductance of an array of N tunnel junctions is determined in dependence on the length N of the array and the environmental impedance. Finally, we consider a single electron transistor and compare our results for large tunneling conductance with experimental findings. Received 2 February 2000     

Ultrasmall nanoscale devices fabricated from compensating-layer GaAs/AlGaAs heterostructures

Nanoscale devices are fabricated from modulation-doped GaAs/AlGaAs heterostructures, where the two-dimensional electron system is initially depleted. Upon removing the p-type capping layer that compensates for the n-type supply layer, the electron system is induced. Arbitrarily shaped areal, line, and dot elements, i.e. the nanostructures and 2D leads, are simultaneously fabricated by patterning a thin resist layer with an atomic force microscope and subsequent selective wet etching. In this way a single-electron transistor (SET) with a 60 nm diameter island, a 60 nm wide electron waveguide (EWG), and an Aharonov–Bohm (AB) loop of 110 nm average diameter are prepared. Measurements at T=1.5 K reveal Coulomb-blockade, quantized conductance and AB-oscillations for the SET, EWG, and AB loop, respectively. Finally, an EWG is demonstrated in split-gate geometry where the compensating layer is used as split gate.     

Parasitic bipolar amplification in single event transient and its temperature dependence

Using three-dimensional technology computer-aided design (TCAD) simulation, parasitic bipolar amplification in single event transient (SET) current of single transistor and its temperature dependence are studied. We quantify the contributions of different current components in SET current pulse, and it is found that the proportion of parasitic bipolar amplification in total collected charge is about 30% in both 130-nm and 90-nm technologies. The temperature dependence of parasitic bipolar amplification and the mechanism of SET pulse are also investigated and quantified. The results show that the proportion of charge induced by parasitic bipolar increases with rising temperature, which illustrates that the parasitic bipolar amplification plays an important role in the charge collection of single transistor.     

Parasitic bipolar amplification in a single event transient and its temperature dependence

Using three-dimensional technology computer-aided design (TCAD) simulation, parasitic bipolar amplification in a single event transient (SET) current of a single transistor and its temperature dependence are studied. We quantify the contributions of different current components in a SET current pulse, and it is found that the proportion of parasitic bipolar amplification in total collected charge is about 30% in both 130-nm and 90-nm technologies. The temperature dependence of parasitic bipolar amplification and the mechanism of the SET pulse are also investigated and quantified. The results show that the proportion of charge induced by parasitic bipolar increases with rising temperature, which illustrates that the parasitic bipolar amplification plays an important role in the charge collection of a single transistor.     

Quantum Efficiency of Charge Qubit Measurements Using a Single Electron Transistor

The quantum efficiency, which characterizes the quality of information gain against information loss, is an important figure of merit for any realistic quantum detectors in the gradual process of collapsing the state being measured. In this work we consider the problem of solid-state charge qubit measurements with a single-electron-transistor (SET). We analyze two models: one corresponds to a strong response SET, and the other is a tunable one in response strength. We find that the response strength would essentially bound the quantum efficiency, making the detector non-quantum-limited. Quantum limited measurements, however, can be achieved in the limits of strong response and asymmetric tunneling. The present study is also associated with appropriate justifications for the measurement and backaction-dephasing rates, which were usually evaluated in controversial methods.     

Low‐voltage,high‐gain,and high‐mobility organic complementary inverters based on N,N′‐ditridecyl‐3,4,9,10‐perylenetetracarboxylic diimide and pentacene

The authors describe an organic complementary inverter with N,N′‐ditridecyl‐3,4,9,10‐perylenetetracarboxylic diimide as an n‐type semiconductor and pentacene as a p‐type semiconductor. Each transistor of the inverter exhibited high carrier mobility: 1.62 cm²/Vs for an n‐type drive transistor and 0.57 cm²/Vs for a p‐type switch transistor. The gain of the inverter reached 125. Another inverter using Ta₂O₅ as a high κ gate dielectric performed well with a gain of 500 and an operation voltage of only 5 V.

     

Optically controlled characteristics of a new heterojunction field effect transistor

A new heterojunction field effect transistor has been proposed and studied analytically to investigate its suitability as a high speed optical detector. The characteristics of the device have been studied in the dark as well as in the illuminated conditions. In the new structure the Schottky gate of a typical high electron mobility transistor (HEMT) has been replaced by a junction gate in order to facilitate the absorption of optical radiation. The performance of the new device has been compared with that of the existing HEMT structure for similar application. It is seen that the absorption of optical radiation of suitable wavelength in the larger bandgap material results into conductivity modulation of the channel giving rise to a considerable change in the saturation drain current in illuminated conditions. This enables the device to respond to an intensity modulated optical signal. It has further been observed that due to an increased optical absorption region the modified structure shows a much better optical sensitivity than the existing one.     

Barrier height and negative tunnel magnetoresistance

Based on the free-electron approximation method proposed by Slonczewski, we substitute the finite magnetic zone by a semi-infinite magnet. On this basis, the relationship between the tunnel magnetoresistance (TMR) and the barrier height of magnetic tunnel junction (MTJ) is studied. We find the TMR at small bias is always positive for various barrier heights when the MTJ has a symmetric configuration and the negative TMR can be observed when MTJ is with lower barrier height in the asymmetric condition.     

A thin-film transistor with a very thin a-Si:H layer and its application for an LC panel

One of the disadvantages of applying an a-Si:H thin-film transistor (TFT) to an active matrix-addressed liquid crystal (LC) panel is that a TFT with an a-Si:H has a very large photo-leakage current because of the high photo-conductivity of an a-Si:H itself.We have tried decreasing the photo-leakage current by varying the thickness of an a-Si:H layer (L) in TFTs and investigated the characteristics of TFTs, mainly drain voltage versus drain current containing photo-leakage current (I _ph).As a result, it is shown that lnI _ph is proportional to InL, and its gradient is 1.5–2.0. We assume that the thinner an a-Si:H layer is, the more effective the recombination of carriers at the interface states is forI _ph.We have applied TFT with a very thin a-Si:H layer (30nm) to a full-color active matrix-addressed LC panel for a moving picture display and realized a display of good quality under illuminated condition of 5×10⁴lx without a shading layer in it.     

Transport Characteristics of Mesoscopic Radio-Frequency Single Electron Transistor

The transport property of a quantum dot under the influence of external time-dependent field is investigated. The mesoscopic device is modelled as semiconductor quantum dot coupled weakly to superconducting leads via asymmetric double tunnel barriers of different heights. An expression for the current is deduced by using the Landauer-Buttiker formula, taking into consideration of both the Coulomb blockade effect and the magnetic field. It is found that the periodic oscillation of the current with the magnetic field is controlled by the ratio of the frequency of the applied ac-field to the electron cyclotron frequency. Our results show that the present device operates as a radio-frequency single electron transistor.     

A single-event transient induced by a pulsed laser in a silicon-germanium heterojunction bipolar transistor

A study on the single event transient (SET) induced by a pulsed laser in a silicon-germanium (SiGe) heterojunction bipolar transistor (HBT) is presented in this work. The impacts of laser energy and collector load resistance on the SET are investigated in detail. The waveform, amplitude, and width of the SET pulse as well as collected charge are used to characterize the SET response. The experimental results are discussed in detail and it is demonstrated that the laser energy and load resistance significantly affect the SET in the SiGe HBT. Furthermore, the underlying physical mechanisms are analyzed and investigated, and a near-ideal exponential model is proposed for the first time to describe the discharge of laser-induced electrons via collector resistance to collector supply when both base-collector and collector-substrate junctions are reverse biased or weakly forward biased. Besides, it is found that an additional multi-path discharge would play an important role in the SET once the base-collector and collector-substrate junctions get strongly forward biased due to a strong transient step charge by the laser pulse.