Dot PIN Photodiodes With a Capacitance Down to 1.14 aF/μm2

Abstract

PIN photodiodes with small half-spherical cathodes are investigated. They combine a large light-sensitive area with a very small capacitance. Dot photodiodes without and with shallow-trench isolation are compared. Also, the influence of an optical window on the responsivity is examined. The dot photodiodes were fabricated in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.18~\mu \text{m}$ </tex-math></inline-formula> high-voltage CMOS without needing process changes. Capacitances down to 0.8 fF are achieved at a light-sensitive area of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$706.9~\mu \text{m}^{2}$ </tex-math></inline-formula> . Responsivities in the range of 0.35 A/W to 0.4 A/W were observed for a wavelength of 635 nm. The −3dB cut-off frequencies for 675 nm light reach up to 300 MHz for reverse voltages up to 30 V. The rise and fall times reduce to about 0.8 ns and 1.0 ns, respectively, also at 30 V reverse bias.