U. Bhattacharya

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. >

Submicron scaling of HBTs

The variation of heterojunction bipolar transistor (HBT) bandwidth with scaling is reviewed. High bandwidths are obtained by thinning the base and collector layers, increasing emitter current density, decreasing emitter contact resistivity, and reducing the emitter and collector junction widths. In mesa HBTs, minimum dimensions required for the base contact impose a minimum width for the collector junction, frustrating device scaling. Narrow collector junctions can be obtained by using substrate transfer or collector-undercut processes or, if contact resistivity is greatly reduced, by reducing the width of the base ohmic contacts in a mesa structure. HBTs with submicron collector junctions exhibit extremely high f/sub max/ and high gains in mm-wave ICs. Transferred-substrate HBTs have obtained 21 dB unilateral power gain at 100 GHz. If extrapolated at -20 dB/decade, the power gain cutoff frequency f/sub max/ is 1.1 THz. f/sub max/ will be less than 1 THz if unmodeled electron transport physics produce a >20 dB/decade variation in power gain at frequencies above 110 GHz. Transferred-substrate HBTs have obtained 295 GHz f/sub T/. The substrate transfer process provides microstrip interconnects on a low-/spl epsiv//sub r/ polymer dielectric with a electroplated gold ground plane. Important wiring parasitics, including wiring capacitance, and ground via inductance are substantially reduced. Demonstrated ICs include lumped and distributed amplifiers with bandwidths to 85 GHz and per-stage gain-bandwidth products over 400 GHz, and master-slave latches operating at 75 GHz.

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>>

SCALING OF InGaAs/InAlAsHBTs FOR HIGH SPEED MIXED-SIGNAL AND mm-WAVE ICs

High bandwidths are obtained with heterojunction bipolar transistors by thinning the base and collector layers, increasing emitter current density, decreasing emitter contact resistivity, and reducing the emitter and collector junction widths. In mesa HBTs, minimum dimensions required for the base contact impose a minimum width for the collector junction, frustrating device scaling. Narrow collector junctions can be obtained by using substrate transfer processes, or -if contact resistivity is greatly reduced -by reducing the width of the base Ohmic contacts in a mesa structure. HBTs with submicron collector junctions exhibit extremely high fmax and high gains in mm-wave ICs. Logic gate delays are primarily set by depletion-layer charging times, and neither fτ nor fmax is indicative of logic speed. For high speed logic, epitaxial layers must be thinned, emitter and collector junction widths reduced, current density increased, and emitter parasitic resistance decreased. Transferred-substrate HBTs have obtained 21 dB unilateral power gain at 100 GHz. If extrapolated at -20 dB/decade, the power gain cutoff frequency fmax is 1.1 THz. Transferred-substrate HBTs have obtained 295 GHz fτ. Demonstrated ICs include lumped and distributed amplifiers with bandwidths to 85 GHz, 66 GHz master-slave flip-flops, and 18 GHz clock rate Δ-Σ ADCs.

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>>

A 277-GHz f/sub max/ transferred-substrate heterojunction bipolar transistor

We report a AlInAs-GaInAs transferred-substrate heterojunction bipolar transistor (HBT). The transferred-substrate process permits fabrication of narrow and aligned collector-base and emitter-base junctions, reducing the collector-base capacitance and increasing the device f/sub max/. A device with aligned 0.7-/spl mu/m emitter and 1.6-/spl mu/m collector stripes has extrapolated 277 GHz f/sub max/ and 127 GHz f/sub /spl tau//, respectively.

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.

Multiplexer/demultiplexer IC technology for 100 Gb/s fiber-optic transmission

We report a new integrated circuit for multiplexing and demultiplexing at rates of 100 Gb/s. In transistor multiplexer/demultiplexer circuits, the operating data rate is limited by transistor bandwidth. The demonstrated circuit, which uses terahertz Schottky diodes, readily attains the necessary bandwidths. The IC, based in the diode nonlinear-transmission line (NLTL) technology, consists of an array of four sample-hold gates driven by NLTL strobe generators. To permit use in multiplexing, the sample-hold gates use a six-diode configuration with 150 GHz output bandwidth. Initial measurements with simple data patterns at 104 Gb/s are demonstrated.

4 THz sidewall-etched varactors for sub-mm-wave sampling circuits

Schottky varactor diodes with 4 THz cutoff frequencies were fabricated using 1 /spl mu/m lithography and self-aligned RIE sidewall etching. A novel microwave characterization technique was used to determine the diode cutoff frequency. Using these devices, subpicosecond pulse generators integrated with 515 GHz bandwidth sampling circuits were fabricated.<<ETX>>

ULTRAHIGH fmax AlInAs/GaInAs TRANSFERRED-SUBSTRATE HETEROJUNCTION BIPOLAR TRANSISTORS FOR INTEGRATED CIRCUITS APPLICATIONS

Transferred-substrate heterojunction bipolar transistors (HBTs) have demonstrated very high bandwidths and are potential candidates for very high speed integrated circuit (IC) applications. The transferred-substrate process permits fabrication of narrow and aligned emitter-base and collector-base junctions, reducing the collector-base capacitance and increasing the device fmax. Unlike conventional double-mesa HBTs, transferred-substrate HBTs can be scaled to submicron dimensions with a consequent increase in bandwidth. This paper introduces the concept of transferred-substrate HBTs. Fabrication process in the AlInAs/GaInAs material system is presented, followed by DC and RF performance. A demonstration IC is shown along with some integrated circuits in development.