Michael Y. Frankel

Subpicosecond carrier lifetime in GaAs grown by molecular beam epitaxy at low temperatures

Epitaxial GaAs grown by molecular beam epitaxy (MBE) at low substrate temperatures is observed to have a significantly shorter carrier lifetime than GaAs grown at normal substrate temperatures. Using femtosecond time‐resolved‐reflectance techniques, a sub‐picosecond (<0.4 ps) carrier lifetime has been measured for GaAs grown by MBE at ∼200°C and annealed at 600u2009°C. With the same material as a photoconductive switch we have measured electrical pulses with a full‐width at half‐maximum of 0.6 ps using the technique of electro‐optic sampling. Good responsivity for a photoconductive switch is observed, corresponding to a mobility of the photoexcited carriers of ∼120–150 cm2/Vu2009s. GaAs grown by MBE at 200u2009°C and annealed at 600u2009°C is also semi‐insulating, which results in a low dark current in the switch application. The combination of fast recombination lifetime, high carrier mobility, and high resistivity makes this material ideal for a number of subpicosecond photoconductive applications.

Terahertz attenuation and dispersion characteristics of coplanar transmission lines

Experimental verification of analytic formulas for the dispersion and the attenuation of electrical transient signals propagating on coplanar transmission lines is presented. The verification is done in the frequency domain over a terahertz range although the experiments are in the time domain. The analytic formulas are obtained from fits to the full-wave analysis results. It is quantitatively verified that the full-wave steady-state solutions can be directly applied to the transient time-domain propagation experiments. Subpicosecond electrical pulses and an external electrooptic sampling technique are used to obtain the time-domain propagation data. From the Fourier transforms of the time-domain data both the attenuation and the phase information as a function of frequency are extracted. The dispersion and the attenuation characteristics are investigated for both coplanar waveguide and coplanar strip transmission lines. The investigation is carried out on both semiinsulating semiconductor and dielectric substrate materials. No observable losses caused by the semiconductor material are indicated. >

Experimental characterization of external electrooptic probes

The accuracy and invasiveness of various external LiTaO/sub 3/ electrooptic probe (EEP) geometries are investigated experimentally. The EEPs studied are used for microwave coplanar transmission-line measurements. It is shown that commonly employed EEP geometries can cause substantial inaccuracies in their measurements and that these inaccuracies can be related to specific geometric parameters. The major cause of distortions in pulse measurements was found to be attributable to electromagnetic radiation coupling into the LiTaO/sub 3/ crystal and resonating between the top and bottom crystal interfaces. The authors suggest eliminating the deleterious bulk resonance effects due to the EEP material bulk by using a thin LiTaO/sub 3/ crystal that will act as a single lumped element. The thickness that is expected to be optimal should be about the extent of the guided mode confinement near the conductors. A thicker than optimum cryst al will exhibit resonance, and a thinner one will have reduced sensitivity due to a reduced signal integration path. Thinned crystals possess the additional advantages of reduced thermal drift and reduced stray signal pickup from adjacent lines.<<ETX>>

High-voltage picosecond photoconductor switch based on low-temperature-grown GaAs

A GaAs material grown by molecular beam epitaxy at a low substrate temperature was used to fabricate a photoconductor switch that produces 6-V picosecond electrical pulses. The pulses were produced on a microwave coplanar-strip transmission line lithographically patterned on the low-temperature (LT) GaAs. A 150-fs laser pulse was used to generate carriers in the LT GaAs gap between the metal strips, partially shorting a high DC voltage placed across the lines. The 6-V magnitude of the electrical pulses obtained is believed to be limited by the laser pulse power and not by the properties of the LT GaAs. Experiments were also performed on a picosecond photoconductor switch fabricated on a conventional ion-damaged silicon-on-sapphire substrate. Although comparable pulse durations were obtained, the highest pulse voltage achieved with the latter device was 0.6 V. >

A wide-band fiber-optic true-time-steered array receiver capable of multiple independent simultaneous beams

A wide-band fiber-optic true-time-delay array receiver capable of receiving multiple independent beams simultaneously is demonstrated for the first time. The receiver is based on a dispersive prism technique and exhibits squint-free /spl plusmn/60/spl deg/ azimuthal steering of two simultaneous beams with no observable beam squint over a microwave component-limited bandwidth of 6-18 GHz.

External electro-optic integrated circuit probing

Abstract An external electro-optic measurement system with subpicosecond resolution has been developed. This electro-optic sampling system is designed to operate as a non-contact probe of voltages in electrical devices and circuits with modified wafer-level test equipment and no special circuit preparation. Measurements demonstrate the systems ability to probe continuous and pulsed signals on microwave integrated circuits on arbitrary substrates with single-micron spatial resolution. We also discuss the application of external electro-optic sampling to various aspects of time-domain circuit studies, including the generation of short electrical test pulses using novel photoconductive techniques and the propagation of pulses on interconnects.

Ultrahigh-bandwidth vector network analyzer based on external electro-optic sampling

Abstract We report the development of an ultrahigh-bandwidth vector network analyzer useful for small-signal characterization of high-speed semiconductor devices. It employs 100-fs optical pulses for making terahertz-bandwidth electro-optic measurements of electrical signals, as well as for sub-picosecond, photoconductive, electrical-stimulus-signal generation. High-bandwidth coplanar strip transmission lines are used for signal transmission. A 0.15 × 50 μm gate AlGaAs/InGaAs/GaAs HFET has been characterized over a bandwidth of 100 GHz using this network analyzer. A comparison with conventional RF network analyzer measurements performed to 40 GHz demonstrated good agreement throughout this bandwidth. Such measurements of the actual device characteristics across their entire operating frequency range should improve device development and incorporation into active circuits.

Reduced loss microwave fiber-optic links by intracavity modulation and carrier suppression

Fiber-optic links are limited by the large V/sub /spl pi// of commercial modulators and by nonlinearities in the photodiodes used to demodulate the RF signals. Intracavity modulation and carrier suppression is shown to enhance the received microwave power in a fiber-optic link for a given optical power at a receiver. The carrier wavelength is automatically locked to the intracavity reflective notch filter that suppresses the carrier relative to the modulation sidebands. Link loss reductions exceeding 10 db were measured for frequencies from 1 to 25 GHz. Harmonic distortions can be decreased by combining this technique with above-quadrature biasing.

Large-signal modeling and study of power saturation mechanisms in heterojunction bipolar transistors

A harmonic-balance-based large-signal model of the heterojunction bipolar transistor (HBT) suitable for microwave circuit design and simulation has been developed. Equivalent circuits were extracted from measured small-signal S-parameters by equivalent circuit fitting and used in a harmonic-balance-based simulator. Modeled power and gain results were in good agreement with directly measured characteristics. The power saturation mechanisms have been established in terms of current/voltage amplitudes and power-dependent equivalent circuit elements. Saturation has been shown to be due to the signal-voltage swing entering the cutoff or saturation regions with the resulting loss of current gain. It is pointed out that the model can accurately predict experimentally measured AlGaAs/GaAs HBT power characteristics.<<ETX>>

A study and optoelectronic verification of AlGaAs/GaAs heterojunction bipolar transistor large-signal characteristics

A hybrid optoelectronic measurement system is constructed and used to obtain the large-signal characteristics of AlGaAs/GaAs heterojunction bipolar transistors. The measurement system utilizes a terahertz-bandwidth electrooptic transducer gated by 100-fs laser pulses to interrogate the time-domain waveforms at the device input and output nodes. A microwave signal phase-locked to the laser pulse-train is used to synchronously excite the device in both small-signal and large-signal regimes. The measurement system is capable of 50-GHz bandwidth and provides time-domain voltage waveforms that can be used directly to verify the time-domain results of the large-signal analysis. >