Thomas Y. Hsiang

Picosecond GaAs‐based photoconductive optoelectronic detectors

A novel material deposited by molecular beam epitaxy at low substrate temperatures using Ga and As4 beam fluxes has been used as the active layer for a high‐speed photoconductive optoelectronic switch. The high‐speed photoconductive performance of the material was assessed by fabricating two devices: an Auston switch and a photoconductive‐gap switch with a coplanar transmission line. In a coplanar transmission line configuration, the speed of response is 1.6 ps (full width at half maximum) and the response is 10 to 100 times greater than that of conventional photoconductive switches. Since the material is compatible with GaAs discrete device and integrated circuit technologies, this photoconductive switch may find extensive applications for high‐speed device and circuit testing.

Spectral dependence of 1ƒγ noise on gate bias in n-MOSFETS

Abstract The modified McWhorter model that has been developed before to explain the 1 ƒ γ noise in Metal-Oxide-Semiconductor-Field-Effect-Transistors was extended to include frequency dependence of the noise power spectrum on operating voltages. Under the assumption of energy and spatial distribution of traps in the oxide, the drain voltage noise spectrum was calculated for MOSFETs operating in the linear region. Experimentally flicker noise measured on n -channel enchancement-mode MOSFETs up to the frequency of 4 kHz. The actual value of the exponent γ in the 1 ƒ γ spectrum was found to increase with gate bias. Furthermore, it was found that the magnitude of the noise power decreased with gate bias. The detail of the calculations and the experimental results are presented.

Propagation model for ultrafast signals on superconducting dispersive striplines

The algorithm suitable for the computer-aided design of transmission lines is used to model the propagation of picosecond and subpicosecond electrical signals on superconducting planar transmission lines. Included in the computation of a complex propagation factor are geometry-dependent modal dispersion and the frequency-dependent attenuation and phase velocity which arise as a result of the presence of a superconductor in the structure. The results of calculations are presented along with a comparison to experimental data. The effects of modal dispersion and the complex surface conductivity of the superconductor are demonstrated, with the conclusion that it is necessary to incorporate both phenomena for accurate modeling of transient propagation in strip transmission lines. >

Study of 1/f noise in N-MOSFET's: Linear region

A modified McWhorter model has been developed to explain the mechanisms involved in the1/fnoise in n-channel metal-oxide, semiconductor field-effect transistors (MOSFETs). Under the assumption of an energy distribution of traps in the bandgap, an expression for the power spectral density of1/fnoise was derived for MOSFETs operating in the linear region. Experimentally, noise measurements were performed on short-channel enhancement-mode MOSFETs with gate widths of 100 µm, and varying gate lengths of 2 to 10 µm. It was found that noise power increased with increasing drain voltage or decreasing gate bias. Quantitative analyses have been done to compare the experimental results with the model calculations.

Ultrafast nanoscale metal-semiconductor-metal photodetectors on bulk and low-temperature grown GaAs

Metal‐semiconductor‐metal photodetectors of finger spacing and width as small as 100 nm have been fabricated on bulk and low‐temperature grown GaAs, and tested using a femtosecond pulse laser and high‐speed electro‐optic sampling. The fastest photodetectors have a measured full width at half maximum impulse response and a 3‐dB bandwidth of 0.87 ps and 510 GHz, respectively, for low‐temperature grown GaAs limited by carrier recombination time; and of 1.5 ps and 295 GHz for bulk GaAs, limited by the RC time constant. To our knowledge, they are the fastest detectors of their kinds reported to date.

A thermal activation model for 1/ƒy noise in Si-MOSFETs

Abstract Detailed measurements of the temperature dependence of noise power spectra in a series of commercial p-channel MOSFETs are presented. It was found that both the magnitude and the functional form of the voltage noise power spectral density varied greatly in the range between 60 and 260 K. The experimental results were compared to first-principle calculations which showed that the noise was due to the capture and emission of carriers by oxide traps through thermal activation. The process caused fluctuations in both the density and the surface mobility of the channel carriers through the modulation of the surface potential and the scattering rate respectively.

Intrinsic picosecond response times of Y–Ba–Cu–O superconducting photodetectors

We report our femtosecond time-resolved measurements on the photoresponse of an epitaxial YBa2Cu3O7−x (YBCO) thin-film photodetector, patterned into a microbridge geometry. By varying the current–voltage biasing conditions between the superconducting and resistive (hot spot) states, we observed transients that correspond to the nonequilibrium kinetic-inductance and the nonequilibrium electron-heating response mechanisms, respectively. The two-temperature model and the Rothwarf–Taylor theory have been used to simulate the measured wave forms and to extract the temporal parameters. The electron thermalization time and the electron–phonon energy relaxation time were determined by the electron temperature rise and decay times, which were found to be 0.56 and 1.1 ps, respectively, in the resistive state. We have also measured the ratio between the phonon and electron specific heats to be 38, which corresponds to a phonon–electron scattering time of 42 ps. No phonon-trapping effect (typical for low-temperature ...

High‐frequency characterization of thin‐film Y‐Ba‐Cu oxide superconducting transmission lines

We report the first measurements of picosecond pulse propagation on transmission lines patterned from YBa2Cu3O7−x films. Distortion‐free propagation of high current density transients is demonstrated. The high‐frequency properties were analyzed by careful study of the relative phase delays of the electrical transients as the temperature of the sample was varied from 1.8 K to Tc. Simulations using Mattis–Bardeen complex conductivities showed good agreement with the measured results. High‐frequency critical current densities in excess of 105 A/cm2 were measured.

Determination of Si-SiO/sub 2/ interface trap density by 1/f noise measurements

The concentration of silicon-silicon dioxide interface traps with energies down to within 20 meV of the majority-carrier band edge was determined using 1/f noise measurements. p-type diffused resistors of four-probe geometry were fabricated in a metal-oxide-silicon structure. Flicker noise measurements were performed on these devices at cryogenic temperatures of 20 to 280 K. Using A. L. McWhorters (1957) 1/f noise model and the calculated position of the Fermi level with respect to the valence band edge at each temperature, the density of interface traps was calculated at energy levels corresponding to the position of the Fermi level at that temperature. This technique is proposed as an alternate method to measure the oxide trap and slow interface-state densities with energies close to the band edges. >

Ultrafast photoresponse in microbridges and pulse propagation in transmission lines made from high-T/sub c/ superconducting Y-Ba-Cu-O thin films

We report our femtosecond time-resolved measurements of the photoresponse of microbridges in YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films, performed using an electrooptic sampling technique. Our test structures consisted of 5-/spl mu/m-wide, 7-/spl mu/m-long microbridges, incorporated in 4-mm-long coplanar waveguides, fabricated in 100-nm-thick, high-quality epitaxial YBCO films grown on LaAlO/sub 3/ substrates by laser deposition. When varying the biasing conditions between the superconducting and switched states, we observed transients of single-picosecond duration that corresponded to the nonequilibrium kinetic-inductance and the electron-heating response mechanisms, respectively. In both cases, experimental waveforms could be accurately simulated using a nonequilibrium (two-temperature) electron-heating model. From the fits, the YBCO intrinsic temporal parameters associated with the nonequilibrium conditions were extracted. The electron thermalization time was found to be 0.56 ps in the state above the materials critical temperature (T/sub c/=89 K) and 0.9/spl plusmn/0.1 ps in the superconducting state at temperatures ranging from 20 to 80 K. The electron-phonon energy relaxation time was found to be 1.1 ps. The single-picosecond pulse distortion due to propagation on a YBCO coplanar waveguide was also studied. Our results show that a YBCO microbridge can intrinsically operate as a photodetector at rates exceeding 100 Gb/s, making it useful as an optical-to-electrical transducer for optoelectronic interfaces in YBCO digital electronics. Simultaneously, YBCO mixers, based on hot-electron effects, should exhibit an intrinsic bandwidth exceeding 100 GHz.