Method of charging the photodiode element in active pixel...

Details Method of charging the photodiode element in active pixel... Method of charging the photodiode element in active pixel...

2002-09-20

2004-09-28

Le, Que T. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C250S2140RC

Reexamination Certificate

active

06797935

ABSTRACT:

FIELD OF THE INVENTION
This invention generally relates to electronic systems and in particular it relates to charging the photodiode element in active pixel arrays.
BACKGROUND OF THE INVENTION
A prior art active pixel light sensing element is shown in FIG.
1
. This state of the art active pixel element consists of three NMOS transistors
20
,
22
, and
24
, and a photodiode
26
. One of these transistors is called a reset transistor
20
and its function is to charge up the photodiode
26
(nwell) to a preset value at the beginning of each exposure cycle. The reset operation is needed in order to eliminate memory effect from prior exposure.
Biasing conditions used in state of the art 0.18 um technology are as follows: Nwell photodiode
26
is charged up to 2 V during reset. In order to pass 2 V through the NMOS transistor
20
, a voltage of 3.3 V (+/−10%) is applied on the gate. When the reset transistor
20
is turned off (gate goes to 0 V), nwell potential at node
32
is pushed down by approximately 100 mV due to charge injection.
Referring to
FIG. 1
showing the conventional pixel architecture, the dynamic range at the column output node
30
is calculated as follows:
(1) nwell
26
is charged to 2000 mV when transistor
20
is turned on;
(2) nwell
26
goes to 1900 mV due to charge injection when reset transistor
20
is turned off;
(3) voltage at the source
34
of sense transistor
22
(sense device) is 800 mV=1900 mV−nmos Vt {1100 mV with back bias} (nmos Vt is the threshold voltage of transistor
22
);
(4) minimum voltage drop allocated for the current source transistor
28
is 200 mV;
(5) dynamic range at column output node
30
is 600 mV=800 mV−200 mV;
(6) there is no significant voltage drop across column select transistor
24
(column select device).
One of the limitations of this prior art approach (combined with high voltage sense transistor
22
) is that dynamic range is limited to 500 mV if the gate voltage on the reset transistor
20
drops to 3.0 V.
SUMMARY OF THE INVENTION
A forward biased diode is used to charge up a photodiode rather than an NMOS transistor. This photodiode charging mechanism increases the dynamic range and optical response of active pixel arrays, and improves the scalability of the pixel element.


REFERENCES:
patent: 6225670 (2001-05-01), Dierickx

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