Wide-band switches and true-time delay (TTD) phase shifters have been developed using distributed microelectromechanical system (MEMS) transmission lines for applications in phased-array and communication systems. The design consists of a coplanar waveguide (CPW) transmission line (W=G=100 /spl mu/m) fabricated on a 500 /spl mu/m quartz substrate with fixed-fixed beam MEMS bridge capacitors placed periodically over the transmission line, thus creating a slow-wave structure. A single analog control voltage applied to the center conductor of the CPW line can vary the phase velocity of the loaded line by pulling down on the MEMS bridges to increase the distributed capacitive loading. The resulting change in the phase velocity yields a TTD phase shift. Alternatively, the control voltage can be increased beyond the pull-down voltage of the MEMS bridges such that the capacitive loading greatly increases and shorts the line to ground. The measured results demonstrate 0-60 GHz TTD phase shifters with 2 dB loss/118/spl deg/ phase shift at 60 GHz (/spl sim/4.5-ps TTD) and 1.8 dB loss/84/spl deg/ phase shift at 40 GHz. Also, switches have been demonstrated with an isolation of better than 40 dB from 21 to 40 and 40 to 60 GHz. In addition, a transmission-line model has been developed, which results in very close agreement with the measured data for both the phase shifters and switches. The pull-down voltage is 10-23 V, depending on the residual stress in the MEMS bridge. To our knowledge, this paper presents the first wide-band TTD MEMS phase shifters and wide-band switches to date.

Distributed MEMS true-time delay phase shifters and wide-band switches

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 (NLTL's) 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 NLTL's 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. >

Active and nonlinear wave propagation devices in ultrafast electronics and optoelectronics

This paper describes the design and fabrication of distributed analog phase-shifter circuits. The phase shifters consist of coplanar-waveguide (CPW) lines that are periodically loaded with varactor diodes. The circuits are fabricated on GaAs using standard monolithic processing techniques. The phase velocity on these varactor diode-loaded CPW lines is a function of applied reverse bias, thus resulting in analog phase-shifting circuits. Optimally designed circuits exhibit 0/spl deg/-360/spl deg/ phase shift at 20 GHz with a maximum insertion loss (IL) of 4.2 dB. To the best of our knowledge, this is the lowest reported IL for a solid-state analog phase shifter operating at 20 GHz.

Distributed analog phase shifters with low insertion loss

The terahertz, or far-infrared, region of the electromagnetic spectrum is of critical importance in the spectroscopy of condensed matter systems. The electronic properties of semiconductors and metals are greatly influenced by bound states (e.g., excitons and Cooper pairs) whose energies are resonant with terahertz photons. The terahertz regime also coincides with the rates of inelastic processes in solids, such as tunneling and quasiparticle scattering. As a final example, confinement energies in artificially synthesized nanostruclures, like quantum wells, lie in the terahertz regime. In spite of its importance, terahertz spectroscopy has been hindered by the lack of suitable tools. As pointed out in the introduction to this book, sweptfrequency synthesizers for millimeterand submillimeter-waves are limited to below roughly 100 CHz, with higher frequencies only available using discrete frequency sources. Fourier Transform InfraRed (FTIR) spectroscopy, on the other hand, is hampered by the lack of brightness of incoherent sources. In addition, FTIR methods are not useful if the real and imaginary part of response functions must be measured at each frequency. Terahertz Time-Domain Spectroscopy (THz-TDS) is a new spectroscopic technique that overcomes these difficulties in a radical way. Its advantages have resulted in rapid proliferation within the last few years from a handful of ultrafast laser experts to researchers in a wide range of disciplines. THz-TDS is based on electromagnetic transients generated opto-electronically with the help of femtosecond (lfs = 10 -15 s) duration laser pulses. These THz transients are single-cycle bursts of electromagnetic radiation of typically less than 1 ps duration. Their spectral density spans the range from below 100 GHz to more than 5 THz. Optically-gated detection allows direct measurement of the terahertz electric field with a time resolution of a fraction of a picosecond. From this measurement both the real and imaginary part of the dielectric function of a medium may be extracted, without having to resort to the Kramers-Kronig relations. Furthermore, the brightness of the THz transients exceeds that of conventional thermal sources, and the gated detection is orders of magnitude more sensitive than bolometric detection. Recent developments have shown that THz-TDS has capabilities that go far beyond linear far-infrared spectroscopy. Because the THz transients are perfectly time-synchronized with the optical pulses that generate them, THz-TDS is ideally suited for "visible-pump, THz-probe" experiments. In

Terahertz time-domain spectroscopy

The design and optimization of distributed micromechanical system (MEMS) transmission-line phase shifters at both U- and W-band is presented in this paper. The phase shifters are fabricated on 500 /spl mu/m quartz with a center conductor thickness of 8000 /spl Aring/ of gold. The U-band design results in 70/spl deg//dB at 40 GHz and 90/spl deg//dB at 60 GHz with a 17% change in the MEMS bridge capacitance. The W-band design results in 70/spl deg//dB from 75 to 110 GHz with a 15% change in the MEMS bridge capacitance. The W-band phase-shifter performance is limited by the series resistance of the MEMS bridge, which is estimated to be 0.15 /spl Omega/. Calculations demonstrate that the performance of the distributed MEMS phase shifter can be greatly increased if the change in the MEMS bridge capacitance can be increased to 30% or 50%. To our knowledge, these results present the best published performance at 60 and 75-110 GHz of any nonwaveguide-based phase shifter.

Optimization of distributed MEMS transmission-line phase shifters-U-band and W-band designs

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

A traveling-wave resonant tunnel diode pulse generator

A millimeter-wave vector network analyzer is implemented with a monolithic GaAs directional time-domain reflectometer integrated circuit mounted directly on a microwave wafer probe. The vector network analyzer performs on-wafer network analysis up to 96 GHz with +or-1 dB accuracy. >

A time-domain millimeter-wave vector network analyzer

We report both broadband monolithic transmitter and receiver IC's for MM-wave electromagnetic measurements. The IC's 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. >

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

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 device's maximum frequency of oscillation. We report measured d.c. and microwave parameters of planar SRTDs fabricated with 1 μm-geometries in AlAs/GaAs.

Y. Konishi

Papers

Active and nonlinear wave propagation devices in ultrafast electronics and optoelectronics

A traveling-wave resonant tunnel diode pulse generator

A time-domain millimeter-wave vector network analyzer

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

AlAs/GaAs Schottky-collector resonant-tunnel-diodes