Joungho Kim

Photoconductive sampling probe with 2.3‐ps temporal resolution and 4‐μV sensitivity

We report on a novel probe technology which is applied to the measurement of high‐speed guided electrical signals. The probe consists of a high‐impedance gate fabricated using an interdigitated electrode structure on semi‐insulating, low‐temperature‐grown GaAs, and its operation is based on the optoelectronic technique of photoconductive sampling. The probe has a dynamic range of ≳106, permitting the linear measurement of short‐duration signals with amplitudes ranging from microvolts up to several volts. Its resistance is 100 MΩ, and its capacitance is less than 0.1 fF, making this probe attractive for noninvasive, external circuit testing of ultrahigh‐speed devices and circuits.

1/f noise in gold nanoparticle chemosensors

ing of electrons between neighboring nanoparticles. 7 The reversible partitioning of the chemical species affects the tunnel coupling, and thus the resistance of the Au nanoparticle film typically changes in the presence of chemical species. Because the tunnel coupling is exponentially dependent on the properties of the medium in between the nanoparticles, they are highly sensitive sensors. Even a small amount of swelling associated with the absorption of chemical species into the thiolate coating can lead to a significant increase in film resistance. Such sensors are currently being developed as the detector elements of a microfabricated gas chromatography system which can analyze complex mixtures of gases for a wide range of applications, including monitoring of industrial emissions, detection of explosives, and medical diagnostics. 8,9

Transient velocity overshoot dynamics in GaAs for electric fields ≤ 200 kV/cm

We have experimentally studied the transient velocity overshoot dynamics of photoexcited carriers in GaAs for electric fields as great as 200 kV/cm. Time domain waveforms proportional to the velocity and the acceleration of carriers have been acquired, respectively, from guided and free‐space radiating signals which contain terahertz frequency components. The measurements demonstrated that the degree of overshoot was maximized for an electric field on the GaAs between 40 and 50 kV/cm when 1.44‐eV photons in an 80‐fs laser pulse excited the sample. For carriers excited with higher initial energy (1.55 eV), the degree of overshoot decreased and the maximum degree of overshoot occurred at a higher electric field.

Transport properties of InAsxSb1−x (0≤x≤0.55) on InP grown by molecular‐beam epitaxy

Molecular‐beam epitaxy has been successfully used to grow InAsxSb1−x on InP substrates with good electrical characteristics. The samples are all n type with electron concentrations varying in the range (3–9)×1015 cm−3. The mobilities are high (70 000 and 110 000 cm2/V s at 300 and 77 K, respectively) in InSb and the alloys. More importantly, the mobilities remain high at the low temperatures in the alloys also, without any type conversion. The mobility data have been analyzed taking into account the appropriate scattering mechanisms. The alloy scattering potential in InAs0.24 Sb0.76 is estimated to be 0.3 V.

Uniformity of optical absorption in HgCdTe epilayer measured by infrared spectromicroscopy

Infrared absorption in HgCdTe epitaxial material has been investigated using infrared spectromicroscopy to study the uniformity at dimensions representative of typical infrared detectors. Infrared transmission measurements were performed on HgCdTe material using an infrared beam diameter of 9 μm. Line scans and area maps of transmission spectra were obtained to investigate statistical variations in infrared absorption. The HgCdTe material demonstrates a high degree of uniformity, with a standard deviation in absorption coefficient near the sensitive turn-on region of less than 3% and standard deviation in extracted Hg1-xCdxTe compositon of 3×10−4.

High-speed metal-semiconductor-metal photodiodes with Er-doped GaAs

Very high-speed MSM photodiodes have been fabricated on Er-doped GaAs over a doping range of 10/sup 18/-10/sup 20/ cm/sup -3/. The impulse response (characterized by photoconductive sampling) of these diodes, with finger widths/spacings of 2 /spl mu/m, has been found to be tunable over a range of about 3 ps-22 ps. Electro-optic sampling was used to characterize MSM diodes with finger widths/spacings of 0.5 /spl mu/m and 1 /spl mu/m on a sample with [Er]=10/sup 19/ cm/sup -3/, resulting in 3-dB bandwidths of 160 GHz and 140 GHz, respectively, corresponding to pulse widths of 2.7 ps and 3.3 ps. Correlation measurements were also done on the GaAs:Er samples, using an all-electronic Sampling Optical Temporal Analyzer (SOTA) structure.<<ETX>>

Purging of dropwise condensate by electrowetting

The onset of motion of a dropwise condensate (on inclined substrate) under applied direct current potential (with an overhanging electrode) is studied experimentally and described based on overcoming of the static three-phase contact line (TCL) friction by electrowetting. Electrowetting phenomenon exerts a radial electrostatic force on the TCL, causing an imbalance in the TCL surface tension forces, thus initiating the surface droplet motion (by an applied threshold, local electric field intensity). The experimental results using overhanging wire electrodes have identified three regimes (gravity dominated, intermediate, and surface force dominated) and show significant lowering of the critical inclination angle for the liquid droplet. In the intermediate Bond number regime (both gravity and surface liquid-gas tension are significant), the critical (for onset of motion) electrostatic potential is predicted and good agreement is found with experiments.

Time-domain network analysis of MM-wave circuits based on a photoconductive probe sampling technique

A photoconductive probe sampling technique with 2-ps temporal resolution and microvolt sensitivity has been developed. The photoconductive probe sampling technique combines the ultrafast optical technology of the 120-fs Ti-sapphire short pulse laser and the microfabrication technology of the silicon-on-sapphire photoconductive sampling probe, which consists of a high-impedance interdigitated photoconductive switch. This technique can be applied to the measurement of the S-parameters of millimeter-wave circuit components with a 120-GHz measurement bandwidth. The probe technology was applied to the characterization of millimeter-wave band-block filters used in InP-based heterostructure MMICs (monolithic microwave integrated circuits) for 90-GHz to 180-GHz frequency doubling. Millimeter-wave delay lines have also been characterized, and the properties of transmission lines on thin semiconductor substrates have been studied.<<ETX>>

Novel high-impedance photoconductive sampling probe for ultra-high speed circuit characterization

Reports on a novel probe technology applied to the measurement of high-speed guided electrical signals. The probe, based on the optoelectronic technique of photoconductive sampling, consists of a high-impedance gate fabricated using an interdigitated electrode structure on semi-insulating low-temperature-MBE (molecular beam epitaxy)-grown GaAs or on silicon-on-sapphire. Its resistance is 100 M Omega and its capacitance is less than 0.1 fF, making this probe very attractive for the noninvasive, external circuit testing of ultrahigh-speed devices and circuits with a 120-GHz measurement bandwidth. A 30-ps switching time was measured for an E/D-mode InAlAs/InGaAs HIGFET inverter using the probe.<<ETX>>

Photoconductive step-function sampling

Measurement of picosecond electrical signals using a photoconductive step-function gate is demonstrated analytically and experimentally. The time resolution of our step-function technique is limited only by the rise time of the step-function, which is approximately the same as the laser pulse width. Also, a regular, undoped semiconductor material, which is essential for the realization of a short-duration gate, can be used instead of the highly defected material. The use of undoped material gives 10 to 100 times higher sensitivity in the measurement than the impulse-function technique because of the high mobility of the undoped material.<<ETX>>