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28 Oct 2002

Volume 81, Issue 18, pp. 3311-3500

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Dependence of the detective quantum efficiency of photoconductive x-ray image detectors on charge transport parameters and exposure: Application to a-Se

Safa O. Kasap, M. Zahangir Kabir, John A. Rowlands, Olivier Tousignant, Jonathan Leboeuf, Luc Laperrière, and Yves Demers

Appl. Phys. Lett. 81, 3482 (2002); http://dx.doi.org/10.1063/1.1517173 (3 pages) | Cited 2 times

Online Publication Date: 22 October 2002

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The effects of charge carrier trapping (i.e., incomplete charge collection) on the detective quantum efficiency (DQE) of a photoconductive detector are studied by using a cascaded linear system model. The model includes signal and noise propagations in the following stages: (1) x-ray attenuation, (2) conversion gain, (3) charge collection, and (4) the addition of electronic noise. We examine and discuss the DQE of an a-Se fluoroscopic detector as a function of charge transport parameters and x-ray exposure. The theoretical model shows a very good agreement with experimental DQE versus exposure characteristics. © 2002 American Institute of Physics.
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07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
29.40.Wk Solid-state detectors
85.60.-q Optoelectronic devices

Fabrication and subpicosecond optical response of low-temperature-grown GaAs freestanding photoconductive devices

R. Adam, M. Mikulics, A. Förster, J. Schelten, M. Siegel, P. Kordoš, X. Zheng, S. Wu, and R. Sobolewski

Appl. Phys. Lett. 81, 3485 (2002); http://dx.doi.org/10.1063/1.1518159 (3 pages) | Cited 9 times

Online Publication Date: 22 October 2002

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We have fabricated and tested freestanding low-temperature-grown GaAs photoconducting devices exhibiting subpicosecond photoresponse. The epitaxially grown thin-film microswitches were transferred on top of sapphire and Si/SiO2 substrates and integrated with Ti/Au coplanar strip transmission lines. Our devices exhibited above 200 V breakdown voltages and dark currents at 100 V below 3×10−7 A. The photoresponse of our microswitches was measured using electro-optic sampling technique with 100-fs-wide laser pulses at wavelengths of 810 nm and 405 nm as the excitation sources. For 810-nm excitation, we measured 0.55-ps-wide electrical transients with voltage amplitudes of up to 1.3 V. The ultraviolet excitation resulted in 1.35-ps-wide transients due to the hot carriers generation and intraband relaxation. © 2002 American Institute of Physics.
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85.60.Bt Optoelectronic device characterization, design, and modeling
73.61.Ey III-V semiconductors
73.50.Fq High-field and nonlinear effects
73.50.Mx High-frequency effects; plasma effects

On–off switching of edge direct tunneling currents in metal-oxide-semiconductor field-effect transistors

Ming-Jer Chen and Ming-Pei Lu

Appl. Phys. Lett. 81, 3488 (2002); http://dx.doi.org/10.1063/1.1518563 (3 pages) | Cited 8 times

Online Publication Date: 22 October 2002

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On–off switching behaviors or two-level random telegraph signals (RTS) are measured in the low voltage (−1.40 V<VG<−0.88 V) edge direct tunneling currents in ultrathin gate stack (10 Å oxide+10 Å nitride) n-channel metal-oxide-semiconductor field-effect transistors. The plausible origin is the process-induced defects in terms of localized gate stack thinning (or equivalently the conductive filament). In such extrinsic case, the current trapping–detrapping theories can adequately elucidate the data, particularly the RTS magnitude as large as 18%. The current–voltage characteristic associated with a certain defective spot is assessed straightforwardly, showing remarkable compatibility with existing oxide thinning case. Systematic measurements of RTS in the terminal currents allow for determining the occurrence probability as well as locations of defects, and may be treated as a sensitive process monitor. © 2002 American Institute of Physics.
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85.30.Tv Field effect devices
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
81.05.Cy Elemental semiconductors

Self-heating effects at high pump currents in deep ultraviolet light-emitting diodes at 324 nm

A. Chitnis, J. Sun, V. Mandavilli, R. Pachipulusu, S. Wu, M. Gaevski, V. Adivarahan, J. P. Zhang, M. Asif Khan, A. Sarua, and M. Kuball

Appl. Phys. Lett. 81, 3491 (2002); http://dx.doi.org/10.1063/1.1518155 (3 pages) | Cited 5 times

Online Publication Date: 22 October 2002

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We present a detailed high-pump-current study of self-heating effects in ultraviolet light-emitting diodes (LEDs) grown on sapphire. For deep ultraviolet LEDs on sapphire, our results establish self-heating to be a primary cause of premature power saturation under dc pumping. Even the flip-chip packaged devices undergo a steady-state temperature rise to about 70 °C at a dc pump current of only 50 mA (at 8 V) resulting in a significant decrease in LED output. Temperature rise values estimated from peak emission wavelength shifts and from micro-Raman mapping of the active devices were in good agreement. © 2002 American Institute of Physics.
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85.60.Jb Light-emitting devices
85.30.De Semiconductor-device characterization, design, and modeling
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