Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
1998-12-01
2002-09-24
Williams, Alexander O. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C257S088000, C257S099000, C257S103000, C257S461000, C257S431000, C257S448000
Reexamination Certificate
active
06455873
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to microelectronic devices and sensors that have a semiconductor/conducting polymer interface.
The operation of traditional semiconductor devices such as diodes and transistors relies on interfacial properties. Such devices are generally based on interfaces or multilayer structures consisting of conducting (e.g. metals such as Au or Ni), semiconducting (e.g. Si, GaAs) and insulating materials (e.g. SiO
2
) (S. M. Sze,
Physics of Semiconductor Devices
, Wiley, New York (1981)). Conducting polymers have been introduced into a number of these devices by simple substitution (see e.g. J. H. Burroughs, C. A. Jones, and R. H. Friend,
Nature
335, 137 (1988); J. H. Burrough et al.,
Nature
347, 539 (1990)). The conducting polymer serves as a replacement for a metal or a semiconductor in traditional device architectures. Although the introduction of a conducting polymer may bring certain advantages in processing and/or chemical diversity, the operational principle of the vast majority of these conducting polymer devices is identical to their more traditional analogues.
There is one notable exception to the simple substitutional approach that has dominated the design of conducting polymer devices. A broad class of devices has been developed whose operation relies on the ability to modulate the conductivity of a conducting polymer through manipulation of its electrochemical potential. This property has served as the basis for electrochemical transistors (see e.g. J. W. Thackeray, H. S. White, M. S. Wrighton
J. Phys. Chem
. 89, 5133 (1985); E. P. Lofton, J. W. Thackeray, and M. S. Wrighton,
J. Phys. Chem
. 90, 6080 (1986)) and a wide range of conductometric sensors (see e.g. Pearce et al.,
Analyst
118, 371-377 (1993), Shurmer et al. Sens. Act. B 4, 29-33 (1991), Y. Miwa et al.,
Bull. Chem. Soc. Jpn
. 67, 2864-6 (1994); A. Talaie,
Polymer
38, 1145-1150 (1997); P. N. Bartlett et al.
Anal. Chem
. 70, 3685-3694 (1998)). Unlike traditional semiconductor devices that rely on the electrical characteristics of interfaces, these devices rely on changes in the bulk electrical characteristics of conjugated polymers. In the case of electrochemical transistors, a “gate” potential is used to control the electrochemical potential of a conducting polymer that is one electrode of an electrochemical cell. A minimum of two electrodes, termed source and drain, contacted to the polymer serve to sense the change in conductivity observed in response to the gate potential. Since the gate potential serves to modulate the current flowing (induced by a constant source-drain potential) across the source and drain electrodes, amplification and logic functions become possible. In the case of sensors, the gate is the environment. If an analyte in the gating environment induces a change in the electrochemical potential of the conducting polymer, its presence will be sensed through a change in conductivity.
This disclosure generally concerns itself with hybrid conducting polymer devices that rely on the electrical properties of semiconductor/conducting polymer interfaces and their response to changes in the bulk electrical characteristics of the conducting polymer. The electrical characteristics of the conducting polymer are either actively controlled using external electronics or controlled by analytes in an environment to which it is contacted, either directly or through a mediating layer. In the case of active electrochemical control, a variable barrier or tunable diode results. In the case of control by analytes in an environment, a general electrochemical transducer for sensing applications results. A method of generating semiconductor diodes with specific electrical characteristics is also disclosed.
For the purpose of this disclosure, a semiconductor diode is defined as an interface between a material that conducts electricity and that can support an electric field through the formation of a depletion region (typically but not limited to an inorganic semiconductor such as Si, GaAs or InP) and another electrical conductor (such as but not limited to a metal or conducting polymer). In general, the current-voltage characteristics of such a semiconductor diode are described by the following equation relating current, I
PS
, to applied potential, V
PS
:
I
ps
=
I
o
[
1
-
exp
(
-
qV
ps
nkT
)
]
(
1
)
where I
0
is the equilibrium exchange current or reverse saturation current, n is the diode quality factor, k is the Boltzmann constant, q is the elementary charge, and T is the temperature. The ps subscripts indicate reference to a conducting polymer/semiconductor interface. Both I
o
and n depend on the details of current flow at the interface with the theoretical minimum of n=1 generally considered ideal. I
o
is given by a number of parameters as described by the following equation, with some loss of generality, for an n-type inorganic semiconductor where majority carrier transfer dominates current flow:
I
o
=
aqk
n
N
C
exp
[
-
q
φ
b
kT
]
(
2
)
where a is the active device area, k
n
is the surface recombination velocity, and N
c
is the effective density of states at the conduction band edge of the n-type inorganic semiconductor, and &phgr;
b
is the Schottky barrier height. It is noted that the barrier height is at times considered as an effective empirical parameter, but its strict definition relates to the magnitude of the interfacial potential barrier. Herein, we used the barrier height in the empirical sense although at times this is equivalent to the stricter definition.
The Schottky barrier height, &phgr;
b
, is a central parameter determining the precise electrical characteristics of a diode where majority carrier transfer dominates current flow. Through choice of materials, it is possible to exert control over the barrier height of a diode. However, for many semiconductors, in particular so-called small band gap semiconductors, only a very small level of control is possible (E. H. Rhoderick and R. H. Williams, Metal-Semiconductor Contacts, P. Hammond and G. L. Grimsdale, Eds. (Monographs in Electrical and Electronic Engineering, Oxford Univ. Press, Oxford, ed. 2, (1988), vol. 19). For instance, a series of semiconductor diodes fabricated from clean n-type indium phosphide (n-InP) and the following metals—Ag, Cr, Cu, Au, Pd, Mn, Sn, Al, and N—allows the effective barrier height to be controlled over a range of only 0.2 eV (N. Newman et al.,
Appl. Phys. Lett
. 46, 1176 (1985)). The tunable diode disclosed herein allows for extensive control over the effective barrier height of semiconductor interfaces. Furthermore, this control is continuous and, if desired, available in a single tunable device rather than in a series of separate devices. For comparison, an embodiment of the tunable diode based on n-InP allows for the effective barrier to be controlled by more than twice that possible with the series of metals described above and again in a single device if so desired.
Certain disclosed devices can serve as general electrochemical transducers. Such transducers can be interfaced to nearly any sensing scheme that relies on potentiometric detection. Classic potentiometric detection schemes measure the electrochemical potential of a material or the junction potential of an interface by comparing the potential signal at one electrode with a second reference electrode such as a saturated calomel electrode (SCE) (A. J. Bard and L. R. Faulkner,
Electrochemical Methods
(Wiley, New York, 1980)) Several alternatives to this classic mode of measuring electrochemical potential based on conducting polymers have been developed and have served as the basis for the development of a wide range of sensors for analytes such as, protons, glucose, and organic vapors (see e.g. Pearce et al.,
Analyst
118, 371-377 (1993), Shurmer et al. Sens. Act. B 4, 29-33 (1991), Y. Miwa et al.,
Bull. Chem. Soc. Jpn
. 67, 2864-6 (1994); A. Talaie,
Polymer
38, 1145-1150 (1997); P. N. Bartlett et al.
Anal. Che
Klarquist & Sparkman, LLP
State of Oregon Acting by and through the State Board of Higher
Williams Alexander O.
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