U.S. patent application number 11/200425 was filed with the patent office on 2005-12-08 for light dependent polymeric field effect transistor.
Invention is credited to Narayan, K. S..
Application Number | 20050269564 11/200425 |
Document ID | / |
Family ID | 26710082 |
Filed Date | 2005-12-08 |
United States Patent
Application |
20050269564 |
Kind Code |
A1 |
Narayan, K. S. |
December 8, 2005 |
Light dependent polymeric field effect transistor
Abstract
A polymer-based field effect transistor photosensitive to
incident light, which may enhance the transistor's characteristics
and controlling parameters of the transistor state. The transistor
is comprised of a metal-insulator-semiconductor structure with the
insulating and semiconducting layers made of a polymeric media. The
semiconducting polymer which also is photoconducting, forms the
charge transport layer between the source and drain. The transistor
exhibits large photosensitivity indicated by the sizable changes in
the drain-source current, by a factor of 100-1000 even at low
levels of light with illumination of approximately 1 mlux. The
photosensitivity of the transistor is further enhanced with
introduction of dilute quantity electron acceptor moieties in the
semiconducting polymer matrix. Several applications of the
light-responsive polymer-transistor are disclosed, such as use as a
logic element and as a backbone of an image sensor.
Inventors: |
Narayan, K. S.; (Bangalore,
IN) |
Correspondence
Address: |
CHOATE, HALL & STEWART LLP
TWO INTERNATIONAL PLACE
BOSTON
MA
02110
US
|
Family ID: |
26710082 |
Appl. No.: |
11/200425 |
Filed: |
August 9, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11200425 |
Aug 9, 2005 |
|
|
|
10033743 |
Dec 28, 2001 |
|
|
|
60259375 |
Jan 2, 2001 |
|
|
|
Current U.S.
Class: |
257/40 |
Current CPC
Class: |
H01L 51/0545 20130101;
H01L 51/4253 20130101; H01L 51/0046 20130101; B82Y 10/00 20130101;
H01L 51/0516 20130101; H01L 51/428 20130101; H01L 51/4226 20130101;
Y02E 10/549 20130101; H01L 51/004 20130101; H01L 51/005 20130101;
H01L 51/0036 20130101 |
Class at
Publication: |
257/040 |
International
Class: |
H01L 029/08 |
Claims
What is claimed is:
1. A photosensing organic field effect transistor (POFET),
comprising: a substrate insulating layer, the insulating layer
having a high relative dielectric constant and a first side and a
second side; a gate electrode, the gate electrode being an
electrical conductor, the gate electrode being positioned adjacent
to the first side of the insulating layer; a semiconducting polymer
layer, the semiconducting polymer layer being responsive to
incident light, the semiconducting polymer layer having a first
side, a second side, a first end and a second end, the second side
of the semiconductor layer being adjacent the second side of the
insulating layer; a source electrode, the source electrode being an
electrical conductor, the source electrode being in electrical
contact with the first end of the semiconductor layer; and a drain
electrode, the drain electrode being an electrical conductor, the
drain being in electrical contact with the second end of the
semiconducting polymer layer.
2. A POFET, comprising: a substrate insulating layer, the
insulating layer having a high relative dielectric constant and a
first side, a second side, a first end and a second end; a gate
electrode, the gate electrode being an electrical conductor, the
gate electrode being positioned adjacent to the first side of the
insulating layer; a source electrode, the source electrode being an
electrical conductor, the source electrode being in electrical
contact with the first end of the second side of the insulating
layer; a drain electrode, the drain electrode being an electrical
conductor, the drain electrode being in electrical contact with the
second end of the second side of the insulating layer; and a
semiconducting polymer layer, the semiconducting polymer layer
being responsive to incident light, the semiconducting polymer
layer being in electrical contact with the second side of the
insulating layer and the source electrode and the drain
electrode.
3. The POFET of claim 1, wherein the semiconducting polymer layer
further comprises a photoconducting polymer having a field effect
mobility of 10.sup.-2 cm.sup.2/V-sec or greater.
4. The POFET of claim 1, wherein the insulating layer has a
dielectric constant of 3.0 or greater.
5. The POFET of claim 1, wherein the insulating layer is further
comprised of a polymeric material.
6. The POFET of claim 5, wherein the polymeric media is polyvinyl
alcohol.
7. The POFET of claim 5, wherein the polymeric media is polymethyl
methacrylate.
8. The POFET of claim 1, wherein the insulating layer is further
comprised of an inorganic material.
9. The POFET of claim 1, wherein the insulating layer is at least
semi-transparent to optical radiation.
10. The POFET of claim 1, wherein the insulating layer is further
comprised of SiO.sub.2.
11. The POFET of claim 1, wherein the gate electrode is partially
transparent.
12. The POFET of claim 1, wherein the semiconducting polymer layer
further comprises a polymer matrix including, in dilute quantities,
one or more electron acceptors selected from the group consisting
of buckministerfullerene C.sub.60 and derivatives thereof,
viologen, dichloro-dicyano-benzoquinone, nanoparticles of titanium
dioxide, nanoparticles of cadmium sulphide and the like, thereby
enabling electron transfer from the polymer matrix upon
photoexcitation in order to obtain a high photo-induced current
between the drain and source electrodes.
13. The POFET of claim 1, wherein a drain current (and transistor
ON state) is independently controllable by a voltage applied to the
gate electrode and by the intensity of light incident upon the
POFET.
14. The POFET of claim 1, wherein the semiconducting polymer layer
further comprises a regioregular polyalkylthiophene with 98.5%
head-to-tail regiospecific conformation.
15. The POFET of claim 14, wherein the regioregular
polyalkylthiophene is Poly (3-octylthiophene).
16. The POFET of claim 14, wherein the regioregular
polyalkylthiophene is Poly (3-hexylthiophene).
17. A method of fabricating a POFET, comprising the steps of:
coating a glass substrate with a semi-transparent gate electrode;
depositing upon the gate electrode an electrically insulating layer
having a first side and a second side, the first side adjacent to
the gate electrode; forming on the second side of the insulating
layer a semiconducting polymer layer comprised of a regioregular
polyalkylthiophene responsive to incident light and having a 98.5%
head-to-tail regiospecific conformation; and forming on the
semiconducting polymer layer electrically conducting source and
drain electrodes.
18. The method of claim 17, wherein the insulating substrate is
comprised of a polymeric media.
19. The method of claim 17, wherein the insulating substrate is
partially transparent.
20. The method of claim 17, wherein the semiconducting polymer
layer further comprises a polymer matrix including, in dilute
quantities, one or more electron acceptors selected from the group
consisting of buckministerfullerene C.sub.60 and derivatives
thereof, viologen, dichloro-dicyano-benzoquinone, nanoparticles of
titanium dioxide, nanoparticles of cadmium sulphide and the like,
thereby enabling electron transfer from the polymer matrix upon
photoexcitation in order to obtain a high photo-induced current
between the drain and source electrodes.
21. The method of claim 17, wherein the regioregular
polyalkylthiophene is Poly (3-octylthiophene).
22. The method of claim 17, wherein the regioregular
polyalkylthiophene is Poly (3-hexylthiophene)
23. A method of using a POFET as a logical element, comprising the
step of: activating a transistor ON state by controlling gate bias
or the intensity of incident light.
24. A method of using a POFET as a logical element, comprising the
step of: activating a transistor ON state by controlling gate bias
and the intensity of incident light.
25. A method of using a POFET as a backbone of a position sensitive
detector, comprising the steps of: positioning one or more
photosensing organic FETs in a beam of light incident from an
object to be imaged; and monitoring the variation of drain
current(s) from the one or more photosensing organic FETs, wherein
the drain current(s) vary with the spatial position of the incident
light beam.
26. A method of controlling the electrical properties of a POFET,
comprising the step of: varying the intensity of light incident
upon the photosensing organic FET, thereby varying the carrier
concentration in the channel region and the drain-source current.
Description
[0001] This application is a continuation of, and claims the
benefit of priority of, U.S. patent application Ser. No. 10/033,743
filed Dec. 28, 2001, which was based on U.S. Provisional
Application No. 60/259,375, filed Jan. 2, 2001, the contents of
which are herein incorporated by reference.
FIELD OF THE INVENTION
[0002] The instant invention is related generally to the field of
transistors. More specifically, the present invention related to
transistors based on semiconducting polymers and the use and
manufacture of said transistors.
BACKGROUND OF THE INVENTION
[0003] Classical MIS structure field-effect transistors, commonly
called MISFETs or MOSFETs, are generally formed on a silicon
substrate strongly doped to make the transistors conduct. A metal
layer is deposited on one surface of the substrate so that the grid
voltage can be applied. An insulating silica layer is grown on the
other surface of the substrate. A semiconducting layer and the two
metal contacts constituting the source and drain are applied to
this silica layer. The source and drain may be in contact with the
insulating layer or disposed above the semiconducting layer. It has
been shown over the last decade that the insulator in the MISFET
structure can be replaced by insulating polymers with dielectric
constants exceeding silica, and that the semiconductor components
in MISFETs can be replaced by a conjugated semiconducting polymer
or organic molecules with aromatic structures of finite molecular
weight. This choice of transistor components has enabled
applications where requirements of flexibility over a large area
are required. A particular need for such organic-FET transistors is
in display applications, for example, as an option to amorphous
silicon based thin film vast transistors for driving the light
emitting diodes (LEDs). Another application is in development of
active-matrix drive circuits printed on plastic media. The
transistors in these circuits are made of plastic materials and are
fabricated with a low-cost printing process that uses
high-resolution rubber stamps. Their switching properties are
similar to typical thin film transistors manufactured from silicon
and through conventional fabrication methods, but they are
mechanically flexible, rugged and lightweight.
[0004] Advances in crystal growth techniques of organic molecules
with much lower degrees of defects have led to tremendous
improvements of material characteristics resulting in enhanced
transistor properties. The improvements in polymer-transistor
characteristics over the last decade were possible due to
improvement of polymers used, in terms of purity and orientation
methods. Regioregular polyalkylthiophenes P3ATs have been shown to
self-assemble into such films, with the molecules adopting a
preferred orientation with respect to the substrate. Consequently,
it has been shown that semiconducting polymers with mobilities as
high as 0.1 cm.sup.2/V-s can be realized.
[0005] The photosensitivity of these polymers significantly
increases upon dispersing electron acceptors such as derivatised
buckminsterfullerene C.sub.60, viologen, nanoparticles of
TiO.sub.2, CdS and other such materials with suitable energy levels
for accepting the photogenerated electron in the polymer matrix.
Device fabrication with composites of conjugated polymers and
C.sub.60 as the active layer with efficient photo-induced charge
transfer preventing the initial e-h recombination has resulted in
efficient organic photodiodes and photovoltaic cells. A
prerequisite for such an enhancement are materials with high
electron affinity with a distribution in the polymer matrix such
that the interparticle distances is on the order of the exciton
diffusion length. In addition, the charge separation process must
be fast enough to compete with the radiative and nonradiative decay
pathways of the excited species, which is in the range of 100 ps to
1 ns.
[0006] One of the applications of silicon technology is in the
field of image sensors. As is well known, an image sensor is a
semiconductor device for sensing a light reflected from an object
to generate image data. A photo-transistor, unlike a photo-diode,
is a high output impedance (light-controlled) current source. Also,
the output impedance of a photo-transistor can be rendered
independent of the size of its photosensing junction, while the
output impedance of a photo-diode cannot. Photo-transistors are
more effective sensors than photo-diodes in certain applications
because of these two properties. Such applications include those in
which the voltage output of the loaded sensor is limited by the
impedance of the sensor rather than the dynamic range of the
load.
[0007] An optically activated FET using organic semiconductors
without an explicit insulating layer but where the aluminum shottky
gate region forms the active region has been reported. High
sensitivity to light in n-channel silicon-on-insulator (SOI) metal
oxide FET which operates in the inversion mode has been
demonstrated. The photodetector comprises a short-channel SOI film
with a negatively biased gate electrode. Under illumination,
electrons generated in the semiconductor flow away from the channel
region and into the drain whereas the photogenerated holes remain
in the channel. Positive charges of the remaining holes bias the
source-channel junction forward, leading to large drain
current.
SUMMARY OF THE INVENTION
[0008] It is an object of the present invention to provide improved
electronic devices using electrically conductive, photosensitive
polymers.
[0009] In one aspect, the present invention is a photosensing
organic field effect transistor (POFET) comprising a substrate
insulating layer having a high relative dielectric constant and a
first side and a second side, an electrically conducting gate
electrode positioned adjacent to the first side of the insulating
layer, a semiconducting polymer layer responsive to incident light
and having first and second sides and first and second ends wherein
the second side is the side adjacent to the second side of the
insulating layer, an electrically conducting source electrode in
electrical contact with the first end of the semiconductor layer,
and an electrically conducting drain electrode in electrical
contact with the second end of the semiconducting polymer
layer.
[0010] The semiconducting polymer layer preferably has a field
effect mobility of 10.sup.-2 cm.sup.2/V-sec or greater, and further
comprises a polymer matrix including regioregular
polyalkylthiophenes with 98.5% head-to-tail regiospecific
conformation and, in dilute quantities, one or more electron
acceptors selected from the group consisting of
buckministerfullerene C.sub.60 and derivatives thereof, viologen,
dichloro-dicyano-benzoquinone, nanoparticles of titanium dioxide,
nanoparticles of cadmium sulphide and the like, thereby enabling
electron transfer from the polymer matrix upon photoexcitation in
order to obtain a high photo-induced current between the drain and
source electrodes.
[0011] The insulating layer preferably is at least partially
transparent to optical radiation and has a dielectric constant of
3.0 or greater. The insulating layer may be comprised of a
polymeric material, such as polyvinyl alcohol (PVA) or polymethyl
methacrylate (PMMA), or may be comprised of an inorganic material.
The gate electrode is also preferably partially transparent.
[0012] One operational characteristic of a transistor in accordance
with the present invention is that the transistor drain current and
ON state are independently controllable by a voltage applied to the
gate electrode and/or by the intensity of light incident upon the
transistor. Transistor saturation current gains of greater than 100
may be achieved by judicious selection of transistor materials,
dimensions, and appropriate photoexcitation and biasing.
[0013] In another aspect, the present invention is a method of
fabricating a photosensing organic field effect transistor,
comprising the steps of: coating a glass substrate with a
semi-transparent gate electrode; depositing upon the gate electrode
an electrically insulating layer having a first side and a second
side, the first side adjacent to the gate electrode; forming on the
second side of the insulating layer a semiconducting polymer layer
comprised of a regioregular polyalkylthiophene responsive to
incident light and having a 98.5% head-to-tail regiospecific
conformation; and forming on the semiconducting polymer layer
electrically conducting source and drain electrodes.
[0014] In yet another aspect, the present invention comprises
applications of the transistor described herein. One such
application is the use of the transistor as a logical element.
Another such application is the use of the transistor as a backbone
of a position sensitive detector. The implementation of these uses
is described below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIGS. 1A, B are schematic diagrams illustrating
top-contacting and bottom-contacting configurations of a photoFET
structure in accordance with the present invention.
[0016] FIG. 2A is a graph upon which is plotted drain current
I.sub.d versus drain-source voltage V.sub.d for different gate bias
V.sub.g with and without incident light of 1 mW/cm.sup.2.
[0017] FIG. 2B is a graph upon which is plotted (in log scale)
I.sub.d with and without incident light of 1 mW/cm.sup.2 at
V.sub.g=0.
[0018] FIG. 2C is a graph upon which is plotted (in log scale)
I.sub.d versus V.sub.d for a device with a thicker P30T layer than
in FIG. 2A and with a light intensity of 20 mW/cm.sup.2.
[0019] FIG. 3A is a graph upon which is plotted I.sub.d versus
V.sub.g at different light intensities with V.sub.d=9V.
[0020] FIG. 3B is a log-log plot of drain current in an initial OFF
state (V.sub.g=-0.5 V) and in an initial ON state (V.sub.g=-20 V)
versus normalized light intensity.
[0021] FIG. 4 is a graph upon which is plotted the normalized
spectral response of drain photocurrent I.sub.d along with the
absorbance of P30T shown as the dashed line.
[0022] FIG. 5 is a graph upon which is plotted decay curves of the
drain current I.sub.d at different V.sub.g, wherein time t=0
indicates the point at which the light is switched off.
DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS
[0023] Preferred embodiments of the invention will now be described
with reference to the accompanying drawings.
[0024] In one aspect, the present invention is directed to a
polymer FET 2 (as depicted in FIGS. 1A and 1B) whose transistor
action is considerably modified upon photoexcitation, with large
changes in the drain-source current I.sub.ds. The saturation value
of I.sub.ds can be independently controlled by the intensity of
incident light 6 and/or a voltage V.sub.g applied to a gate 8. The
large photo-induced drain current is a consequence of an internal
amplification process resulting from the photogenerated carriers
that is possible only in this transistor configuration.
[0025] In another aspect, the present invention is a method to
improve polymer-transistor characteristics, and in another, a novel
low-noise, sensitive photodetector device. The applicant
demonstrates that in an organic FET 2, the incident light 6
intensity can act as a an added control parameter in the FET's
operation, thereby adding another functional aspect to the variety
of applications of polymer transistors. The applicant herein
suggests methods to enhance the performance of the light responding
transistor. The polymer components in the FET are deposited at room
temperature using a simple solution deposition method employing
common organic solvents. Potential applications for FETs in
accordance with the present invention include their use as flexible
image sensors, photoactivated switches and photogenerated-charge
storage devices.
[0026] In one embodiment, a photosensing 3-terminal polymer based
FET 2 is comprised of an insulating polymer layer 10 with a high
relative dielectric constant, a semiconducting-photoconducting
polymer layer 12, and metal electrodes 14, 16, 18 in either
top-contacting or bottom-contacting transistor element
configurations, as shown in FIGS. 1A and 1B respectively. The
semiconducting-photoconducting layer 12 is comprised of a polymer
matrix including regioregular polyakyl thiophene polymers P3ATs
with regiospecific configurations optimal for semiconducting
polymers.
[0027] Electron acceptors such as derivitised C.sub.60, TiO.sub.2
nanoparticles, dye-coated TiO.sub.2 and
2,3-dichloro-5,6-dicyano-1,4 benzoquinone (DDQ) can also be
incorporated in the semiconducting polymer matrix at appropriate
concentration levels to obtain a high photo-induced response.
[0028] More specifically, the polymer matrix includes
semiconducting polymer regioregular Poly(3-octylthiophene), P30T,
and Poly(3-hexylthiophene) with 98.5% head-to-tail regiospecific
conformation. The polymer matrix can be coated on insulating,
transparent, high dielectric constant insulators such as
polyvinylalcohol (PVA) or polymethylmethacrylate (PMMA).
[0029] The photosensing 3-terminal polymer based FET 2 described
herein may be used as a logical element whereby the transistor can
be switched to an ON state either by the incident light 6 or by
applying a voltage to gate electrode 14. In addition, the FET 2 can
be used as the backbone of an image sensor with large patterning
possible due to an expected strong variation of the drain current
with the spatial position of the incident light beam. The charge
storage capability of the structure with further modifications
known to one skilled in the art of conventional semiconductors can
be exploited for memory related applications.
[0030] The photosensing 3-terminal polymer based FET 2 may also be
used to control other phenomena, such as superconductivity, which
arise from the gate-induced carriers in the channel region 20. The
ability of incident light 6 to modulate the carriers at the
interface 22 is maintained down to the superconducting transition
temperature.
[0031] The insulator medium for the insulating layer 10 can be
polymeric media such as PMMA or PVA, or conventional insulator
media such as SiO.sub.2. Transparent insulators offer the advantage
of having light 6 incident from the gate electrode 14 side of the
FET 2, thereby increasing the sensitivity of the device. The
solubility of the dielectric polymer is an important processing
factor. Soluble insulating dielectric polymers such as PVA and PMMA
can be cast on a glass substrate coated with a partially
transparent gate metal electrode 14 to form layers of controlled
thickness in the range 0.2-1.0 micron. The nature of the
semiconductor polymer-insulator polymer interface 22 plays a
decisive role in the FET characteristics. Regioregular
polyalkythiophene polymers P3ATs, with 98.5% head to tail
regiospecific conformation, which are reported to have mobilities
greater than 0.01 cm.sup.2/V-sec can be used in the
semiconducting-photoconducting polymer layer 12. The introduction
of electron acceptors such as C.sub.60, TiO.sub.2 nanoparticles,
dye-coated TiO.sub.2, and DDQ, even in extremely dilute quantities,
can result in tremendously high photo-induced response. P3ATs may
be dissolved in chloroform and spin coated on the insulator to
yield a 100 nm thick film. Uniform bilayers of the polymers with a
sharp interface 22 is possible with use of compatible solvents,
thermal treatment procedures and an electrostatic-surface treatment
of the insulator layer 10.
[0032] Gold electrodes, in the range of 0.1 mm-1 mm wide with an
inter-electrode spacing of 10 microns-70 microns, determine the
carrier channel length, with a typical channel width/channel length
ratio of .about.100. The gold electrodes of thickness t.about.100
nm (t<<channel length) form the source electrode 18 and drain
electrode 16 and are deposited on the P3AT polymer layer 12.
Referring to the structure depicted in FIG. 1A, it is to be noted
that the source electrode 18 and drain electrode 16 are on top of
the thin, active semiconductor layer 12 for these devices, instead
of having the semiconductor layer on top of the drain-source
electrodes. This arrangement results in a more homogenous P3AT
layer along with additional physical effects which assist the light
induced processes. This configuration permits the design of an
array of FETs with a desired patterning of gate electrode 14 or the
gate and source electrodes in the initial and the final stages
using conventional metal masks. The light 6 can access the
photosensitive semiconductor layer 12 from both the gate side as
well as the source and drain side directly into the semiconductor
layer. The majority of organic semiconductors exhibit p-type
behavior; i.e., the majority carriers are holes (h+). Their I-V
characteristics can be adequately described by models developed for
inorganic semiconductors. However, there are modifications which
arise from the differences in the transport mechanism in the
polymer-semiconductors compared to conventional semiconductors.
[0033] Upon illumination, the drain-source current I.sub.ds can
increase by orders of magnitude. FIGS. 2A and 2B depict the
characteristics of a polymer transistor I.sub.ds-V.sub.ds at
different V.sub.g. Light incident on the region between the drain
and source significantly increases I.sub.ds. As seen from the
results in FIG. 2B, the effect of light 6 on a polymer FET 2 can be
viewed simplistically as an equivalent to a large gate bias, with
an enhanced value of I.sub.ds. Other effects such as the decrease
of threshold V.sub.ds necessary to drive I.sub.d to saturation with
light is also observed. The gate bias has been viewed as a method
of doping the organic semiconductor, n=CV.sub.g/e where n is the
charge carrier density. For example, a gate bias in the range of
200 V, for a p-channel FET comprised of P3HT, can be viewed as the
equivalent of increasing the sheet density to 10.sup.13-10.sup.14
per cm.sup.2, depending on the geometry of the structure. In fact,
it has been shown that at temperatures below 2.35 K and sheet
carrier densities exceeding 2.5.times.10.sup.14 per cm.sup.2 a
polythiophene film becomes superconducting. (See "Gate Induced
Superconductivity in a Solution Processed Polymer Film", Schon, et
al., Nature, Vol. 410, 189-192, 2001.) The effect of incident light
can similarly result in an equivalent action, similar to the large
gate bias in terms of increasing the carrier density depending on
the degree of photogenerated electron-hole separation.
[0034] This photoeffect is substantially more pronounced than a
mere linear process involving photo-induced charge carrier
generation in presence of a lateral electric field. The substantial
increase in I.sub.ds upon illumination needs to be understood in
the context of the following observed features in P3AT
top-contacting FETs: (i) unipolar (hole) transport in the channel;
(ii) accumulation of charges beneath the drain and source contacts;
(iii) minority carrier (electron) vertical diffusion process; (iv)
strong non-linear dependence of the drain source current on the
light intensity (below the saturation threshold); and (v) increases
in I.sub.ds at moderately-high light-intensity regions, both in
enhancement and depletion modes, with a marginally higher value of
near-saturated light induced I.sub.d.sup.light in the enhancement
mode. I.sub.d.sup.light in this light-intensity range is dependent
on V.sub.g.
[0035] The additional factors present for I.sub.d.sup.light besides
the lateral field become more obvious upon comparison with a
planar, surface configured, 2-terminal photodetector device. The
scenario upon illumination of the device is the generation of e-h
pairs in the channel region of the P3OT layer as well as in the
bulk region. The typical transistor type I.sub.d.sup.light-V.sub.d
behavior with the presence of saturation region indicates that the
I.sub.d.sup.light is also channel restricted. The electrons diffuse
away from the channel causing a largely vertical electron-hole
separation. The potential has a minimum at the location where the
holes can flow between source and drain. The holes from the bulk,
which are created by electric field and thermally assisted
processes, drift towards the channel. Electrons remaining in the
bulk can bias the source channel forward leading to a large
I.sub.ds with a self-biased base structure under illumination. In
other words, the amplification in I.sub.ds due to light arises from
these independent pathways for the holes and the electrons.
[0036] In the present case, it is clear that there are different
mechanisms, i.e., at low light level where the V.sub.g is a
controlling factor for I.sub.ds and at higher light intensities
where I.sub.ds is weakly dependent on V.sub.g. The switching
response due to photoexcitation, indicated by the time constant for
rise and decay, are related to the charging and discharging process
of the bulk P3OT and gate, with the electron accumulation and
removal from the bulk. The I.sub.ds decay is clearly faster in the
depletion mode than in the accumulation mode, indicating a gate
dependent factor in I.sub.d.sup.light. The decay rates are also
influenced by parameters such as the initial light intensity,
temperature, insulator and P3OT thickness (not shown). The ability
to control the decay rates should also lead to the use of such FETs
in memory related applications.
[0037] It is expected that the photo-responsivity at low-light
levels in the transistor devices, which is in the order of 1 A/W,
can easily be increased to 100 A/W by optimizing the transistor
geometry parameters, increasing the degree of order of the active
polymer, improving the photosensitivity by introducing dilute
quantity of electron acceptors, without significantly altering the
interface homogenity, tailored to facilitate photoinduced electron
transfer.
EXAMPLE
[0038] A FET 2 with P3OT comprising the
semiconducting-photoconducting layer 12 and PVA comprising the
insulating layer 10 was fabricated. PVA, which is soluble in warm
water (50.degree. C.) and transparent, was cast on a glass
substrate coated with a partially transparent (10%) gate aluminum
electrode 14 to form a uniform layer. The insulating layer coated
on the substrate was thoroughly dried. Regioregular Poly
(3-octylthiophene), P3OT, and Poly (3-hexylthiophene) with a 98.5%
head to tail regiospecific conformation was obtained commercially
from Aldrich Inc., USA and used as received. P3OT was dissolved in
chloroform and spin coated on the insulator layer 10 to yield a 100
nm thick layer 12. Uniform bilayer of the polymers with a sharp
interface 22 was possible partly due to different solvents. Gold
electrodes, 3 mm wide with an inter-electrode spacing of 70
microns, forming the channel length, was deposited on the P3OT
layer 12 to form the source electrode 18 and drain electrode 16.
The leakage current in the drain-gate and gate-source circuits was
insignificant and was monitored to ensure that they did not affect
the photo-FET characteristics. The consistency of the results was
established by fabricating and studying many such devices.
[0039] FIG. 2A shows the FETs characteristics, which essentially
represent a typical p-channel enhancement type metal-insulator
(PVA)-semiconductor (P3OT) FET, with increasing I.sub.ds as V.sub.g
is biased more negative. The field effect mobility of P3OT was
estimated to be in the range of 10.sup.-2-10.sup.-3 cm.sup.2V-sec
on the basis of I.sub.ds-V.sub.ds measurements carried on several
such devices. The value obtained for regioregular
polyhexylthiophene, which has shorter side chain length, is
marginally higher.
[0040] FIG. 2B shows the I.sub.ds response to light 6 incident from
the gate side (1 mW/cm.sup.2, 532 nm) without any gate bias. As can
be seen, the effect of light can be viewed simplistically as an
equivalent to a large gate bias, with an enhanced value of
I.sub.ds. It was observed that the threshold V.sub.ds necessary to
drive I.sub.d to saturation also decreases with light. The current
gain, I.sub.d.sup.light/I.sub.d, due to light (photon flux rate 1
W) in this case is 100. It was also observed that the gain
(I.sub.d.sup.Iight/I.sub.d) can be further increased to as high as
1000 for certain devices, with higher flux rates and a thicker P3OT
(150 nm) layer as shown in FIG. 2C. The dependence of
I.sub.d.sup.light on the gate bias V.sub.g is significant only at
low light intensity, as shown in FIG. 3A, with a weak linear
dependence on V.sub.g at higher incident intensity. The large
I.sub.d.sup.light was observed both for the enhanced and depleted
modes of the FET. The effectiveness of light as a gate element is
more obviously illustrated in FIG. 3B. The dependence of I.sub.ds,
which is initially in the OFF state (Vg=-0.5 V), on the light
intensity is shown in FIG. 3B. The I.sub.d-Intensity response is
non-linear in this region and highlights the sensitivity of the
device at low levels of light flux. To the best of the applicant's
knowledge, the results reported here include the highest
responsivity (1 A/W) at low light levels for polymer/organic based
photodetectors, which until now have all been based on
non-amplifying 2-terminal configurations.
[0041] The control aspect of the light, similar to the gate bias in
a standard FET, gives rise to interesting features as highlighted
in these results. The ON state in this particular device, for
instance, can be attained by different mechanisms: (i) normal FET
mode without light and with only gate bias; (ii) FET mode at low
light intensity with smaller gate bias values; and (iii) FET mode
at high illumination without any gate bias, along with attributes
of a multi-level logic system. A single FET with these two
independent control options can function as an OR gate.
[0042] The present invention differs from a two terminal surface
cell polymer-photodetector. This was verified by fabricating the
2-terminal device on a quartz substrate with the same
inter-electrode distance as that of the drain-source length
(implying similar lateral electric field for charge separation) in
the 3-terminal device. I.sub.light-V characteristics in the two
terminal structure are practically linear at voltage bias (0-30 V)
and similar to the dark I-V response with a maximum ratio of
I.sub.light/I.sub.dark.apprxeq.1.4.
[0043] The process of illuminating the device results in the
generation of e-h pairs in the channel region of P30T layer as well
as in the bulk. FIG. 4 illustrates the spectral response of
I.sub.ds and its similarity to the absorption spectra of the
semiconducting polymer. This is evidence of P30T being the source
of photogenerated carriers. The typical transistor type
I.sub.d.sup.light-V.sub.d behavior with a presence of saturation
region indicates that the I.sub.d.sup.light is also channel
restricted. The electrons diffuse away from the channel causing a
largely vertical electron-hole separation. In the present case, it
is clear that there are different processes contributing to charge
transport, i.e., at low light level where the V.sub.g is a
controlling factor for I.sub.ds and at higher light intensities
where I.sub.ds is weakly dependent on V.sub.g. The switching
response due to photoexcitation, indicated by the time constant for
rise and decay, are related to the charging and discharging process
of the bulk P30T and gate, with the electron accumulation and
removal from the bulk. FIG. 5 shows the current decay when the
light is switched off at different V.sub.g. The I.sub.ds decay is
clearly faster in the depletion mode than in the accumulation mode,
indicating the gate dependent factor in I.sub.d.sup.light. The
decay rates are also influenced by parameters such as the initial
light intensity, temperature, insulator and P30T thickness (not
shown). Experiments with light sources with beam size less than the
channel length indicates the sensitivity of the current to the
light-position. The strong variation of the drain current with the
position of the incident light beam in the region between the drain
and source electrode can be exploited in position sensitive
photodetector applications.
[0044] The spectral response of I.sub.d shown in FIG. 4 essentially
covers the entire visible spectral range. The response also reveals
the sizable I.sub.d at low-absorption (wavelength>600 nm),
indicating the field assisted electron-hole separation processes.
It is expected that the photo-responsivity at low-light levels in
the transistor devices which is in the order of 1 A/W can be
increased to 100 A/W by optimizing the geometry and improving the
photosensitivity of the active semiconducting media.
[0045] Other embodiments of the invention will be apparent to those
skilled in the art from a consideration of the specification or
practice of the invention disclosed herein. It is intended that the
specification and examples be considered as exemplary only, with
the true scope and spirit of the invention being indicated by the
following claims.
* * * * *