U.S. patent application number 11/956551 was filed with the patent office on 2008-06-19 for highly sensitive photo-sensing element and photo-sensing device using the same.
Invention is credited to Mutsuko Hatano, Masayoshi Kinoshita, Hideo Sato, Mitsuharu Tai.
Application Number | 20080142920 11/956551 |
Document ID | / |
Family ID | 39526114 |
Filed Date | 2008-06-19 |
United States Patent
Application |
20080142920 |
Kind Code |
A1 |
Tai; Mitsuharu ; et
al. |
June 19, 2008 |
HIGHLY SENSITIVE PHOTO-SENSING ELEMENT AND PHOTO-SENSING DEVICE
USING THE SAME
Abstract
According to the present invention, a highly sensitive
photo-sensing element and a sensor driver circuit are prepared by
planer process on an insulating substrate by using only
polycrystalline material. Both the photo-sensing element and the
sensor driver circuit are made of polycrystalline silicon film. As
the photo-sensing element, a photo transistor is formed by using
TFT, which comprises a first electrode 11 prepared on an insulating
substrate 10, a photoelectric conversion region 14 and a second
electrode 12, and a third electrode 13 disposed above the
photoelectric conversion region 14. An impurity layer positioned
closer to an intrinsic layer (density of active impurities is
10.sup.17 cm.sup.-3 or lower) is provided on the regions 15 and 16
on both sides under the third electrode 13 or on one of the regions
15 or 16 on one side.
Inventors: |
Tai; Mitsuharu; (Kokubunji,
JP) ; Sato; Hideo; (Hitachi, JP) ; Hatano;
Mutsuko; (Kokubunji, JP) ; Kinoshita; Masayoshi;
(Hachioji, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET, SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
39526114 |
Appl. No.: |
11/956551 |
Filed: |
December 14, 2007 |
Current U.S.
Class: |
257/463 ;
257/E31.057; 345/207 |
Current CPC
Class: |
H01L 27/1214 20130101;
H01L 31/1136 20130101; G09G 2360/14 20130101 |
Class at
Publication: |
257/463 ;
345/207; 257/E31.057 |
International
Class: |
G09G 5/00 20060101
G09G005/00; H01L 31/103 20060101 H01L031/103 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 18, 2006 |
JP |
2006-339745 |
Claims
1. A photo-sensing element prepared on an insulating substrate,
said photo-sensing element comprises a first electrode and a second
electrode disposed by introducing highly-doped impurities on a
first semiconductor layer, a photoelectric conversion region
prepared by introducing an intrinsic layer or lowly-doped
impurities on said first semiconductor layer is positioned between
said first electrode and said second electrode, and a third
electrode is arranged above said photoelectric conversion region
via an insulating film.
2. A photo-sensing element according to claim 1, wherein a third
electrode is disposed via an insulating film above a partial region
of said photoelectric conversion region except a region in contact
with said first electrode and a region in contact with said second
electrode.
3. A photo-sensing element according to claim 2, wherein density of
majority carriers in the semiconductor layer to form the first
electrode and the second electrode is 1.times.10.sup.19/cm.sup.3 or
higher and density of majority carriers in the semiconductor layer
to form the photoelectric conversion region is
1.times.10.sup.17/cm.sup.3 or lower under the conditions with no
light projected and with no voltage applied.
4. A photo-sensing element according to claim 3, wherein density of
majority carriers in the semiconductor layer to form a region in
contact with the first electrode and a region in contact with the
second electrode in the photoelectric conversion region is in the
range from 1.times.10.sup.17/cm.sup.3 to
1.times.10.sup.19/cm.sup.3.
5. A photo-sensing element according to claim 4, wherein, when the
sensor is in operation, the voltage on the second electrode is set
to a value higher than the voltage on the first electrode, and the
voltage applied on the third electrode is set to a value lower than
the voltage on the first electrode.
6. A photo-sensing element according to claim 4, wherein, when the
sensor is in operation, the voltage applied on the second electrode
is higher than the voltage applied on the first and the third
electrodes.
7. A photo-sensing element according to claim 2, wherein, when the
first, the second and the third electrodes are projected vertically
on the surface of the insulating substrate, each of a distance
between the first electrode and the third electrode and a distance
between the second electrode and the third electrode is 1 .mu.m or
more.
8. A photo-sensing element according to claim 1, wherein a third
electrode is disposed via an insulating film above a partial region
of the photoelectric conversion region except a region in contact
with the first electrode and a region in contact with the second
electrode.
9. A photo-sensing element according to claim 8, wherein, when the
sensor is in operation, the voltage applied on the second electrode
is higher than the voltage on the first electrode, and the voltage
applied on the third electrode is lower than the voltage on the
first electrode.
10. A photo-sensing element according to claim 8, wherein, when the
sensor is in operation, the voltage applied on the second electrode
is higher than the voltage applied on the first electrode and the
third electrode.
11. A photo-sensing device, comprising a photo-sensing element
provided on an insulating substrate and a photo-sensor driver
processing circuit for processing the output from the photo-sensing
element, wherein: said photo-sensing element comprises a first
electrode and a second electrode prepared by introducing
highly-doped impurities to a first semiconductor layer, there is
provided a photoelectric conversion region prepared by introducing
an intrinsic layer or lowly-doped impurities to said first
semiconductor layer, and a third electrode is disposed above the
photoelectric conversion region via an insulating film; a switching
element to constitute the photo-sensor driver processing circuit
comprises a first electrode and a second electrode prepared by
introducing highly-doped impurities to a first semiconductor layer,
said switching element comprises an active layer region disposed
between the first electrode and the second electrode on the first
semiconductor layer, a third electrode is disposed via an
insulating film above a partial region of the active layer region
except a region in contact with the first electrode and the region
in contact with the second electrode, said partial region is an
intrinsic layer or a lowly-doped impurity layer, and, into the
region in contact with the first electrode in the active layer
region and into the region in contact with the second electrode,
impurities are introduced in such quantity that the quantity of the
impurities is fewer than the quantity of the impurities introduced
into the first electrode and the second electrode and more than the
quantity of the impurities introduced into the partial region.
12. A photo-sensing device according to claim 11, wherein, under
the conditions with no light projected and with no voltage applied,
density of majority carriers in the semiconductor layer to form the
first electrode and the second electrode of the photo-sensing
element and the switching element is 1.times.10.sup.19/cm.sup.3 or
higher, density of majority carriers in the semiconductor layer to
make up photoelectric conversion region of the photo-sensing
element and partial region of the switching element is
1.times.10.sup.17/cm.sup.3 or lower, and density of majority
carriers in the semiconductor layer at two points to form the
active layer region in contact with the first electrode and the
second electrode of the switching element is in the range from
1.times.10.sup.17/cm.sup.3 to 1.times.10.sup.19/cm.sup.3.
13. A photo-sensing device according to claim 12, wherein density
of majority carriers in the semiconductor layer to form
photoelectric conversion region in contact with the first electrode
or the second electrode of the photo-sensing element is in the
range from 1.times.10.sup.17/cm.sup.3 to
1.times.10.sup.19/cm.sup.3.
14. An image display unit, comprising a photo-sensor disposed on an
insulating substrate, a photo-sensor driver processing circuit for
processing sensor signal from the photo-sensor, and a peripheral
circuit for driving a plurality of pixels in response to the sensor
signal, wherein: said photo-sensing element comprises a first
electrode and a second electrode prepared by introducing
highly-doped impurities to a first semiconductor layer, there is
provided a photoelectric conversion region prepared by introducing
an intrinsic layer or lowly-doped impurities to said first
semiconductor layer, and a third electrode is disposed above the
photoelectric conversion region via an insulating film; a switching
element to constitute the photo-sensor driver processing circuit
comprises a first electrode and a second electrode prepared by
introducing highly-doped impurities to a first semiconductor layer,
said switching element comprises an active layer region disposed
between the first electrode and the second electrode on the first
semiconductor layer, a third electrode is disposed via an
insulating film above a partial region of the active layer region
except a region in contact with the first electrode and the region
in contact with the second electrode, said partial region is an
intrinsic layer or a lowly-doped impurity layer, and, into the
region in contact with the first electrode in the active layer
region and into the region in contact with the second electrode,
impurities are introduced in such quantity that the quantity of the
impurities is fewer than the quantity of the impurities introduced
into the first electrode and the second electrode and more than the
quantity of the impurities introduced into the partial region.
15. An image display unit according to claim 14, wherein, under the
conditions with no light projected and with no voltage applied,
density of majority carriers in the semiconductor layer to form the
first electrode and the second electrode of the photo-sensing
element and the switching element is 1.times.10.sup.19/cm.sup.3 or
higher, density of majority carriers in the semiconductor layer to
make up photoelectric conversion region of the photo-sensing
element and partial region of the switching element is
1.times.10.sup.17/cm.sup.3 or lower, and density of majority
carriers in the semiconductor layer at two points to form the
active layer region in contact with the first electrode and the
second electrode of the switching element is in the range from
1.times.10.sup.17/cm.sup.3 to 1.times.10.sup.19/cm.sup.3.
16. An image display unit according to claim 15, wherein density of
majority carriers in the semiconductor layer to form photoelectric
conversion region in contact with the first electrode or the second
electrode of the photo-sensing element is in the range from
1.times.10.sup.17/cm.sup.3 to 1.times.10.sup.19/cm.sup.3.
Description
[0001] The present application claims priority from Japanese
application JP 2006-339745
[0002] filed on Dec. 18, 2006, the content of which is hereby
incorporated by reference into this application
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to a thin-film photo-sensing
element formed on an insulating film substrate and to a
photo-sensing device using the same. In particular, the invention
relates to an optical sensor array such as an X-ray imaging device,
a near-infrared light detector for biometrics, etc. and an image
display unit integrated with a display panel with touch panel
function, ambient light detecting function, and input function
using photo-sensor, e.g. low temperature process semiconductor
thin-film transistor used in liquid crystal display, organic
electroluminescence display, inorganic electroluminescence display,
and electro chromic display, and low temperature process
photoconductive element or low temperature process photo-diode
element.
[0005] 2. Description of the Prior Art
[0006] X-ray imaging device is now indispensable as a medical
treatment device, and there are strong and continuous demands to
simplify the operation of the device and to produce it at lower
cost. Also, in recent years, special notice has been given on the
means for biometrics to obtain information from finger vein or palm
vein, and it is an imminent problem to have a device or a system
for reading the information of this type. In such device or system,
a sensor array for optical detection in a certain area, i.e. the
so-called area sensor, for reading these types of information, and
this area sensor must be provided at lower cost. To satisfy these
requirements, a method has been proposed in the Non-Patent Document
1 as given below, according to which an area sensor is prepared by
semiconductor forming process (planer process) on an inexpensive
insulating substrate typically represented by glass substrate.
[0007] In the field of the products different from the area sensor,
the photo-sensor is also required on a medium-to-small size
display. The medium-to-small size display is applied for display
purpose in mobile devices such as handy phone, digital still
camera, PDA (personal digital assistant), or display on board.
Multiple functions and high performance characteristics are
required on these types of devices and systems. Attention is now
focused on the photo-sensor as effective means for adding ambient
light detecting characteristics (see the Non-Patent Document 2
given below) and touch panel functions. However, unlike the large
size display, panel cost is low in the medium-to-small size
display. This means that the cost is increased for mounting the
photo-sensor or the sensor driver. Therefore, when a pixel circuit
is prepared on a glass substrate by semiconductor forming process
(planer process), special notice is now given on the technique to
prepare the photo-sensor or the sensor driver and on the method to
manufacture them at lower cost.
[0008] The important issues in the groups of products as described
above are to prepare a photo-sensing element or a sensor driver on
an inexpensive insulating substrate. The sensor driver typically
comprises LSI, and it usually requires MOS transistor deposited on
monocrystalline silicon wafer or a switching element with high
performance characteristics of similar type. To solve such
problems, the technique as described below seems to be
essential.
[0009] As pixel and pixel driver circuit element for an active
matrix type liquid crystal display, an organic electroluminescence
display, or an image sensor, the thin-film transistor (hereinafter
referred as "polycrystalline semiconductor TFT") has been
developed, which is made up by polycrystalline semiconductor.
Compared with other types of driver circuit elements, the
polycrystalline semiconductor TFT is advantageous in that it has
higher driving ability. Peripheral driver circuit can be prepared
on the same glass substrate as pixel. As a result, this is
convenient for attaining the customization of circuit specification
and low cost production by simultaneously performing pixel
designing and preparation process and for achieving high
reliability by avoiding mechanical fragility of the connections of
the driving LSIs and pixels.
[0010] The polycrystalline semiconductor TFT for liquid crystal
display is prepared on a glass substrate for the purpose of
reducing the manufacturing cost. In the process to prepare TFT on
the glass substrate, process temperature is determined by
heat-resistant temperature of the glass. As a method to prepare
polycrystalline semiconductor thin-film of high quality without
giving thermal damage to the glass substrate, ELA method (excimer
laser annealing method) is known, according to which the
semiconductor layer is molten and re-crystallized. The
polycrystalline semiconductor TFT obtained by this method has
driving ability more than 100 times as high as that of TFT (with
the channel made of amorphous semiconductor) as used in the
conventional type liquid crystal display, and some of the circuits
such as driver circuit can be mounted on the glass substrate.
[0011] With regard to the photo-sensor, a method to use the
polycrystalline semiconductor TFT and a method to use a PIN type
diode in addition to pixel circuit and driver circuit are described
in the Patent Document 1 as given below. The characteristics
required for the photo-sensor are high sensitivity and low noise.
If it is limited to the important characteristics of the
photo-sensing element, "high sensitivity" means to issue as high
signal as possible with respect to a light with certain intensity.
This means that a material and an element structure with high
light-to-current conversion efficiency are required. "Low noise"
means that the signal is as low as possible when the light is not
projected.
[0012] FIG. 12 represents cross-sectional views each showing a
conventional type photo-sensing element. FIG. 12 (a) shows a PIN
type diode element of vertical structure using amorphous silicon
layer as a photoelectric conversion layer 113. FIG. 12 (b) shows a
TFT element of lateral structure, which uses amorphous silicon film
as the photoelectric conversion layer 113 and in which electric
charge flows in parallel to the connected surface. Both of these
serve as photo-sensing elements.
[0013] The photo-sensing element as shown in FIG. 12 (a) comprises
a photoelectric conversion layer 113 of amorphous silicon film
interposed between a first metal electrode layer 111 and a second
metal electrode layer 112, and an impurity induced layer 120, which
is prepared between the photoelectric conversion layer 113 and each
of the electrode layers. This photo-sensing element is disposed on
an insulating substrate 110. Each of the electrode layers is
connected to an electrode line 114 insulated by an interlayer
insulating film 115 and is covered with a passivation film 117.
[0014] The photo-sensing element shown in FIG. 12 (b) comprises a
source electrode 131, a gate electrode 132, a drain electrode 133
and a photoelectric conversion layer 113 made of amorphous silicon
film. Further, it comprises an impurity induced layer 120 disposed
on boundary surface between the photoelectric conversion layer 113
and each of the electrodes. This photo-sensing element is mounted
on the insulating substrate 110 and is covered with a passivation
film 117.
[0015] In FIG. 12, as a semiconductor material to be used in the
photoelectric conversion layer 113 of the sensor element disposed
on the insulating substrate 110, a silicon type material such as
silicon, silicon-germanium, etc. should be used because due
consideration must be given on environmental problem or process
coordination when driver circuit (or pixel circuit) is formed at
the same time. When the silicon type material is used, among the
light components absorbed in the wavelength range from infrared
light to visible light, almost all of the light components are
converted to electric current. This means that a material having
higher absorption coefficient is suitable as the material for the
photo-sensing element.
[0016] Also, when attention is given on solid phase status
(hereinafter referred as "phase status") of semiconductor such as
amorphous, crystalline or polycrystalline semiconductor, absorption
coefficient of the amorphous material is at the highest for the
entire wavelength range and this has high resistance. In this
respect, amorphous material is advantageous and suitable as the
material of the sensor element.
[0017] However, when the amorphous material is used in the sensor
element, the performance characteristics of the switching element
are not sufficient, and it is not possible to have the driver
circuit at the same time. For instance, when TFT is made of
amorphous silicon material, which is optimal as the material for
the sensor element, field effect mobility is 1 cm.sup.2/Vs or
lower. For this reason, high sensor characteristics can be attained
by preparing the sensor array as the structure shown in FIG. 12,
while switching characteristics can be provided by mounting the
driver LSI and by connecting via the means such as FPC.
[0018] When the material is monocrystalline, it is possible to make
up the sensor element and the circuit at the same time. This
manufacturing process is a process requiring the temperature as
high as 1000.degree. C. or higher. Thus, it cannot be prepared on
an inexpensive insulating substrate such as glass substrate.
[0019] When the switching element and the sensor element to
constitute the driver circuit are made of polycrystalline
semiconductor film, the driver circuit (and also, pixel circuit)
and the sensor element can be prepared at the same time on the same
insulating substrate. In case of the polycrystalline semiconductor
film produced by ELA method, TFT with high quality can be obtained,
which can be used for the driver circuit. [0020] [Non-Patent
Document 1] "Technology and Applications of Amorphous Silicon; pp.
204-221. [0021] [Non-Patent Document 2] SHARP Technical Journal,
Vol. 92 (2005); pp. 35-39. [0022] [Patent Document 1]
JP-A-2006-3857.
SUMMARY OF THE INVENTION
[0023] When the PIN type diode described in the Patent Document 1
is compared with an amorphous vertical (laminated) type element,
the former is lower in sensitivity than the latter but has higher
sensitivity among the sensor elements made of polycrystalline film.
However, intrinsic region (I region), P region and N region must be
separately provided, and this means that the number of photo-masks
and the number of processes are increased. This results in higher
manufacturing cost compared with other types of sensor
elements.
[0024] As another type of sensor element prepared by using
polycrystalline semiconductor film, TFT is known. Because the
structure is the same as that of the switching element, which makes
up the circuit, it is advantageous in that the number of processes
can be decreased and the manufacturing cost can be reduced.
However, there are problems in the maintenance and the improvement
of sensitivity of the element. Normally, in the channel region,
which serves as photoelectric conversion region, thin impurity
layer is introduced for the control of threshold voltage. As a
result, the depletion region is short, and service life of the
electron-hole pair is shortened. Accordingly, the photocurrent to
be detected is low, and the sensitivity is worsened. Also, when the
gate is positioned closer to the photoelectric conversion surface
with respect to the channel, the sensitivity is reduced further due
to the light shielding effect of the gate.
[0025] It is an object of the present invention to provide a sensor
driver circuit (and pixel circuit and other circuits if necessary)
and a photo-sensing element with high performance characteristics
by preparing it on an insulating substrate through planer process
using only polycrystalline semiconductor material, and also to
provide a lost-cost area sensor integrated with driver or an image
display unit integrated with a photo-sensor at lower cost by
maintaining the manufacturing cost of the driver itself and the
manufacturing cost of pixel circuit of the image display unit at a
low level.
[0026] The sensor driver circuit (and pixel circuit and other
circuits if necessary) and a photo-sensing element are manufactured
by using polycrystalline silicon film or polycrystalline
silicon-germanium film. A diode with a gate using TFT is prepared
as the photo-sensing element, and an impurity layer closer to
intrinsic layer (the density of activated impurities is 10.sup.17
cm.sup.-3 or lower) is provided on both sides or on one side of the
gate. In so doing, it is possible to maintain or reduce the number
of masking processes and the number of photolithographic processes.
As a result, a low-cost area sensor integrated with driver or an
image display unit integrated with photo-sensing element can be
provided by maintaining the manufacturing cost of the driver itself
and the manufacturing cost of the pixel circuit of the image
display unit on a low level.
[0027] According to the present invention, it is possible to
provide an area sensor with sensor driver circuit (and pixel
circuit and other circuits if necessary) and a photo-sensing
element with high performance characteristics prepared on
inexpensive insulating substrate. Also, it is possible to provide
an image display unit integrated with the photo-sensing
element.
[0028] To give additional values to the display driven by TFT, it
is essential to add higher functions and characteristics. As the
means to attain this purpose, the integration or the incorporation
of the photo-sensing element is very useful from the viewpoint of
the wide scope of functions, which can be added by its use.
Further, the area sensor, in which the photo-sensing element is
provided in array, is essential and useful in the applications such
as devices for medical treatment or for biometrics. In this sense,
it is important to manufacture these components at lower cost. As a
result, photo-sensing element with high performance characteristics
and sensor processing circuit can be prepared on inexpensive glass
substrate and the products with high reliability can be
manufactured at lower cost.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 represents conceptual drawings, each showing a
photo-sensing element according to the present invention;
[0030] FIG. 2 represents cross-sectional drawings, each showing a
photo-sensing element of the prior art and a photo-sensing element
according to the present invention respectively;
[0031] FIG. 3 is a diagram showing the relation between output
current and illuminance of the photo-sensing element of the prior
art and that of the present invention respectively;
[0032] FIG. 4 is a cross-sectional view of a switching element
manufactured at the same time as the photo-sensing element of the
present invention;
[0033] FIG. 5 represents cross-sectional views, each showing
another structural example of the photo-sensing element of the
present invention;
[0034] FIG. 6 (a) represents drawings, each showing manufacturing
process of the photo-sensing element and the switching element;
[0035] FIG. 6 (b) represents drawings, each showing manufacturing
process of the photo-sensing element and the switching element;
[0036] FIG. 6 (c) represents drawings, each showing manufacturing
process of the photo-sensing element and the switching element;
[0037] FIG. 7 is a cross-sectional view of the photo-sensing
element and the switching element (P-type TFT and N-type TFT);
[0038] FIG. 8 represents a layout drawing, a cross-sectional view
and an equivalent circuit diagram of one pixel of an area
sensor;
[0039] FIG. 9 represents a layout drawing, a cross-sectional view
and an equivalent circuit diagram of one pixel of another area
sensor;
[0040] FIG. 10 represents equivalent circuit diagrams, each for one
pixel of another area sensor;
[0041] FIG. 11 represents a rear view, a side view and a front view
of an image display unit with the photo-sensing element integrated
in it; and
[0042] FIG. 12 represents cross-sectional views, each showing a
conventional type photo-sensing element.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0043] Detailed description will be given below on embodiments of
the present invention referring to the attached drawings.
Embodiment 1
[0044] FIG. 1 represents conceptual drawings, each showing a
photo-sensing element according to the present invention. FIG. 1
(a) is a cross-sectional view of a photo-sensing element prepared
on an insulating substrate, and FIG. 1 (b) is a top view of the
photo-sensing element. As shown in FIG. 1, a first electrode 11 and
a second electrode 12 are disposed on a first semiconductor layer
on an insulating substrate 10 by introducing highly-doped
impurities. A photoelectric conversion region 14 prepared by
introducing an intrinsic layer or highly-doped impurities on the
first semiconductor layer is disposed between the first and the
second electrodes. Above an area of the photoelectric conversion
region 14 except a region 15 in contact with the first electrode 11
and except a region 16 in contact with the second electrode 12, a
third electrode 13 is arranged via an interlayer insulating film
17. On and above the interlayer insulating film 17, an interconnect
18, an interlayer insulating film 19 and a passivation film
connected with the first electrode 11 and the second electrode 12
via a contact hole 21 are provided respectively.
[0045] In the first semiconductor layer, a layer between the first
electrode 11 and the second electrode 12 is a layer where an
intrinsic layer or very lowly-doped impurities (with the density of
majority carriers in the semiconductor layer being
1.times.10.sup.17/cm.sup.3 or lower under the conditions with no
light projected and with no voltage applied) are introduced, and
this layer fulfills the function as a photoelectric conversion
layer. The functions as electrodes are given to the first electrode
11 and the second electrode 12 by introducing highly-doped
impurities (the density of majority carriers in the semiconductor
layer being 1.times.10.sup.19/cm.sup.3 under the conditions with no
light projected and with no voltage applied).
[0046] In FIG. 2, a photo-sensing element using a conventional type
switching TFT as shown in FIG. 2 (a) is compared with a
photo-sensing element according to the present invention as shown
in FIG. 2 (b). In FIG. 2, when the photo-sensors are in operation,
a depletion region 25 is formed on high voltage side (on the second
electrode side in FIG. 2), and an electron-hole pair provided
primarily on the depletion layer 25. Therefore, when a light to be
detected is projected to the region of the depletion layer 25,
considerable effect can be given on the sensitivity of the
sensor.
[0047] In the photo-sensor using the conventional type TFT, for the
purpose of maintaining the reliability of the switching
characteristics, a moderately-doped impurity layer 26 is provided
between the first electrode 11 and the second electrode 12 on the
first semiconductor layer by introducing the impurities of the same
type as the first electrode 11 and the second electrode 12 (with
the density of majority carriers being in the range from
1.times.10.sup.17/cm.sup.3 to 1.times.10.sup.19/cm.sup.3). In this
case, the depletion layer 25 is disposed below the third electrode
13, and the light coming from above cannot be absorbed when the
third electrode 13 is non-transparent to the light to be detected
(i.e. when the third electrode 13 does not allow the light to
pass).
[0048] In contrast, in the photo-sensor according to the present
invention, the depletion layer 25 is not covered by the third
electrode 13 because it has no moderately-doped impurity layer.
Also, in the photo-sensor of the present invention, leakage occurs
rarely when the light is not projected because of the photoelectric
conversion layer 14. As a result, the sensitivity is increased more
in the photo-sensing element of the present invention compared with
the conventional type TFT.
[0049] FIG. 3 is a diagram to show the relation between output
current of photo-sensing elements (i.e. the output of photo-sensing
element using the conventional type TFT and the output current of
the photo-sensor of the present invention) and illuminance. Each of
these photo-sensing elements outputs electric current with a value
to match the illuminance. When the values of these outputs are
compared with each other, it is found that the output current when
external light of 100 lx is projected is increased by 43% compared
with the photo-sensing element using the conventional type TFT, and
that leakage current (noise) when the light is not projected is
decreased by 67%. These results reveal that the sensitivity of the
photo-sensing element according to the present invention is higher
and it has superior characteristics as the photo-sensing
element.
[0050] As shown in FIG. 1 (a), when the length of the region 15 in
contact with the first electrode 11 in the photoelectric conversion
region 14 and the length of the region 16 in contact with the
second electrode 12 are longer, the light output is increased more,
and the leakage current (noise) when the light is not projected can
be reduced. This length is a distance from a line obtained by
projecting the end of the third electrode 14 on the first
semiconductor layer to each of the first electrode 11 and the
second electrode 12 respectively. These are the lengths each shown
by arrows in FIG. 1 (a). This length can be maintained to a length
of 1/2 to 1/10 of the scale of the element. In the photo-sensor
integrated with the photo-sensor array or the switching element,
the sensor element and the switching element of several .mu.m in
scale are prepared. In this case, it is desirable that each of the
length of the region 15 in contact with the first electrode 11 and
the length of the region 16 in contact with the second electrode 12
is 1 .mu.m or more.
[0051] FIG. 4 is a cross-sectional view of a switching element
(poly-crystalline silicon TFT) prepared at the same time as the
photo-sensor of the present invention. In FIG. 4, it is
characterized in that a first electrode (source or drain) 41 of a
plurality of switching elements 40 in a photo-sensor driver 50, an
active layer region (channel) 44 immediately below a third
electrode (gate) 43, a second electrode (drain or source) 42, and
the first electrode 11, the second electrode 12 and the third
electrode 13, and photoelectric conversion layer 14 of a
photo-sensing element 60 are made of polycrystalline silicon film
(a first semiconductor film). Because these are made of common
material, the manufacturing process can be simplified. At the same
time, it is possible to prepare the high-performance switching
element 40 by using the polycrystalline silicon TFT, and the
high-performance photo-sensing element 60 according to the present
invention can be prepared on the same insulating substrate 10
through common manufacturing process. In this case, the regions 15
and 16, which come into contact with the first electrode 11 and the
second electrode 12 of the photo-sensing element 60 respectively.
are intrinsic layer or very lowly-doped impurity induced layer (the
density of majority carriers in the semiconductor layer under the
conditions with no light projected or with no voltage applied being
1.times.10.sup.17/cm.sup.3 or lower). On the other hand, the
regions 45 and 46 of the first electrode 41 and the second
electrode 42 of the switching element 40 are moderately-doped
impurity induced layer of the same type as the first electrode 41
and the second electrode 42 of the photo-sensing element 60 (the
density of majority carriers under the conditions with no voltage
applied being in the range from 1.times.10.sup.17/cm.sup.3 to
1.times.10.sup.19/cm.sup.3).
[0052] FIGS. 5 (a) and (b) each represents a cross-sectional view
of another structural example of the photo-sensing element of the
present invention. The first electrode 11 and the second electrode
12 are prepared on the first semiconductor layer by introducing
highly-doped impurities, and a photoelectric conversion region 14
prepared by introducing intrinsic layer or lowly-doped impurities
to the first semiconductor layer is disposed between the first
electrode and the second electrode. A third electrode 13 is
arranged above the photoelectric conversion region 14 via an
interlayer insulating film 17 except a region 16, which comes into
contact with the second electrode 12 in the photoelectric
conversion region 14. The difference of FIG. 1 (a) from FIG. 5 (a)
and FIG. 5 (b) is as follows: In FIG. 1 (a), the region 15 in
contact with the first electrode 11 and the region 16 in contact
with the photoelectric conversion region 14 and the second
electrode 12 are made of the same intrinsic layer or the same very
lowly-doped impurity induced layer. On the other hand, in FIG. 5
(a), the region 15 in contact with the first electrode 11 is a
moderately-doped impurity induced layer (the density of majority
carriers under the conditions with no light projected and with no
voltage applied being in the range from 1.times.10.sup.17/cm.sup.3
to 1.times.10.sup.19/cm.sup.3). In FIG. 5 (b), the region 15 in
contact with the first electrode 11 is a highly-doped impurity
induced layer (the density of majority carriers in the
semiconductor layer under the conditions with no light projected
and with no voltage applied being 1.times.10.sup.19/cm.sup.3 or
more). As explained in connection with FIG. 2, when the sensor is
in operation, if a depletion layer 25 is formed on high voltage
side (e.g. on the second electrode side), the sensitivity of the
sensor can be maintained. Thus, highly sensitive photo-sensing
element can be provided even in the structure as shown in FIG. 5
similarly to the first electrode 11 and the second electrode 12.
Because resistance between the electrodes is decreased, this is an
effective structure in case the photo-sensing element also serves
as the switching element.
[0053] Next, referring to FIGS. 6 (a), (b) and (c), description
will be given on the process to prepare the photo-sensing element
and the switching element. First, as shown in FIG. 6 (a) (1), an
insulating substrate 10 is prepared. Here, an example is given on
an inexpensive glass substrate used as the insulating substrate 10,
while a plastic substrate typically represented by PET, an
expensive quartz substrate or a metal substrate may be used. In
case of the glass substrate, sodium, boron, etc. are contained in
the substrate, and this may cause contamination to the
semiconductor layer. In this respect, it is desirable to deposit an
undercoating film such as silicon oxide film or silicon nitride
film. On upper surface of it, an amorphous silicon film or a
microcrystalline silicon film 61 is deposited by chemical vapor
deposition (CVD). Then, excimer laser 62 is irradiated to the
silicon film 61 to turn it to polycrystalline, and a
polycrystalline silicon film 63 is prepared.
[0054] Next, as shown in FIG. 6 (a) (2), the polycrystalline
silicon film 63 is processed by photolithographic process to
prepare a polycrystalline silicon film 64 of island-like shape.
Then, as shown in FIG. 6 (a) (3), a gate insulating film 65 made of
silicon oxide film is deposited by chemical vapor deposition. The
material of the gate insulating film 65 is not limited to silicon
oxide film, while it is preferable to select a material having high
dielectric constant, high insulating property, low fixed charge,
low interface trapped charge, low density of trapping state, and
high process coordination. By implanting ions 66 to the island-like
polycrystalline silicon film via the gate insulating film 65, boron
is introduced, and a low density boron injection layer (NE layer)
67 is prepared. In this case, non-injection region of the
photoelectric conversion layer of photo-sensing element is
determined by a photo-resist 68 in the photolithographic process so
that impurities may not be intermingled. In case the photoelectric
conversion layer is a very thin impurity induced layer, very
lowly-doped impurities are introduced in advance. (As the methods
to introduce the impurities, there are: a method to intermingle
with impurity gas at the time of deposition, a method to introduce
impurities by ion implantation to the polycrystalline silicon film
via the gate insulating film, etc., while there is no limitation on
the selection of the method.)
[0055] Further, as shown in FIG. 6 (a) (4), non-injection regions
such as an N-type TFT region 70, a photo-sensor region 71, etc. are
determined by using a photo-resist 69 in the photolithographic
process. By introducing phosphorus through implantation of ions 72,
a lowly-doped phosphorus injection layer (PE layer) 73 is prepared.
The purpose of the use of impurities in the PE layer 73 and the NE
layer is to adjust threshold value of TFT. As the dosage at the
time of ion implantation, the optimal value is in the range from
1.times.10.sup.11 cm.sup.-2 to 1.times.10.sup.13 cm.sup.-2. In this
case, it is known that the density of majority carriers in the NE
layer 66 and the PE layer 72 is in the range from 1.times.10.sup.15
to 1.times.10.sup.17/cm.sup.3. The optimal value of boron injection
quantity is determined by the threshold value of the N-type TFT,
and the optimal value of phosphorus injection quantity is
determined by the threshold value of P-type TFT.
[0056] Next, as shown in FIG. 6 (a) (5), metal film for gate
electrode is deposited by CVD or sputtering. The metal film for
gate electrode is processed by using photo-resist in the
photolithographic process, and a gate electrode 76 is prepared. The
metal film for the gate electrode may not necessarily be a metal
film. It may be a polycrystalline silicon film prepared by
introducing highly-doped impurities with lower resistance.
[0057] Next, as shown in FIG. 6 (b) (6), by using a mask used in
FIG. 6 (a) (3), ions 78 are injected by using photo-resist 77 in
the photolithographic process, and phosphorus is introduced to both
sides of the gate electrode 76 of TFT, and a moderately-doped
phosphorus injection layer (N-layer) 79 is prepared. This
introduction of impurities has the purpose to improve the
reliability of the N-type TFT. As the dosage in ion implantation,
the optimal value to be injected is between 1.times.10.sup.11
cm.sup.-2 to 1.times.10.sup.15 cm.sup.-2. In this case, the density
of majority carriers in the N-layer 79 will be in the range from
1.times.10.sup.15/cm.sup.3 to 1.times.10.sup.19/cm.sup.3.
[0058] Next, as shown in FIG. 6 (b) (7), a non-injection region is
determined by using a photo-resist 80 in the photolithographic
process. By implanting ions 81 to electrode regions of TFT and of
the photo-sensor, phosphorus is introduced, and a highly-doped
phosphorus injection layer (N+ layer) 82 is prepared. The dosage of
phosphorus in the ion implantation layer is preferably
1.times.10.sup.15 cm.sup.-2 or more because it is necessary to
sufficiently decrease the resistance of the electrode. In this
case, the density of majority carriers in the N+ layer can be
1.times.10.sup.19/cm.sup.3 or higher.
[0059] Next, as shown in FIG. 6 (b) (8), a non-injection region of
the N-type TFT and the photo-sensor are determined by using a mask
used in FIG. 6 (a) (4) and by using a photo-resist 83 in the
photolithographic process. By implanting ions 84 to the electrode
region of the P-type TFT, boron is introduced, and a highly-doped
boron injection layer (P+ layer) 85 is prepared. The dosage at the
time of ion implantation is preferably 1.times.10.sup.15 cm.sup.-2
or more because the resistance of the electrode must be
sufficiently decreased. In this case, the density of majority
carriers in the P+ layer will be 10.sup.19/cm.sup.3 or more. By the
processes as described above, electrodes for TFT and photo-sensor
can be prepared.
[0060] In this embodiment, care must be taken that boron in the
same quantity as in the NE layer 67 is introduced into the PE layer
73, and that phosphorus in the same quantity of the N+ layer is
introduced into the P+ layer. These are the impurities, which are
not initially needed. For the purpose of maintaining the type of
majority carriers of TFT and the photo-sensor, it is necessary to
introduce phosphorus and boron in such quantities as to offset each
other into each of the layers.
[0061] In the present embodiment, it is advantageous in that the
photolithographic process can be simplified and the use of
photo-masks can be eliminated, while it is disadvantageous in that
many defects occur in the active layer of the P-type TFT. In case
the characteristics of the P-type TFT cannot be maintained, it is
desirable to increase the number of photo-masks used and the number
of the photolithographic processes to block the introduction of the
impurities, which need not be introduced, by covering the PE layer
73 and the P+ layer 85.
[0062] Next, as shown in FIG. 6 (b) (9), an interlayer insulating
film 86 is deposited above the gate electrode 76 by CVD using TEOS
(tetraethoxy silane) as raw material. Then, annealing is performed
for activation of the introduced impurities. Next, a contact hole
88 is prepared on electrode portion by using photo-resist 87 in the
photolithographic process. The interlayer insulating film 86 is
used to insulate the interconnects as prepared later from the gate
electrode of the lower layer and polycrystalline semiconductor
layer. In this respect, any type of film may be used so far as it
has insulating property. However, parasitic capacitance must be
reduced, and it is desirable to use a material, which has low
specific dielectric constant and low film stress so that it has
good process coordination to the thickening of the film. Further,
to be compatible with display function, the transparency of the
film is important, and it is desirable to use a material, which has
high transmittance to the visible light. In the present embodiment,
silicon oxide film using TEOS gas as raw material is used as an
example.
[0063] Next, as shown in FIG. 6 (c) (10), the materials for
interconnects are deposited, and interconnects 80 are prepared by
the photolithographic process. Further, as shown in FIG. 6 (c)
(11), a passivation film 90 is prepared by CVD. If necessary, after
the passivation film 90 is prepared, additional annealing is
performed to improve the characteristics of TFT. Any type of film
may be used so far as it has insulating property as in the case of
the interlayer insulating film 86. If necessary, as shown in FIG. 6
(c) (12), a flattened insulating film 91 is prepared by using an
insulating layer formed with paste method or insulating resistant
material. Then, by using a photo-resist 92 in the photolithographic
process, a contact hole 93 is formed for the contact between the
interconnects 89 and ITO in the subsequent process.
[0064] Next, as shown in FIG. 6 (c) (13), a transparent electrode
film such as ITO is prepared. Then, using a photo-resist 94 in the
photo-lithographic process, a transparent electrode 95 is prepared.
Then, a passivation film is prepared on it if necessary, and a
contact can be provided in the photolithographic process.
[0065] FIG. 7 shows an example of a P-type TFT 701, an N-type TFT
702 and a photo-sensing element 703 as prepared in the present
embodiment. Here, a photo-sensing element as shown in FIG. 1 (a) is
prepared. By the processes described in the present embodiment, the
TFT to constitute the circuit, and the photo-sensing element of all
of the structures shown in FIG. 5 can be prepared at the same
time.
[0066] FIGS. 8 (a), (b) and (c) represent a layout of one pixel of
an area sensor using a photo-sensor TFT according to the present
invention, its equivalent circuit diagram and an operation timing
chart respectively. In FIG. 8, the voltage of a bias line 801 (a
second electrode 812) of a photo-sensor TFT 800 is set to a value
lower than the voltage of a sensor node 802 (a first electrode 811
of the photo-sensor TFT 800), and the voltage of a sensor gate line
803 (a third electrode 813 of the photo-sensor TFT 800) is set to a
higher value, and voltage of the sensor node 802 (the first
electrode 811) is reset. When the sensor is in operation, the
voltage of the bias line 801 (the second electrode 812) is set to a
value considerably higher than the voltage of the sensor node 802
(the first electrode 811), and the voltage of the sensor gate line
803 (the third electrode 813) is decreased. In this case, the
photo-sensor TFT 800 is in "off" state, and only very slight
electric current flows to the photo-sensor TFT 800. When light is
projected to the photo-sensor TFT 800, more electric current flows
than the time when light is not projected, and the voltage of the
sensor node (the first electrode 811) is increased. When voltage is
applied on a gate electrode 814 of a switching TFT 820 from the
gate line 804 at a certain time, electric charge proportional to
the illuminance of incident light is given to the data line 805,
and the voltage on the data line 805 is increased. This voltage is
read by a sensor driver, which is provided outside the region of
the area sensor. The electric charge accumulated on the sensor node
802 (the first electrode 811) is maintained by parasitic
capacitance, and auxiliary storage capacitance may be added if
necessary. The sensor operation time should be set so that the
voltage of the sensor node 802 (the first electrode 811) does not
exceed the voltage of the bias line 801 (the second electrode 812).
In this case, accumulated electric charge on the sensor node 802 is
proportional to illuminance. Reference numeral 815 in FIG. 8 (a)
represents a contact hole to connect the data line 805 to the first
electrode (or the second electrode) of the switching TFT 820, and
the numeral 816 denotes a contact hole to connect the bias line 801
to the second electrode 812 of the photo-sensor TFT 800.
[0067] FIGS. 9 (a), (b) and (c) represent another example of a
layout drawing of one pixel of an area sensor using a photo-sensor
TFT according to the present invention, its equivalent circuit
diagram, and an operation timing chart respectively. In this
example, it is characterized in that the third electrode 813 of the
photo-sensor TFT 800 and the first electrode 811 are
short-circuited. In FIG. 9, the voltage of the bias line 801 (the
second electrode 812) is set to a lower value than the voltage of
the sensor node 802 (the first electrode 811 and the third
electrode 813) before the operation of the sensor, and the voltage
on the sensor node 802 is reset. When the sensor is in operation,
the voltage of the bias line 801 (the second electrode 812) is set
to a value considerably higher than the voltage on the sensor node
802. In this case, the photo-sensor TFT 800 is in "off" state, and
only very slight electric current flows to the photo-sensor TFT
800. When light is projected to the photo-sensor TFT 800, more
electric current than that of the time when light is not projected
flows, and the voltage on the sensor node 802 is increased. When
voltage is applied on the gate electrode 814 of the switching TFT
820 from the gate line 804 at a certain time and the switching TFT
820 is set into operation, electric charge proportional to the
illuminance of incident light is given to the data line 805, and
the voltage of the data line 805 is increased. This voltage is read
by a sensor driver provided outside the region of the area sensor.
The electric charge accumulated on the sensor node 802 is
maintained by parasitic capacitance similarly to the case of FIG.
8, and auxiliary storage capacitance may be added if necessary.
Sensor operation time should be set so that the voltage of the
sensor node 802 (the first electrode 811 and the third electrode
813) does not exceed the voltage of the bias line 801 (the second
electrode 812). In this case, the accumulated electric charge on
the sensor node 802 is proportional to the illuminance.
[0068] FIG. 10 shows another example of equivalent circuit diagram
of one pixel of an area sensor using the photo-sensor TFT according
to the present invention. FIG. 10 (a) shows that the third
electrode 813 of the photo-sensor TFT 800 can be controlled by an
independent line. In FIG. 10 (b), the third electrode 813 of the
photo-sensor TFT 800 is short-circuited to the first electrode 811.
In FIG. 10, the voltage of the bias line 801 (the second electrode
812) is set to a value lower than the voltage of the sensor node
802, and the voltage on the sensor node 802 is reset. When the
sensor is in operation, the voltage of the bias line 801 (the
second electrode 812) is set to a value considerably higher than
the voltage on the sensor node 802. In this case, only very slight
electric current flows to the photo-sensor TFT 800 due to
rectifying effect. When light is projected to the photo-sensor TFT
800, more electric current flows than the time when light is not
projected, and the voltage on the sensor node 802 is increased. In
this case, the voltage on the data line 805 is set to a value lower
than the voltage on the gate line 804 in advance (or it may be set
reversely). When the voltage on the sensor node 802 reaches a value
higher than the sum of the voltage of the data line 805 (the
voltage of the gate line in reverse case) and the threshold value
of the switching TFT 820, the switching TFT 820 is turned to "on"
state, and the voltage on the data line 805 reaches. The voltage
approximately equal to the voltage on the gate line 804. The change
of the voltage is read by the sensor driver, which is provided
outside the region of the area sensor. Therefore, if the switching
TFT 820 can be turned on within the operation time of the sensor,
signal can be outputted regardless of the illuminance. As a result,
by changing the sensor operation time, gray scale can be
detected.
Embodiment 2
[0069] The sensors applied in FIG. 8, FIG. 9 and FIG. 10 may be any
of the sensors shown in FIG. 1 (a) and FIGS. 5 (a) and (b).
However, FIGS. 5 (a) and (b) are asymmetrical, and care must be
taken on the arrangement of the electrodes. In these figures,
examples of the area sensors are shown. If pixel circuits are
arranged for each pixel at the same time as the sensor, an image
display unit with photo-sensor functions can be provided. Signal
line to send signal to pixel, gate line, etc. may be added newly.
Or, the bias line, the data line or the gate line may be used in
common by adjusting the timing of signal line.
[0070] FIG. 11 represents schematical drawings of an image display
unit integrated with the photo-sensing element of the present
invention. FIG. 11 (a) is a rear view of an image display unit. On
a glass substrate 101, a printed board 103 for driver LSI
comprising a driver LSI 102 is disposed. Via an FPC 104, a
plurality of pixels formed on the front side of the image display
unit are driven. FIG. 11 (b) is a side view of the image display
unit. On front side of the image display unit, a photo-sensor 105
comprising the photo-sensing element of the present invention and a
plurality of pixels 106 arranged on an image display region are
disposed. FIG. 11 (c) is a front view of the image display unit. On
a glass substrate 101, a peripheral driver circuit 107 for driving
pixels 106, a photo-sensor driver processing circuit 108 for
processing the output of the photo-sensor 105, a backlight, and
other control circuits 109 are disposed.
[0071] In FIG. 11, sensor signals to correspond to external light
from the photo-sensor 105 are processed by the photo-sensor driver
processing circuit 108, and the signals are sent to the peripheral
driver circuit 107, which drives the pixels 106. At the peripheral
driver circuit 107, image quality such as luminance, contrast, etc.
of the image display unit are controlled, depending on the sensor
signals.
[0072] In FIG. 11, a part of the driver is composed of LSIs and
these are mounted on rear surface via FPC. To meet the required
performance characteristics, TFT arranged on the glass substrate
can be used sequentially. In so doing, LSIs and the cost for
mounting them can be reduced, and the decrease of mechanical
reliability due to the mounting can be avoided. Also, the driver
can be designed at the time of the designing of pixels, and this
facilitates the customization of the components. According to the
present invention, the sensor and its driver can be integrated on
the glass substrate, and this makes it possible to arrange and
mount the sensor and processing circuits at any desired position in
compact arrangement.
* * * * *