Thomas A. Plut

p‐channel, strained quantum well, field‐effect transistor

A p‐channel field‐effect transistor with a 3.5 μm Cr/Au gate was fabricated from a modulation‐doped GaAs/In0.2Ga0.8As/GaAs quantum well structure. Well‐behaved transistor action was observed at both 300 and 77 K with extrinsic transconductances of 6.2 and 11.3 mS/mm, respectively. Shubnikov–deHaas measurements prove the existence of a two‐dimensional hole gas with a strain‐shifted light‐hole ground state associated with a light‐hole mass of 0.154m0.

A low loss RF MEMS Ku-band integrated switched filter bank

A switched Ku-band filter bank has been developed using two single-pole triple-throw (SP3T) microelectromechanical systems (MEMS) switching networks, and three fixed three-pole end-coupled bandpass filters. A tuning range of 17.7% from 14.9 to 17.8 GHz was achieved with a fractional bandwidth of 7.7 /spl plusmn/2.9%, and mid-band insertion loss ranging from 1.7 to 2.0 dB.

Flux flow microelectronics

Flux-flow-based devices such as the superconducting flux flow transistor and magnetically controlled long junctions have been made from thin films of TlCaBaCuO and YBaCuO. The devices are based on the magnetic control of flux flow in their respective structures: a long junction or an array of weak links. The equivalent circuits of the two devices are similar: a low-impedance input control line, an output impedance of 3-20 Omega , and an active current-controlled element. The long junctions have tended to be slower, to have lower gain, and to be somewhat less noisy than their counterparts. Circuits such as narrowband and distributed amplifiers (50-GHz bandwidths, noise figures <3 dB), phase shifters (continuous with <2-dB loss, 4-40 GHz), logic gates (2-3-ps gate delays). and memories made using these devices are compared and analyzed in terms of performance.<<ETX>>

YBa2Cu3O7 nanobridges fabricated by direct‐write electron beam lithography

A direct method for nondamaging, nanometer‐scale patterning of high Tc superconductor thin films is presented. We have fabricated superconducting nanobridges in high‐quality, epitaxial thin‐film YBa2Cu3O7 (YBCO) by combining direct‐write electron beam lithography and an improved aqueous etchant. Weak links with both length and width dimensions less than 20 nm have exhibited critical currents at 77 K of 4–20 μA and IcRn products of 10–100 μV which compare favorably with results for other YBCO junction technologies. We have used this technique in the fabrication of a shock‐wave pulse former as an initial demonstration of its applicability to monolithic superconductive electronics.

An X-band to Ku-band RF MEMS switched coplanar strip filter

Radio frequency microelectromechanical systems (RF MEMS) are key enabling technologies for miniature reconfigurable circuits such as microwave filters. We present a two-pole monolithic RF MEMS switched filter, fabricated on GaAs, that employs surface-micromachined capacitors to present a variable capacitance to a coupled coplanar strip filter, thereby switching the filter center frequency 37% between 10.7 GHz and 15.5 GHz with voltages of 20 and 0 V, respectively. This 15% bandwidth filter occupies a chip area of 2.2 /spl times/1.5 mm and demonstrates less than 2-dB of loss, making it promising for numerous applications within these critical frequency bands.

Improved aqueous etchant for high Tc superconductor materials

We have developed an aqueous etchant suitable for producing feature sizes of 1 μm or less in YBa2Cu3O7 and TlBaCaCuO while not degrading high‐frequency surface resistance. The 10%–300% increase in surface resistance previously seen in aqueous solutions is largely due to the presence of dissolved CO2; decarbonation of the water by bubbling N2 before and during etching can produce a better than 50% reduction in the surface‐resistance degradation. Surface‐resistance degradation can be further reduced by selecting the appropriate solution pH for ethylenediaminetetraacetic acid (EDTA)‐based etchants. We have minimized increases in surface resistance to less than 5% following etching in decarbonated disodium EDTA solutions for both YBa2Cu3O7 and TlBaCaCuO superconductors. Similar improvements can be expected from decarbonation and pH control of other aqueous processing solutions.

Fabrication and characterization of ohmic contacting RF MEMS switches

We have fabricated and characterized radio frequency microelectromechanical systems (RF MEMS) ohmic switches for applications in discrete tunable filters and phase shifters over a frequency range of 0 to 20 GHz. Our previously reported cantilever switches have been redesigned for higher isolation and are now achieving 22 dB of isolation at 10 GHz. The measured insertion loss is 0.15 dB at 10 GHz. We have also fabricated and characterized new devices, designated “crab” switches, to increase isolation and contact forces relative to the cantilever design. The measured insertion loss and isolation are 0.1 dB per switch at 20 GHz and 22 dB at 10 GHz, respectively. A simple and accurate equivalent model has been developed, consisting of a transmission line segment and either a series capacitor to represent the blocking state or a series resistor to represent the passing state. Experimental analysis of the switch shows that high contact and substrate capacitive coupling degrades the isolation performance. Simulations indicate that the isolation improves to 30 dB at 10 GHz by reducing these capacitances. The crab switch design has a measured contact force of 120 μN, which represents a factor of four increase over the cantilever switch contact force and results in consistent, low-loss performance.

Fabrication of TlCaBaCuO step‐edge Josephson junctions with hysteretic behavior

One way to create hysteretic Josephson junctions from the currently available nonhysteretic high temperature superconducting junctions is to artificially add capacitance. We have adapted a multilayer technique for artificial capacitance addition/grain boundary modification to produce TlCaBaCuO step‐edge junctions exhibiting large amounts of hysteresis at 77 K. A gaplike structure is present at 32–36 mV that has a temperature dependence that does not contradict that predicted by the Bardeen‐Cooper‐Schrieffer theory. In addition, the junctions show fast switching times, less than a fixture limited 20 ps, and Fraunhofer‐like dependence of critical current on magnetic field.

HTS-based switched filter banks and delay lines

A thermal switch has been developed that provides good isolation and insertion loss with adequate switching times to allow a monolithic approach to the switched lines and filter banks. Filter banks in the 8-11-GHz range have been demonstrated with insertion losses of < 1 dB and out-of-band rejection greater than a package-limited 50 dB. Switched delay lines have been fabricated with insertion losses less than 0.3 dB/b and peak phase deviations from linearity of less than 5 degrees over 30-GHz bandwidths. The materials and substrate dependencies of the performance are analyzed, and there generally is a trade-off between RF power handling and switching speed/control power requirements. Devices made with TlCaBaCuO tend to switch faster and consume less power than YBaCuO devices because of the effects of flux dissipation. For the same reason, the RF power handling ability of the TlCaBaCuO devices is worse. YBaCuO devices grown on sapphire switch faster than do those on LaAlO/sub 3/ because of substrate properties but do require additional DC power.<<ETX>>

The effects of processing sequences on the microwave surface resistance of TlCaBaCuO

The effects of several microelectronic processing sequences on the high‐frequency surface resistance of the high‐temperature superconducting thin films in the TlCaBaCuO system have been examined. These processes include an acid etch, Br/alcohol etches, positive and negative photoresist sequences, and exposure to de‐ionized water. The surface resistance decreases during the Br etch, remains constant during the negative photoresist process, and increases moderately during the positive photoresist sequence and on exposure to water. The surface resistance increases dramatically on exposure to the acid solution as might be expected from other work. The effects of extended exposures to de‐ionized water and to Br etches on surface resistance are also presented.