Programmable ion-sensitive transistor interfaces. I. Electrochemical gating

Krishna Jayant, Kshitij Auluck, Mary Funke, Sharlin Anwar, Joshua B. Phelps, Philip H. Gordon, Shantanu R. Rajwade, and Edwin C. Kan

Phys. Rev. E 88, 012801 – Published 1 July 2013

Abstract

Electrochemical gating is the process by which an electric field normal to the insulator electrolyte interface shifts the surface chemical equilibrium and further affects the charge in solution [Jiang and Stein, Langmuir 26, 8161 (2010)]. The surface chemical reactivity and double-layer charging at the interface of electrolyte-oxide-semiconductor (EOS) capacitors is investigated. We find a strong $p$ H-dependent hysteresis upon dc potential cycling. Varying salinity at a constant $p$ H does not change the hysteretic window, implying that field-induced surface $p$ H regulation is the dominant cause of hysteresis. We propose and investigate this mechanism in foundry-made floating-gate ion-sensitive field-effect transistors, which can serve as both an ionic sensor and an actuator. Termed the chemoreceptive neuron metal-oxide-semiconductor (CνMOS) transistor, it features independently driven control gates (CGs) and sensing gates (SGs) that are capacitively coupled to an extended floating gate (FG). The SG is exposed to fluid, the CG is independently driven, and the FG is capable of storing charge $Q_{F G}$ of either polarity. Asymmetric capacitive coupling between the CG and SG to FG results in intrinsic amplification of the measured surface potential shifts and influences the FG charge injection mechanism. This modified SG surface condition was monitored through transient recordings of the output current, performed under alternate positive and negative CG pulses. Transient recordings revealed a hysteresis where the current was enhanced under negative pulsing and reduced after positive pulsing. This hysteresis effect is similar to that observed with EOS capacitors, suggesting a field-dependent surface charge regulation mechanism at play. At high CG biases, nonvolatile charge $Q_{F G}$ tunneling into the FG occurs, which creates a larger field and tunes the $p$ H response and the point of zero charge. This mechanism gives rise to surface programmability. In this paper we describe the operational principles, tunneling mechanism, and role of electrolyte composition under field modulation. The experimental findings are then modeled by a Poisson-Boltzmann formulation with surface $p$ H regulation. We find that surface ionization constants play a dominant role in determining the $p$ H tuning effect. In the following paper [K. Jayant et al., Phys. Rev. E 88, 012802 (2013)] we extend the dual-gate operation to molecular sensing and demonstrate the use of $Q_{F G}$ to achieve manipulation of surface-adsorbed DNA.

Received 26 February 2013

DOI:https://doi.org/10.1103/PhysRevE.88.012801

Authors & Affiliations

Krishna Jayant^1,*, Kshitij Auluck¹, Mary Funke^2,†, Sharlin Anwar^3,†, Joshua B. Phelps¹, Philip H. Gordon¹, Shantanu R. Rajwade¹, and Edwin C. Kan¹

¹School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853, USA
²Department of Chemistry, High Point University, North Carolina 27262, USA
³Department of Biomedical Engineering, City College of New York, New York 10031, USA

^*Corresponding author: kj75@cornell.edu
^†Contributed equally to this work.

Programmable ion-sensitive transistor interfaces. II. Biomolecular sensing and manipulation

Krishna Jayant, Kshitij Auluck, Mary Funke, Sharlin Anwar, Joshua B. Phelps, Philip H. Gordon, Shantanu R. Rajwade, and Edwin C. Kan

Phys. Rev. E 88, 012802 (2013)

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Vol. 88, Iss. 1 — July 2013

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Figure 1
(a) The EOS capacitor used in the $C V$ analysis. (b) The CνMOS transistor with two independently driven gates: (control) CG and (sensing) SG coupled to a common (floating gate) FG. The FG to electrolyte capacitive coupling is mimicked by the EOS structure. The CG is shielded from the solution via a thick oxide (2-μm) passivation. (c) A SEM image of the fabricated transistors showing the SG, CG, and transistor regions, respectively.Reuse & Permissions
Figure 2
(a) Variation in $ψ_{O}$ as a function of $p H_{B}$ for variations in $Δ p K$ . As $Δ p K$ increases (arrow) the slope becomes more non-Nernstian (lower than 60 mV/ $p$ H). (b) $ψ_{O}$ vs $p H_{B}$ for variations in surface site density in m $^{- 2}$ . The relative flattening in the response increases as $N_{S}$ decreases. The arrow indicates the direction of the $N_{S}$ decrease. (c) $ψ_{O}$ vs $E_{o x}$ for varying $p H_{B}$ . The solid arrow represents the direction of decreasing $p H_{B}$ . When $p H_{B}$ is in the range between the 2 $p K$ 's, the surface buffering is low with $p H_{P Z C} \sim 7$ . Here $p K_{a} = 10$ and $p K_{b} = 5$ . The region between the dashed lines represents the $E_{o x}$ range used in this study and the dark shaded region represents the fields applicable during readout. (d) $ψ_{O}$ vs $E_{o x}$ for varying salinity $n_{O}$ . The shaded region (gray) represents the field during readout. The solid arrow represents the direction of increasing $n_{O}$ . The maximum modulation in $ψ_{O}$ occurs for lower $n_{O}$ . At the zero $E_{o x}$ condition, the surface sensitivity to varying $n_{O}$ is negligible. An applied positive or negative $E_{o x}$ can tune $ψ_{O}$ to be sensitive to changes in $n_{O}$ . (e) $ψ_{O}$ vs $E_{o x}$ for varying $p K_{b}$ . Here $ψ_{O}$ is affected only in the $- E_{o x}$ region. The solid arrow represents the direction of increasing $p K_{b}$ . (f) $ψ_{O}$ vs $E_{o x}$ for varying $p H_{B}$ with $p K_{a}$ and $p K_{b}$ flipped. The solid arrow represents the direction of increasing $p H_{B}$ . Maximal buffering (dashed arrow) is observed in the range between the 2 $p K$ 's.Reuse & Permissions
Figure 3
Experimental $C V$ analyses depicting $V_{F B}$ shifts for (a) varying $p H_{B}$ . A strong $p H_{B}$ -dependent hysteresis is observed while performing cyclic sweeps. A lowering in $V_{F B}$ is observed when the reference electrode is swept from positive to negative voltages (indicated by the arrow and the ellipse) implying a net positive remnant surface charge for (b) varying bulk ion concentration $n_{O}$ . The hysteretic window at constant $p H_{B}$ is insensitive to $n_{O}$ , while the accumulation region capacitance is dependent on $n_{O}$ . (c) Varying cationic valence $z$ . Varying $z$ influences the double-layer composition, which further influences $σ_{O}$ . Divalent cations shift the $V_{F B}$ lower, while trivalent ions induce a slight increase. Here $C_{D L}$ is lower for the trivalent cations. Here (R) denotes the reverse sweep in (a)–(c).Reuse & Permissions
Figure 4
Transient recordings under CG pulse trains: drain current output as a function of varying $n_{O}$ at (a) $p H_{B} = 11$ , (b) $p H_{B} = 7$ , and (c) $p H_{B} = 3$ . The pulse train amplitude and duration are shown under each figure. Calculated $ψ_{O}$ as a function of $E_{o x}$ for varying $n_{O}$ for (d) $p H_{B} = 11$ , (e) $p H_{B} = 7$ , and (f) $p H_{B} = 3$ , using $p K_{a} = 10$ and $p K_{b} = 5$ . At $p H_{B} = 11$ , $ψ_{O}$ is net negative for $E_{S G_o x}$ close to $0 MV / cm$ and becomes more negative with decreasing $n_{O}$ . This is reflected in the current levels during the transient recordings. At $p H_{B} = 7$ the current levels flip when $E_{S G_o x}$ is switched from positive to negative since $ψ_{O}$ is positive at positive $E_{o x}$ and negative at negative $E_{o x}$ . At $p H_{B} = 3$ , $ψ_{O}$ is net positive and increases with decreasing $n_{O}$ . In all three cases, the drain current is higher between 230 and 285 sec than the initial state between 0 and 60 sec. This is attributed to a net positive charge due to the field-induced protonation that remains after the negative gating pulse is relaxed. This is similar to the observed hysteresis in Fig. 3. The regions between the dashed lines in (d)–(f) represent the fields during readout. Solid arrows in (d)–(f) represent the direction of increasing $n_{O}$ .Reuse & Permissions
Figure 5
(a) $Δ V_{t h}$ (representative of $ψ_{O}$ ) as a function of $p H_{B}$ for varying $n_{O}$ . The slope of the $p H_{B}$ response reduces in the range between $p K$ 's, while it increases at extreme $p H_{B}$ values. Cations are presumed to contribute to the slight increase in $ψ_{O}$ at high $p H_{B}$ . The arrow indicates increasing order of $n_{O}$ . (b) Theoretical fit to the experimental $p H_{B}$ response reveals a $Δ p K$ of 5 for a given surface site density of $10^{17} m^{- 2}$ . $ψ_{O}$ vs $p H_{B}$ for varying $E_{o x}$ with (c) $p K_{a} < p K_{b}$ and (d) $p K_{a} > p K_{b}$ . Both responses indicate a shift in $p H_{P Z C}$ (star) towards higher $p H_{B}$ as $E_{o x}$ is increased, while the $p$ H-insensitive region shifts towards (c) higher $p H_{B}$ and (d) lower $p H_{B}$ , with increasing $E_{o x}$ , respectively. This is primarily due to the different ionization states of the surface dependent on the choice of $p K_{a}$ and $p K_{b}$ . (e) Experimentally extracted $p H_{B}$ response as a function of positive $E_{o x}$ in the SG oxide, achieved by $Q_{F G}$ . Results show the $p$ H-insensitive region shifts towards lower $p H_{B}$ . Error bars represent an average over three experimental runs. (f) $ψ_{O}$ vs $p H_{B}$ for varying $E_{o x}$ with $p K_{a} > p K_{b}$ and lower $N_{s}$ ( $5 \times 10^{16} m^{- 2}$ ). The modulation in $p H_{P Z C}$ is much more exaggerated.Reuse & Permissions
Figure 6
$Δ V_{t h_C G}$ as a function of the CG pulse amplitude for variations in (a) $n_{O}$ for a NaCl electrolyte and (b) cationic valence for CνMOS with an amplification ratio of 20 at a bulk $n_{O}$ of 20 mM. An initial increase in $Δ V_{t h}$ at low to moderate CG voltages is attributed to surface deprotonation and a net remnant negative $ψ_{O}$ . Due to asymmetric CG and SG capacitances, $V_{F G}$ is pulled closer to $V_{r e f}$ . This ensures that at sufficiently high $V_{C G}$ the $V_{F G}$ does not increase much, which leads to large $E_{C G_o x}$ for FN tunneling. Reduction in $n_{O}$ and $C_{D L}$ weakens the coupling between the FG and $V_{r e f}$ , causing $E_{C G_o x}$ and net $Q_{F G}$ to decrease. Varying cationic valence indicates more pronounced shifts in $Δ V_{t h_C G}$ around the knee point (i.e., where tunneling begins) (shown by the dashed arrow) especially with trivalent salts in comparison to monovalent and divalent salts. A decrease in the overall $Δ V_{t h_C G}$ with trivalent salts upon tunneling is consistent with the notion that $C_{D L}$ is also decreasing. Error bars represent an average over three experimental runs.Reuse & Permissions

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