ECE 1352 Presentation Active Pixel Imaging Circuits By : Ashkan Olyaei

Outline: Active Pixel Sensors (APS) Vs. Charge Coupled Devices (CCD) APS Design Issues Dynamic Range Noise

Defining Some Concepts Quantum Efficiency QE = Electrical Energy / Radiant Energy Fill Factor FF = Active Area in Pixel / Total Pixel Area Microlenses improve the effective fill factor 2 or 3 times

APS Vs. CCD CCD requires specialized expensive processes; not easily integrable with CMOS has high Quantum Efficiency, high fill factor and low noise lacks random access and fast readouts needs multiple voltages on chip for efficient charge transfer APS is lower voltage and lower-power achieves random access and faster readout can yield low noise with peripheral circuitry compatible with CMOS process

APS Design A Simple Photodiode APS Cell Described by Noble in 1968 Three transistors per pixel High quantum efficiency (no overlying polysilicon) E. R. Fossum, “CMOS image sensors: Electronic camera-on-a-chip,” in IEEE IEDM Tech. Dig., 1995.

Dynamic Range DRpixel (dB) = 20 log (Vmax/Vnoise) Rail-to-rail input swing Vdsat+ Vtp Vtn + = Vin Wider output swing Vdsat Vtp Vtn + = Vout Chen Xu, M. Chan “The Approach to Rail-to-Rail CMOS APS for Portable Applications ,” in IEEE Tencon. 2001.

Dynamic Range Parameters Rail to Rail Photodiode APS Architecture Normal Photodiode APS Architecture Technology 0.25 um 0.8 um Operating Voltage 1.2 V 5 V Pixel Size 12*10 um 16*16 um Fill Factor 30% 35% Dynamic Range 67 dB 68 dB

Noise Analysis Temporal Noise Spatial Noise (FPN) Time-dependent fluctuations in the signal level of fundamental origins. Pixel Noise Photon shot noise (Photon detection a Poisson process, Noise = N^0.5) Reset (kT/C) noise associated with reset level Dark current shot noise proportional to leakage current and exposure time MOS device noise (flicker 1/f noise and Thermal noise) Column Noise Thermal kT/C noise associated with the sampling process Thermal and 1/f noise of the column amplifier MOS devices Spatial Noise (FPN) Refers to a non-temporal spatial noise and is due to device mismatches in the pixels & color filters, and variations in column amplifiers.

Noise Reduction Correlated Double Sampling (Level 1) Reduce FPN and Temporal noise in pixel Reset Level Transferred to CR Signal Level Transferred to CS CMOS Active Pixel Image Sensors for Highly Integrated Imaging Systems Mendis, S.K.; Kemeny, S.E.; Gee, R.C.; Pain, B.; Staller, C.O.; Quiesup Kim; Fossum, E.R.; Solid-State Circuits, IEEE Journal of , Vol. 32 Issue: 2 , Feb. 1997 Page(s): 187 -197

Noise Reduction Correlated Double Sampling (Level 2) AJ Blanksby, MJ Loinaz, “Performance Analysis of a Color CMOS Photogate Image Sensor,” IEEE Transactions on Electron Devices, Vol. 47, No. 1, Jan 2000.

Noise Reduction

Conclusion Two generation of imagers: CCD and APS Dynamic range critical as technology scales Noise an important impediment of APS

References Two generation of imagers: CCD and APS CMOS image sensors: electronic camera on a chip Fossum, E.R.; Electron Devices Meeting, 1995., International , 10-13 Dec. 1995 The approach to rail-to-rail CMOS active pixel sensor for portable applications Chen Xu; Mansun Chan; Electrical and Electronic Technology, 2001. TENCON. Proceedings of IEEE Region 10 International Conference on , Volume: 2 , 19-22 Aug. 2001 Page(s): 834 -837 vol.2 CMOS Active Pixel Image Sensors for Highly Integrated Imaging Systems Mendis, S.K.; Kemeny, S.E.; Gee, R.C.; Pain, B.; Staller, C.O.; Quiesup Kim; Fossum, E.R.; Solid-State Circuits, IEEE Journal of , Vol. 32 Issue: 2 , Feb. 1997 Page(s): 187 -197 Performance analysis of a color CMOS photogate image sensor Blanksby, A.J.; Loinaz, M.J.; Electron Devices, IEEE Transactions on , Volume: 47 Issue: 1 , Jan. 2000 Page(s): 55 -64 Two generation of imagers: CCD and APS Dynamic range critical as technology