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Modelling the gas-sensing behaviour of SnO2-gate FETs
540
Sensors and Actuators 8, 18-H (1994) 540-542
Modelling the gas-sensing behaviour of SnO,-gate FETs SK. Audreev, L.I. Popova, V.K. Gueorguiev and N.D. Stoyanov Instituteof Soiid State Physics, BulgarianAcademy of Sciences, 7.2 ‘TsarigmdskoChawsee’ bivd., Sofia 1784 (Bulgaria)
Abstract An attempt to create a first-order model of the operation mechanisms of SnOx-gate FETs sensitive to ammonia is reported in this work. In previous work a strong dependence of the sensing effect on the relative humidity of the ambient has been reported. On these grounds, the presence of adsorbed positive NH+ ions and negative OH- ions on the SnO, surface is assumed and the model is built up on the lateral transportation of these ions under the combined effect of the transversal gate-to-channel electric field and the surface source-drain field. The transversal field is supposed to weaken electrostatically the adsorption bonds of the OH- ions over the channel and of the NH+ ions over the saturated surface area. This enables lateral transportation and redistribution of the ions along the surface source-drain geld. Dtle to the strong non-unifo~ity of the channel this redist~~ution results in equivalent addition to the gate voltage applied and, thus, in channel current modulation. Expressions for the floods of positive and negative ions over the channel are derived. The calculated curves for the gate voltage variation according to these expressions are in good agreement with experimental data.
Intruduction
First-order modeliing
During the last decade MOSFETs have been extensively studied as gas sensors. Numerous MOSFET structures have been reported for a multitude of chemical measurements [l-6]. All of them have in common a silicon substrate with diffused source and drain regions, and the channel area covered with a thin or thick SiOz layer. They differ mainly in the gate structure - catalytic metal gate, suspended gate, open gate area, etc. Several mechanisms appear to take part in the operation of the different structures: modulation of the dielectric constant of the gate dielectric, transportation of charged particles along the surface under the influence of the source-drain electric field, diffusion of particles into the SiOz insulator layer, direct channel current modulation by the fringing fieid of the adsorbed molecules. It is reported [7] that the operation mechanism of SnO,gate FETs sensitive to ammonia, exhibits an overall dependence on the transversal gate-to-channel electric field. However, the additional dependence on the drainvoltage observed suggests that the operation of such devices cannot be explained by some work function variations or polarization effects or charge storage effects dependent on the transversal electric field individually. In this work an attempt to create a first-order model of the operating mechanism of such devices is rep resented.
Two basic propositions are made as a starting point: - The structure discussed is an n-type Iong-channel silicon FET with positive drain voltage (Vu) applied and grounded source (V,=O). - The gate electrode is a semiconductor tin oxide layer separated from the silicon surface by an SiOa dielectric layer. Bearing in mind that the sensing behaviour reported is essentially dependent on the relative humidi~ of the ambient f7] one could assume that, in the case with water and ammonia molecules adsorbed from the air and at v,=O, positive NH,+ ions and negative OHions are homogeneously distributed on the SnO,-gate surface (Fig. l(a)). Then, with positive V, applied and assuming a saturation mode of operation, the inversion channel has the profile shown schematically in Fig. l(b). The pinchoff end of the channel is denoted by A. The channel potential V, at point A is given by V,= VGo- V,, where V,, is the gate voltage corresponding to the measured current voltage under clean air ~nditions~ V, is the FET’s threshold voltage. The average transversal electric field between the gate and the silicon surface is given by
09254005/94/$7.000 1994Elsevier Sequoia. All rights reserved SSDI OY25-4UU5(93)U1079-5
541 Finally
where the constants p, W and do, are lumped into k=pWld,. In the case with E,
(4
@I
(4
AV,,=nS,E,
(4
where n is a constant coefficient (specific for given surface and type of ion), S, is the channel area (practically constant at long-channel conditions). Finally
0 SnO, PIIdsio, I Al Fig. 1. Ion distribution on the Sn02 surface in the Snq-gate FET: (a) at V,=O; (b) with E,>O, E,O, E,>O; (d) with E,
AV,=I(&V,)
(5)
(6)
where where EP, E, are the transversal electric fields over the saturated region and over the channel, respectively; li,,, VC,,are the average surface potentials of the saturated region and of the channel, respectively; Vo is the Vo applied electrically to the gate when testing the device in ammonia containing ambient; do, is the thickness of the dielectric SiO, layer. vC,, is computed making use of the basic FET’s equations and I?&,is given by v_, = (V, + VJ2 in this work. In the case with Ep> 0 one could assume that the adsorption bonds between the SnO, surface and the positive NH+ ions are weakened electrostatically, so that the ions could travel along the surface to more negative areas (over the channel). Thus, an equivalent positive addition AV, to VG is obtained, this addition giving rise to the drain current I,,. At a given surface density of adsorbed molecules the flow of positive ions over the channel area would be proportional to the average repulsive field E, and the area of the saturation region S, (source of ions). Thus AV, =pE,S,
(2)
where p is a constant coefficient (specific for a given surface and type of ion) and s, = W(2p*-I-v, - V*)l12
(3)
where W is the channel width, 2Qa= inversion wne bending at the silicon surface (2Qs = 0.56 V in this work). The term in parentheses in eqn. (3) accounts for the V,, dependence of the length of the saturation region.
I= 2 ox is a lumped constant. Thus, at the combination E,> 0, E, <0 (Fig. l(b)) the entire contribution AVc to V, amounts to Al’, = AV, - AV”
(7)
Two other combinations E,, E, are possible (Fig. l(c) and (d)). At E,>O, E,>O AV,=AV, and at E,
Experimental verification Experimental verification of the validity of expressions (4), (6) and (7) is carried out on the basis of the experimental work reported in ref. 7. Samples with VT=0 were measured in that work and the character of their current response to 100 ppm ammonia in air was demonstrated. With VT=0 for the entire actual working area (VD, Vc) the case EP> 0, E,
542
References P. Bergveld, The impact of MOSFBT-based
sensors, Sensors
and Actuators,8 (1985) 109-127. I. Lundstr&n, M. Armgarth, A. Spetz and F. Winquist, Gas sensors based on catalytic metal-gate field-effect devices, Sensors and Actuators,10 (1986) 399-421. I. Cassidy, S. Pons and J. Janata, Hydrogen response of palladium coated suspended gate field-effect transistors,Anal Chenr, 58 (1986) 1757-1761. I. Lundsttim, M. Armgarth, A. Spetz and F. Winquist, Physics of ammonia sensitive metal oxide semiconductor structures, Proc. 2nd ht. Meet. Chemical Sensors,Bordeaur, France, July 7-14 1986, pp. 387-390. M. Peschke, H. Lorenz, H. Riess and I. Eisele, Recognition of hydrogen and ammonia by modified gate metallization of the suspended-gate FET, Senrors and Actuators,Bl (1990)
21-24. S. Andreev, L. Popova, V. Gueorguiev
Fig. 2. Caldated curves Ak’o (Vo) for the SnOz-gate FET according to the model compared with experimental data
and G. Beshkov, Characteristics and gas-sensing bebaviour of a tin-oxide-gate FET, Sensors and ActuatorsB, 8 (1992) W-91. L. Popova, S. Andreev, V. Gueorguiev and N. Stoyanov, Voltage dependence of gas-sensing behaviour of Sn02-gate FETs, Sensors and ActuatorsB, 18-19 (1994) 543-545.
These values vary but are of the same order for different samples. Biographies Conclusions A first-order model of the ammonia-sensing behaviour of SnO,-gate FETs is proposed, based on the lateral transportation of NH, + and OH- ions along the surface initiated by the combined effect of the transversal gateto-channel electric field and the source-drain field along the silicon surface. The effect of the transversal field is expressed mainly by a weakening of the adsorption bonds between the adsorbed ions and the SnO, surface. Under conventional bias conditions the adsorption bonds of the negative OH- ions are weakened over the channel and the bonds of the positive NH,+ ions are weakened over the saturated area between the pinched-off end of the channel and the drain region. Due to the strong non-uniformity of the channel in the saturation mode of operation this redistribution of positive and negative ions on the surface is equivalent to variation AV, of the gate voltage V,, this variation resulting in channel current modulation. The experimental verification represented demonstrates a good agreement between the calculated variations AV, according to expressions (4), (6) and (7) in this paper and the experimental data. Acknowledgement Financial support from the Scientific Commission of the National Science Fund (contract 243/92) for this work is gratefully acknowledged.
Liliana I. Popova was born in Sofia in 1937. She graduated from the Faculty of Physics of Sofia University in 1960 and received a Ph.D. with a thesis on the physics and technology of MNOS memories. She has been an associate professor since 1974 and her field of research covers microelectronic physics and technology, especially thin films for sensors and ICs. St#an K. Andreev was born in Sofia in 1942 and graduated in 1967 from the Faculty of Physics of Sofia University. He received a Ph.D. in 1980 with a thesis on the quasilinear analysis of MOSICs, and has been an associate professor since then. His field of research is the design, measurement and analysis of MOSICs and he is currently working in the field of semiconductor sensors. Valentin K. Gueotguiev was born in Bulgaria in 1941. He graduated in semiconductor physics from Sofia University in 1967 and received a Ph.D. in 1987 with a thesis on the quantum model of non-uniformly doped semiconductor surfaces. His current research is in the physics and technology of polycrystalline and amorphous materials for sensors and ICs. Nikolai D. Stoyanow was born in Bulgaria in 1965 and graduated in microelectronics from St. Petersburg Electrical University. He is currently working in the field of polycrystalline and amorphous materials for microelectronics and the physics of semiconductor sensors.
Sensors and Actuators 8, 18-H (1994) 540-542
Modelling the gas-sensing behaviour of SnO,-gate FETs SK. Audreev, L.I. Popova, V.K. Gueorguiev and N.D. Stoyanov Instituteof Soiid State Physics, BulgarianAcademy of Sciences, 7.2 ‘TsarigmdskoChawsee’ bivd., Sofia 1784 (Bulgaria)
Abstract An attempt to create a first-order model of the operation mechanisms of SnOx-gate FETs sensitive to ammonia is reported in this work. In previous work a strong dependence of the sensing effect on the relative humidity of the ambient has been reported. On these grounds, the presence of adsorbed positive NH+ ions and negative OH- ions on the SnO, surface is assumed and the model is built up on the lateral transportation of these ions under the combined effect of the transversal gate-to-channel electric field and the surface source-drain field. The transversal field is supposed to weaken electrostatically the adsorption bonds of the OH- ions over the channel and of the NH+ ions over the saturated surface area. This enables lateral transportation and redistribution of the ions along the surface source-drain geld. Dtle to the strong non-unifo~ity of the channel this redist~~ution results in equivalent addition to the gate voltage applied and, thus, in channel current modulation. Expressions for the floods of positive and negative ions over the channel are derived. The calculated curves for the gate voltage variation according to these expressions are in good agreement with experimental data.
Intruduction
First-order modeliing
During the last decade MOSFETs have been extensively studied as gas sensors. Numerous MOSFET structures have been reported for a multitude of chemical measurements [l-6]. All of them have in common a silicon substrate with diffused source and drain regions, and the channel area covered with a thin or thick SiOz layer. They differ mainly in the gate structure - catalytic metal gate, suspended gate, open gate area, etc. Several mechanisms appear to take part in the operation of the different structures: modulation of the dielectric constant of the gate dielectric, transportation of charged particles along the surface under the influence of the source-drain electric field, diffusion of particles into the SiOz insulator layer, direct channel current modulation by the fringing fieid of the adsorbed molecules. It is reported [7] that the operation mechanism of SnO,gate FETs sensitive to ammonia, exhibits an overall dependence on the transversal gate-to-channel electric field. However, the additional dependence on the drainvoltage observed suggests that the operation of such devices cannot be explained by some work function variations or polarization effects or charge storage effects dependent on the transversal electric field individually. In this work an attempt to create a first-order model of the operating mechanism of such devices is rep resented.
Two basic propositions are made as a starting point: - The structure discussed is an n-type Iong-channel silicon FET with positive drain voltage (Vu) applied and grounded source (V,=O). - The gate electrode is a semiconductor tin oxide layer separated from the silicon surface by an SiOa dielectric layer. Bearing in mind that the sensing behaviour reported is essentially dependent on the relative humidi~ of the ambient f7] one could assume that, in the case with water and ammonia molecules adsorbed from the air and at v,=O, positive NH,+ ions and negative OHions are homogeneously distributed on the SnO,-gate surface (Fig. l(a)). Then, with positive V, applied and assuming a saturation mode of operation, the inversion channel has the profile shown schematically in Fig. l(b). The pinchoff end of the channel is denoted by A. The channel potential V, at point A is given by V,= VGo- V,, where V,, is the gate voltage corresponding to the measured current voltage under clean air ~nditions~ V, is the FET’s threshold voltage. The average transversal electric field between the gate and the silicon surface is given by
09254005/94/$7.000 1994Elsevier Sequoia. All rights reserved SSDI OY25-4UU5(93)U1079-5
541 Finally
where the constants p, W and do, are lumped into k=pWld,. In the case with E,
(4
@I
(4
AV,,=nS,E,
(4
where n is a constant coefficient (specific for given surface and type of ion), S, is the channel area (practically constant at long-channel conditions). Finally
0 SnO, PIIdsio, I Al Fig. 1. Ion distribution on the Sn02 surface in the Snq-gate FET: (a) at V,=O; (b) with E,>O, E,
AV,=I(&V,)
(5)
(6)
where where EP, E, are the transversal electric fields over the saturated region and over the channel, respectively; li,,, VC,,are the average surface potentials of the saturated region and of the channel, respectively; Vo is the Vo applied electrically to the gate when testing the device in ammonia containing ambient; do, is the thickness of the dielectric SiO, layer. vC,, is computed making use of the basic FET’s equations and I?&,is given by v_, = (V, + VJ2 in this work. In the case with Ep> 0 one could assume that the adsorption bonds between the SnO, surface and the positive NH+ ions are weakened electrostatically, so that the ions could travel along the surface to more negative areas (over the channel). Thus, an equivalent positive addition AV, to VG is obtained, this addition giving rise to the drain current I,,. At a given surface density of adsorbed molecules the flow of positive ions over the channel area would be proportional to the average repulsive field E, and the area of the saturation region S, (source of ions). Thus AV, =pE,S,
(2)
where p is a constant coefficient (specific for a given surface and type of ion) and s, = W(2p*-I-v, - V*)l12
(3)
where W is the channel width, 2Qa= inversion wne bending at the silicon surface (2Qs = 0.56 V in this work). The term in parentheses in eqn. (3) accounts for the V,, dependence of the length of the saturation region.
I= 2 ox is a lumped constant. Thus, at the combination E,> 0, E, <0 (Fig. l(b)) the entire contribution AVc to V, amounts to Al’, = AV, - AV”
(7)
Two other combinations E,, E, are possible (Fig. l(c) and (d)). At E,>O, E,>O AV,=AV, and at E,
Experimental verification Experimental verification of the validity of expressions (4), (6) and (7) is carried out on the basis of the experimental work reported in ref. 7. Samples with VT=0 were measured in that work and the character of their current response to 100 ppm ammonia in air was demonstrated. With VT=0 for the entire actual working area (VD, Vc) the case EP> 0, E,
542
References P. Bergveld, The impact of MOSFBT-based
sensors, Sensors
and Actuators,8 (1985) 109-127. I. Lundstr&n, M. Armgarth, A. Spetz and F. Winquist, Gas sensors based on catalytic metal-gate field-effect devices, Sensors and Actuators,10 (1986) 399-421. I. Cassidy, S. Pons and J. Janata, Hydrogen response of palladium coated suspended gate field-effect transistors,Anal Chenr, 58 (1986) 1757-1761. I. Lundsttim, M. Armgarth, A. Spetz and F. Winquist, Physics of ammonia sensitive metal oxide semiconductor structures, Proc. 2nd ht. Meet. Chemical Sensors,Bordeaur, France, July 7-14 1986, pp. 387-390. M. Peschke, H. Lorenz, H. Riess and I. Eisele, Recognition of hydrogen and ammonia by modified gate metallization of the suspended-gate FET, Senrors and Actuators,Bl (1990)
21-24. S. Andreev, L. Popova, V. Gueorguiev
Fig. 2. Caldated curves Ak’o (Vo) for the SnOz-gate FET according to the model compared with experimental data
and G. Beshkov, Characteristics and gas-sensing bebaviour of a tin-oxide-gate FET, Sensors and ActuatorsB, 8 (1992) W-91. L. Popova, S. Andreev, V. Gueorguiev and N. Stoyanov, Voltage dependence of gas-sensing behaviour of Sn02-gate FETs, Sensors and ActuatorsB, 18-19 (1994) 543-545.
These values vary but are of the same order for different samples. Biographies Conclusions A first-order model of the ammonia-sensing behaviour of SnO,-gate FETs is proposed, based on the lateral transportation of NH, + and OH- ions along the surface initiated by the combined effect of the transversal gateto-channel electric field and the source-drain field along the silicon surface. The effect of the transversal field is expressed mainly by a weakening of the adsorption bonds between the adsorbed ions and the SnO, surface. Under conventional bias conditions the adsorption bonds of the negative OH- ions are weakened over the channel and the bonds of the positive NH,+ ions are weakened over the saturated area between the pinched-off end of the channel and the drain region. Due to the strong non-uniformity of the channel in the saturation mode of operation this redistribution of positive and negative ions on the surface is equivalent to variation AV, of the gate voltage V,, this variation resulting in channel current modulation. The experimental verification represented demonstrates a good agreement between the calculated variations AV, according to expressions (4), (6) and (7) in this paper and the experimental data. Acknowledgement Financial support from the Scientific Commission of the National Science Fund (contract 243/92) for this work is gratefully acknowledged.
Liliana I. Popova was born in Sofia in 1937. She graduated from the Faculty of Physics of Sofia University in 1960 and received a Ph.D. with a thesis on the physics and technology of MNOS memories. She has been an associate professor since 1974 and her field of research covers microelectronic physics and technology, especially thin films for sensors and ICs. St#an K. Andreev was born in Sofia in 1942 and graduated in 1967 from the Faculty of Physics of Sofia University. He received a Ph.D. in 1980 with a thesis on the quasilinear analysis of MOSICs, and has been an associate professor since then. His field of research is the design, measurement and analysis of MOSICs and he is currently working in the field of semiconductor sensors. Valentin K. Gueotguiev was born in Bulgaria in 1941. He graduated in semiconductor physics from Sofia University in 1967 and received a Ph.D. in 1987 with a thesis on the quantum model of non-uniformly doped semiconductor surfaces. His current research is in the physics and technology of polycrystalline and amorphous materials for sensors and ICs. Nikolai D. Stoyanow was born in Bulgaria in 1965 and graduated in microelectronics from St. Petersburg Electrical University. He is currently working in the field of polycrystalline and amorphous materials for microelectronics and the physics of semiconductor sensors.