Search

A theory of resonant tunneling in the presence of an ac field is presented. An analytical high frequency model is formulated, and the device frequency characteristics are discussed.

The noise characteristics of double-barrier resonant tunneling structures are measured in three samples with different barrier structures. In the resonant tunneling regime, shot noise of full or reduced strengths is observed. The results provide direct evidence of incoherent tunneling and can be qualitatively understood if phase incoherence of the tunneling processes is taken into account. In the regime where assisted tunneling processes dominate, the noise is generally not linear in current and can be much larger than the full shot noise. In particular, it displays pronounced peaks in a quantizing magnetic field, at biases where the I-V curve exhibits negative differential resistances due to Landau quantization.

The peculiar differences in tunneling characteristics measured from a number of asymmetric GaAs/ AlAs double-barrier resonant-tunneling structures under two opposite biases have systematically been studied. It is found that LO phonon-assisted tunneling in two opposite directions may possibly be controlled by the different dwell times of traversing electrons in the well as the structure is positively or negatively biased.

The effect of a large magnetic field, B, applied parallel to the plane of the barriers on the resonant current-voltage characteristics of n-type and p-type double barrier structures is described. The effect of the magnetic field on the electron resonances in n-type structures can be understood using the effective mass approximation. The results for the p-type structures provide a novel magneto-tunneling spectroscopy technique for probing the complicated e(k‖) dispersion curves of hole states in a quantum well. These experiments reveal directly the strong admixing of the light and heavy hole states and that some of the states correspond to negative hole effective mass for motion in the plane of the quantum well.

Double barrier structures of nanometric lateral dimensions have been fabricated. Transport measurements show evidence of resonant tunnelling through the discrete levels of quantum dots. The effect of lateral confinement was investigated as a function of size and temperature.

The nearly lattice—matched InAs/GaSb/AlSb system offers tremendous flexibility in designing novel heterostructures due to the wide range of available band alignments. We have recently exploited this advantage to demonstrate several different devices exhibiting negative differential resistance (NDR) based on interband tunneling. These devices show a wide range of different characteristics including very high peak current densities (1.6 × 105 A/cm2) or peak to valley current ratios (20:1 at 300K and 88:1 at 77K). We have also studied “traditional” double barrier (resonant) tunneling in the InAs/GaSb/AlSb system. In particular, extremely high peak current densities in InAs/AlSb double barrier devices have been exploited to fabricate oscillators operating at the highest frequencies yet reported. Two and three terminal tunneling devices in this material system show great promise for use in high frequency analog and digital applications.