E. Wolak

Elastic scattering centers in resonant tunneling diodes

The effect of impurities placed in the wells of double‐barrier resonant tunneling diodes on the current‐voltage characteristics was experimentally determined. Four different double‐barrier structures were grown by molecular beam epitaxy with n‐type, p‐type, undoped, and highly compensated doping in the center of the well. Resonant tunneling devices of various sizes were fabricated, and measured at 77 K. Systematic shifts in the peak position and peak to valley ratios were observed for the different dopant profiles. The shifts in peak position are correctly predicted by a ballistic model which includes the effects of band bending due to ionized impurities in the well. The doped devices showed a systematic decrease in the peak to valley ratio which is not predicted by the ballistic model. By scaling our results, it is apparent that in most cases unintentional background impurities are not sufficient to significantly degrade the current‐voltage characteristics of resonant tunneling diodes.

Resonant tunneling of electrons of one or two degrees of freedom

Analytical expressions for the tunneling current of electrons with one or two degrees of freedom (DOF), due to additional quantum confinement transverse to the electron transport direction, are explicitly derived, analyzed, and implemented into computer simulations. The results are compared with the well‐known case in which 3‐DOF electrons tunnel through a one‐dimensional double‐barrier well. The results show that the singularity of the density of states in a one‐dimensional system will not manifest sharp features in tunneling current, and that when the spacing between the Fermi energy and bottom of conduction band is the same, the tunneling current peak becomes broader and the peak‐to‐valley ratio becomes smaller as the number of degrees of freedom of the electrons is reduced. The results also show that when scattering is neglected, the energy quantization due to transverse confinement in 1‐ or 2‐DOF systems will not contribute any additional peaks to the tunneling current.

The design of GaAs/AlAs resonant tunneling diodes with peak current densities over 2×105 A cm−2

A coherent transport model is described which accommodates bandstructure nonparabolicity by using a ‘‘local energy parabolic band approximation.’’ The model and a knowledge of its limitations is used to design resonant tunneling diodes in the GaAs/AlAs material system with measured peak current densities of 2.5(2.8)×105 A cm−2 concurrent with peak‐to‐valley ratios as high as 1.8 (3.1) at room temperature (77 K).

Variation of the spacer layer between two resonant tunneling diodes

The effect of changing the length of the spacer layer between two vertically integrated resonant tunneling diodes (RTDs) is experimentally determined. Three different wafers, each containing two RTDs, were grown by molecular beam epitaxy, with spacer layers of 1200, 700, and 200 A, respectively. A fourth wafer with a single such RTD was grown as a control sample. Two of the control samples wired in series show two current peaks, the temperature dependence of the current‐voltage (I‐V) curves being correctly predicted by a nonlinear load model. The I‐V characteristics of the stacked devices with 1200 and 700 A quantum wells between the RTDs also show two current peaks, confirming that the bulk of electrons lose longitudinal wave function coherence between the two double‐barrier structures. The first derivative of the I‐V curve for the samples with 700 and 1200 A spacers displays evidence of quantum interference between the cathode well and the central spacer as a second‐order effect. The first major peak in...

Influence of ballistic electrons on the device characteristics of vertically integrated resonant tunneling diodes

We present a systematic study of the ballistic electron contribution to the current‐voltage (I‐V) characteristics of vertically integrated resonant tunneling diodes (RTDs) separated by doped spacer layers (Wsp). A magnetic field (B) transverse to the tunneling direction was used to tune the electron’s longitudinal energy. The results confirm the isolated circuit element picture of the Wsp=1000 A sample and the strongly coupled description of the 0 A sample. This work shows that even for some nominally isolated RTDs (in this work for Wsp= 400 and 500 A), the I‐V characteristics can undergo striking B‐induced changes. This effect is due to resonant charge buildup in the well of the collector RTD from the relatively weak ballistic component of the current traversing the doped spacer region. A simple model that includes a calculation of the conduction‐band profile and quantum well energy levels under bias gives good agreement with the data.

Improved vertically integrated resonant tunneling diodes

A lumped-element circuit model is developed for vertically integrated resonant tunneling diodes (RTDs), assuming that the individual RTDs behave as independent circuit elements in vertically integrated RTDs. The limit of this model is determined and the transition between incoherent and coherent transport is observed by systematic variation of separation layer thickness between diodes. The limit of applicability of this model is determined to be devices with 500 AA separation for 10/sup 18/ cm/sup -3/ doping. High current density is identified as a critical parameter for high-speed operation, and a peak current density of 2.5*10/sup 4/ A/cm/sup 2/ was achieved with little degradation in the peak to valley ratio by using thin barriers. Approaches for further improvement are suggested.<<ETX>>

Elastic scattering in resonant tunneling devices with one degree of freedom

Abstract The effect of elastic scattering centers on the current-voltage (I–V) characteristics of double barrier resonant tunneling diodes with transverse confinement is examined. It is shown that elastic scattering centers break the separation of variables condition, and allow transitions between states of different transverse confinement. The changes to the transmission coefficients through the structure due to elastic scattering in the well of a resonant tunneling device are modeled by using perturbation theory. It is shown that additional structure in the I–V characteristics may result.

A lateral resonant tunneling FET

Abstract A lateral resonant tunneling FET (RTFET) is proposed. The RTFET has three closely spaced gates. The outer gates control the barrier heights, and the inner gate controls the potential of the quantum well. These gates are capacitively coupled to the barriers and the well, therefore, the gate currents are very small. Modeling and computer simulation show that the RTFET should have an improved peak-to-valley ratio, narrower current peak widths, and more uniform distribution of peak currents than that of a conventional resonant tunneling diode with the same structure. Furthermore, a unique feature of this device is that the barrier height can be adjusted, which allows the current peak, the peak-to-valley ratio, and the peak positions to be tuned.

Effect of high current density and doping concentration on the characteristics of GaAs/AlAs vertically integrated resonant tunneling diodes

The influence of high current density and doping concentration on the current‐voltage (I‐V) characteristics of vertically integrated resonant tunneling diodes (VIRTDs) is experimentally determined. Room‐temperature peak current densities as high as 2.7×104 A/cm2 and first and second peak‐to‐valley ratios of 3.6 and 2, respectively, are achieved in VIRTDs with 6‐monolayer (ML) (17 A) barrier RTDs and 600 A separation between them. Symptoms of degradation in the I‐V characteristics of these devices, which are attributed to insufficient longitudinal momentum relaxation in the region between RTDs, turn into a serious problem when the separation between RTDs is decreased to 500 A. Through the variation of doping in the separation region, higher doping (3×1018 cm−3) between RTDs is proposed to remedy this problem and demonstrated to be quite effective in restoring the I‐V characteristics.

Ballistic Electron Contributions in Vertically Integrated Resonant Tunneling Diodes

Abstract We present a systematic study of the ballistic electron contribution to the current-voltage (I–V) characteristics of vertically integrated resonant tunneling diodes (RTDs). The doped spacer layer separating the two RTDs was varied from 0 to 1000 A. The conduction band profile and quantum well energy levels were determined from a self-consistent model as a function of bias voltage. A magnetic field transverse to the tunneling direction was used to tune the electrons longitudinal energy. The results confirm the isolated RTD picture of the 1000 and 500 A spacer samples, and the strongly coupled description of the 0 A sample. The data also show that under certain circumstances, even for isolated RTDs, the overall I–V characteristics can be very sensitive to resonant charge buildup from a weak ballistic component of the current. We observed Γ - X interband tunneling contributions by ballistic electrons with high kinetic energies.