3rd International Conference NANOCON 014 Nanotechnology - Smart Materials, Composites, Applications and New Inventions - Date : 14th, 15th October,

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2 insulator of JL transistor and the electrolyte varies with the specific ion activity in the electrolyte causing a shift in threshold voltage and hence the drain current of the device. Currently, research interest has been focused upon overcoming the challenges associated with conventional CMOS technology especially when dimensions scale down to tens of nanometers. At such scaled dimensions, leakage current and short channel effects (SCEs) becomes very crucial. Moreover, the formation of the ultra sharp junctions imposes severe challenges on doping techniques due to the difficulty to control the distribution of dopants at the metallurgical junctions and the intrinsic discreteness of the dopant itself. Recently, the Junctionless (JL) MOSFET is proposed as one of the most promising alternative device architecture for CMOS technology as it is highly immune to short channel effects and delivers outstanding characteristics [-4]. The Junctionless MOSFET is free from such severe doping issues and hence provides a simplified fabrication process, and so on []. In this series, we have demonstrated n-type Silicon on insulator Junctionless ISFET based ph-sensor through simulation technique by using commercial device simulator, SENTAURUS [3], which can deal with D as well as 3D structures. The simulation method deals with a simulation domain that includes both semiconductor and electrolyte regions. The electrolyte solution is considered as the type of the semiconductor material in which the hole and electron charges represent the mobile ions in the solution [5, 6]. The modulation of drain current in SOI JL ISFET based ph sensor is due to change in the ionic concentration of the electrolyte and hence, hydrogen ions (H + ions) and hydroxyl ions (OH - ions) of the solution. Therefore, various ph sensor characteristics can be evaluated using this simulation technique, such as the ph sensitivity as well as the drain current fluctuation.. Device Architecture (a) ) Reference Electrode Electrolyte Contact Source Gate insulator (SiO ) Channel Contact Drain t ox t si Buried Oxide Reference Electrode (b) ) Gate Insulator (SiO) Electrolyte n+ Source p-type Channel n+ Drain Buried Oxide Fig.. Schematic diagram of the ph sensors. (a) n-type Silicon on Insulator Junctionless ISFET, and (b) n-type conventional ISFET.

3 Table. Simulation parameters of both devices Parameters ph-isfet ph-soi JL ISFET Channel Doping 5x m -3 (p-type) 5x m -3 (n-type) Source/Drain Doping Channel length (L) Channel Width (W) Gate oxide Thickness (t ox) Site binding charge (σ) x 6 m -3 (n-type) 35nm µm nm -x 6 m - 5x m -3 (n-type) 35nm µm nm -x 6 m - The cross-sectional view of the conventional ph-isfet and ph-soi JL ISFET are shown in Fig.. From the figures it is clear that in the case of ph-soi JL ISFET design the channel region has the same doping concentration as in the source and drain regions. The device is dived in the electrolyte solution and electrically characterized using the reference electrode. The insulator (i.e., the gate oxide material) is exposed to the aqueous solution and reacts with it by developing interfacial charges layers. The actual amount of charges depends on the concentration of specific ions present in the solution, on the H + ion concentration, i.e., on the ph of the solution that modulates consequently the surface charges at the insulator/semiconducting interface. This results in shift of the threshold voltage of the device. The main behavior is dominated by the electrochemical processes of ion exchanges, described by Nernst equation and the maximum Nernstian sensitivity is limited by 59. mv/ph at 7 C [7, 8]. In the conventional ISFET structure, the reference electrode only fixes the potential of the homogeneous solution, and leads to homogeneous distribution of the H + ions. The numerical studies are performed to compare the characteristics of the proposed SOI JL ISFET structure with conventional ISFET on the basis of electrical and sensitivity parameter. The device parameters used in our study for both structures are given in Table. 3. Simulation And Calibration: A. Simulation Methodology The simulation was carried out with help of commercial 3D TCAD tool (Sentaurus, Synopsys Inc.) []. TCAD simulator is commonly used to characterize the electrical properties of the semiconductor devices and dielectric materials whereas it cannot deal with the ionic solution. In this paper, the ionic solution is defined as an intrinsic semiconductor material with the dielectric constant of water (i.e., 78). In a real ionic solution, the charge distribution is represented by the Poisson Boltzmann (PB) equation [9]. In this work we assume a : electrolyte (e.g. H + Cl - ) and the original form of the PB equation can be written as below: q q q H Cl C e C e x w () H Where, C Cl and C denote the H + and Cl - ion concentrations at the electrically neutral condition, having the same value between them. This PB equation is very close to the semiconductor equation except for the Fermi Dirac distribution of the hole and electron charges in the semiconductor. The semiconductor equation for an intrinsic material can be rearranged as follows: Ei Ev Ec Ei q e e p n E i Ev q Ec Ei q x si e e e e ()

4 Drain Current (A) In above equation, p and n respectively denote the hole and electron concentrations in the equilibrium condition. Equation () accords very well with equation () if (E g / - qψ) is greater than thermal energy (), an assumption which is always valid under this simulation condition. If the ionic solution is replaced with an intrinsic semiconductor material, the electrostatic solution of the aqueous region can be determined by solving the semiconductor equation in that region. Since an intrinsic semiconductor material is considered as the ionic solution, some of the physical parameters for the semiconductor material needs to be decided. First the bandgap of the semiconductor is chosen equal to the silicon bandgap (. ev) because the silicon bandgap has to satisfy only the condition (E g / - qψ). Second, the equivalent density of state (DOS) of the semiconductor (N c, N v ) is obtained such that the number of the hole and electron charge is equal to the molal concentration of solution ions. Third, the electron affinity (χ e ) should be determined such that the simulation reproduces the real I V characteristics of the SOI JL ISFET device. The electrochemical meaning of electron affinity is related to the standard reduction potential between the silicon and the ionic solution. B. Calibration Calibration of model parameters used in the simulation has been performed according to the experimental results [5]. Various models used in simulation are as follows: concentration dependent mobility, field dependent mobility, Fermi-Dirac model, and Shockley-Read-Hall recombination model. Closed proximity of simulated results with experimental results as shown in Fig. validates the choice of parameters taken in the simulation..e-6.e-7 Experimental (Ref. [5]) Simulated.E-8.E-9.E-.E-.E- N d = 5x 7 cm -3 W/L= 75nm/.µm t si = 5nm V d =.V χe = 3.9 ev mm KCl Solution.E E-5. Reference gate Voltage (V) Fig.. Validation of simulation results with the experimental results [5] for p-type SiNWs. C. Charge Distribution In Electrolyte The charge distribution in the electrolyte is represented by the PB equation which describes the charge density distribution. Charge concentration depends on the concentration of specific ions in the solution. Fig. 3 gives a representation of the position of positive and negative charges in electrolyte at ph 7 (i.e. with the same concentration of H+ and OH ions) and null voltage. If the positive and negative charges are non-uniformly distributed then the potential is not constant in the electrolyte region. The positive charges are attracted toward the silicon dioxide side and the negative charges toward the reference electrode side at thermodynamic equilibrium.

5 Density of State (cm -3 ) Reference Electrode Side Silicon Dioxide (Insulator) Side Fig. 3. Schematic representation of charge position at equilibrium (V g = ). As seen earlier there is an equality between the equations which describes positive and negative ions in the electrolyte and holes and electrons in a semiconductor. From this a : electrolyte can be described in simulator as a semiconductor which is called electrolyte material. The ionic charge concentration in the electrolyte which is given as: C N N N C (3) i V C A where N A is Avogadro s number (/mol) and C the ion molar concentration (M = mol/m 3 ) in the bulk of the solution and C i is the ionic charge concentration in the electrolyte which is equal to the density of state of intrinsic semiconductor material. From the above Eq. (3), ph can be calculated as given below: ph 3 Ci log N A (4) The above relationship between ph value and the density of state of intrinsic semiconductor material is shows in the Fig. 4..E+.E+.E+9.E+8.E+7.E+6.E+5.E+4.E+3 6.x 3 cm -3 = ph 7.E+.E+.E+.E+9.E+8.E+7.E+6.E+5.E Fig. 4. Variation of density of state of intrinsic semiconductor with respect to The concentration of H + ions, in pure water is -7 moles/liter and the number of molecules per mole is 6.x 3 (Avogadro s number), the concentration of H + is around 6.x 3 / cm 3. OH - concentration in the pure water is the same as H +. The mass action law states that [OH - ] [H + ] in pure water is 3-4x 6 / cm 3 at room temperature, and the

6 Drain Current (A) number is preserved if the impurity molecules are added to change either [H + ] or [OH - ]. Notice that the mass action law of water is similar to the mass action law in the semiconductor where n-p product correspond to [OH - ] [H + ]; n-p in silicon is.x /cm 3 at room temperature. If [H + ] is larger (smaller) than [OH - ], the solution is called an acid (base). Note the similarity in the mass action law between electron and hole concentrations in the semiconductor materials and [H + ] and [OH - ] concentration in the solution. The concentration of electrons and holes are denoted by number/cm 3 whereas the ion concentration in an electrolyte is expressed by mol/l. For conversion of mol/l to number/cm 3 is simply to multiply 6.x /cm 3 / (mol/l). D. Insulator/Electrolyte Interface The shift in the threshold voltage depends primarily on the amount of surface charge at the insulator/semiconducting interface of the device. In order to evaluate this effect in the simulation, concentration of negative fixed charges at the interface of the SiO and Silicon channel is varied. The Drain current characteristics are shown in Fig. 5 for a fixed amount of charges ranging from -x 9 to -x cm 3 and there is great influence of ions present at the SiO surface. The amount of charges depends on the concentration of specific ions present in the solution (H + ionic concentration) and modulates consequently the surface charge at the insulator semiconductor interface..5e-6.e-6.5e-6.e-6 5.E-7 -x Nf = -e9 9 cm - -x Nf= -e cm - -x Nf= -e cm - -5x Nf=-5e cm - -x Nf=-e cm -.E Gate Voltage (V) Fig. 5. Drain current for different concentration of negative charges at electrolyte/sio interface. Interface reactions can be taken into account by the so-called site-binding model for silicon nitride surfaces where the adsorption and dissociation of H+ and OH ions at the interface between the electrolyte and the nitride leads to interface charge densities. 4. Results And Discussion A. Threshold Voltage The ph response is defined as the amount of threshold voltage shift when the ph in the injected solution is varied from to ph 7. Fig. 6(a) shows when the ph value is increased by increasing the ionic concentration in the solvent the threshold voltage shifts for particular channel thickness of the device. The threshold voltage of SOI JL ISFET ph sensor increase as ph value increases. It can also seen from the Fig. 6(a) that the threshold voltage decreases as the channel thickness of the device increases for particular ph value of the electrolyte. Fig. 6(b) shows the comparison between the conventional SOI ISFET ph sensor and the proposed SOI JL ISFET ph sensor. At particular channel thickness of both devices the threshold voltage increases when the ph value of the device increases. But at particular ph value of the ionic solution, when the channel thickness of the device is increased the threshold voltage increases for the conventional SOI ISFET ph sensor whereas the threshold voltage decreases for the proposed SOI JL ISFET ph sensor. The shift in threshold voltage for conventional SOI ISFET ph sensor is 59

7 Threshold Voltage (V) Threshold Voltage (V) Threshold Voltage (V) Threshold Voltage (V) mv/ph which is close to the Nernst sensitivity limit whereas for the proposed SOI JL ISFET is 6.7 mv/ph which is beyond to the Nernst sensitivity limit of ΔV th = 59. mv/ph at room temperature (a) Tsi= t =nm Tsi= t =3nm Tsi= t si =4nm Tsi= t si =5nm.5 (b) Hollow: SOI JL ISFET Solid: SOI ISFET Tsi= t =nm Tsi= t =nm Tsi= t =5nm Tsi= t =5nm Fig. 6. (a) threshold volatge variation with respect to ph value as a function of channel thickness and (b) threshold volatge comparison of convential SOI ISFET ph sensor and proposed SOI JL ISFET ph sensor (a) SiO AlO3 O 3 HfO SOI JL ISFET (b) SiO AlO3 O 3 HfO SOI ISFET Fig. 7. Threshold voltage variation for different site binding layer. (a) SOI JL ISFET ph sensor, and (b) Conventional SOI ISFET ph sensor. Fig. 7 shows the impact of different adhesion layers (gate oxide layer) on the threshold volatge of both type of devices. In both Fig. 7(a) and 7(b) threshold voltage decreases for Al O 3 and HfO adhesion layer in comparison to SiO layer at particular ph value of the solution. But the magnitude of the threshold voltage for particular ph value is much higher of the proposed device in comparison to the conventional SOI ISFET ph sensor. The threshold volatge shift for SiO, Al O 3, and HfO are 6.7 mv/ph, 4. mv/ph, and 38 mv/ph respectively for the proposed SOI JL ISFET ph sensor whereas for conventional SOI ISFET ph sensor are 59 mv/ph, 69.4 mv/ph, and 88 mv/ph. Although, high-k adhesion layer increases the sensitivity for conventional ISFET based ph sensor [], but the high-k and silicon interface is prone to degradation due to high traps density [, ]. Therefore, SOI JL ISFET based ph sensor with SiO gate oxide material can be used as an adhesive layer having a minimum sensitivity of 6.7 mv/ph. In this way, the performance degradation can be avoided which arises due to poor interface quality of high-k and silicon.

8 Drain Current (A) Transconductance (A/V) Threshold Volatge (V).5 Te t e =5nm= Te t e =3nm= Te t e =4nm= Te t e =5nm =.5.5 Fig. 8. Threshold voltage variation with respect to the ph value as function of electrolyte thickness of SOI JL ISFET ph sensor. Fig. 8 shows the impact of electrolyte thickness on the threhold voltage of the proposed device. It can be seen from that as the thickness of the electrolyte region is increased there is negligible change in the threhold voltage for the lower ph in comparison to the higher ph value (from ph = 5). For higher ph value the threshold voltage increases as the electrolyte thickness increases. Therefore, in the proposed device there is no need to have bigger electrolyte region to detect the ph value of the electrolyte. B. Drain Current And Transconductance The transfer characteristics i.e., drain-source current I ds versus the gate voltage V gs is evaluated when the drain voltage V d is kept constant at mv. Fig. 9(a) shows the transfer characteristics I ds -V gs of the n-type SOI JL ISFET ph sensor dipped in the electrolyte with different ph values at room temperature. Drain current characteristics shifts toward right side when the ph value increases. Fig. 9(b) shows the transconductance of SOI JL ISFET ph sensor versus gate voltage for different ph value. The transconductance peak decreases and shifts toward the right hand side as ph value of the solution increases. For ph = the transconductance peak voltage is V but for ph =7 the peak voltage.8 V..5E-6.E-6.5E-6.E-6 5.E-7 (a) ph = ph = ph = ph = 3 ph = 4 ph = 5 ph = 6 ph = 7 7.E-6 6.E-6 5.E-6 4.E-6 3.E-6.E-6.E-6 (b) ph = ph = ph = ph = 3 ph = 4 ph = 5 ph = 6 ph = 7.E+ 3 Gate Voltage (V).E+ 3 Gate Voltage (V) Fig. 9. (a) Transfer characteristics of SOI JL ISFET with different ph increases at room temperature, (b) Transconductance with gate bias for different ph value.

9 Sensitivity (V/pH) C. Sensitivity The shift of the threshold voltage versus ph, from ph 7 to lower ph value is shown in Fig. (a) and (b) (shift relative to ph curve as reference). Fig. (a) shows that dip in the ph value of the solution induces a significant increase in the threshold voltage of the SOI JL ISFET. Therefore, the sensitivity of the device increases if the ph value of the solution is decreased. It is also seen that as the channel thickness increases the sensitivity decreases for the proposed device. Fig. (b) shows the comparison of the conventional SOI ISFET based ph sensor and proposed SOI JL ISFET based ph sensor in terms of sensitivity. In Fig. (b) sensitivity is also increased when ph value of the device decreases for both devices. But as the channel thickness increases for both devices the sensitivity increases for conventional SOI ISFET in comparison to the proposed device. Therefore, the proposed device shows good sensitivity for smaller channel thickness whereas conventional device shows good sensitivity at larger channel thickness. ΔVth = Vth(pH = 7)- Vth(pH < 7) (V) (a) Tsi= t si =nm Tsi= t si =3nm Tsi= t si =4nm Tsi= t si =5nm ΔVth = Vth(pH = 7) - Vth(pH < 7) (V) (b) Hollow: SOI JL ISFET Solid: SOI ISFET ttsi= =nm ttsi= =5nm ttsi= =nm ttsi= =5nm Fig.. Variation of the threshold voltage as a function of ph (shift relative to a refernce at ph = 7), (a) SOI JL ISFET based ph sensor, and (b) comparison of conventional and proposed devices Electrolyte Thickness (nm) Fig.. Senitivity for various values of the electrolyte thickness of SOI JL ISFET based ph sensor

10 The effect of electrolyte thickness on the ph sensitivity is also evaluated for the SOI JL ISFET based ph sensor. Fig. shows the sensitivity for different value of the electrolyte thickness between the reference gate and the insulator layer. It shows that the sensitivity of the device is maximum between 35nm and 45nm. Moreover, adsorption of the charges on silicon dioxide surface is strongly coupled with the effect of the strong electric field in the electrolyte, which has a significant influence on the site-binding phenomenon. 5. Conclusion In this paper, Silicon on Insulator Junctionless ion sensitive field effect transistor (SOI JL ISFET)based ph-sensor has been proposed. The proposed sensor (SOI JL ISFET) has no source and drain junctions, where the concentration and the doping type is the same in channel region and in the source and drain. The proposed sensor shows better sensitivity for the shorter channel thickness whereas the conventional sensor shows the better sensitivity at longer channel thickness. The SOI JL ISFET based ph sensor shows 6.3 mv/ph shift in the threshold voltage which is greater than the Nernst sensitivity 59. mv/ph in comparison to the conventional SOI ISFET based ph sensor got the threshold voltage shift is 59 mv/ph. 6. Acknowledgment Authors would like to thank Ministry of Science and Technology, Department of Science and Technology, Government of India and University of Delhi. Ajay would like to thank University Grants Commission, Government of India, for providing the necessary financial assistance during the course of this research work. 7. References [] Y. Cheng, P. Xiong, C. S. Yun, G. Strouse, J. Zheng, R. S. Yang, and Z. L. Wang, Nano letters, 8, , (8). [] X.-j. Li and M. Schick, Biophysical journal, 8, 73-7, (). [3] N. Nakamura, S. Tanaka, Y. Teko, K. Mitsui, and H. Kanazawa, Journal of Biological Chemistry, 8, 56-57, (5). [4] Y. Chen, X. Wang, M. Hong, S. Erramilli, and P. Mohanty, Sensors and Actuators B: Chemical, 33, , (8). [5] U. Kummer, J. Zobeley, J. C. Brasen, R. Fahmy, A. L. Kindzelskii, A. R. Petty,A. J. Clark, and H. R. Petty, Biophysical journal, 9, , (7). [6] A.-S. Yang and B. Honig, Journal of molecular biology, 3, , (993). [7] R. Frost and R. Griffin, Soil Science Society of America Journal, 4, 53-57, (977). [8] P. N. Royce, Critical reviews in biotechnology, 3, 7-49, (993). [9] P. Bergveld, Biomedical engineering, IEEE Transactions on. 7-7, (97). [] S. Martinoia, G. Massobrio, and L. Lorenzelli, Sensors and Actuators B: Chemical, 5, 4-7, (5). [] P. Bergveld, Biomedical Engineering, IEEE Transactions on, 34-35, (97). [] J.-P. Colinge, C.-W. Lee, A. Afzalian, N. D. Akhavan, R. Yan, I. Ferain,, P. Razavi, B. O'Neill, A. Blake, M. White, A.-M. Kelleher, B. McCarthy, and R. Murph, Nature nanotechnology, 5, 5-9, (). [3] A. M. Ionescu, Nature nanotechnology, 5, 78-79, (). [4] C.-W. Lee, I. Ferain, A. Afzalian, R. Yan, N. D. Akhavan, P. Razavi,and J.-P. Colinge, Solid-State Electronics, 54, 97-3, (). [5] I.-Y. Chung, H. Jang, J. Lee, H. Moon, S. M. Seo, and D. H. Kim, Nanotechnology, 3, 65, (). [6] F. Pittino, P. Palestri, P. Scarbolo, D. Esseni, and L. Selmi, Solid-State Electronics, (4). [7] T. Hizawa, K. Sawada, H. Takao, and M. Ishida, Japanese journal of applied physics, 45, 959, (6). [8] O. Knopfmacher, A. Tarasov, W. Fu, M. Wipf, B. Niesen, M. Calame,and C. Schönenberger, Nano letters,, 68-74, (). [9] D. C. Grahame, Chemical Reviews, 4, 44-5, (947). [] S. Zafar, C. D Emic, A. Afzali, B. Fletcher, Y. Zhu, and T. Ning, Nanotechnology,, 455, (). [] S. Mohapatra, K. Pradhan, and P. Sahu, International Journal of Advanced Science & Technology, 65, (4). [] E. Amat, T. Kauerauf, R. Degraeve, R. Rodríguez, M. Nafría, X. Aymerich,and G. Groeseneken, Microelectronic Engineering, 87, 47-5, (). [3] TCAD Sentaurus Device User Manual, Synopsys, CA, (3).

11 Ajay received B.Sc. (Hons.) and M. Sc. degree in electronics from University of Delhi, New Delhi, in and respectively. He is currently working toward the Ph. D. Degree in Semiconductor Device Research Laboratory, Department of Electronic Science, University of Delhi, South Campus. His research interest includes modeling and Simulation study of BioFETs for label free electrical detection of the BioMolecules. Rakhi Narang received B.Sc., M. Sc. and Ph.D. degree in electronics from University of Delhi, New Delhi, in 5, 7 and 4 respectively. She is currently an Assistant Professor in the Department of Electronics, Sri Venkateswara College, University of Delhi. Her research interests include modeling and Simulation of novel device architectures like Tunnel Field Effect Transistor and FET based biosensors. She has authored/co-authored 3 technical papers in the international journal and conference proceedings. Manoj Saxena received the B.Sc. (Hons.), M.Sc., and Ph.D. degree in electronics from the University of Delhi, New Delhi, India. He is currently an Associate Professor in the Department of Electronics, Deen Dayal Upadhyaya College, University of Delhi. He has authored/co-authored more than 95 technical papers in international journals and conference proceedings. His current research interests are in the areas of analytical modeling, design, and simulation of Optically controlled MESFET/MOSFET, silicon-on-nothing, insulated-shallow-extension, cylindrical gate MOSFET and Tunnel FET. Mridula Gupta received the B.Sc. degree in physics, M.Sc. degree in electronics, the M.Tech. degree in microwave electronics, and Ph.D. degree in optoelectronics from University of Delhi, Delhi, India, in 984, 986, 988, and 998, respectively. Since 989, she has been with the Department of Electronic Science, University of Delhi South Campus, New Delhi, India, where she is currently Professor and with the Semiconductor Devices Research Laboratory. She has authored or co-authored approximately 377 publications in international and national journals and conference proceedings. Her current research interests include modeling and simulation of MOSFETs, MESFETs, and HEMTs for microwave-frequency applications.