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Adapting CMOS Active Pixel Sensors for HgCdTe Photodiodes

This paper discusses the adaptation of CMOS Active Pixel Sensors (APS) for HgCdTe photodiodes. It explores the SFAPS circuit, dark current compensation methods, and future possibilities for improving sensor performance.

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Adapting CMOS Active Pixel Sensors for HgCdTe Photodiodes

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  1. Adapting CMOS Active Pixel Sensors for HgCdTe Photodiodes Zeynep Dilli, Neil Goldsman, Martin Peckerar, Nibir Dhar

  2. Active Pixel Sensors (adapted from El Gamal et al. and Yadid-Pecht et. al.)

  3. Source-Follower Active Pixel Sensor (SFAPS) Circuit • Simple version: 3 transistors (pixel reset, source follower, access), one photodiode • Not shown: Row read circuitry, a timed buffer connected to v_out and activated a set time after pixel access

  4. SFAPS Circuit • Operation: • Reset transistor gate pulsed, photodiode junction cap. charged up, source follower output follows • Photocurrent starts discharging, SF output follows • Access transistor gate pulsed, output cap.charged up • Bias current discharges output voltage linearly until the set read time for v_out

  5. SFAPS Circuit • Operation: • Reset transistor gate pulsed, photodiode junction cap. charged up, source follower output follows • Photocurrent starts discharging, SF output follows • Access transistor gate pulsed, output cap.charged up • Bias current discharges output voltage linearly until the set read time for v_out

  6. Operation: • Reset gate pulsed, photodiode charged up, SF output follows • i_pf discharges diode cap., SF output follows • Access gate pulsed, output cap. charged up • i_bias discharges output until the set read time for v_out

  7. APS Circuit + Output Transmission Gate • Modeling: • Photodiode modeled with capacitor in parallel with n p current sources • Dark current is modeled as a fixed DC current

  8. HgCdTe Photodiodes • Diode responsivity: ~4-5.5 for 10 μ radiation [1] • For a 40 μ by 40 μ diode, photocurrent~10s of μA, for light intensity ~1e-9 W/μ2 (sunlight in visible; source intensity for 10 μ radiation likely to be lower.) • Dark current ~4 nA for 100K and ~500 nA for 150 K (ARL data) • Diode needs to be reverse biased; however, higher reverse bias increases dark current significantly as well [1] Fukuda, Matsuura & Mori, 1984

  9. SFAPS circuit with HgCdTe diode; Idark=~4nA (T=100K)

  10. SFAPS circuit with HgCdTe diode; Idark=500nA(T=150K)

  11. SFAPS circuit operation with HgCdTe diode; Effect of Idark • If next stage is A/D converter, resolution depends on output voltage separation • Low Id: 1.26V-912mV=348 mV • High Id: 1.08V-812V=268mV • Highest detectable illumination range is also reduced.

  12. SFAPS Circuit • Advantages: • Small circuit, just three transistors per pixel: Large fill factor possible • Photodiode operation isolated from readout circuit • Flexibility in design with respect to timing design vs. power consumption vs. readout speed vs. resolution • DISADVANTAGE: Diode is reverse biased quite highly (at around 3.5V for out previous example)---not usable for HgCdTe diodes in basic form

  13. Approaches for Dark Current Compensation • Reduce the dark current of the diode itself: • Cool the diodes (not applicable in this project) • Reduce the bias across the diode with the design of the input and readout circuits • Transimpedance amplifiers [1] • Direct injection [2,3] • Pseudoactive pixel sensors [4] • Cancel the dark current of each diode during operation: • Frame differencing schemes [5] • Extra unilluminated diodes and current subtraction [4,6] • Extra unilluminated diodes and differential amplification [1] D’Souza, Wijewarnasuriya, Poksheva [2] Scribner, Kruer, Killiany, 1990 [3] Bailey, Kozlowski, Chen et al, 1991 [4] Shih, Wu, 2004 [5] Ma, Chen, 1999 [6] Abdallah, Dubaric, Nilsson et al., 2001

  14. Transimpedance Amplifier: Reducing Dark Current • Converts diode current input into voltage level at output of common-source amplifier PMOS • Sets diode bias by the source bias of the gate-drain shortedbiasing PMOS diode_n diode_p

  15. Transimpedance Amplifier---Selecting Bias Point Diode bias set around -300 mV---better than SFAPS, still not low enough for HgCdTe. Working on improving this circuit.

  16. Transimpedance Amplifier---Operation

  17. Dark Current Cancellation Method: Current Subtraction Two diodes per pixel; one shielded from light.

  18. Dark Current Cancellation Method: Differential Amplification Two diodes per pixel; one shielded from light; extra access signal.

  19. Dark Current Cancellation Method: Differential Mode Output Requires shutter control and extra timing signals.

  20. Future Work • Investigate • Transimpedance amplifiers (further) • Direct injection circuits • Different biasing schemes • Unilluminated extra pixels and dark current cancellation • Consider • HgCdTe requirement: Very low reverse bias • Hybrid integration [1] allows more space for readout circuitry per pixel; still, optimize circuits for power consumption • Timing requirements and tradeoffs? (Extrapolate from SFAPS work) • Sensitivity requirements and tradeoffs? (Extrapolate from SFAPS work) [1] Bailey, Kozlowski, Chen et.al., 1991 and later work

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