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19 th International Symposium on Space Terahertz Technology State-of-the-Art Quasi-Vertical Schottky Diodes for THz-Applications O. Cojocari (1) , C. Sydlo (1) , I. Oprea (2) ,R. Zimmermann (3) , A. Walber (3) , R. Henneberger (3) P. Meissner (2) and H.-L. Hartnagel (2) (1) ACST Advanced Compound Semiconductor Technology GmbH, Merckstr. 25, 64283 Darmstadt, GERMANY. (2) Technical University of Darmstadt, Dep of Microwave Engineering, Merckstr. 25, 64283 Darmstadt, GERMANY. (3) RPG Radiometer Physics GmbH - Birkenmaarstraße 10, 53340 Meckenheim, GERMANY. 8-3 Abstract: Schottky is a generic technology, needed not only in space instruments but in practically all mm/sub-mm equipment, with the imaging evolving fast to become the primary area of application. Commercial applications of Schottky devices have, until very recently, been limited to frequencies below 100 GHz. Making available device technologies for higher frequencies at a reasonable cost will stimulate the development of existing and the appearance of new applications. Quasi-vertical diode (QVD) is a Schottky structure suitable for hybrid and monolithic integration in THz –circuitry. Low loss in semiconductor substrate and a good heatsink of a QVD is ensured by its specific design features as are membrane-substrate and very thin mesa enclosed between the Schottky and ohmic contacts, which are situated on its different sides. These represent significant advantages over conventional planar diodes for particular applications at high frequencies. Various diode structures were fabricated and tested. Varistor diodes repeatedly show values of the ideality factor η≈1.25 and an unprecedented low value of series resistance of R s ≈4Ω for sub-micrometer anodes with junction capacitance C 0j ≈2fF. Using the simplest formula for the calculation as f cut-off =1/(2π C 0j R s ) these data result in a cut-off-frequency of up to 20THz. Subharmonically-pumped mixers were fabricated using waveguide technology and available anti-parallel diode pairs, and tested at different frequencies. They show values of the mixer temperature as low as 450K, 500K, 500K, and 700K (DSB) at frequencies of 150GHz, 183GHz, 220GHz, and 280GHz, respectively. These results are well comparable with state-of-the-art mixer performance at millimeter waves. Varactor diodes exhibit DC-parameters in the ragge of η≈1.08, R s ≈3Ω, C 0j ≈22fF, breakdown voltage U bd =13.5V and capacitance modulation factor M= C 0j / C Ubd =4.5. Such diodes were mounted in waveguide doubler to 208±10GHz and exhibited up to 30% conversion efficiency. Industry experts appreciated these results as extremely encouraging. Zero-bias detector diodes exhibit a low video-resistance of below 10kΩ at 0V and a current responsivity of above 15A/W for a very small anode area with a junction capacitance of below 2fF. The total capacitance of such a diode is around 3fF. These parameters ensure high performance of a Zero-Bias Schottky-based detector at frequencies beyond 1THz. Technology issues as well as recent measurement results will be presented in the full paper. 70
19 th International Symposium on Space Terahertz Technology 8-4 Schottky Diode Mixers on Gallium Arsenide Antimonide? Erich Schlecht and Robert Lin Jet Propulsion Laboratory, California Institute of Technology Increasing interest in deep space missions using submillimeter and terahertz spectrometers increases has recently pushed the development of Schottky mixers in this frequency range. Due to the extreme requirements for low mass and power in deep space instruments, these mixers are quite attractive because they have good sensitivity, and do not need cryogenic cooling for operation. One drawback is that, compared to other mixers, they have fairly high LO power requirements, in the range of a milliwatts or more. Generally, this is attributed to the fairly high barrier of the usual Schottky contact of Au, Pt or Ti on GaAs. Fig. 1. Dependence of barrier height on mobility for GaAsSb and GaInAs for different thirdelement fractions. Because of this, it is reasonable to explore the use of alternate semiconductor systems that have lower barriers, and at the same time higher mobilities to offset the reduced non-linearity of the lower barrier materials. Usually, ternary compounds have been grown on GaAs or InP wafers, and required lattice matched mixtures in order to prevent development of dislocations that would greatly reduce mobility. However, the use of special techniques such as strained superlattice buffer layers allows a wider variation of materials to be used for devices. In this work we will use a harmonic balance simulator coupled to our recently developed hot-electron diode model [1] to analyze Schottky mixers fabricated using the Gallium Arsenide Antimony (GaAsSb) ternary alloy, and compare the results with those fabricated using GaAs and Gallium Indium Arsenide for comparison. Figure 1 shows a comparison of barrier height versus mobility for GaAsSb and GaInAs for various Antimony and Gallium fraction. The results of the simulations will be presented at the conference. 71
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