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Microprocessor designs are increasingly moving towards multiple cores on a single die. Validating memory consistency, coherency, ordering, and atomicity is crucial. X86 microprocessors are prevalent at most levels of computing. Thus, new x86 microprocessors undergo extensive compatibility testing. Being a high volume product, the economic and logistical repercussions of a functional deficiency escaping into the production cycle and beyond are humbling. The first silicon functional validation and debug of multicore microprocessors are constrained by design complexity, compatibility with existing hardware and software, and time-to-market pressures. This paper describes microprocessor debug features and their use in debugging functional failures. An encompassing overview of the microprocessor's first silicon validation is presented. Emphasis is put on validation and debug of multicore microprocessors targeting multinode systems. This paper presents a novel method to validate and debug intra-node and inter-node communication traffic. This paper also develops an analysis to determine optimal on die debug resources. Finally, data from an 8-node system is presented to demonstrate the extent of intrusiveness of a coherent and noncoherent traffic debug feature

The d.c. and a.c. characteristics of double-barrier diodes have been used to probe the large-scale annealing of defects within ion-implanted GaAs. Multi-stage annealing of defects was elucidated by the staged recovery of resonant tunnelling through ion-implanted diodes. Surprisingly, the current - voltage characteristics of some ion-implanted diodes after very rapid anneals were qualitatively identical to the as-grown characteristics, but the peak-current density was two orders of magnitude lower. This is explained by the creation of small percolation paths of as-grown material during the initial stages of annealing, where these paths surrounded clusters of defects. A simple capacitance model based on the creation of these percolation paths is described, and is consistent with the capacitance - voltage data. The idea that small-area percolation paths of as-grown material were created within the ion-implanted DBDs during very rapid annealing was supported by the observation of very-low bias resonant tunnelling and single-electron switching at 4.2 K. Both these phenomena are usually observed only within small-area pristine DBDs.

We report on the motion of two-dimensional electrons in a magnetic field which is periodic on the scale of a few hundred nanometres. Such a system exhibits novel magnetoresistance oscillations that we unambiguously identify as due to a commensurability effect between the period of the magnetic field and the diameter of the cyclotron orbit. We find that a semiclassical picture of electron motion can quantitatively describe all of the observed experimental features. We also propose methods of optimizing the effects of the magnetic modulation in hybrid semiconductor-superconductor devices.

We report on tunnelling transport through a novel quantum device consisting of a double barrier structure fabricated as a submicron square loop in the plane of growth. We find two kinds of quantum confinement coexist in such a device: near the current onset, tunelling starts through zero-dimensional states located in the four corners of the loop, whereas at higher bias the resonant current exhibits kinks due to one-dimensional confinement in the arms of the loop. The use of a high magnetic field permits us to measure the binding energy of the dot ground state to the first one-dimensional subband in the connecting wires. This value is then compared to existing theory. Moreover, we show that the different kinds of lateral confinement are not only evidenced in the resonant structure, but also in the scattering assisted replica peaks.

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.

Tunnelling through highly localised impurity states in the quantum well of a resonant tunnelling diode allows us to study the properties of the two-dimensional electron gas formed at the emitter barrier of the device. In high magnetic fields, the electrons form quantum dots in the disordered potential due to unscreened donors in the depleted collector contact.

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 mesoscopic conductance fluctuations in a GaAs quantum well with five electrically quantized two-dimensional (2D) subbands occupied. In contrast to the case of a single occupied subband, where the fluctuations depend only on the component of magnetic field perpendicular to the plane of the electrons, in our multi-subband structures the conductance fluctuations occur even in a magnetic field parallel to the plane. We attribute the fluctuations to rapid electron-impurity scattering between different 2D subbands which leads to quasi-3D motion of electrons within the quantum well. We are able to confirm the 3D character of the fluctuations by correlations between the magnetoresistances observed when the magnetic field is applied at various angles relative to the plane of the quantum well.

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 report the observation of the fractional quantum Hall effect in single GaAs-(Ga,Al)As quantum well structures with well widths of 102 A and 68 A. In both samples we have observed a strong v= 4 3 fraction in both the longitudinal resistivity ϱ xx and the Hall resistivity ϱ xy . The surprizing result in our data (in comparison with the FQHE in conventional single heterojunctions) is that the 5 3 is heavily suppressed or possibly absent. In a tilted magnetic field the energy gap of 4 3 depends only on the perpendicular field. We associate this difference between our samples and the conventional heterojunctions with the energy (well width) dependence of the Lande g ∗ factor and the influence of the Zeeman energy on the (partially) spin-polarized energy levels and hence the activation energies.

We have observed a high-field transition into a high-resistance state at filling factor v just below 2/7 in a high -density, high-mobility two-dimensional hole gas (2DHG). The state shows clear threshold behaviour consistent with a strongly pinned Wigner solid. The transition is at lower filling factor than the value of ∼0.37 recently reported by Santos et al. [7] for a low -density 2DHG, and higher than the value of ∼0.22 usually found in equivalent 2D electron systems. Our experiment shows no indication of a re-entrant behaviour of the insulating phase. We believe that this may be the highest-carrier-density Wigner-transition observed.

We have observed remarkable new structure in the source-drain I(V) characteristics of a symmetric double barrier resonant tunneling device in which the cross sectional area may be varied from ≅ 1μm2 to ≅0.1 μm2 by applying a voltage to a gate. In the source-drain voltage range close to, but slightly below, the threshold for current flow, peaks are observed with a peak value of ≅ 20 pA, and peak/valley ratio up to 10 1 . The structure in I(V) is independent of temperature between 35 mK and 10 K, and of gate voltage between 0 and −2 V. The first peak in I(V) in each polarity is unaffected by a magnetic field applied parallel to the current flow for fields up to 6 T. This structure cannot be explained by lateral quantisation or Coulomb blockade arising from the confinement. We propose that it is due to resonant tunneling between zero-dimensional states formed in the active region by the Coulombic potential of a single ionised donor, combined with the barrier potential.

Hydrostatic pressure is used to investigate two types of device based upon the n-type GaAs/(AlGa)As material system: an asymmetric double barrier resonant tunnelling device and a device incorporating two superlattices separated by a single barrier. In the double barrier structure, the pressure-induced increase in Gamma -X tunnelling and scattering reduces the build-up of electronic space charge in the GaAs quantum well. This allows one to study the transition from intrinsic bistability to ordinary negative differential conductivity in I(V). In the superlattice/barrier/superlattice structure the authors have evidence for a pressure-induced decrease in the free-electron density of the heavily doped n+ contact regions due to DX centres.

Intense, pulsed magnetic field studies of 2D hole gas systems at low temperatures examine competition of fractional QHE ground states with an insulating phase in the region ν

We have used resonant tunnelling spectroscopy to investigate the energy level spectrum of a wide 60 nm potential well with a strong magnetic field applied at an angle θ to the normal to the barriers. In this geometry, the current-voltage characteristic I(V) reveal distinct series of resonances which change dramatically with both V and θ. In a classical picture, the electron orbits in the well are strongly chaotic for 15° ≤ θ ≤ 75° and voltages V ≤ 1.2 V. However, we have identified a hierarchy of unstable but periodic orbits whose calculated properties explain the origin and θ-dependence of the resonant structure. We show that the classical motion becomes non-chaotic at high bias voltages and that the changeover to stable orbits is characteriaed by the appearance of widely-spaced resonances in I(V) .

The conductance of a mesoscopic device is usually described in terms of the transmission of electrons between electrical contacts. Implicit in this description is the assumption that contacts can be approximated as ideal thermal reservoirs which act solely to thermalise incident electrons before re-injecting them into the device. We describe two classes of experiment where this assumption is not valid. First, when the dissipation in the system is strongly dependent on an external parameter, such as temperature or magnetic field, the physical position of the contact may itself depend the parameter. We study this by investigating the transport properties of quantum wires with disordered contacts. Second, when there is the possibility of some sort of interaction between the electrons in the contact and those in the device, as we find in resonant tunnelling from a 2D contact into a 0D state. In both cases we find that the conductance properties of the device are strongly modified by the nature of the contact. PACS: 73.40.Kp, 73.40.Hm, 73.40Gk

Shubnikov-de Haas and persistent photoconductivity measurements are used to study the effect of hydrostatic pressure on the free electron concentration, mobility, and the occupancy of the DX centre in MBE grown n + GaAs heavily doped with either Si or Sn. The results show that the DX centre produces a resonant donor level between the Γ- and L- conduction band minima at a concentration comparable with the doping level. The position and occupancy of the DX centre are calculated using Fermi-Dirac statistics. For the Si-doped samples comparison with local vibrational mode measurements indicate that the DX level can be identified with Si Ga .

Hydrostatic pressure is used to investigate band structure effects in double barrier resonant tunnelling devices based on n-type (AlGa)As/GaAs. The pressure-induced increase in Γ-X tunnelling and scattering reduces the space charge build-up in the quantum well at resonance and allows us to study the transition from intrinsic bistability to ordinary negative differential conductivity in the I(V) characteristics. In the limit of fast Γ-X transfer the resonant peak in I(V) is lost.

Resonant tunneling spectroscopy is used to study the electronic energy levels of a wide potential well when a strong magnetic field is applied at an angle θ to the normal to the plane of the well. Even though the classical motion of electrons in the potential well is chaotic in the tilted field geometry, the current-voltage characteristics reveal distinct series of quasi-periodic resonances. The voltage range and period of each series of resonances change dramatically with θ. We explain this behavior by considering the unstable periodic orbits within the chaotic sea of the potential well.

Resonant tunnelling through a GaAs-AlAs triple-barrier structure with coupled quantum wells of unequal widths has been studied experimentally. Peaks in the current-voltage characteristic due to tunnelling via the lowest subband of each well were observed. Application of a magnetic field parallel to the interfaces broadens the peaks and at a certain critical field an additional resonance peak appears at a bias voltage between the original resonances. The field-induced peak is interpreted as a double-resonance effect, even though the subbands are not energetically aligned. This interpretation is supported by detailed calculations of current-voltage characteristics using a transfer-matrix approach.

A single-sided modulation-doped 90 A quantum well with Al0.45Ga0.55As barriers has been realised using metalorganic vapour-phase epitaxy with selenium doping. Hall effect measurements give a sheet carrier density ns= 2.27 × 1012cm−2 and a mobility μ = 6050cm2V−1s−1 at 300 K. At 4.2 K, Shubnilcov-de Haas and quantum Hall effect measurements give ns = 1.52 × 1012cm−2 and μ = 1.28 × 105cm2V−1s−1. A persistent increase in ns to 2.00 × 1012cm−2 is observed at 4.2K by passing a large current through the sample for a short period of time.

The authors have investigated the I(V) characteristics of a GaAs/(AlGa)As resonant tunnelling diode in which the effective cross-sectional area, A, may be varied using a gate. As A is progressively reduced I(V) becomes asymmetric. In particular the peak to valley ratio in forward bias is decreased from a value approximately=20 to approximately=1, but in reverse bias remains constant at approximately=20. They propose that this arises from a lateral variation of the voltage drop across the tunnel barriers which leads to a smearing of the resonance. As A is reduced they also observe additional maxima and minima in I(V).

Resonant tunnelling spectroscopy is used to investigate the energy level spectrum of a wide potential well in the presence of a large magnetic field oriented at angles theta between 0 degrees and 90 degrees to the normal to the plane of the well. In the tilted field geometry, the current-voltage characteristics exhibit a large number of quasiperiodic resonant peaks, even though the classical motion of electrons in the potential well is chaotic. The voltage range and spacing of the resonances both change dramatically with theta . We give a quantitative explanation for this behaviour by considering the classical period of unstable periodic orbits within the chaotic sea of the potential well.

We have measured the low temperature current-voltage characteristics ( I ( V )) of GaAs/A1As resonant tunnelling diodes with sub-micron lateral dimensions. Additional peaks in I ( V ) are observed due to resonant tunnelling via one-dimensional quantum wire states. In the presence of a magnetic field oriented perpendicular to the current and parallel to the wire the peaks show a complex splitting evolving into a regular series at high field with upt to 20 resonances. For the smallest device we are able to deduce the probability density of the lowest three bound states from the magnetic field dependence of the current and show that the confining potential is close to parabolic. For a magnetic field which is perpendicular to both the current and the wire a much weaker dependence on magnetic field is observed confirming the one-dimensional nature of our device. Finally, in the presence of a field oriented parallel to the current a continuous transition from electrostatic (at low field) to magnetic confinement (at high field) is observed.