G.A. Toombs

Space‐charge buildup and bistability in resonant‐tunneling double‐barrier structures

Using the sequential theory of resonant tunneling, 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. These parameters are directly related to the periodicity of magnetoquantum oscillations in the current.

Sequential tunneling due to intersubband scattering in double‐barrier resonant tunneling devices

Magnetoquantum oscillations in the tunnel current of double‐barrier n‐GaAs/(AlGa)As/GaAs/(AlGa)As/GaAs resonant tunneling devices reveal evidence of sequential tunneling in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the GaAs quantum well. The sequential tunneling arises from intersubband scattering between two quasi‐bound states of the well. Near this resonance, the charge buildup in the well can be estimated from the magnetoquantum oscillations.

Electrical and spectroscopic studies of space-charged buildup, energy relaxation and magnetically enhanced bistability in resonant-tunneling structures

Abstract Photoluminescence measurements and magnetoquantum oscillations in the differential capacitance are used to measure space-charged buildup and study electron thermalization in a double-barrier resonant tunneling structure based on n-type (AlGa)As. The intrinsic bistability observed in the I(V) characteristics is also seen in the linewidth and photon energy of the photoluminescence. The spectroscopic data reveal clearly the importance of intersubband transitions in the voltage range at which electrons tunnel resonantly into the second bound state of the quantum well. A novel field-induced enhancement of the intrinsic bistability effect is reported for B ‖ J .

Observation of space-charge bulk-up and thermalisation in an asymmetric double-barrier resonant tunnelling structure

By means of a study of magnetoquantum oscillations in the differential capacitance, the authors have observed the thermalisation of the space charge stored dynamically in the quantum well of an asymmetric double-barrier resonant tunnelling heterostructure based on n-GaAs/(AlGa)As. Fourier analysis of the oscillations was used to monitor the charge build-up in both the emitter accumulation layer and in the well. The storage time of an electron in the well was found to be approximately=0.5 mu s. The resonant tunnelling is truly sequential rather than coherent.

Magnetic field studies of negative differential conductivity in double barrier resonant tunnelling structures based on n-InP/(InGa)As

Abstract Negative differential conductivity (NDC) with a peak/valley ratio of 4.5:1 (4 K) and 2:1 (150 K) is observed in double barrier resonant tunnelling devices based on n-InP/(InGa)As. A transverse magnetic field applied in the plane of the tunnelling barriers ( J ¦ B ) significantly changes the current-voltage characteristics and eliminates the NDC for fields above −10 T. This behaviour is explained qualitatively in terms of the effect of the magnetic vector potential on the tunnelling electrons. The magneto-oscillations in the tunnelling current for J ‖ B are discussed in terms of a simple model of resonant tunnelling.

Charge build-up and intrinsic bistability in an asymmetric resonant-tunnelling structure

Intrinsic bistability is observed in an asymmetric resonant-tunnelling structure based on n-GaAs/(AlGa)As, incorporating a thin emitter barrier and a thick collector barrier. The resonant charge build-up in the quantum well which gives rise to the bistability is monitored by the Landau level structure in the magneto-capacitance.

Space-charge effects and ac response of resonant tunneling double-barrier diodes

Abstract We analyse theoretically the small-signal ac response of a resonant tunneling double-barrier semiconductor diode using the sequential tunneling approach. Electrostatic feedback effects due to space charge buildup in the quantum well are included in the analysis. The results are expressed in terms of an equivalent circuit which is used to interpret measurements of the capacitance of an asymmetric double-barrier structure at low frequencies (

The oscillatory magnetoresistance of electrons in a square superlattice potential

The transverse magnetoresistance of a two-dimensional electron gas in an n-type GaAs/(AlGa)As heterostructure subjected to a square superlattice potential is investigated. Magneto-oscillations are observed at low field (B ≤ 0.4 T) with period Δ(1/B) = ea/2kF, where a = 145 nm is the superlattice constant. At higher fields the magneto-resistance is dominated by Shubnikov-de Haas oscillations. A comparison is made with experiments on a one-dimensional superlattice.

Evidence for sequential tunnelling and charge build-up in double barrier resonant tunnelling devices

Abstract Magnetoquantum oscillations in the tunnel current of double barrier n-GaAs/(AlGa)As/GaAs/ (AlGa)As/GaAs resonant tunnelling devices are shown to contain both resonant and non-resonant contributions. The former demonstrates the build-up of charge in the GaAs quantum well when electrons are resonant with the first subband of the well. A theoretical analysis of resonant tunnelling is in excellent agreement with the experimental data. Evidence of sequential tunnelling is found in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the well. Near this resonance, the charge build-up in the well is estimated from the magnetotunnelling oscillations.

Investigations of double barrier resonant tunneling devices based on (AlGa)As/GaAs

A series of electrical measurements on double barrier resonant tunneling structures is described. It is shown that the bistability effect in the current–voltage characteristics of a double barrier structure can be removed by connecting a suitable capacitor to the external circuit. These measurements cast serious doubt on the recent interpretation of the bistability as an intrinsic space‐charge effect. The temperature dependence of the tunneling current at voltages below threshold is also investigated. An expression for the temperature and voltage dependence of the thermally activated resonant tunneling process is compared with experiment. Finally, the effect of hydrostatic pressure (up to 11 kbar) on I(V) characteristics is studied. It is found that, whereas the pressure has little effect on the low‐temperature I(V) characteristics, the peak/valley ratio at room temperature is significantly reduced. This behavior is attributed to the effect of the higher conduction band minima.