KJ 8056 CHAPTER 1. ELECTROCHEMICAL SENSORS. Part B. Semiconductor devices as chemical sensors

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1 NTNUet. of Chemistry KJ 8056 CHAPTER 1. ELECTROCHEMICAL SENSORS Part B. Semicoductor devices as chemical sesors CONTENTS By F. G. Baica, August 2006 B.1. Semicoductors devices a) Silico ad Germaium semicoductors b) Semicoductor diodes c) Field effect trasistors (FET) B.2. Ioselective fieldeffect trasistors (ISFET) a) Hsesitive sesors (HISFET) b) Polymer membraes for ISFET c) Heterogeeous membraes d) Multile fuctio ISFET e) Referece FET B.3. Gas sesitive FET B.4. Ezymebased FET sesors B.5. Electroic devices for the oeratio of the FET chemical sesor. INTROUCTION Semicoductor devices (like diodes ad trasistors) are essetial comoets of ay kid of electroic equimet. Such device allows cotrollig the curret flow i electric circuits i order to do iformatio covey ad rocessig. A io sesig layer ca be combied with a semicoductor device. The electric charge which develos at the solutio iterface (FRAME 1) will exert a effect o the electric roerties of that device. This is the uderlyig ricile of the semicoductorbased io sesors. The recogitio riciles ad methods are mostly alike to that ecoutered with the membrae io sesors (Chater 1, Part A). The semicoductor device itself lays the role of the trasductio art. The reset Part of Chater 1 starts u with a resetatio of semicoductor materials ad semicoductor devices. Further, various alicatios of such devices i sesor sciece ad techology are reseted. Created by F. G. Baica Created o 9/7/2006 8:27 AM

2 FRAME 1 ION SENSORS The solutio /membrae iterface Charge distributio H Si O Io Exchager Solutio A io trasfer rocess occurs at the membraesolutio iterface (e.g. ositive ios from the solutio are boud to the egative recetor sites at the membrae surface). Cosequetly, a ouiform charge distributio develos, with a excess ositive charge i the solutio hase. The excess charge ca rovide iformatio about the io cocetratio i the solutio if a suitable method for charge assessmet is available. Assessmet of the iterface electrical charge ca be doe by: Potetiometry (usig membrae sesors, Chater 1, art A), or Electrostatic effects usig semicoductor device (Chater 1, art B) Created by F. G. Baica Created o 9/7/2006 8:27 AM

3 FRAME 2 SEMICONUCTOR EVICES The doig of semicoductors Semicoductor devices (diodes, trasistors...) are based o doed silico materials. oig cosists of addig a very small ercetage of foreig atoms i the regular crystal lattice of silico. It roduces dramatic chages i its electrical roerties. Imurity atoms with 5 valece electros roduce tye semicoductors by cotributig extra electros. Imurity atoms with 3 valece electros roduce tye semicoductors by roducig a hole or electro vacacy (i.e. a ositively charged site). I each case, the electric coductivity of the semicoductor icreases very much. Electricity trasort is doe by free electros movemet i tye semicoductors ad by holes drift i the tye semicoductors. Created by F. G. Baica Created o 9/7/2006 8:27 AM

4 FRAME 3 SEMICONUCTOR EVICES The Semicoductor iode Forward biasig I 0 I Reverse biasig I = 0 I = 0 Created by F. G. Baica Created o 9/7/2006 8:27 AM

5 FRAME 4 SEMICONUCTOR EVICES The Field Effect Trasistor (FET) Metal layer (gate) S I=0 G Oxide Isulator (SiO 2 ) I Semicoductor U S = Source = rai G = Gate A FET cosists of a small iece of tye silico with two tye regios called source ad drai. The surface is covered by a isulatig layer (SiO 2 ) ad a thi metal layer is deosited above to for the gate. No curret ca flow betwe the source ad the drai because a reverse biasig occurs at oe of the juctio (FRAME 3). However, if a ositive electric charge is loadet to the gate (by meas f the voltage source), free electro may aear betwee the source ad the drai, immediately uder the gate/isulator makeu. Thus, a iversio layer aears. I this regio (also called chael) the tye semicoductor is coverted ito a tye ad o reverse biasig occurs ay loger. A curret (I ) ca flow betwee the source ad the drai i this istace. I deeds o the free electro desity i the chael, which is dictated by. It deeds also o electro velocity, which is a fuctio of the sourcedrai voltage, U. Small chages i result i large modificatios of I. Therefore, the curret flowig through the FET ca be cotrolled by the gate voltage. See FRAME 5 for details. Created by F. G. Baica Created o 9/7/2006 8:27 AM

6 FRAME 5 Curret flow through the FET: the I relatio Curret starts to flow betwee source ad drai whe the chael begis to form. This occurs whe the gate voltage rises above a critical value (threshold voltage, U T, Fig. 1). I UV TT UV GG Fig. 1. U is the voltage betwee source ad drai; is the voltage alied to the gate; I is the curret flowig betwee source ad drai. The gate/isulator/substrate assembly behaves as a caacitor with a caacitace C G. It stores a chael charge Q give by : (1) Q= C ( U U ) G G T The drai curret reresets the amout of electric charge that asses across the chael er time uit: (2) I = Q/ t Where: chael legth L (3) t = = electro velocity v ad: v = ( electro mobility)x (4) (stregth of the electric field) = µ ( U / L) So: (5) Fially: 2 t= L µ U µ CG (6) I = U ( ) 2 UG UT L Equatio (6) holds at low values, whe the otetial alog the chael is aroximately costat. It shows that at a costat, I is roortioal to U. Also, at a costat U, I is roortioal to. This coclusio is very imortat to FET alicatios i sesor devices. At high values, the I U relatio is more comlicated tha the liear relatioshi (6) (see Fig. 2). I, ma Fig. 2,V U,V Created by F. G. Baica Created o 9/7/2006 8:27 AM

7 FRAME 6 FET based H sesors (H ISFET) Structure ad riciles i G =0 Referece electrode Solutio H selective solid layer S B i Tye Semicoductor U I order to build u a H sesitive FET, the isulator layer should be relaced by a H bidig material (e.g, a SiO cotaiig material). This oe is i cotact with the samle solutio ad a referece electrode assumes the role of the gate. A otetial differece develos at the solutio iterface as a result of io trasfer (FRAME 1): RT RT (7) Ei = k' 2.303s log a = k' 2.303s H H zf zf k is a costat ad s is the sesitivity factor (s = ) The iterface otetial differece adds to ad iflueces the drai curret (I )which becomes Hdeedet (FRAME 5, eq. (6)). This forms the basis for H determiatio by meas of the FET sessor. The icture to the left dislays a miiature H ISFET. Miiaturizatio is the mai advatage of the FET sesors (see also FRAME 10). Created by F. G. Baica Created o 9/7/2006 8:27 AM

8 FRAME 7 The H ISFET Oeratio at costat I I H 2 < H 1 I = cost U(H 1 ) U(H 2 ) Eq. (7) alog with eq. (6) suggest that the drai curret is a H fuctio ad ca be emloyed as aalytical sigal for H determiatio at a costat gate voltage. However, for ractical reasos it is more suitable to oerate the sesor at a costat I. The Figure above demostrates the shift of the I curve whe the H chages from H 1 to H 2. A automatic devices adjusts so as to kee I costat. The icremet i ( UG = U( H1) U( H2) comesates the cotributio of the iterface otetial (E i, FRAME 6) i.e. U = E Therefore, UG is a liear fuctio of H ad a liear calibratio fuctio ( UG vs. H ) is the basis for sesor calibratio. G i Created by F. G. Baica Created o 9/7/2006 8:27 AM

9 FRAME 8 The H ISFET: Several versios STANAR Stadard Coical CONIC PLAT Flat Lace LANCE The H ISFET: figures of merit Sesitivity: mv H rage: 0 14 No glass Robust Fast resose Easy maiulatio Ca be stored i dry state Easy cleaig (tooth brush soa!) The shae ca be adated for various alicatios H ISFET sesors are commercially available Created by F. G. Baica Created o 9/7/2006 8:27 AM

10 FRAME 9 The Io Selective FET Sesor ( ISFET ) I G =0 Referece electrode Solutio Ioselective layer S B Isulator (SiO 2 ) I tye semicoductor U The io selective layer (which forms the sesig art) cosists of a lastic (PVC) membrae which icludes a io recetor. The recetor ca be either a liquid io exchager or a eutral macrocyclic ligad. Measurig riciles are similar to that reseted for the H ISFET (FRAME 7 ad FRAME 1). Created by F. G. Baica Created o 9/7/2006 8:27 AM

11 FRAME 10 ISFET as a flow directio ad flow rate trasducer Reactie Aodic aodica reactio (Pt):2H 2 O 4H 4e O 2 Cathodic Reactie catodica reactio: 2 H 2 O 2e 2OH H 2 The geerator electrode (Pt) roduces H io by water electrolysis (the cathode is ot show). The H sesors idicate the fluid flow directio ad seed by meas of H differece betwee the test oits. ISFET Miiature Multile Sesor Referece Flow direct. Flow rate Temerature H Peicilli Various chemical ad hysical arameters ca be measured by the above multile sesor which was desiged for moitorig eicilli fabricatio. Created by F. G. Baica Created o 9/7/2006 8:27 AM

12 FRAME 11 Schematics of a ISFET measurig riciles As a result of io exchage at the sesorsolutio iterface, a iterface membrae E i develos ((FRAME 7 ad FRAME 1). I order to kee I costat, is automatically adjusted to ( UG UG). As UG = E i, it deeds o the io cocetratio accordig to a logarithm low (oa otetial, see Chater 1, art A). Therefore, UG rovides the exected aalytical iformatio. U =cost (adjustable) ISFET I =cost Referece Electrode Samle I the Fig. above, the sesig device (leftdow) is oversized for the sake of clarity Created by F. G. Baica Created o 9/7/2006 8:27 AM

13 FRAME 12 ISFET Sesors A overview of the sesor structure Metal layer (gate) S I=0 G Oxide Isulator (SiO 2 ) MOSFET I Semicoductor U i G =0 Referece electrode Solutio H ISFET S B H selective solid layer i Tye Semicoductor U i G =0 Referece electrode Solutio Ioselective Membrae ISFET S B Isulator (SiO 2 ) i Tye Semicoductor U Created by F. G. Baica Created o 9/7/2006 8:27 AM