Diamond P-i-N and Schottky P-i-N Diodes for High Power Limiters

Diamond P-i-N and Schottky P-i-N diodes are a promising new technology for high power limiters. Receivers, solid-state amplifiers, and detectors commonly use P-i-N and/or Schottky diodes for protection from high power incident signals. Limiters reflect the majority of excess RF power, but inevitably absorb some portion of the incoming power as well. As dissipated power converts to heat, the thermal impedance and temperature resilience of the limiter diode becomes the primary factor determining device lifetime, reliability, and power handling. Si and GaAs-based P-i-N and Schottky diodes and even SiC-based Schottky diodes suffer non-recoverable thermal overload upon excess power dissipation. Diamond, by contrast, offers superior thermal conductivity and temperature resilience. Diamond also provides good charge carrier mobility and high electric breakdown field making it an ideal material for next generation high power RF limiters.<br/><br/>Power limiter circuits frequently rely on solid-state P-i-N and/or Schottky diodes to act as a protective buffer for microwave components that are susceptible to damage from high-power signals. Limiters must transmit low power signals with minimal attenuation or distortion, while attenuating high power signals so as not to exceed a prescribed maximum. While solid state power limiters offer the advantages of a long operating life, small size, and flexible use, including integration with other MMIC components, they often require a pre-limiter stage due to limitations in power handling and spike leakage. Diamond diodes with high power handling and low spike leakage promise to eliminate the need for a prelimiter stage by offering significantly enhanced power handling and reduced spike leakage.<br/><br/>In a shunt-connected diode power limiter circuit, the diode remains off for low input signals, with any diode off-capacitance (C_off) contributing to insertion loss (attenuation of low power signals) and limiting the frequency bandwidth. When the magnitude of the input signal exceeds a threshold, the shunt-connected diode turns on. In the on state, the diode acts as a short to ground, resulting in reflection of most incoming RF power. However, on-resistance (R_on) in the diode results in dissipation of some portion of incident power through the limiter circuit. As incident power increases, the power absorbed by the power limiter circuit increases as well. Device overheating from this dissipated power is the primary failure mechanism that sets the maximum power handling of a diode. Using a larger diode can increase power handling at the expense of greater C_off and thus greater insertion loss.<br/><br/>The cutoff frequency figure of merit captures the tradeoff between C_off and R_on, and can serve as a useful metric for evaluating the suitability of limiter device technologies for RF applications. Commercially available high power Si P-i-N diodes have an F_CO of around 700GHz. The F_CO figure of merit, however, does not account for the power dissipation capability of a device. Two devices with the same F_CO may dissipate the same power under the same incident conditions, but if one device can handle a greater heat load, it will have better power handling. Therefore, fabricating diodes in a material such as diamond offers a route to dramatically increasing power handling without insertion loss penalty. Compared to the conventional materials systems used for diodes—including Si, GaAs, and SiC—diamond offers over 20x higher thermal conductivity and 2-3x higher maximum operating temperature. While significant material growth and processing challenges remain before diamond matures as an electronics material, the advent of single-crystal substrates and breakthroughs in bipolar dopant incorporation have spurred rapid advances in recent years. If diamond diodes can be achieved with F_CO of several hundred GHz, they could be a viable high power-handling alternative for the Si or GaAs diodes in use today.