# Optimum Design of Organic Electrochemical Type Transistors for Applications in Biochemical Sensing

Authors: Shruti Badhwar; K. S. Narayan
Journal: Journal of Sensors
 (2008)

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## Abstract

This paper addresses the issue of optimizing various performance parameters involved in the design of organic electrochemical type transistors based on the conducting polymer, poly (3,4-ethylenedioxythiophene): poly(styrene sulfonate)(PEDOT:PSS) for applications in biochemical sensing. We report the effect of device contact geometry, gate to channel length ratio “Lg/L_, and analyte distance from the source electrode “x_, on the device sensitivity and response time.


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## Body

## 1. Introduction

The
discovery  of conducting  polymers by Shirakawa et al. in 1970s and the
subsequent Nobel Prize in Chemistry in 2000 have spurned a huge interest in the
field of conducting polymer devices [1].
Electrochemical transistors based on conducting polymers such as poly (3,4-ethylenedioxythiophene):
poly(styrene sulfonate) (PEDOT:PSS ) switch  between different redox states
when a gate voltage is applied through an electrolyte and have shown great
promise in the sensing of biochemicals such as glucose [
2, 3] and DNA [4].
Further, the sensor when operated as a transistor has shown to be more
sensitive than a chemiresistor by an order of magnitude [3]. It has also been shown that the gate electrode in
an electrochemical transistor can be integrated into the ceiling of a
microfluidic channel to realize the “lab-on-chip” concept [5]. However, a major performance
constraint of these electrochemical sensors is the response time which ranges
from ~ 60 seconds to 30 minutes [3, 6]. This feature is attributed to the large ionic diffusion
time scales which are involved in the electrochemical dedoping process. The
conducting polymer gets modified from its conducting (oxidized) state to its
less conducting (reduced) state, according to the following general electrochemical
scheme.Polymer (Oxidized
State)+M++e−↔ Polymer
(Reduced State), where M+ represents cation and e− represents
electron [7].
The decrease in conductivity has also been interpreted in terms of an ion-leveraged
mechanism, where the disorder driven metal-insulator-transition is
characterized by charge localization which is controlled by the ionic environment
[8, 9]. This model interprets the slow response to the percolation of the ions
from the electrolyte into the organic film, under the influence of the vertical
electric field, affecting the conductance of PEDOT: PSS [
8, 9].In
this work, we show the effect of additional (parasitic) capacitance, which is
prevalent in certain transistor geometries. This factor appears to dominate
over the ionic diffusion process and is detrimental to the sensor performance. The
performance of these devices is explained in terms of the following parameters:
(i) the device sensitivity(S) and (ii)
response time (τ). A phenomenological model built on
the basis of the observation indicates an origin to the microscopic structure
of the spin-coated PEDOT: PSS blend films. These results can identify and
optimize appropriate electrochemical transistor geometry for a given
application.It
has been demonstrated in 1980s by Wrigthon et al. that lower source-drain
separation or channel lengthL (50–100 nm) leads to
higher sensitivity and faster response time in gas sensors using solid-state
microelectrochemical transistors with conducting polymer
Poly(3-methylthiophene) as the active material [10, 11].
In another device configuration, it has been shown by Berggren et al. that
electrochemical transistors designed with (channel width W/channel length L)~1
and gate area equal to 10 times WL gives highest on-off ratio and switching
speeds [12].
Recent work on modeling of the transistor steady state and transient behavior by Bernards and Malliaras [13]
suggests that the gate  thickness Hg,
channel length L, and the drain voltage Vds can modify the transient
response by the changing the electronic transit time, defined as the τe=L2/μVds and the
ionic transit time, defined as τi=CdRs~Hg/N1/2, (Cd and Rs are the charging
capacitance and resistance, N is the
ionic concentration) which appear in the following relation: (1)Ids(t)=Ioff+ΔIds(1−f*τeτi)exp(−tτi), where Ids is the drain-source
current, Ioff is the
steady-state drain-source current at a gate voltage Vgs,ΔIds=(Ids,Vgs=0−Ioff) and f are
the proportionality constant to account for nonuniformity of the dedoping
process. The above model assumes constant carrier mobility and ignores its
dependence on electric field and carrier concentration in the polymer. Further,
the model attributes the charging capacitance Cd to the polarization at PEDOT: PSS-electrolyte
interface and the electrolyte-gate electrode interface, but neglects the effect
of additional capacitance factors and anisotropic conductivity of PEDOT: PSS in
the bulk, interface, and the surface. This simplified first-order model works
well with bottom-contact, BC, device geometry (Figure 1(a), inset) in
predicting the extent of gating and response times with varying gate properties
(size, materials), but may not explain the transistor transfer and transient
characteristics with the top-contact geometry, TC (Figure 2). The BC geometry
has the drain-source electrodes staggered with respect to the
electrolyte/conducting polymer interface, while the TC geometry has the
coplanar structure of drain-source/conducting polymer interface and the
electrolyte/conducting polymer interface. It may also be mentioned that the
organic electrochemical transistor used commonly for biological sensing [2, 5] employs TC geometry (Figure 1(b), inset).(a) Transient characteristics of electrochemical
transistor of0.1<Lg/L<1.0, with BC geometry, when a Vgs pulse of 5 V, duration 50 seconds, is applied.
Inset shows the BC device geometry, with channel length L=12mm, electrolyte thickness Hg=1μm, PEDOT: PSS thickness HPEDOT:PSS=200nm,
ITO thickness HITO=150nm, and Lelec=Lg. (b) Transient
characteristics of electrochemical transistor of 0.1<Lg/L<1.0, with TC
geometry, when a Vgs pulse of 5 V duration
50 seconds, is applied. Inset shows the TC device geometry, with channel
length L=12mm, electrolyte
thickness Hg=1μm, PEDOT: PSS thickness HPEDOT:PSS=150nm, Au
thickness HAu=70nm, and electrolyte length Lelec=Lg.
(a)(b)Figure 2
Transconductance characteristics of electrochemical
transistor of0.1<Lg/L<1.0, with TC geometry. Inset shows the steady state, Ids versus Vds characteristic of an electrochemical transistor at varying Vgs. The transistor is
normally “on” at Vgs=0V.In
this work, we study the effect of varying (gate lengthLg/channel length L),
where L~10mm, and
varying analyte position (distance from source x/channel length L) on these
two different geometries; top contact, TC, and bottom contact, BC. We show that
as Lg/L is increased; the
parasitic capacitance starts to compete with the charging capacitance, observed
most prominently in the TC structure. A larger
variation of the current density J(z) is expected in TC compared to BC, in the off-state (reduced) of the polymer.
The thickness of the polymer and anisotropy in σ (bulk
& surface) are factors which decide the magnitude of the off-state
conductance. We take these factors into account for interpreting our
observations.


## 2. Experimental Methods

The BC and TC
device structures are shown in Figures1 and 2, inset. A set of 6–8 electrochemical
transistors with L~12mm and width W~3mm was fabricated. Commercially
available Baytron (P) PEDOT: PSS was spin-coated on precleaned glass substrates
following standard procedures. PVA (~ molecular weight ~130000) and KCl (250 mM)
in the  ratio (1:9), baked at 90° for 4 hours, were used as the
solid-electrolyte. Further fabrication details for these solid-state devices
are mentioned in [8, 14]. In all our devices, the length of
the electrolyte, Lelec, is equal to the length of gate
electrode, Lg. To study
the sensor performance on the position of the liquid analyte, (Figure 5,
inset), a~70nl drop of 250 mM KCl solution was positioned on the PEDOT: PSS channel
at varying x using Auto Nanojet
injector and micromanipulator from Warner instruments. The gate voltage was
applied through the electrolyte using a gold-coated probe tip. All electrical
measurements were carried out using Kiethley Sourcemeter, Model 2400 and
Electrometer, Model 6514, and controlled
by Lab view interface software.


## 3. Results and Discussion

Transistor  characteristics were obtained on a set of devices with similar initial drain-source
conductance values and differentLg/L ratios. The Ids−Vds response, transfer, or curve
(Ids versus Vgs)
and transient behavior Ids(t) were determined for each of the device. With Vds swept from 0 to  positive 1 V and Vgs varied in
discrete steps from 0 to +5V, the PEDOT: PSS channel was observed to switch
from its normally on to the off state (see Figure 2, inset). Igs magnitude was negligible
compared to Ids. The transfer
or transconductance characteristics were obtained by keeping Vds constant at positive 1 V
and by sweeping the gate voltage cyclically from 0 to +5V and then to −5V.
The hysteresis in the transfer characteristics can be attributed to asymmetric
charging and discharging rates of the electrolyte capacitance. The hysteresis
and the slope of transfer curve (i.e., the transconductance Gm) were observed to be a
function of the Lg/L, with Gm increasing
monotonically with Lg/L(0.1to1.0) for BC geometry with a maxima at 0.2<Lg/L<0.5 for the TC geometry. Figure 2 shows the
transconductance characteristics when Vgs was swept from 0 to +5V for a device with TC geometry.The
transient characteristics (Figure1) were obtained by applying a gate voltage
pulse Vgs=5V, for a duration
of 100 seconds to the gate electrode, at a constant drain voltage, Vds=1V. The change in the Ids from on to the off state
is depicted as ΔIds. The current drop is
normalized with the initial Ids(Vgs=0V) and
defined as β=ΔIds/Ids. A scaling parameter for the sensitivity S defined as the ratio of β to Lg/L, that is, S=β/(Lg/L) is useful in
characterizing the device response to the electrolyte and device dimensions. The
time constant τ (see Figure 4) can be extracted by fitting (1)
to the experimentally observed transient characteristics.Figure4 represents τ and S (indicated by the color scale) dependence on the Lg/L ratios. In the BC device geometry, τ decreases by more than an order of magnitude (60
seconds to 0.5 second), with increase in Lg/L from 0.1 to 1.0. The decrease in τ can be better expressed in terms of the slope Δτ/Δ(Lg/L)~66. The sensitivity, depicted
in blue in Figure 4, is almost constant at S~1 for the entire range of Lg/L. The
operating window of the BC device for optimum performance (high S and low τ) is then observed to be 0.9<Lg/L<1.0. For the entire
range of the Lg/L, the S of TC>S of BC. In case of TC device geometry, high sensitivity (S~2.5) is achieved in the range 0.2<Lg/L<0.5, with a
corresponding τ value of 6 seconds. It is to be noted that
this suggested operating window for BC and TC geometries was obtained
statistically by repeating the measurements over a set of 8 batches (1 batch = 6 devices with 0.1<Lg/L<1.0).The response function
ofτ(Lg/L) and the higher magnitude
of τ in
the TC geometry as compared to BC geometry seem to suggest a competing factor
from the additional parasitic capacitance in determining the device
performance. The parasitic capacitance in TC geometry (Figure 2) essentially
comprises of two components: (i) outer fringing capacitance, Cfgs and Cfgd (between outer sides of the gate to source/drain
electrode across the air-dielectric) and (ii) outer fringing capacitance Cfgsp and Cfgdp (between outer sides of
the gate electrode to the PEDOT: PSS film on either side of the electrolyte,
across the air-dielectric). To account for the parasitic capacitance in the
electrochemical transistors, (1) can be modified to Ids(t)=Ioff+ΔIdsexp(−t/RCeff),
for f=0, where τ=RCeff is the effective response time. The response time τ for TC configuration is a nonlinear function
and can be represented by an expression consisting of contributions from the
intrinsic factors, f(Lg/L),
and the extrinsic parasitic factors, g(Lg/L).These
results can also be understood from microscopic aspects of the conducting
polymer film organization. It has been reported [14] that the bulk conductivity in spin-coated PEDOT: PSS
thin films can be three orders in magnitude less than the lateral conductivity
in the plane of the film. This is because in the perpendicular direction, the
PEDOT-rich domains are separated by thick barriers formed by PSS lamella,
whereas in the parallel direction, within the PEDOT-rich lamella, conduction
can take place with a thin or no barrier at all by 3D variable-range hopping. The
current (when the polymer is in the reduced nonconducting form at the interface)
through the lower bulk conductivity is lowered more effectively. In the off state
of the TC device, the leakage pathways are clearly reduced compared to the BC
device and can explain the higher S values. The effect of parasitic capacitance
and the anisotropic PEDOT: PSS conductivity on the performance parameters of
the electrochemical transistor can be better understood in terms of an
equivalent (lumped) circuit model, is shown in Figure 3. The lateral
resistance, Rbulk< the vertical resistance, Rver.
The interface resistance Rint is represented as a variable resistor. The resistance of the PEDOT: PSS film at
the surface, on either side of the electrolyte, is denoted as R1. The fringing
capacitances, defined previously, are accounted in the circuit diagram.Figure 3
Equivalent circuit representation of an
electrochemical transistor, including the effect of additional (parasitic)
capacitance and the nonuniform conductivity of PEDOT: PSS.Figure 4
Response time,τ, shown on the y axis and sensitivity, S, shown as the color bar versus Lg/L of the electrochemical
transistor with BC and TC device geometry.Figure 5
Sensitivity,S versus x/L for an electrochemical
transistor, TC Geometry, plotted for different positions of the analyte (a)
at Vgs=0V (b) at Vgs=0.2V, duration
100 seconds. The inset shows the drop-potentiometry setup.The charging
capacitanceCd and resistance Rs for
electrochemical transistors in BC and TC geometry, 0.1<Lg/L<1.0, were measured using the 4200-CVU
(multifrequency impedance card) inserted in Kiethley parameter analyzer, in a
series RC configuration. The transistors were driven at 10 KHz, 30 mV rms Vgs signal riding over a DC
bias of −5.0V<Vgs<5.0V, with drain-source electrodes shorted
(figure not included). Unlike the conventional silicon-based MOSFET, where CV
measurements are made to extract useful device parameters such as oxide capacitance,
flat band capacitance, and oxide thickness, the inference from CV measurements
of electrochemical transistors is more involved and must be drawn with care.
This is because in an electrochemical transistor, an array of charged species
and oriented dipoles exists at the metal-electrolyte interface and forms an
electrical double layer [15]. At a given potential Vgs, the
metal-electrolyte interface is characterized by a double layer capacitance Cd. However, unlike
real capacitors, whose capacitances are independent of the voltage across them, Cd is often a function of
potential, and hence the device geometry which is an important criteria in the
electric field distribution. Furthermore, the exact distribution profile of the
ions in the bulk, their transport, and influence on the potential must be
known, to obtain a suitable value of Cd.From our
measurements, the general trend thatCd (when Vgs~−3.0V, PEDOT:
PSS is in oxidized state, and the transistor is on) >Cd (when Vgs~+5.0V, PEDOT: PSS is in reduced state, and the transistor is off) is
observed. The  magnitudes of Ci=(∫CddVgs)/(∫dVgs), where Ci is the integral
capacitance averaged over changes in potential, are consistent with that
evaluated from geometric parameters and reasonably assumed value of dielectric
constant (C=Aε/t~0.01to0.15nF). The scaling
trend in CV measurement is also qualitatively in agreement with the S and τ measurements. The specifics involving
measurement of individual components of capacitance and resistance, their
relationship to S(Lg/L) and τ(Lg/L) are
quite complex owing to factors like geometrical imprecision, film morphology,
humidity, irreversibility and so forth and require separate effort. An accurate
electrostatic modeling which is underway in our laboratory can possibly arrive
at a form of S(Lg/L) and τ(Lg/L) , where the minima
correspond to the experimentally observed optimum value of(Lg/L).These
observations, results, and interpretation are also consistent with
drop-potentiometry type measurements, where the electrolyte is in form of a
liquid drop with a thin wire electrode immersed in it. The transient response
of the electrochemical transistor to a liquid electrolyte gated, with a pulse
ofVgs=0.2V, duration
100 seconds, and constant Vds=1V, was studied. The drop was precisely positioned at distance x from the source electrode, for 6
devices with 0.1<x/L<1.0. The
rate of droplet evaporation, being a constant for each of the “x” positions, did not affect the relative
measurements. Vgs of sufficiently low magnitude was used to prevent
hydrolysis. The sensitivity, defined as S=β/(area of drop/channel area), is
observed to follow similar qualitative trend (Figures 4 and 5), with S attaining a maxima at x/L~0.4 for an electrochemical
transistor with a TC geometry. These results also indicate the requirement of
appropriate location for dispensing the drop to yield high performance.


## 4. Summary

We
report observations which indicate the requirement for optimizing the geometrical
factors in designing polymer-based electrochemical transistors for sensor
applications. These results were analyzed in terms of the switching response
and a sensitivity parameter represented by a scaled-dimensional factor. The
results were qualitatively interpreted in terms of additional parasitic capacitance
factors and the anisotropy in the conductivity of the conductivity polymer
medium.



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