Submitted to 14th Int. Symp. on Space THz Technology  Tucson  22 April 2003 Design and operational considerations for robust planar GaAs varactors: A reliability study Frank Maiwald, Erich Schlecht, John Ward, Robert Lin, Rosa Leon, John Pearson, and Imran Mehdi Jet Propulsion Laboratory, MS 168-314, 4800 Oak Grove Drive, Pasadena, CA 91109 Abstract — Fix-tuned, broadband local oscillator (LO) sources to 1900 GHz have been developed for the HIFI instrument on the Herschel Space Observatory. Each LO chain consists of cascaded multipliers (doublers or triplers) being pumped with high-power (>100 mW) power amplifier modules in the 70 to 110 GHz frequency range. For long-term mission reliability it is important to quantify the safe operating conditions for these multi- pliers, especially when pumped with high input power. This paper will describe on-going investigations into the ef- fects of excessive reverse currents in Schottky diodes along with presenting a methodology for determining safe bias conditions for a given multiplier. I. INTRODUCTION In recent years tremendous progress has been made in demonstrating broadband all planar solid-state LO chains to 1.9 THz [1,2,3] for the heterodyne instrument on the Herschel Space Observatory [4]. In order to produce sufficient out- put power at frequencies above 1 THz, the first stage multipliers are being pumped with power levels of more than 100 mW at W-band with power-combined GaAs based power amplifiers [5]. The multipliers must be designed and oper- ated keeping this high RF input power requirement in mind. Though there have been some general studies regarding the reliability of GaAs Schottky diodes [6,7,8] we are not aware of any systematic study on the reliability of planar high power GaAs varactors. The situation is further complicated due to the fact that reliability of the devices is intimately de- pendent on the fabrication technology as well as the RF design. The goal of the present study is to determine what con- stitutes a safe RF and bias range for a given multiplier. II. ELECTRICAL STRESS Excessive amounts of either forward or reverse currents can cause the GaAs/metal contact to degrade. Catastrophic fail- ure of GaAs Schottky contacts with forward current of 14mA/mm 2 has been reported in [8] for whisker-contacted diodes. For planar diodes it is observed that the forward current limit is often imposed by the metal trace connecting the anode (diode finger), which acts as a fuse. However, the effect of reverse currents on diode characteristics is more difficult to predict. In the presence of constant reverse currents we have observed the diode bias voltage at constant current degrade even before there is avalanche breakdown at room temperature. Degradation of the Schottky contact due to reverse current is investigated by passing a constant reverse current through a device while monitoring the required voltage over an extended period of time. This is depicted for three different current % change in voltage for constant reverse current -40 -30 -20 -10 0 10 0 200 400 600 800 1000 Time (min) % change 295K -1uA 295K -5uA 295K -10uA Figure 1: Measured reverse bias voltage for constant reverse currents of -1mA, -5mA, and -10mA. The diodes used have anode areas of 1.5 mm 2 with 2*10 17 cm -3 doping.