A Review of Recent Developments in Resonant Tunnelling

Abstract

This chapter reviews some recent developments in our understanding of the physical properties of double barrier resonant tunnelling structures. Using the sequential theory of resonant tunnelling, the DC current-voltage characteristic of a double-barrier structure is calculated, taking into account the effect of space charge in the quantum well. A region of current bistability is found over a voltage range which is determined by the maximum space charge and the capacitance of the structure. Although there are good theoretical reasons to suggest that space charge build-up can cause intrinsic bistability, it is shown that the commonly observed bistability effect in the current-voltage characteristics of a typical resonant tunnelling device can be removed by connecting a suitable capacitance or resistance to the device. These measurements cast serious doubt on the recent observation and interpretation of a bistability in I(V) as an intrinsic space-charge effect. In the stabilised section of the I(V) curve, at voltages above the main resonant peak, the magnetoquantum oscillations observed with B| |J are used to investigate tunnelling assisted by LO phonon emission and by elastic scattering processes. Such processes have a deleterious effect on the peak/valley ratio which is commonly used as a figure of merit for resonant tunnelling devices. Resonant tunnelling devices with wide wells (~60 nm–1200 nm) exhibit a large number (≳ 20) of regions of negative differential conductivity. The effect of a transverse magnetic field J⊥B on the resonances in the I(V) characteristics of these wide well structures is investigated. At sufficiently high magnetic field, a transition is observed from tunnelling into magneto-electric box-quantised states to tunnelling into magnetically quantised cycloidal skipping states involving only the emitter barrier interface.