F. A. Buot

Simulation of resonant tunneling structures: Origin of the I–V hysteresis and plateau-like structure

Hysteresis and plateau-like behavior of the I–V curves of a double-barrier resonant tunneling structure are simulated in the negative differential resistance region. Our simulation results show that the creation of an emitter quantum well after the current passes its maximum value is the key point in understanding the origin of the I–V plateau-like structure. It is demonstrated that the plateau-like behavior of the I–V curves is produced by the coupling between the energy level in the emitter quantum well and that in the main quantum well. The hysteresis is a manifestation of the above-mentioned energy level coupling, the accumulation and distribution of electrons in the emitter, and the coupling between the energy level in the quantum well and the conduction band edge or the three-dimensional continuum states in the emitter. The effects of the structural parameters on the bistability of the I–V curves of resonant tunneling devices are discussed. The creation and disappearance mechanism of the emitter qua...

The effects of scattering on current‐voltage characteristics, transient response, and particle trajectories in the numerical simulation of resonant tunneling diodes

The relaxation‐time approximation is used in the numerical simulation of the Wigner distribution function to incorporate scattering. The effects within the constant relaxation‐time approximation are (a) a decrease in the peak‐to‐valley ratio of the current‐voltage curve; (b) a reduction in the oscillations of the Wigner distribution function, especially at resonance bias; (c) a suppression of the decay time of current oscillations after a sudden bias shift, indicating a smaller switching time than for no scattering; (d) a degradation in the resonant tunneling trajectories towards the characteristics of nonresonant trajectories; (e) a decrease in the spatial range of the quantum influences near resonance; and (f) ballistic transport sets in [i.e., the mean free path of the electrons is greater than the barrier region (110 A)] for temperatures less than 74 K.

Numerical aspects on the simulation of I‐V characteristics and switching times of resonant tunneling diodes

The development of a more accurate numerical scheme for simulating double‐barrier semiconductor structures has highlighted sensitivities of the computational results to numerical parameters for the different approximation schemes. In numerically evaluating the time evolution of the Wigner function, a second‐order differencing scheme (SDS) was used instead of a simple up/down wind differencing scheme (UDS). In our investigations of the numerical aspects of these schemes, we have found: (a) the proximity of the ‘‘computational box’’ boundaries to the double‐barrier region affects the peak‐to‐valley ratio of the I‐V curve and the value of the bias at peak current; (b) the peak‐to‐valley ratio is larger for the SDS than it is for the UDS; (c) the current at the resonant bias for SDS is larger than that calculated using UDS; (d) the rise in the current in the nonresonant regions for both SDS and UDS is dependent on how the bias is applied; and (e) the presence of an accumulation of electrons in the first heter...

Numerical simulation of transient response and resonant-tunneling characteristics of double-barrier semiconductor structures as a function of experimental parameters

The tunneling current characteristics and transient response of double‐barrier semiconductor structures are simulated for different barrier and quantum‐well widths, barrier heights, operating bias voltage, and ambient temperatures, using the equation for the Wigner distribution function. The numerical results suggest the following: (a) There is a particle buildup inside the quantum well prior to the resonant current peak as the applied bias is varied: (b) the number of resonant energy levels seen in the simulation agrees with its proportionality to the square root of the product of the barrier height and quantum‐well width: (c) the resonant peak width is larger for higher resonant energy levels than for the lower resonant energy levels in agreement with the different degree of localization of these levels; (d) at T=77 K, the current slowly increases with bias at lower bias than for T=300 K, with higher peak‐to‐valley ratio at T=77 K, presumably due to a much sharper convolution of the tunneling density an...

A metal/oxide tunnelling transistor

We have fabricated a new type of nanometre-scale transistor that operates by using a gate field to modulate the tunnelling probability of electrons through a lateral metal/oxide tunnel junction. Computer simulations predict that such a tunnelling transistor should have operating characteristics similar to those of a Si MOSFET but should be scalable to gate lengths. The device is composed entirely of noncrystalline materials, thus facilitating fabrication on a variety of substrates and multilayer stacking of devices for three-dimensional circuit architectures. Our initial devices have a 40 nm wide tunnel junction on top of a planar buried gate. Application of gate bias results in an order of magnitude modulation of the source-drain tunnelling current at 77 K. However, the device transconductance is smaller than predicted by modelling, which we attribute to the gate field not fully penetrating to the active region of the tunnel junction.

On the different physical roles of hysteresis and intrinsic oscillations in resonant tunneling structures

Electronic sources based upon resonant tunneling diodes (RTDs) usually generate power by establishing limit cycles which exchange energy with storage elements in an external biasing circuit; hence, the output power in this type of implementation will always be limited by extrinsic effects. We verify the presence of multiple energy‐storage mechanisms solely within the RTD and characterizes the interdependencies necessary to induce intrinsic oscillations observed in quantum mechanical simulations. Specifically, we show that a nonlinear ‘‘access’’ resistance and quantum‐well inductance is responsible for the hysteresis, ‘‘plateaulike’’ behavior, and bistability associated with the intrinsic current–voltage (I–V) characteristic. Furthermore, a new circuit‐level representation which accurately incorporates the nonlinear dependencies into these heretofore ‘‘linear’’ equivalent‐circuit elements is used to demonstrate the different roles, as well as the degree of cooperative interplay, of the intrinsic oscillatio...

Equivalent circuit parameters of resonant tunneling diodes extracted from self-consistent Wigner-Poisson simulation

The equivalent circuit parameters of resonant tunneling diodes (RTD) are extracted from numerical simulation results for RTDs. The RTD models used in this paper are double barrier structures. The influence of the resonant tunneling structure (RTS) parameters, such as the height of barriers, the width of the quantum well, the width of the spacers, and the width of the barriers, on the device parameters are systematically discussed. The effects of device temperature on device parameters are also discussed. Scattering between electrons and phonons greatly affects device parameters and thereby the function of the RTDs. Physical explanations about how the structure parameters and device temperature influence the device parameters are provided. Based on the analysis results, a general way to get an RTD oscillator with a higher maximum frequency is suggested.

Ensemble Monte Carlo particle investigation of hot electron induced source‐drain burnout characteristics of GaAs field‐effect transistors

The lattice heating rate has been calculated for GaAs field‐effect transistors of different source‐drain channel design by means of the ensemble Monte Carlo particle model. Transport of carriers in the substrate and the presence of free surface charges are also included in our simulation. The actual heat generation was obtained by accounting for the energy exchanged with the lattice of the semiconductor during phonon scattering. It was found that the maximum heating rate takes place below the surface near the drain end of the gate. The results correlate well with a previous hydrodynamic energy transport estimate of the electronic energy density, but shifted slightly more towards the drain. These results further emphasize the adverse effects of hot electrons on the Ohmic contacts.

Binary information storage at zero bias in quantum‐well diodes

It is argued, based on the intrinsic time‐dependent behavior of double‐barrier structures, that a modification of a conventional quantum‐well diode with special spacer‐layer structure in the source and/or the drain region will lead to two stable current‐voltage and charge state behaviors all the way down to zero bias. This viewpoint explains the salient features of a recent experimental observation on quantum‐well diodes with n−‐n+‐n− spacer layers. We substantiate this with a simple theory of self‐consistent charge buildup and bistability, and show that a limited supply or highly altered distribution of electrons from the emitter at high bias leads to fractional recharging of the quantum well and fractional current values, during the decreasing voltage sweep portion of a ‘‘closed‐loop’’ voltage sweep. This is in contrast with previous theories based on numerical simulations which allow for more than two current states, by virtue of the use of time‐independent analysis and/or the use of ‘‘open‐multibranch...

Self‐consistent Monte Carlo particle transport with model quantum tunneling dynamics: Application to the intrinsic bistability of a symmetric double‐barrier structure

The intrinsic bistability in a symmetric resonant tunneling device (RTD) is simulated by the ensemble particle Monte Carlo technique, coupled with a simple model of the space‐ and time‐dependent particle quantum dynamics inside the double‐barrier region of the RTD. This model particle quantum dynamics is based upon the phase‐time delay, which is obtained from a piecewise‐linear‐potential Airy function approach to the calculation of the transmission amplitude. An unambiguous hysteresis in the negative differential resistance (NDR) region of the current‐voltage (I‐V) characteristic is observed for a symmetric AlGaAs/GaAs double‐barrier structure. The dynamical accumulation of carriers in the well is seen to be the cause of this marked bistability/hysteresis. However, the plateau‐like features of the I‐V curve are not resolved, although oscillations in the quantum well carrier density in the NDR are prominent. This article strongly suggests that a more accurate treatment of the space‐ and time‐dependent part...