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We have studied fluctuations in the differential conductance of resonant tunnelling diodes which incorporate a δ-layer of donor impurities in the centre plane of the quantum well. The system is well-suited to studying this type of conductance fluctuation, as we can control the number and position of the impurity states and also vary their effective linewidth by the application of a magnetic field perpendicular to the current direction.

Electron tunnelling through donor-related states is discussed. This tunnelling process, which occurs well below the threshold voltage for conventional resonant tunnelling into the two-dimensional continuum states of the quantum well, reveals a new type of Fermi edge singularity effect which arises from the Coulomb interaction between the tunnelling electron on the localized site and the Fermi sea of electrons in the emitter layer. A new means of forming laterally confined resonant tunnelling devices is also described. By studying the effect of an applied magnetic field, the additional structure that appears in the current-voltage characteristics of these devices can be unambiguously associated with a lateral quantum mechanical confinement effect.

We have investigated resonant tunnelling in GaAs/(AlGa)As heterostructures which have been fabricated into square mesas 6 x 6 mum. A delta-layer of donors (n approximately 2 x 10(9) cm-2) has been incorporated at the centre of the quantum well which is 9 nm wide. The I(V) characteristics show a feature at approximately 70 mV, which is below the threshold for the main resonance and is due to resonant tunnelling through single donor states in the well. This feature is also present in large area mesas. At lower biases and at low temperatures we see a new set of resonances which, although they occur in all small area mesas, differ in detail between devices with regard to their strength and bias position. The form of the low-bias structure is strongly dependent on temperature, T, below 4 K where several very sharp steps appear, becoming sharper as T is decreased. We have also investigated the dependence of the new structure on magnetic field, B, parallel to the current direction. We attribute the new features to tunnelling through potential fluctuations on the mesoscopic scale due to donor clustering.

We have investigated the I ( V ) characteristics of a sub-micron gated GaAs/ (AlGa)As resonant tunnelling diode. We find that, relative to a large area device, there is additional structure at both high and low source-drain bias. The low bias features are shown to be due to the presence of impurities in the quantum well. This is confirmed by experiments on δ-doped large area devices. Using the gate on the sub-micron devices we are able to control the current through a single donor state. The high voltage structure occurs within the main resonance and only in forward bias where the peak-to-valley ratio (PVR) is poor. We ascribe the poor PVR to a lateral variation of the resonance condition and discuss the high voltage structure in these devices, and in similar two-terminal devices, in the light of this hypothesis.

We have observed a new sub-threshold peak in the I(V) characteristics of double-barrier resonant tunneling structures. By investigating a range of intentionally δ-doped and control layers we can relate this peak to resonant tunneling through the bound state of a shallow donor impurity in the quantum well. We have studied the dependence of the voltage position of this peak on the magnitude of a magnetic field applied either parallel or perpendicular to the plane of the quantum well. In the parallel orientation the donor peak shifts slightly to lower voltage whereas the threshold of the continuum resonance shows the usual large quadratic shift to higher voltage. The amplitude of the donor-related peak is reduced in the presence of a magnetic field. We present a theoretical model to explain this behaviour and show how the results can be used to probe the spatial form of the impurity wavefunction in the quantum well.

Photoluminescence spectroscopy is used to investigate the donor-assisted resonant tunnelling processes in double-barrier structures which incorporate a low-density delta -doped donor layer in the centre of the quantum well. A quantum well luminescence line corresponding to hole-neutral donor recombination is observed, along with two other lines at higher photon energy. The possible origin of these two lines is discussed.