Adapting CMOS Active Pixel
Sensors for HgCdTe Photodiodes
Zeynep Dilli, Neil Goldsman,
Martin Peckerar, Nibir Dhar
Active Pixel Sensors
(adapted from El Gamal et al. and Yadid-Pecht et. al.)
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
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
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
•
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
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
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
SFAPS circuit with HgCdTe diode; Idark=~4nA (T=100K)
SFAPS circuit with HgCdTe diode; Idark=500nA(T=150K)
SFAPS circuit operation with HgCdTe diode; Effect of Idark
• If next stage is
A/D converter,
resolution
depends on
output voltage
separation
• Low Id: 1.26V912mV=348
mV
• High Id: 1.08V812V=268mV
• Highest
detectable
illumination
range is also
reduced.
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
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
Transimpedance Amplifier: Reducing Dark Current
diode_n
diode_p
• 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
shorted biasing
PMOS
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.
Transimpedance Amplifier---Operation
Dark Current Cancellation Method: Current
Subtraction
Two diodes per pixel; one shielded from light.
Dark Current Cancellation Method: Differential
Amplification
Two diodes per pixel; one shielded from light; extra access signal.
Dark Current Cancellation Method: Differential
Mode Output
Requires shutter control and extra timing signals.
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