Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2002-08-15
2003-11-11
Munson, Gene M. (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S188000, C257S189000, C257S441000, C257S616000
Reexamination Certificate
active
06646318
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to imager cells and, more particularly, to a bandgap tuned vertical color imager cell.
2. Description of the Related Art
A standard imager cell is a device that utilizes a photodiode to convert light energy into photo-generated electrons, and a number of transistors to control the operation of the photodiode and sense the number of photo-generated electrons that were produced by the light energy.
A color imager cell is a cell that produces photo information for more than one color, typically red, green, and blue. Digital cameras utilize millions of color imager cells that capture images based on the amount of reflected red, green, and blue light energy that strikes each imager cell.
One approach to implementing a color imager cell is to utilize three standard imager cells, and vary the depth of the sense point. Blue photons are high-energy photons that are absorbed at the surface of the photodiode, while green photons have less energy and are absorbed at a first depth that lies below the surface of the photodiode. Red photons have even less energy and are absorbed at a second depth that lies below the first depth.
Thus, by sensing the number of photo-generated electrons at the surface of a first cell, at the first depth in a second cell, and at the second depth in a third cell, blue, green, and red photo information, respectively, can be obtained from the three standared imager cells. One drawback of this approach, however, is that a color imager cell is three times the size of a standard imager cell.
Another approach to implementing a color imager cell that addresses the size problems of the three-cell approach is a vertical color imager cell. A vertical color imager cell is an imager cell that collects photo-generated electrons that represent different colors, such as red, green, and blue, at different depths in the same photodiode.
In an imager cell, the transistors required to operate the photodiode and sense the number of photo-generated electrons utilize a relatively small amount of space with respect to the amount of space utilized by the photodiode. Thus, one of the advantages of a vertical color imager cell is that since the cell collects multiple colors with the same photodiode, a vertical color imager cell is only slightly larger than a standard imager cell.
U.S. Patent Application Publication US 2002/0058353 A 1 published on May 16, 2002 describes a vertical color imager cell.
FIG. 1
shows a combined cross-sectional and schematic diagram that illustrates a prior art color imager cell
100
. Cell
100
is substantially the same as the cell shown in
FIG. 2A
of the '353 published application.
As shown in
FIG. 1
, imager cell
100
includes a first p− region
110
, a first n+ region
112
that contacts p− region
110
, and a first depletion region
114
that is formed at the junction between regions
110
and
112
. Imager cell
100
also includes a second p− region
120
that contacts n+ region
112
, a second n+ region
122
that contacts p− region
120
, and a second depletion region
124
that is formed at the junction between regions
120
and
122
. In addition, imager cell
100
further includes a third p-type region
130
that contacts n+ region
122
, a third n+ region
132
that contacts p− region
130
, and a third depletion region
134
that is formed at the junction between regions
130
and
132
.
In operation, as shown in
FIG. 1
, p− regions
110
,
120
, and
130
are connected to ground. In addition, n+ regions
112
,
122
, and
132
are connected to first, second, and third reset transistors
150
,
152
, and
154
, respectively. Prior to collecting photo information, reset transistors
150
,
152
, and
154
are pulsed on which, in turn, places a positive potential on n+ regions
112
,
122
, and
132
.
The positive potential reverse biases the pn junction of regions
110
and
112
, thereby forming a red collecting photodiode, and the pn junction of regions
120
and
122
, thereby forming a green collecting photodiode. The positive potential also reverse biases the pn junction of regions
130
and
132
, thereby forming a blue collecting photodiode.
Once the positive potentials have been placed on n+ regions
112
,
122
, and
132
, light energy, in the form of photons, is collected by the red, green, and blue photodiodes. The red photons are absorbed by the red photodiode which, in turn, forms a number of red electron-hole pairs, while the green photons are absorbed by the green photodiode which, in turn, forms a number of green electron-hole pairs. Similarly, the blue photons are absorbed by the blue photodiode which, in turn, forms a number of blue electron-hole pairs.
The red electrons from the electron-hole pairs that are formed in depletion region
114
move under the influence of the electric field towards n+ region
112
, where each additional electron collected by n+ region
112
reduces the positive potential that was placed on n+ region
112
by reset transistor
150
. On the other hand, the holes formed in depletion region
114
move under the influence of the electric field towards p− region
110
.
In addition, the electrons, which are from the electron-hole pairs that are formed in p− region
110
within a diffusion length of depletion region
114
, diffuse to depletion region
114
and are swept to n+ region
112
under the influence of the electric field. Further, the electrons that are formed in n+ region
112
remain in n+ region
112
.
Similarly, the green electrons from the electron-hole pairs that are formed in depletion region
124
move under the influence of the electric field towards n+ region
122
, where each additional electron collected by n+ region
122
reduces the positive potential that was placed on n+ region
122
by reset transistor
152
. On the other hand, the holes formed in depletion region
124
move under the influence of the electric field towards p− region
120
.
In addition, the electrons, which are from the electron-hole pairs that are formed in p− region
120
within a diffusion length of depletion region
124
, diffuse to depletion region
124
and are swept to n+ region
122
under the influence of the electric field. Further, the electrons that are formed in n+ region
122
remain in n+ region
122
.
As with the red and green electrons, the blue electrons from the electron-hole pairs that are formed in depletion region
134
move under the influence of the electric field towards n+ region
132
, where each additional electron collected by n+ region
132
reduces the positive potential that was placed on n+ region
132
by reset transistor
154
. On the other hand, the holes formed in depletion region
134
move under the influence of the electric field towards p− region
130
.
In addition, the electrons, which are from the electron-hole pairs that are formed in p− region
130
within a diffusion length of depletion region
134
, diffuse to depletion region
134
and are swept to n+ region
132
under the influence of the electric field. Further, the electrons that are formed in n+ region
132
remain in n+ region
132
.
After the red, green, and blue photodiodes have collected light energy for a period of time, known as the integration period, sense circuitry associated with the photodiodes detects the change in potential on n+ regions
112
,
122
, and
132
. Specifically, in addition to a reset transistor, each photodiode also has an associated source follower transistor SF and a row select transistor RS.
The change in potentials on an n+ region is present on the gate of the associated source follower transistor SF, while the source of the source follower transistor SF is one diode drop below the potential. Thus, when the gate of the row select transistor RS is pulsed, an output potential equal
Hopper Peter J.
Lindorfer Philipp
Munson Gene M.
National Semiconductor Corporation
Pickering Mark C.
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