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Velocity Saturation Image Sensor

Velocity Saturation Image Sensor: We have designed a novel current-mode active pixel sensor (APS) imager where conversion of photodiode voltage to output current is done using transistors operating in velocity saturation region. The high output impedance of this region makes it more suitable for current-sourcing operation than the linear region. The transistors also exhibit high linearity, allowing us to suppress fixed pattern noise (FPN) by correcting for both offset and gain variations among pixels. With conventional correlated double sampling (CDS), FPN is reduced from 3.8% to 0.85%. Further reduction requires compensation of gain variations, and results in a final FPN of 0.19%.

Velocity saturation is a short-channel effect. It occurs when the electric field along the channel (lateral E-field) increases beyond a critical value EC, so that the carrier velocity v approached a saturated value vsat. The critical value EC is about 1.5×104 V/cm for p-type silicon. Further increase in the lateral E-field will only decrease the mobility, but won’t increase the velocity. As the saturation current is limited, it is generally undesirable to enter velocity saturation.

Our imager was fabricated in a standard 0.5μm CMOS process. It contains a 110×200 array of pixels designated for velocity saturated operation. The pixels have a pitch of 12μm and a fill factor of 31.25%. The readout transistors are minimum-sized, with W=0.9μm and L=0.6μm.

The figure bellow shows a sample picture taken with the imager before and after FPN correction. With digital CDS, most of the FPN is suppressed. However, pixels near the borders appear darker, where the photodiodes have a lower quantum efficiency. With gain correction, the edge effect is eliminated

References:
[1] Z. Yang, V. Gruev and Van der Spiegel, "Current-mode Image Sensor with 1.5 Transistors per Pixel and Improved Dynamic Range," Proc. IEEE ISCAS, Seattle, USA, May 2008.
[2] Z. Yang, V. Gruev and J. Van der Spiegel, "Low Fixed Pattern Noise Current-Mode Imager Using Velocity Saturated Readout Transistors," Proc. IEEE ISCAS, New Orleans, USA, May 2007 (best paper in Sensory Systems Track). (Upenn, IEEExplore)