Characteristics Of Resonant Tunneling Structures Research Articles

Quantum hydrodynamic equations and quantum-hierarchy decoupling scheme.

There is a need to extract the relevant device physics from exact quantum transport formulations, and hence to reduce the complexity of quantum transport simulations for practical applications. We use a Hermite polynomial expansion to drastically reduce the number of degrees of freedom associated with the momentum variables. The result is a quantum hierarchy in real space. We also give a general procedure for the quantum-hierarchy decoupling scheme to derive the quantum hydrodynamic (QHD) and quantum drift-diffusion transport equations. We present some numerical results for the quantum hierarchy. A rigorous foundation of a decoupling procedure is given whereby the lower-order equations are renormalized in terms of a self-consistent effective potential, quantum diffusion coefficient, and moments, endowed with all the quantum corrections to order variant Planck's over h(2). Our decoupling scheme is based on the general expression of Tr H(n) to order variant Planck's over h(2), valid at all temperatures without the need for expansion in terms of the small parameter and high temperature assumption. This is very important conceptually since existing QHD formulations, using expansion to order h(2), are based on a Boltzmann distribution with the restrictive assumption of a small parameter, which is not valid in abrupt heterojunction semiconductor devices. They also fail to account for important quantum nonlinearity in the form of nonequilibrium quantum corrections. These nonequilibrium quantum corrections are expected to play a major role in approximating the coherence manifested by the highly nonlinear current-voltage characteristics of resonant tunneling structures.

Self-consistent modeling of the current–voltage characteristics of resonant tunneling structures with type II heterojunctions

We present a model and results of self-consistent calculation of current–voltage (I–V) characteristics of the InAs/AlSb/InAs, InAs/AlSb/GaSb resonant tunneling structures with type II heterojunctions. The different current components, charge accumulation in the quantum well and quasi-Fermi level variation in contacts and spacers due to the drift-diffusion processes, are taken into account. The kp band model is used to describe the interband and intraband tunneling processes. The transfer Hamiltonian approach is employed to obtain the resonant tunneling current density and charge density in the quantum well. A good quantitative agreement with the experiment is obtained for both structures including the agreement for the values of peak-to-valley (P/V) current ratio.

Piezoelectric effects in (001)-oriented double barrier resonant tunneling structures

We show that piezoelectric effects can give rise to internal electric fields that modify the current–voltage characteristics of double barrier resonant tunneling devices, if suitable stresses are applied. Electric polarization fields in the direction perpendicular to the interfaces arise in heterostructures on (001)-oriented substrates under uniaxial stress parallel to the (110) or (11̄0) orientations. We measured current–voltage characteristics of (001)-oriented AlAs/GaAs/AlAs double barrier structures as a function of uniaxial external stress applied parallel to the (110) and the (11̄0) orientations. The substrate contact was grounded in all measurements. Under (110) stress, the resonance peaks shift to more positive voltages for both positive and negative bias. For (11̄0) stress, the peaks shift toward more negative voltages. We also calculated the current–voltage characteristics of resonant tunneling structures under uniaxial stress, taking into account stress effects on the band alignment and piezoelectric effects. We obtain satisfactory agreement between our calculations which contain no adjusted parameters and the measured data.

The different characteristics for heavy- and light-hole resonant tunneling in Si1−xGex/Si double barrier structures

A new approach to analyze the transport characteristics of resonant tunneling structures has been developed. It is shown that the Fermi-level Ef position per se, the temperature variations of the Fermi-level, the effective mass, and the mobility, have important influences on the resonant tunneling transport properties. Both calculations and experiments have shown very different features for heavy-hole and light-hole resonant tunneling in SiGe/Si resonant tunneling structures. These different features can for the first time be explained coherently. The calculated subband energy at current resonance can be much higher or lower than the Fermi level and the peak current can decrease or increase with temperature, depending on the device structure, the carrier type, and the working conditions. The calculations have predicted an interesting result that, in the low-temperature region, the valley current for the first heavy-hole resonance decreases with temperature, and that is confirmed by our experiments. The calculated temperature dependences of heavy- and light-hole transport, such as peak and valley currents/voltages and their variations with the well width, agree very well with our observations and other published results.

Influence of impurity scattering on the traversal time and current-voltage characteristics of resonant tunneling structures

The transmission line technique to solve the time independent Schrödinger equation is modified to include the effect of elastic scattering by introducing a scattering impedance Z s = 4 jV 0/ℏ. The method is applied to calculate the current-voltage and traversal time characteristics of double barrier resonant tunneling structures. The peak current density for a random distribution of scatterers in the quantum well decreases while it increases for scattering at a fixed location compared to its value in the absence of any scatterers. Traversal time decreases with applied bias and shows distinct dips at resonance. The traversal time through an asymmetric resonant tunneling structure is also investigated.

Effects of carrier mass differences on the current-voltage characteristics of resonant tunneling structures

We show that the current-voltage characteristics of resonant tunneling structures are drastically influenced by the difference of electronic effective masses between the electrodes and the quantum well. In particular, if the mass in the well is larger than that in the emitter, the current peak is shifted to lower voltages, relative to the more conventional case of equal masses. This situation is illustrated experimentally with GaAs/AlAs/GaAs heterostructures, in which Γ electrons emitted from GaAs tunnel resonantly through the AlAs X point, where the mass is considerably heavier.