Enhanced Sensitivity of Small-Size (With 1-$\mu \hbox{m}$ Gate Length) Junction-Field-Effect-Transistor-Based Germanium Photodetector Using Two-Step Germanium Epitaxy by Ultrahigh Vacuum Chemical Vapor Deposition

In this letter, we demonstrate a scalable (with gate length of 1 mum) Ge photodetector based on a junction field-effect-transistor (JFET) structure with high sensitivity and improved response time. To overcome the low-detection-efficiency issue of typical JFET photodetectors, a high-quality Ge epilayer, as the gate of JFET, was achieved using a novel epigrowth technique. By laser surface illumination of 3 mW on the Ge gate, an
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ratio up to 185 was achieved at a wavelength of 1550 nm for the first time. In addition, the device shows a temporal response time of 110 ps with a rise time of 10 ps, indicating that the scalable Ge JFET photodetector is a promising candidate to replace large-size photodiodes in future optoelectronic integrated circuits and as an image sensor integrated with a CMOS circuit for its comparable size in respect to modern MOSFETs.