Which photodiode to use: A comparison of CMOS-compatible structures

Kartikeya Murari, Ralph Etienne-Cummings, Nitish Thakor, Gert Cauwenberghs

Research output: Contribution to journalArticlepeer-review


While great advances have been made in optimizing fabrication process technologies for solid state image sensors, the need remains to be able to fabricate high quality photosensors in standard CMOS processes. The quality metrics depend on both the pixel architecture and the photosensitive structure. This paper presents a comparison of three photodiode structures in terms of spectral sensitivity, noise and dark current. The three structures are n +/p-sub, n-well/p-sub and p+ /n-well/p-sub. All structures were fabricated in a 0.5 μm 3-metal, 2-poly, n-well process and shared the same pixel and readout architectures. Two pixel structures were fabricated - the standard three transistor active pixel sensor, where the output depends on the photodiode capacitance, and one incorporating an in-pixel capacitive tran-simpedance amplifier where the output is dependent only on a designed feedback capacitor. The n-well/p-sub diode performed best in terms of sensitivity (an improvement of 3.5 × and 1.6 × over the n +/p-sub and p+/n-well/p-sub diodes, respectively) and signal-to-noise ratio (1.5 × and 1.2 × improvement over the n +/p-sub and p+/n-well/p-sub diodes, respectively) while the p+/n-well/p-sub diode had the minimum (33% compared to other two structures) dark current for a given sensitivity.

Original languageEnglish (US)
Article number4957497
Pages (from-to)752-760
Number of pages9
JournalIEEE Sensors Journal
Issue number7
StatePublished - Jul 2009


  • Active pixel sensors
  • CMOS
  • Photodiodes

ASJC Scopus subject areas

  • Instrumentation
  • Electrical and Electronic Engineering


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