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We demonstrate single-photon detection with a GaAs/AlGaAs modulation-doped field effect transistor. High sensitivity of the transistor to light is obtained by incorporating in its structure a layer of self-organized InAs quantum dots in close proximity to the conducting channel but separated by a thin AlGaAs barrier. We show that capture of a single photoexcited carrier by a quantum dot results in a sizeable change in the source–drain current of the transistor, allowing the detection of a single photon.

We report an experimental study of GaAs/Al 0.33 Ga 0.67 As modulation doped field effect (MODFET) transistors, in which an InAs layer of self-assembled quantum dots is placed in one of the Al 0.33 Ga 0.67 As barrier layers close to the two-dimensional electron gas (2DEG). We find the source–drain resistance is bistable with the two states controlled by illumination and applied gate bias. Brief illumination induces a large, persistent drop in the resistance, which can be recovered by applying a positive gate bias. Magneto-transport measurements show that while illumination causes only a relatively small change in the 2DEG density, it can greatly enhance its mobility. We suggest this is because the 2DEG mobility is limited by percolation of the electrons through the rough electrostatic potential induced by the charged dots. Illumination reduces the negative charge trapped in the dots, thus smoothing the conduction band potential, which produces a large increase in the mobility.

We have regrown, on ex situ patterned GaAs substrates, using hydrogen radical decontamination, two-dimensional electron gases (2DEGs) confined in 15 nm quantum wells (QWs) with the inverted AlGaAs/GaAs interface 30 nm from the regrowth interface (RI), which conduct before illumination at 1.5 K. Mobilities in excess of 1×10 6 cm 2 V −1 s −1 have been achieved. We have not observed a degradation in the mobility for a constant carrier concentration between 2DEGs, 200 and 30 nm from the RI. A control structure with the 2DEG 30 nm from the RI was also grown on an un-patterned GaAs substrate that was only thermally cleaned, which had a maximum mobility of 3×10 4 cm 2 V −1 s −1 after illumination at 1.5 K.

This audit analysed the Tanner and Whitehouse II twenty bone (TW2) method of bone age assessment which was used in our department, and compared it with the Greulich and Pyle (GP) method. 50 previous bone ages were independently re-calculated by each of three registrars using both techniques, with the time taken to perform each assessment being recorded. For each method the interobserver variation was analysed in terms of the spread of results. The intraobserver variation in TW2 was determined by comparing the bone age originally reported with that subsequently calculated on the same film by the same registrar. The average spread of results was 0.74 years for TW2 method, and 0.96 years for the GP method and this difference is not statistically significant at the 5% level. The average intraobserver variation to TW2 was 0.33 years, but with 95% confidence limits of -0.87 to +1.53 years. The average time taken was 7.9 min for TW2 and 1.4 min for GP assessments. It was concluded that the GP method gave similar reproducibility and was faster than the TW2 method. Following clinical discussion the routine departmental bone age assessment method was changed from the TW2 to the GP method.

Birefringent pulmonary talc granuloma are often found in lung biopsies from intravenous drug abusers (IVDA) but their clinico-pathological significance remains undefined.

We report a bistability in the resistance of a GaAs/AlGaAs modulation doped field effect transistor in which a layer of InAs self-organized quantum dots has been grown near the electron channel. Brief optical illumination causes a large, persistent drop in the two-dimensional electron gas (2DEG) resistance which can be recovered by allowing a current to flow through the Schottky gate. We demonstrate that illumination reduces the number of electrons trapped in the quantum dots, lowering their potential and thereby enhancing the 2DEG mobility. This bistability could be the basis of an optical memory or sensitive phototransistor.

Summary form only given. Single photon detectors find a diverse range of applications for measurement of very weak optical signals at the shot noise limit. Future applications such as quantum cryptography and computing also require devices for detection of single photons. Conventionally single photons are detected by multiplying a photo-generated electron using an avalanche process, either in a vacuum photomultiplier tube (PMT), or, in the case of the semiconductor avalanche photodiode (APD), a reverse biased p-n junction. We propose and demonstrate a novel device for detection of single photons based on a GaAs/AlGaAs modulation doped field effect transistor (MODFET) which does not rely on avalanche processes.

Two methods for diagnosing radiological osteopenia in thoracic (TS) and lumbar (LS) spine radiographs were assessed: a subjective conventional method (A) and a semiquantitative method (B), by comparing them with bone mineral density (BMD) measured by dual energy X-ray absorptiometry (DEXA), in a population of "normal" women aged 45-70 years (n = 818). For both methods there was good intraobserver and interobserver reproducibility. BMDs were significantly lower with increasing radiological osteopenia grades (p0.001), and remained lower after adjustment for age and body mass index (p0.01). The proportion of subjects with DEXA-defined osteoporosis rose with increasing radiological osteopenia grades for both methods. The worst osteopenia categories identified 29.7-55.3% of women with DEXA-defined osteoporosis, compared with 6.1-11.7% in the "normal" categories. Both methods, however, showed a large degree of overlap of BMDs between the various radiological osteopenia grades. The sensitivity and specificity of method A in diagnosing osteoporosis were 45.3% and 78.4%, respectively, for the TS and 19.0% and 94.3%, respectively, for the LS. For method B the sensitivities and specificities were 8.8% and 96.1%, respectively (TS), and 10.2% and 95.6%, respectively (LS). Although both methods have poor sensitivities, "definite" or "high" grade osteopenia should be an indication for bone densitometry. The high specificities suggest that a "normal" (no osteopenia) X-ray is unlikely to have a significantly low BMD.

We propose and demonstrate a type of GaAs/AlGaAs modulation-doped field effect transistor (FET) which is sensitive to single photons. The FET contains a layer of InAs quantum dots formed using an in-situ, self-organising method, adjacent to the channel and separated from it by a thin AlGaAs barrier. Capture of a single photo-excited carrier by a quantum dot leads to a sizeable change in the source-drain current through the transistor, allowing the detection of a single photon. We show this is because the mobility of the electron channel is extremely sensitive to the charge trapped in the dots. This discovery may allow a new type of single photon detector to be developed which does not rely upon avalanche processes.

We demonstrate that the conductance of a field-effect transistor (FET) gated by a layer of nanometer-sized quantum dots is sensitive to the absorption of single photons. Rather than relying upon an avalanche process, as in conventional semiconductor single-photon detectors, the gain in this device derives from the fact that the conductivity of the FET channel is very sensitive to the photoexcited charge trapped in the dots. This phenomenon may allow a type of three-terminal single-photon detector to be developed based upon FET technology.