BJT induced dark current in CMOS image sensors

Abstract

This paper demonstrates that cut-off pnp Bipolar Junction Transistors (BJTs) can induce visible dark current to their nearby CMOS (Complementary Metal Oxide Semiconductor) image sensor pixels, supported with experimental results. Previously, it has been shown that active BJTs and forward-biased PN junctions can incur dark shading to nearby CMOS image sensor pixels through electroluminescence (EL). However, in our experiments, when the BJTs are in their cut-off region, they can induce dark current to nearby image pixels at a similar distance but the dark current level declines much faster. To figure out the causes of dark current induced by the cut-off BJTs, the thermal coefficient of their nearby image pixels has been measured and the current is found to double for every 5 °C temperature rise. This indicates the cut-off BJT induced dark current is mainly due to the diffusion current generated from the silicon neutral regions in the pinned photodiodes (PPDs). This is in contrast to the hot pixels’ dark current incurred by thermal generation, that doubles for every 10 °C increase in temperature, or EL that has a very small thermal coefficient.