Planar Varactor Multiplier and Mixer Diodes for Submillimeter and THz Receivers

Graduate Students: P. Marsh and K. Hong
Professor D. Pavlidis
National Aeronautics and Space Administration NAGW-1334 and U.S. Army Research Office DAAL03-92-G-0109

Space based and other high-reliability terahertz and submillimeter wave heterodyne receivers require the reliability and compactness afforded by all solid-state construction. At frequencies exceeding 250GHz, varactor multipliers offer the highest solid-state power output, making them promising candidates as reference LO sources. Schottky diode mixers are also demonstrating very promising characteristics. Planar technology is very attractive as a replacement for conventional but unreliable whisker contact technologies.

Our varactor research address the following points: (a) improvement of fundamental to harmonic conversion efficiency, (b) improvement in the understanding of the internal operation and performance-limiting mechanisms of varactors and (c) use of information acquired in the pursuit of (b) above to guide design of the varactor layer structures.

To date, we have fabricated varactors using in-house MOCVD-grown i-InAlAs/i-InGaAs/n+InGaAs layers on S.I. InP wafers. This varactor structure takes advantage of the excellent electron transport properties of InGaAs, along with the high barrier provided by the InAlAs barrier layer. Additionally, the InAlAs/InGaAs heterojunction serves to increase the C-V slope, thus avoiding the need for large drive powers and attendant efficiency decline due to electron velocity saturation.

CNAX/CMIN ratios of 1.9:1 to 3.1:1 and estimated series resistance, in a 1um diameter varactor, of 6.4 Ohm to 11 Ohm have been demonstrated using these designs. At zero bias, 1 um varactors had capacitances from 6.2fF (1000A active layer) to 14.4fF (600A active layer). It has been demonstrated that the use of aluminum-rich (60%Al in barrier) barrier layers will significantly reduce both forward and reverse leakage current. Al-rich varactors exhibited effective Schottky barriers (of 0.52eV vs. lattice-matched varactors of 0.44eV.

Mixer research addresses the following aspects: (1) Reduction of equivalent noise temperature at the RF input port, (2) reduction of the required LO power needed to minimize mixer noise temperature and (3) reduction of conversion loss. Due to its high mobility and high saturation velocity, InGaAs and InP are explored as active layers for the new Schottky diodes. Due to the low barrier height (FI(b)) of InGaAs, studies will be conducted regarding the use of p+ InGaAs, InP, and InAlAs layers to enhance FIb.

Our planar diode process makes use of Au-plated airbridges to reduce stray capacitance and series resistance. A novel airbridge process that eliminates the need to undercut the airbridge during the isolation etch, is being investigated.

Current Research Programs: THz Technology

Current Research Programs

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