Scott T. Allen

Active and nonlinear wave propagation devices in ultrafast electronics and optoelectronics

We describe active and nonlinear wave propagation devices for generation and detection of (sub)millimeter wave and (sub)picosecond signals. Shock-wave nonlinear transmission lines (NLTLs) generate /spl sim/4-V step functions with less than 0.7-ps fall times. NLTL-gated sampling circuits for signal measurement have attained over 700-GHz bandwidth. Soliton propagation on NLTLs is used for picosecond impulse generation and broadband millimeter-wave frequency multiplication. Picosecond pulses can also be generated on traveling-wave structures loaded by resonant tunneling diodes. Applications include integration of photodetectors with sampling circuits for picosecond optical waveform measurements and instrumentation for millimeter-wave waveform and network (circuit) measurements both on-wafer and in free space. General properties of linear and nonlinear distributed devices and circuits are reviewed, including gain-bandwidth limits, dispersive and nondispersive propagation, shock-wave formation, and soliton propagation. >

Travelling-wave photodetectors with 172-GHz bandwidth and 76-GHz bandwidth-efficiency product

Results of the first fabrication and measurement of travelling-wave photodetectors are reported. The devices have bandwidths as high as 172 GHz, the highest reported for a p-i-n photodetector, and bandwidth-efficiency products as large as 76 GHz, the largest reported for any photodetector without gain. Comparisons with vertically illuminated and waveguide photodetectors fabricated on the same wafer establish the superior performance of travelling-wave photodetectors. Microwave loss on the travelling-wave photodetector structure is identified as a bandwidth limitation.<<ETX>>

DC - 725 GHz sampling circuits and subpicosecond nonlinear transmission lines using elevated coplanar waveguide

Nonlinear transmission lines (NLTLs) fabricated with Schottky diodes on GaAs were used to electrically generate 3.7-V step functions that had a measured 10%-90% fall time of 0.68 ps. These NLTLs were integrated on wafer with sampling circuits that had a measured 3-dB bandwidth of 725 GHz. Key to circuit performance are the use of low-loss, high-wave-velocity elevated coplanar waveguide transmission lines and the elimination of active device pad parasitics by contacting devices above the plane of the wafer.<<ETX>>

A traveling-wave resonant tunnel diode pulse generator

Traveling-wave resonant funnel diode (TWRTD) pulse generators comprising transmission lines periodically loaded by GaAs/AlAs resonant tunnel diodes (RTDs) are fabricated. The TWRTD pulse generators have convolved transition times of 3.5 ps when measured with an active probe. Using identical RTDs, TWRTD pulse generators can attain smaller transition times than those obtained with switching circuits employing a single RTD.<<ETX>>

Millimeter-wave on-wafer waveform and network measurements using active probes

We have fabricated active probes for on-wafer waveform and network measurements. The probes incorporate GaAs nonlinear transmission line (NLTL) based network analyzer (NWA) integrated circuits and low-loss quartz coplanar-waveguide probe tips. The active probes show step response falltimes of 2.7 ps when excited by a 0.7-ps falltime input, Using these active probes, we demonstrate both waveform measurements with 2.7-ps risetime and network measurements to 200 GHz. We discuss the probe tip and NWA IC design, the hybrid assembly and mechanical design, and system design considerations. On-wafer waveform and S-parameter measurements of monolithic millimeter-wave integrated circuits are demonstrated. >

A broadband free-space millimeter-wave vector transmission measurement system

We report both broadband monolithic transmitter and receiver ICs for MM-wave electromagnetic measurements. The ICs use a nonlinear transmission line (NLTL) and a sampling circuit as a picosecond pulse generator and detector. The pulses are radiated and received by planar monolithic bow-tie antennas, collimated with silicon substrate lenses and off-axis parabolic reflectors. Through Fourier transformation of the received pulse, accurate 30-250 GHz free space gain-frequency and phase-frequency measurements are demonstrated. Systems design considerations are discussed, and a variety of MM-wave broadband transmission measurements are demonstrated. >

Picosecond measurements by free-running electro-optic sampling

Timing jitter in passively mode-locked lasers is an important limit in electro-optic measurements. We demonstrate picosecond resolution electro-optic sampling measurements in the presence of nanosecond-scale timing jitter. Timing-jitter distortion is eliminated by scanning at a large offset frequency and triggering on the resulting waveform. Frequency jitter and the effects of triggering on a noisy signal are analyzed. Using this method, 1.5 ps fall times are measured on electronically generated shock waveforms.<<ETX>>

Picosecond GaAs monolithic optoelectronic sampling circuit

An optoelectronic sampling circuit has been fabricated for direct measurement of picosecond optical waveforms. The monolithic device incorporates a GaAs Schottky photodetector and a high-speed sampling circuit gated by a nonlinear transmission line strobe pulse generator. Excited by a 850-nm mode-locked dye laser, a 5.6-ps FWHM impulse response is measured; the authors estimate a deconvolved impulse response of approximately 4.5 ps FWHM.<<ETX>>

AlAs/GaAs Schottky-collector resonant-tunnel-diodes

Abstract The Schottky-collector resonant-tunnel-diode (SRTD) is an resonant-tunnel-diode with the normal N + collector layer and ohmic contact replaced by direct Schottky contact to the space-charge layer, thereby eliminating the associated parasitic series resistance R ins. By scaling the Schottky collector contact to submicron dimensions, the device periphery-to-area ratio is increased, decreasing the periphery-dependent components of the parasitic resistance, and substantially increasing the devices maximum frequency of oscillation. We report measured d.c. and microwave parameters of planar SRTDs fabricated with 1 μm-geometries in AlAs/GaAs.

Full two-port on-wafer vector network analysis to 120 GHz using active probes

A millimeter-wave full two-port on-wafer vector network analyzer (VNA) is implemented with monolithic GaAs directional time-domain reflectometer integrated circuits mounted directly on low-loss microwave wafer probes. The VNA performs S-parameter measurements to 120 GHz with +or- 0.2 dB repeatability using a line-reflect-match calibration method.<<ETX>>