Abstract
Summary form only given: There is a need for high power, high current, high speed photodetectors, for efficient, high gain RF photonic links, which can operate with 1-5 W RF power in the DC to 20 GHz range. This need is currently being met by state-of-art InGaAs photodetectors, but there are several material-dependent limitations. It is well known that InGaAs is a very poor thermal conductor. As observed, based strictly on thermal considerations since Ge is a 9.5× better thermal conductor than InGaAs, Ge p-i-n detectors should be able to operate at 2× the RF power, especially at high frequencies. Not as well known is the effect of impact ionization as a limiting nonlinear mechanism for these photodetectors. In the low field regime (5V/μm-10V/μm), which is the typical operating range for p-i-n diodes, the electron ionization coefficient of InGaAs is more than an order of magnitude greater than that of Ge. The voltage dependent responsivity of the InGaAs detector results in increased 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> and 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> order harmonic distortion. It has been calculated that there should be 100× less 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> order harmonic distortion, and 1000× less 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> order harmonic distortion with a Ge photodetector. This allows the Ge detector to be used in a broad-band receiver to detect weak signals without being affected by harmonic distortion (spurs) from stronger signals. Heterogeneous integration with Si permits backside illumination, which is the optimum configuration for thermal control allowing the use of front-side heatsinks. In addition, there are the advantages of inexpensive, large Si substrates, compatibility with the multi-B$ Si industry, and potential monolithic integration with Si circuits.
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