A system-level analysis of Schottky diodes for incoherent THz imaging arrays E.R. Brown University of California, Santa Barbara, CA 90095, USA Received 10 December 2003; accepted 15 March 2004 Abstract GaAs Schottky diodes are analyzed as upper-mm-wave or THz direct-detectors in passive imaging arrays. Standard models are used for the current–voltage curve and small-signal responsivity with an assumed ideality factor of 1.2. The 1=f and burst noise properties are adopted from available empirical data, and a small-signal circuit model is used to compute the power delivered by the antenna to the diode. For a 4 sq lm diode area and typical modulation frequencies up to about 100 Hz, the noise-equivalent power (NEP) is found to be limited primarily by the 1=f and burst noise to values above 1 · 10 10 W/Hz 1=2 . If the modulation frequency could be increased to 1 MHz or above, or if the 1=f and burst noise mechanism could be greatly reduced, the analysis predicts that the Schottky NEP would drop to 3 · 10 12 W/Hz 1=2 at room temperature. At video sampling rates (30 s 1 ), the corresponding noise-equivalent delta temperature (NEDT) would fall in the range 1–10 K depending on the RF bandwidth. Ó 2004 Elsevier Ltd. All rights reserved. The challenges of concealed-weapons detection, all- weather imaging, and bioparticle remote sensing are providing new system pull in the upper millimeter and THz bands, particularly for room-temperature systems that are portable. One of the most useful semiconductor devices, the Schottky diode (SD), is presently a corner- stone for large-signal THz applications such as mixers and frequency multipliers, but is not as common in small-signal applications such as square-law and enve- lope detection. This paper examines modern SDs as di- rect detectors in a focal-plane imaging array using a model that accounts for the following: small-signal impedance and THz coupling; square-law rectification parameterized by ideality factor and temperature; ther- mal, shot, flicker, and burst noise; and post-detection center frequency and integration time. The model com- putes the noise-equivalent power (NEP), noise-equiva- lent delta temperature (NEDT), and image acquisition time (IAT). The basis for the model is an old but practical ana- lytic SD expression after Schneider [1]. Fig. 1 plots the Schneider expression for the dc I –V curve at three temperatures (T ¼ 300, 200, and 77 K) for a typical GaAs SD having ideality factor of 1.2, area ¼ 4 lm 2 and built-in potential of 0.665 eV. The doping in the GaAs is assumed to be 1.0 · 10 17 cm 3 (n-type) such that the device capacitance is 4.6 fF at a forward bias of 0.6 V. The RF equivalent circuit of the Schottky diode consists of the non-linear conductance dI =dV in parallel with the junction capacitance. This circuit is used to calculate the RF-to-low-frequency responsivity R (in units of A/W) based on a 100 X wideband source an- tenna and Torrey–Whitmer rectification theory [2]. Impedance mismatch is thus accounted for, and the resulting responsivity curves for 300 GHz operation and at 77, 200, and 300 K junction temperatures are plotted in Fig. 1. Series resistance is not yet included in this modeling since it is so dependent on device geometry and fabrication technology. On the other hand, the ac- tive area (4 lm 2 ) is chosen large relative to typical THz Schottky diodes, which tends to mitigate the effect of series resistance anyway. The combined effects of device E-mail address: erbrown@ee.ucla.edu (E.R. Brown). 0038-1101/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2004.05.074 Solid-State Electronics 48 (2004) 2051–2053 www.elsevier.com/locate/sse