Electrochemistry of chemically sensitive field effect transistors

Sensors

and Actuators,

4 (1983)

ELECTROCHEMISTRY TRANSISTORS*

255 - 265

255

OF CHEMICALLY

SENSITIVE FIELD EFFECT

J JANATA Department

of Bloengrneenng,

Unwerszty

of Utah, Salt Lake City,

Utah 84112

(US A )

Introduction A field-effect transistor 1s pmmanly a charge-measurmg device and a preamplifier with low and well-defined mput capacitance, high input lmpedante and which can be made very small. For this reason, new types of physical, electrochemrcal and blomedlcal measurements are possible with this device In this paper a few electrochemical aspects of ISFETs are pointed out The ion-selective electrode (ISE) 1s the closest relative of the ISFET It 1s customary to discuss their performance m lsolatlon With ISFETs it 1s necessary to consider the whole measurmg system, mcludmg the sohdstate part of the devtce, the reference electrode and even the connections between these components From this point of vzew we need to discuss two groups of devices one m which the ion-sensitive membrane IS part of a symmetrical arrangement (I e , solutlonlmembrane/solutlon) and devices wth the arrangement, solutlon/membrane/sohd contact Conventional ISEs mth internal filling solution/reference electrode fall into the first category, while coated wire electrodes, hybrid devices and ISFETs fall mto the second. The potential profiles m these two structures are shown m Fig 1 The equivalent electrical clrcult of the membrane shown m Fig l(a) (symmetrical case) 1s m Fig 2 If this membrane 1s ion selective, it 1s always possible to design the composltlon of the solutions m such a way that the mterfaclal resistances R1 and R2 are small The value of the bulk resistance Rb 1s usually dictated by the composltlon and geometry of the membrane Although it 1s of secondary importance, it is advantageous to keep Rb as small as possible In terms of electrode kinetics, the mterfaclal charge transfer resistance (RCT = RI, R,) 1s related to the exchange current (zO) at the interface by the relatlonshlp RCT = RT/nFz, m which R, T, n and F have their usual meanmgs The partial ionic currents due to all charged species that can cross the interface contribute to z. If one speczes alone carries most of the current reZatzue to the others, the *Based on an Invited Paper Netherlands, May 31 -June 3,1983 0250-6874/83/$3

00

presented

at Solid-State

Transducers

@ Elsevler Sequola/Prmted

83, Delft, The

m The Netherlands

256

REFERENCE ELECTRODE

ION SELECTIVE

ELECTRODE

REFERENCE ELECTRODE

ION SELECTIVE

ELECTRODE

(4

METAL

SoilrrloN

l_J

3ArlPLe

XEMERbNz

!ia_I0 (MfTAL~

METhAL

(b) Fig 1 Potential proflle through symmetrlcal ion-selectlve device

(a)

symmetrlcal

ion-selectwe

electrode

(b)

non-

membrane 1s said to be selective to that particular ion If, at the same time, the absolute magrutude of this current 1s high (> 10e5 A cm-2), the electrode 1s pragmatically labelled as ‘good’ because it 1s not affected by motion of the electrolyte or adsorption Conversely, if z. 1s equally high but there IS no dominating smgle lonlc species responsible, the interface 1s non-selective and for all purposes behaves as a liquid Junction On the other hand, if the exchange current density 1s low (< low9 A cme2), it follows from eqn (I) that the interface at which the analytical slgnal 1s generated 1s essentially a high (output) unpedance source As an ion-selective membrane, such a device would earn the label ‘bad’ because it would ‘drift’ m Its response to adsorptlon of lomc species and to motion of the electrolyte Clearly, the exchange current density 1s one of the most important parameters which determine the behavlour of ion-selective membranes This fact has been recognized [l] and a semlquantltatlve mterpretatlon of

257

'b II

FOR

A

DC, R=R,+

Fig 2 Equivalent arrangement

_

RESPONSE Rb+

electrical

R2

clrcu~t of the ion-selectlve

membrane

m the symmetrical

the response of ‘bad’ ion-selective electrodes and certam ISFETs has been given 12, 3) Iromcally, a close coupling of the ‘bad’ membrane to the preamplifier can seemingly ‘improve the sltuatlon The equivalent clrcult m Fig 3 describes the asymmetrical device (Fig l(b)) The reference electrode, liquid Junction and the sample solution of Fig l(b) are lumped together as EREF The section SOLID can be a conductor, such as m coated wire electrodes, hybrid sensors or ISFETs with a thm metallic coating, or it can be an insulator as m other ISFETs The VOLTMETER can be an electrometer (for coated wire electrodes), or an integrated circuit (for a hybnd) or the solid-state portion of the ISFET The two interfaces (Impedances) m question are 1 and 2 Let us first assume that the exchange current den&y at interface 1 1s high (a ‘good’ ISE) If lo at mterface 2 1s also high, the inner electrical potential of the solid contact 1s uniquely determined by the equality of the electrochemical potential (1 e , the Fermi level) of the exchanging charged species m the bulk of the membrane and m the bulk of the conductor As long as the leakage reslstance (shown m the circle) 1s higher than the lowest of R 1, R2 or R,, the potential profile throughout this structure will be stable Several coated wire electrodes and hybnds fit this description, an example (the solution F-/LaF,/AgF/Ag/Cu) being given m 143 Let us now consider the case when the charge-transfer resistance R, IS very high, so much so that this interface becomes capacltlve In that case the parasitic capacitance (shown m the circle m Fig 3) must be small and mvarlable and the parasltlc resistance must be mfmltely high m order to obtam a stable output from the voltmeter It 1s obvious that the shorter

258 ISE

MEMBRANE -

j

/VOLTMEl I

/f-h. E

CI RE:F

-’ I-

x2--

RI

Fig 3 Equivalent

I

electrical

1

cwcult of a non-symmetrical

Ion-selectwe

device

the sohd conductor, the better satlsfled are these condltlons In other words, If the mterfuce between the membrane and the sohd 1s capacltwe, the problems zncrease with the length of the lead An ISFET m which the membrane is placed directly at the input znsulator of the voltmeter is an extreme case m which the length of the conductor 1s zero The corollary of this statement 1s that there IS no difference m prmclple between all the sensors discussed above In practice, however, the differences can be substantlal Finally, we need to consider the case of a ‘bad’ membrane, that IS one with a low exchange-current density at interface 1 whrch 1s a leaking capacltance In a fixed geometry arrangement, such as m an ISFET or a hybrid with a short lead, a deceptively stable and reproducible signal can be obtamed [3 3 Such a device is, however, nearly non-selective and all the problems discussed above ~11 be added to the compllcatrons that are caused by interface 2 In short, a good sensor cannot be made out of a bad membrane Time response of ISFETs In the equivalent circuit m Fig, 3 the capacitors C,, Cb, C, and the input capacitor of the voltmeter are m series, whereas the parasitic capacltor (m the circle) IS m parallel If this last capacitor IS very small the system ~11 respond to a rapld change of applied potential as a capacitor divider, The slmphfIed formula for the time response of the le, mstantaneously gate-to-source voltage, VGS, of an ISFET IS [5, 6 ] exp[-

t/WC +

CGdl

(2)

GS

where V, IS the applied voltage step, Cos 1s the input capacitance and R and C are the lumped paralIel resistance and capacitance of the membrane,

259

respectively It follows from eqn (2) that d C > 0.1 CGs, as commonly happens m ISFETs, a finite voltage V&t = 0) appears across VGs at t = 0 and the time response to electrical step of the gate voltage becomes V&t

= 0) =

C

vo

C+CGS

as shown m Fig 4 In practice, however, the time response to a concentration step 1s always limited by the diffusion of the species to the surface of the electrode (Warburg impedance) and only the exponential response IS seen For polymenc membrane ISFETs, this time constant IS typically of the order of mllhseconds [7, S] and for mlcroISFETs it 1s hundreds of mllhseconds [ 91,

oc -0

___~_~_________--_-’

’ 8’

b

.

i

’

’

’

’ IO

’

’

’

’

’ 15

’ t (ms)

Fig 4 Computer slmulatlon of the time response of an ISFET (Reprinted with permlsslon from [ 5 ] )

with different geometry

Determmatlon of exchange current density It follows from the previous quahtatlve dlscusslon that the exchange current at each mterface of any ion-selective sensor 1s of paramount nnportance This parameter can be obtamed from complex plane analysis of the nnpedance of electrical equivalent clrcults of these devrces, as has been used by Buck [lo] and others for analysis of ISE response We have adopted a so-called eqmhbnum nozse analysis [5] which 1s perhaps more sensitive to the values of partial impedances shown m Fig 3

260

The instantaneous In(t)

value of the dram current In(t)

can be expressed

= Q) +- z(t)

as (4)

where & 1s the mean value of the dram current and z(t) is its random fluctuatlon (noise) There are several ways m which z(t) can be studied We orlgmally adopted [5] a so-called spectral density analysis [ll] and are currently mvestlgatmg the correlation analysis [ 121 The latter techmque enables us to dlstmgulsh between the device noise and the electrochemical noise The procedure for spectral analysis 1s as follows the time record of the equlhbnum ISFET noise IS converted mto the frequency domam using the Founer transformation, yleldmg a so-called power spectrum At this point we again use the equivalent clrcult approach, but now we separate resistors R , - R3 the noise sources V, - V, (Fig 5(a)) from the ‘noiseless The pa&al power spectra and the over-all theoretlcal spectrum corresponding to this sltuatlon are shown m Fig 5(b) The shape, magmtude and posltlon on the frequency axis are very sensltlve to the values of the C’s and Rs It 1s then necessary to match the calculated spectrum with the experimental one (Fig 6) If the Impedances m Pig 5(a) represent 1, the 2, the bulk, and 3, the membrane/msulator solution/membrane interface, mterface, the values of the correspondmg exchange-current densities can be calculated from eqn (1) We have followed this procedure for a K+ ISFET, an Na” ISFET and a pH ISFET In the first two devrces the NOISE

MODELING

(a)

RESULTlNG SPECTTRUM

------------

w

f

(b) Fig 5 Electrical equivalent cn-cult (a) (Reprmted with permlsslon from [ 8 ] )

and

calculated

noise

spectra

for

ISFET

(b)

261

10-231n

’,

k^

’_

’-

Fig 6 Noise spectra of 0 Na+ ISFET, 0 K+ ISFET, l pH ISFET The contmuous lmes correspond to the computer model (Reprinted with permission from [ 51 )

exchangecurrent densities at to be m the range of ‘good’ not be made because of the chemical noise generated by noise and for this reason the used

the solutlonjmembrane interface were found membranes, although an exact estimate could uncertamty of the surface area The electrothe pH ISFET was obscured by the device cross-correlation technique [12] 1s now being

Direct measurement of mterfacml charge Let us assume that interface 1 m Fig 3 IS ideally polarized, z e , capacltlve (R 1 = 00) Provided that the parasitic capacitance 1s small, the excess charge at this interface can m prmclple be measured directly [ 131 We have explored this posslblllty using a CHEMFET with a thm layer of gold deposited over the gate Gold/sodmm fluoride solution 1s a good approxlmatlon of a polaruzed mterface that 1s described by the Gibbs-Llppmann equation dy=qmdE’

+~l?,d,u,

where r 1s the’ surface energy, qm 1s the charge on the metal, E’ IS the mterfaclal potential, rt IS the so-called surface excess of adsorbed ions and

262 IS the chemical potentlal of the adsorbmg species The change of the mterfaclal potential with the actrvlty of adsorbing ion m solution, a,, at constant charge on the metal, the so-called Esm-Markov coefflclent, can be denved from eqn (5)

,q

The condltlon of constant metal charge can be easily achieved by using the feedback clrcult shown m Fig 7 It 1s known that iodide ions adsorb strongly at the gold mterface, yleldmg a value of the Esm-Markov coefflclent of 59 mV/decade In our mltlal experiments [14] we could obtam only a transient response of the mterfaclal potential to the addition of lodlde, which indicated that the charge-transfer resistance at the gold mterface had a finite value The orlgm of this problem has been traced to the presence of residual oxygen m solutron, to the presence of tltamum m the surface of the gold and to the surface conductlvlty of the surroundmg sllrcon mtnde [ 151 The first problem has been circumvented by thorough deaeratlon of the solution followed by cathodic prepolarlzatlon of the gold The presence of vanable amounts of tltamum m thm films of gold could not be elimlnated, however Tltamum 1s used as an underlay m order to improve the adhesion of gold to slllcon mtnde It has been found that T1 diffuses along the gram boundanes [ 161 over a few thousand angstroms m a few days, even at room temperature When it reaches the metal/solution interface it substantially reduces the charge-transfer resistance We found the third problem, the surface conductlvlty of &con mtnde, totally surpnsmg In the study of that problem we have taken the advantage of the dual CHEMFET conflguratlon of our transistor chip By charging up one of the gold gates m air we could observe the effect (crosstalk) of mlgratmg charge on the other gate, which was mtentlonally left electntally floatmg When the devices were placed m dry nitrogen, essentially no

IOK

+

+

-10

*5

v

IOK

Fig 7 Clrcult for direct measurement permission from 1141 )

of the Esm-Markov

coeffxlent

(Reprmted

with

263

crosstalk was observed over a period of 24 hours Under those condltlons the surface reslstlvlty has been estimated to be 4 X 1018 a/Cl Slmllar results have been obtained for devices covered with a thick layer of cured Epon 826 epoxy, even when encapsulated devices were unmersed m water for several hours However, even a bnef exposure of bare slhcon mtmde to humidity decreased this value Fully hydrated s&con nitride was found to have a surface reslstlvlty of 1 3 X 1Ol6 a/Cl which dissipated the deposited charge wAh a time constant of 20 - 30 mmutes For the direct measurement of the Esm-Markov coefficient both the surface contammatlon with titanium and the surface conductlvlty of silicon mtnde have been overcome by attaching a short (- 2 mm) piece of gold wire to the gate and then totally encapsulatmg the whole chip With this arrangement we have determined the Esm-Markov coefflcrent of the adsorption of Iodide on gold to be 53 6 mV/decade The Gibbs-Llppmann equation (eqn (5)) LSthe equation of state In that case the followmg relatlonshlp holds for the state varnables (7) It ~111 be noted that the first term 1s the Esm-Markov coefficient (eqn. (6)) and the third term 1s the dlfferentlal capacitance Cd Provided that the above partial differentials are smooth, contmuous and non-singular functions of I-(~,the second term can be obtained expenmentally from its inverse [17]. The relatlonshlp between this parameter and the Esm-Markov coefficient 1s then

Expelnmentally this means that we have substituted the measurement of mterfaclal potential at constant charge for the measurement of mterfaclal charge at constant potential The experimental arrangement for this measurement 1s shown m Fig. 8. In this clrcult the potential apphed to the reference electrode 1s V, while the transitor gate 1s connected to the feedback voltage Vf through a 10’ a resistor The potential difference Vf - V, IS then the constant mterfaclal potential E+ m eqn (7) If an adsorbmg amon 1s added to the interface, the induced negative charge 1s compensated by the feedback circuit m order to mamtam the set value of the dram current. The signal LS, therefore, the transient current flowing through the lo8 a resistor, which when integrated yields the supplied amount of charge By this techmque we have estimated the slope of the dependence of adsorbed Iodide on solution concentration to be 2 9 X 10e6 C cmY2 mole-’ 1 Equation (7) automatically offers a selfconslstency check of these measurements with Cd measured independently to be 50 X 10m6 F cmd2, the which 1s m good agreement with the product of the three terms 1s -0.93, theoretical value of -1 0

264

Wg 8 Cmxut for the measurement permrsslon from [15] )

of the Esm -Markov

coeffuxent

(Reprinted

with

Conclusions Undoubtedly, the mterest m CHEMFETs IS largely due to then potential as mmlature multlsensors urlth excellent signal-to-noise ratio. Their relatlonshlp to other potentlometrlc sensors LS relatively straghtforward the underlymg prmclples are the same and any differences are mamly practical Because CHEMFETs as well as other potentlometrlc sensors with a solid mternal contact are multllayer structures, It 1s Important that the exchange-current densltles at the boundarles of these layers are as high as possible In other words, it IS preferable that these surfaces are ohmic The prehmmary results show that CHEMFETs can be used for basic electrochemlcaI studies, such as mvestlgatlons of stochastic events at mterfaces and for measurement of mterfaclal charge Our expenence mth these devices also shows that most problems are related to the choice of materials, m particular the encapsulatxon

Acknowledgements Most of the work mentloned m this paper, which has been done at the University of Utah, was supported by the National Science Foundatron (CHE 7800637), National Institutes of Health (NIGMS 22952) and by the Office of Naval Research This support 1s gratefully acknowledged

265

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Cammann,

Exchange kmetlcs of potassmm-selective membrane electrodes, Anal 936 - 940 R P Buck, Kinetics and drift of gate voltages for electrolyte-bathed chemically sensitive semiconductor devices, IEEE Trans Electron Dev , ED-29 (1982) 108 - 115 S Collms and J Janata, A crltrcal evaluation of the mechanism of potential response of antigen polymer membranes to the correspondmg antiserum, Anal Chzm Acta, 136 (1982) 93 99 T A Fleldly and K Nagy, Fluoride electrodes with reversible solid state contacts, J Electrochem Sot, 127 (1980) 1299 - 1303 A Haemmerll, J Janata and J Brophy, Equlhbrlum noise m ion selective field effect Sac , 129 (1982) 2306 - 2312 transistors, J Electrochem A Haemmerll, J Janata and H M Brown, Electrical characterlstlcs of K+ and ClFET mlcroelectrodes, Sensors and Actuators, 3 (1982/83) 149 - 158 R L Smith, J Janata and R J Huber, Transient phenomena in ion sensitive field effect transistors, J Electrochem Sac , 127 (1980) 1599 - 1603 U Oesch, S Caras and J Janata, Field effect transistors sensitive to sodium and ammonium ions, Anal Chem , 52 (1981) 1983 - 1986 A Haemmerh, J Janata and H M Brown, A flow mJectlon system for measurement of chemical response time of mlcroelectrodes, Anal Chzm Acta, 144 (1982) 115 121 R P Buck, Impedance diagrams applied to the mvestlgatlons of ion selective electrodes, Hung Scz Znst, 49 (1980) 7 - 23 L J DeFelrce and B A Sokol, A new analysis for membrane noise The integral spectrum, Bzophys J, 16 (1976) 827 - 838 J M RelJn, J Janata and J J Brophy, Noise m ion selective field effect transistors Electrochemzcal Socrety Meetzng, May 9 12, under non-equlhbrlum condltlons, 1983, San Francisco, Abstract No 617 J Janata and R J Huber, Chemically sensitive field effect transistors, in H Frelser (ed ), Ion Selectrue Electrodes zn Analytzcal Chemzstry, Vol 2, 1980, Plenum Press, New York R M Cohen and J Janata, Measurement of excess charge at polarized electrodes with field effect transistors Part I Direct determmatlon of the Esm-Markov coefficient., J Electroanal Chem , 151 (1983) 33 - 39 R M Cohen and J Janata, The surface conductlvlty of slhcon oxymtrrde, Thzn Solzd Fzlms, (1983) m press T C Tlsone and J Drobek, Diffusion m thm film Tl-Au, Tl-Pd, and TI-Pt couples, J Vat Scr Technol, 9 (1971) 271 - 275 R M Cohen and J Janata, Measurement of excess charge at polarized electrodes with field effect transistors Part II Indlrect determmatlon of the Esn-Markov coefficient, J Electroanal Chem, 151 (1983) 41 - 45 Chem , 50 (1978)

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