M. Reddy

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. >

Monolithic Schottky-collector resonant tunnel diode oscillator arrays to 650 GHz

We report monolithic array oscillators incorporating Schottky-collector resonant tunnel diodes (SRTDs). In the SRTD, a 0.1-/spl mu/m width Schottky collector contact provides a greatly reduced device series resistance, resulting in an estimated 2.2 THz maximum frequency of oscillation. A 64-element oscillator array oscillated at 650 GHz while a 16-element array produced 28 /spl mu/W at 290 GHz.

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>>

Bias stabilization for resonant tunnel diode oscillators

While resonant tunnel diodes (RTDs) are useful as submillimeter-wave oscillators, circuit design constraints imposed to suppress parasitic bias circuit oscillations have limited output powers to well below 1 mW. We report a 7-GHz RTD oscillator with a shunt regulator for bias circuit stabilization. With regulation, oscillator power is not limited by stability constraints. Regulation elements are readily integrated with RTDs to construct monolithic RTD oscillator arrays. >

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. >

0.1 /spl mu/m Schottky-collector AlAs/GaAs resonant tunneling diodes

The Schottky-collector resonant tunneling diode (RTD) is an RTD with the normal N+ collector and ohmic contact replaced by a Schottky contact, thereby eliminating the associated parasitic resistance. With submicron Schottky contact dimensions, the remaining components of the parasitic series resistance can be greatly reduced, resulting in an increased maximum frequency of oscillation, f/sub max/. AlAs/GaAs Schottky-collector RTDs were fabricated using 0.1 /spl mu/m T-gate technology developed for high electron mobility transistors. From their measured dc and microwave parameters, and including the effect of the quantum well lifetime, f/sub max/=900 GHz is computed.<<ETX>>

Transferred substrate Schottky-collector heterojunction bipolar transistors: first results and scaling laws for high f/sub max/

Unlike normal heterojunction bipolar transistors (HBTs), transferred substrate Schottky-collector HBTs (SCHBTs) exhibit substantial increases in f/sub max/ as the emitter and collector stripes are scaled to deep submicron dimensions. First generation InAlAs/InGaAs SCHBTs with aligned 1-/spl mu/m emitter and collector stripes have been fabricated.<<ETX>>

Electron Beam Lithography for the Fabrication of Air-bridged, Submicron Schottky Collectors

T‐gate technology as is commonly used for field‐effect transistors and high electron mobility transistors has been adapted for use in Schottky‐collector resonant tunneling diodes (SRTDs) devices in which it is necessary for the footprint to be extremely small in both dimensions. By air bridging the contact, GaAs‐based RTDs with projected cutoff frequencies of nearly 1 THz have been fabricated. The process is advantageous for the fabrication of terahertz diodes because of the large periphery to area ratio associated with the small footprint (which reduces the parasitic resistance), because small areas provide better impedances, and because the air bridge both reduces parasitic capacitances and provides certain processing advantages. The process is also inherently planar in contrast with other diode implementations for use at submillimeter wave frequencies. In addition to the GaAs‐based RTDs, the process is also being used for the fabrication of GaAs Mott diodes, which have cutoff frequencies of 12.5 THz and InGaAs/AlAs RTDs, which appear to have cutoff frequencies of 2.5–3 THz.

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.

Fabrication and dc, microwave characteristics of submicron Schottky‐collector AlAs/In0.53Ga0.47As/InP resonant tunneling diodes

We report the fabrication and dc, microwave characteristics of 0.1 μm, Schottky‐collector resonant tunnel diodes (SRTDs) in the AlAs/In0.53Ga0.47As/InP material system. Devices with contact areas as small as 0.05 μm2 have been fabricated using electron beam lithography with an interrupted footprint T‐gate process. SRTD’s fabricated with 1.4 nm AlAs barriers exhibited a 5×105 A/cm2 peak current density at 0.95 V and a −19 mS/μm2 peak negative conductance. The devices incorporate fully depleted P‐doped cap layers to suppress surface leakage currents. From the measured dc and microwave characteristics, a maximum frequency of oscillation fmax=2.2 THz is estimated.