Sensor pixel with linear and logarithmic response

Details Sensor pixel with linear and logarithmic response Sensor pixel with linear and logarithmic response

1999-09-10

2001-11-27

Allen, Stephone B. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C250S208100, C348S308000

Reexamination Certificate

active

06323479

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pixel sensor that has a linear response to low light intensity and a logarithmic response to high light intensity. In particular, the invention relates to the use of a reset gate to pre-bias an accumulation node of the pixel so as to cut-off current through a logarithmic response transistor and thereby provide a linear response region.
2. Description of Related Art
A standard log response pixel is based on the notion of sub-threshold conduction. In
FIG. 7
, a gate of the field effect transistor (FET), denoted Q
1
, is connected to a first predetermined potential, denoted V
DD
. In this configuration, the voltage at node
1
, denoted {circle around (
1
)}, will have a logarithmic dependence on a photon-induced signal current from the photo-diode. In practice, the voltage output to the readout portion tracks the input illumination and thus this pixel does not require a reset FET. The equivalent resistance through the FET denoted Q
1
permits charge to build up and diminish on the parasitic capacitance at node
1
(e.g., due to readout structure). This design achieves a dynamic range of many orders of magnitude. Since the human eye has the same or similar log response, the log response of the pixel is a desired characteristic in electronic vision systems.
In
FIG. 8
, another known log pixel (referred to as the Chamberlain pixel) connects the gate of transistor Q
1
, to the transistor's source (instead of its drain). See Chamberlain and Lee, “A Novel Wide Dynamic Range Silicon, Photodetector and Linear Image Array”,
IEEE Journal Of Solid-State Circuits
, Volume SC-19, No. 1, February 1984, pages 41-48. This pixel works on a sub-threshold logarithmic voltage response to current, and it is described in U.S. Pat. No. 4,473,836 granted to Chamberlain, incorporated herein by reference.
In FIG.
9
and in U.S. Pat. No. 4,794,247 to Stineman, Jr. there is disclosed photo-diode
201
coupled to integrating amplifier
203
,
230
with reset transistor
202
to preset the integration constant. However, this pixel does not include a transistor in series with photo-diode
201
to provide the pixel with a log response region.
In FIG.
10
and in U.S. Pat. No. 5,742,047 to Buhler, et al. there is disclosed a photo-diode D
1
coupled to a diode reset transistor M
1
and, through a pass transistor M
2
, coupled to a “fat zero” transistor M
3
where an integration node is at the junction between M
2
and M
3
. However, this pixel does not include a transistor in series with photo-diode
201
to provide the pixel with a log response region.
The pixel of
FIG. 7
or
FIG. 8
typically produce signal swings (in response to light to dark swings) in the order of 100 millivolts. With such small signals, it is difficult to distinguish photon induced signals from dark current and other noise signals, and therefore, it is difficult to achieve high signal to noise sensors, especially a low light levels.
Furthermore, the signal response time to a photon signal input can be long, especially at low light levels where the photo-current is small. The effective resistance across the FET is large at low light levels, and the time constant to achieve equilibrium of the voltage stored on the parasitic capacitance at node
1
is long. This leads to an image lag phenomenon called ghosting (e.g., where a brightly illuminated pixel takes a prolonged time to output a dark signal when the bright illumination is removed).
SUMMARY OF THE INVENTION
It is an object to the present invention to provide a pixel with linear response and logarithmic response regions. It is a further object to provide a pixel with a high speed response to light in the linear response region. It is a further object to provide a pixel with an increased output signal swing in the logarithmic response region.
These and other objects are achieved in a pixel that includes a photon detecting element coupled between a node and a ground, a transistor structure coupled between the node and a first predetermined voltage to provide a logarithmic response region, and a reset transistor coupled between the node and a second predetermined voltage to provide a linear response region where the second predetermined voltage is greater than the first predetermined voltage.


REFERENCES:
patent: 3770967 (1973-11-01), Hanna et al.
patent: 3770968 (1973-11-01), Hession et al.
patent: 4794247 (1988-12-01), Stineman, Jr.
patent: 5296697 (1994-03-01), Moses, Jr.
patent: 5608204 (1997-03-01), Hofflinger et al.
patent: 5742047 (1998-04-01), Buhler et al.
patent: 6191408 (2001-02-01), Shinotsuka et al.
Savvas G. Chamberlain, et al., “A Novel Wide Dynamic Range Silicon Photodetector And Linear Imaging Array”, IEEE Journal Of Solid-State Circuits, vol. SC-19, No. 1, Feb. 1994, pp. 41-48.
W. D. Washkurak et al., “A Floating Gate Wide Dynamic Range Photodetector”, Dalsa, Mar. 1994, pp. 2-34.

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