Mark A. Hollis

Arrays of gated field‐emitter cones having 0.32 μm tip‐to‐tip spacing

We have reduced the gate voltage required to achieve a given emission current density in field‐emitter arrays by scaling down the gate‐to‐tip and tip‐to‐tip spacing to the unprecedented levels of 0.08 and 0.32 μm, respectively. The submicrometer features of our arrays are patterned using interferometric lithography. Electrical tests of arrays we have fabricated have shown a record low turn‐on voltage of 8 V for cesiated molybdenum emitters. Emission current densities of 1600 A/cm2 have been obtained, which is also a record for such structures. These arrays provide large advantages for applications such as flat panel displays and microwave devices.

Bright‐field analysis of field‐emission cones using high‐resolution transmission electron microscopy and the effect of structural properties on current stability

High‐resolution transmission electron microscopy has been used to analyze 150 nm diameter by 150 nm high polycrystalline molybdenum field‐emission cones. The analysis shows that the cones comprise 5 to 10 nm thick grains with tips having gross radii of curvature of about 5 nm and protrusions having radii of curvature of about 1 nm. Such small protrusions may explain why analysis of experimental emission data indicates that the effective emission area of such tips is only 0.1 to 0.5 nm.2 Furthermore, the fact that the structure is composed of small grains indicates that there is a substantial number of molybdenum atoms at grain boundaries and that many configurations of grains and boundaries are possible with minimal free energy. A qualitative model is proposed which links the structural properties to current stabilization and hydrogen passivation effects.

Secondary ion mass spectrometry study of Pd‐based ohmic contacts to GaAs and AlGaAs/GaAs

Secondary ion mass spectrometry (SIMS) has been used to study nonalloyed Zn/Pd/Au (p‐type) and Ge/Pd/Au (n‐type) ohmic contacts to GaAs. Both contacts have very low contact resistances and smooth surface morphologies. It appears that Pd, a fast diffuser in GaAs, helps the diffusion of Zn into GaAs in the Zn/Pd/Au contact. On the other hand, it was found that Pd and Ge diffused together during the heat treatment of the Pd/Ge/Au contact. SIMS profiles of a Pd/Ge/Au ohmic contact fabricated upon a high‐electron mobility transistor (HEMT) structure provide two possible explanations for its significantly higher contact resistance than a conventional Ni/Ge/Au alloyed contact. The SIMS data indicates that Pd may have caused Al and Ga interdiffusion at the AlGaAs/GaAs interface and that the metallization failed to make contact to the electron gas because of the interdiffusion.

Genosensors: microfabricated devices for automated DNA sequence analysis

A new technology is introduced for developing potentially low cost, high throughput DNA sequence analysis. This approach utilizes novel bioelectronic genosensor devices to rapidly detect hybridization events across a DNA probe array. Detection of DNA probe/target hybridization has been achieved by two electronic methods. The first method utilizes a permittivity chip which interrogates the miniature test fixtures with a low voltage alternating electric field. The second method, which is the emphasis of this paper, utilizes a charge- coupled device (CCD) to detect the hybridization of appropriately tagged (radioisotope, fluorescent, or chemiluminescent labels) target DNA to an array of DNA probes immobilized above the pixels. Such direct electronic-biologic coupling is shown to provide a tenfold sensitivity improvement over conventional lens-based detection systems.

High Power-Added Efficiency Measured At 1.3 And 20 Ghz Using a GaAs Permeable Base Transistor*

Excellent power-added efficiencies of 66%, 41%, and 45% have been obtained from GaAs permeable base transistors (PBTs) operating at 1.3, 20, and 22 GHz, respectively. A gain of over 13 dB was measured at 1.3 GHz when the transistor was operating at its peak power-added eff iciency of 66%, which is roughly 5% better than that for the power transistors commonly used at these frequencies. Power measurements at 20 GHz have shown a power-added efficiency of 41 % with an associated gain of 7.3 dB and at 22 GHz a power-added efficiency of 45% with an associated gain of 6.2 dB. These power-added efficiencies, which have been obtained for PBTs not optimized for power, match the best results near 20 GHz for any device.

Gated field-emitter arrays for microwave-tube applications

As reported in the past, we have developed a process based on laser-interferometric lithography that can pattern an array of gated molybdenum cones having 0.32-/spl mu/m tip-tip and 0.08-/spl mu/m gate-tip spacings. These remain the smallest dimensions and highest density for such cone arrays reported. This geometry offers substantial advantages for achieving lower gate operating voltages, higher transconductance, and higher frequency operation. We are now working as part of a team led by Varian to apply this and related technology in a practical demonstration of a 10-GHz klystrode. Our first major activity in this area has involved the re-engineering of our emitter fabrication process to make it more robust. Gate-film thicknesses have been more than doubled to 700 /spl Aring/ to minimize fragility and lower the RF parasitic gate resistance. It appears this may also reduce the severity of burnout phenomena in these devices. Comprehensive reliability tests have yet to be performed: however, devices have been baked at 400/spl deg/C in a pressure of 10/sup -5/ Torr of H/sub 2/ for 37 hours with no ill effects. To meet the klystrode cathode requirements, we have designed an emitter array comprising four 30-/spl mu/m-long by 240-/spl mu/m-wide annular array segments that are disposed around a 0.024-in.-diameter circle. This structure is intended to deliver 160 mA at a tip loading of 0.57 /spl mu/A/tip. Phase-delay and attenuation effects in the gate and space-charge limitations in the emission current have all been taken into account in the design of these devices and are not expected to present a problem. We have also contributed heavily to the design of the packaging and matching scheme for the field-emitter cathode in the klystrode electron gun. A chip-carrier concept is used to ease the assembly/disassembly, and the microwave matching is done via microstrip circuits. General design of the matching circuits is discussed, and cases for both conjugate and resonant matches are shown for the Lincoln array. Provided that the necessary emitter yield, reliability, and emission current can be obtained, klystrode operation having useful output powers and reasonable gains at 10 GHz should be achievable.

GaAs permeable base transistors fabricated with 240-mm-periodicity tungsten base gratings

The microwave performance of the first GaAs permeable base transistors (PBTs) fabricated with 240-nm-periodicity tungsten base gratings is described. These PBTs have demonstrated a maximum stable gain of 20.1 dB at 26.5 GHz, which is believed to be the largest small-signal power gain for any microwave transistor at this frequency. Extrapolation of this maximum stable gain using a 6 dB/octave rolloff yields a maximum frequency of oscillation (f/sub max/) of 265 GHz. The breakdown voltage is approximately 20 V, which should make these devices excellent microwave power sources. Extrapolation microwave measurements on these devices from 3 GHz using a 6 dB/octave rolloff yields a unity-current-gain frequency (f/sub T/) of 38 GHz, which matches the best prior result for a PBT. These 240-nm-periodicity PBTs that have given the best f/sub T/ have been depletion-mode devices, suggesting that 240-nm-periodicity depletion-mode PBTs should have f/sub T/ values above 38 GHz. >

Resistive arc protection for field-emitter-array cold cathodes used in X-band inductive output amplifiers

Field-emitter arrays (FEAs) are desirable for use as cold cathodes for X-band inductive output amplifiers because they can provide higher efficiency and faster turn-on than their thermionic counterparts. Their major drawback is premature failure due to arcing. Display manufacturers have solved this problem by incorporating a resistive layer under the emitters, which limits the current at each tip. However, this high series resistance limits the frequency at which the gate-to-emitter voltage can be modulated, making it undesirable for RF applications. Fortunately, because the resistors under the tips are effectively in parallel, the total emitter series impedance of the array can be reduced by increasing the number of tips in the array. With proper design, the highly resistive layer can provide enough capacitance to act as a bypass at X-band, while still allowing enough resistance at lower frequencies to suppress arc currents. This paper will present design and test results from resistively protected field emitter arrays with a range of resistive layer parameters.

22 GHz performance of the permeable base transistor

Small-signal performance and power performance of GaAs permeable-base transistors (PBTs) at 22 GHz are reported. A small-signal gain of 14.5 dB was demonstrated over a 1-GHz bandwidth from a device having a 3200-AA-periodicity base grating and an 8*20 mu m/sup 2/ active area. A similar device, biased for class AB operation, achieved 45% power-added efficiency with an output power of 83 mW, and an associated gain of 5.7 dB. An output power of 210 mW with 4.7-dB of gain and 37% efficiency were obtained from a larger device having an 8*40 mu m/sup 2/ active area. A pair of these large devices in parallel delivered 370 mW with 3.7 dB gain and 33% efficiency, and 410 mW with 3.1 dB gain and 31% efficiency.<<ETX>>

Space-Charge Impedance in Photodiodes

High levels of injected space charge can strongly modify both the resistance and the reactance of photodiodes over their operating frequency range, but no substantive consideration of this effect has been published for p-i-n or uni-traveling-carrier (UTC) photodiodes. This letter describes the importance of the effect using a model that assumes a constant electron velocity, and relates published UTC photodiode results to the model. Inclusion of this effect in device design will benefit both linear and nonlinear (e.g., mixing) applications.