Joshua B. Phelps

A Ferroelectric and Charge Hybrid Nonvolatile Memory—Part II: Experimental Validation and Analysis

Part I of this article introduced the concept and operation of the novel hybrid memory, integrating ferroelectric polarization and nonvolatile charge injection. In Part II, we demonstrate the experimental validation of this hybrid design. One-transistor memory cells were fabricated with polyvinylidene fluoride-trifluoroethylene [P(VDF-TrFE)] as the ferroelectric and HfO2 as the charge trap layer. Hybrid devices showed larger memory window and longer retention time compared to conventional FE-FETs with the same effective oxide thickness. Pulsed measurements were performed on metal-ferroelectric-metal capacitors to estimate switching delay in the P(VDF-TrFE) thin film. Field enhancement in the tunnel oxide resulted in pronounced electron injection from the gate compared with gate injection Flash memory cells. Hybrid devices also exhibited higher program efficiencies against the FE-FET due to the contribution from these injected electrons. The presence of the tunnel oxide in hybrid devices showed over 20× reduction in gate leakage, which resulted in 100 × improvement in cycling endurance against FE-FETs.

A Ferroelectric and Charge Hybrid Nonvolatile Memory—Part I: Device Concept and Modeling

We present a new one-transistor hybrid nonvolatile memory based on the combination of two distinctive mechanisms, namely, remanent polarization in ferroelectrics and charge injection into floating nodes. The gate stack design and the memory operation of the hybrid device are aimed to offer mutually complementing benefits between the two mechanisms, thereby presenting superior performance over conventional ferroelectric (FE) FET and gate injection-based Flash memory. During program operation, a high negative bias at the gate orients the ferroelectric polarization to the applied field. In addition, electrons at the gate electrode also tunnel into the floating nodes located between the ferroelectric thin film and the thin top tunnel dielectric and increase the total memory window. High electric displacement in the ferroelectric enables field enhancement in the tunnel dielectric for faster program and erase operations. During retention, the injected electrons reduce the depolarization field in the ferroelectrics, and the remanent polarization reduces the electric field in the tunnel oxide, which helps in the longer retention of the programmed state by the two additive memory mechanisms. Part I evaluates the benefits of the hybrid gate stack through 1-D simulations incorporating the polarization-field (P-E) hysteresis in the ferroelectric layer. The simulations provide a guideline for optimal gate stack design of the proposed hybrid memory. The following Part II then discusses the fabrication and experimental validation.

Non-Faradaic Electrochemical Detection of Exocytosis from Mast and Chromaffin Cells Using Floating-Gate MOS Transistors

We present non-faradaic electrochemical recordings of exocytosis from populations of mast and chromaffin cells using chemoreceptive neuron MOS (CνMOS) transistors. In comparison to previous cell-FET-biosensors, the CνMOS features control (CG), sensing (SG) and floating gates (FG), allows the quiescent point to be independently controlled, is CMOS compatible and physically isolates the transistor channel from the electrolyte for stable long-term recordings. We measured exocytosis from RBL-2H3 mast cells sensitized by IgE (bound to high-affinity surface receptors FcεRI) and stimulated using the antigen DNP-BSA. Quasi-static I-V measurements reflected a slow shift in surface potential () which was dependent on extracellular calcium ([Ca]o) and buffer strength, which suggests sensitivity to protons released during exocytosis. Fluorescent imaging of dextran-labeled vesicle release showed evidence of a similar time course, while un-sensitized cells showed no response to stimulation. Transient recordings revealed fluctuations with a rapid rise and slow decay. Chromaffin cells stimulated with high KCl showed both slow shifts and extracellular action potentials exhibiting biphasic and inverted capacitive waveforms, indicative of varying ion-channel distributions across the cell-transistor junction. Our approach presents a facile method to simultaneously monitor exocytosis and ion channel activity with high temporal sensitivity without the need for redox chemistry.

Capacitive control of an ISFET using dielectric coated electrodes

Reliable control of ISFETs encapsulated in microfluidic environments is required to achieve portable and reliable biosensors. Conventionally ISFETs maintain the stable electrochemical potential through a reference electrode, which can be consumed and is difficult to miniaturize. In this paper, to facilitate microfluidic packaging by using an on-chip capacitive reference electrode, we employ a modified CνMOS approach by connecting the sensing gate (SG) of the CνMOS to an uncoated platinum electrode, while another large-area gate (Si3N4 covered platinum) in the fluidic chamber serves as the electrolytic control gate (ECG). The capacitance ratio between the ECG to the extended SG needs to be engineered to efficiently modulate the transconductance. We observe an Ion/Ioff ratio of 109 and a subthreshold slope of ~80mV/decade when the ISFET is controlled using the ECG. We further observe that the extended platinum SG is relatively insensitive to saline concentration which we attribute to the lack of ion specificity at the extended platinum electrode. The pH response, on the other hand, is anomalous. We attribute this to the opposing proton-dependent sensitivities at the exposed Pt and Si3N4 coating of the ECG. Impedance spectroscopy however shows a clear molarity dependent RC time constant shift indicative of clear conductivity dependence. Our results suggest that non-faradaic capacitive control of ISFETs are more suitable for applications involving impedance detection and not ionic sensing.

Electrolytic charge inversion at programmable CMOS sensor interfaces

Electrochemical interface layer overcharging is experimentally demonstrated at planar MOS sensor interfaces by controlling the surface charge through nonvolatile charge injection. The electric field across the solid-fluid interface is modulated upon floating-gate program/erase and leads to electrolytic charge reversal, for which an analytical model is derived. This electrofluidic gating effect is further used to repel adsorbed DNA, realizing an electrical surface refreshable biosensor. Quasi-static and impedimetric measurements are presented for validation.

Critical Assessment on Modeling and Design of Nonfaradaic CMOS Electrochemical Sensing

To assist the design process of nonfaradaic electrochemical sensors in realistic biological media, we examine multiple types of sensing operations in polyelectrolytes. By comparing the quasi-static transconductance, impedance spectroscopy, and capacitance-voltage measurements, we assess the physical contributions from the double-layer composition, overall solution resistance, and sensing surface potential under various polyelectrolytic molarities. The mixture of NaCl and MgCl2 is chosen for illustration to provide insight into circuit model parameters for nonfaradaic sensing. Our finding also shed light on the dynamics of double-layer competition and correlation, which is critical for understanding the physical phenomena occurring at the sensing interface, and accurately interpreting sensor data.

Circuit models for non-faradaic CMOS electrochemical sensing

To improve field use of ISFET devices for biological and chemical sensing, we present a parameter extraction model based on the correlation between multiple experiments in polyelectrolyte media. By correlating the quasistatic transconductance, impedance spectroscopy, transient current readout and capacitance-voltage (CV) measurements, we can decouple the physical contributions of the immobile/diffusive layer composition, overall solution resistance, surface potential drift under various electrolytic molarities, and reference electrode configurations. Hierarchical parameter extraction of the circuit components is demonstrated for mixtures of NaCl and MgCl2. This method also sheds light the dynamics of double-layer competition and correlation, which is critical for biological and biochemical sensing.

Electrochemical Detection of Signalling Responses in Excitatory and Non Excitatory Cells using Chemoreceptive Neuron MOS Transistors(CVMOS)

Transistor based techniques show tremendous potential to detect cellular events with high temporal resolution at the single cell level. We report on a label-free electronic technique using Chemoreceptive MOS transistors (CVMOS) to study the response to stimulation of excitable and non excitable cells. CVMOS charge sensors provide independent gate bias control facilitating capacitive amplification and reference electrode less operation. As a proof of concept, we use this CMOS platform to detect the response of RBL-2H3 mast cells to stimulation mediated through IgE and its high affinity cell surface receptor, FceRI, using the antigen DNP-BSA on a population of cells. I -V characteristics of the transistor and constant voltage recordings at high temporal resolution suggest changes of extracellular charge and/or capacitance upon stimulation. We observe a shift in the drain current as stimulation is initiated, followed later by current fluctuations that show a time course similar to those of amperometric recordings. The responses are dependent on the presence of extracellular calcium, suggesting that the observed changes may be linked to exocytosis. Unsensitized cells show no detectable response to antigen stimulation. Using adrenal chromaffin cells, we observed rapid current fluctuations in response to stimulation with both ionomycin and high KCl. Experiments are underway to determine whether these responses reflect stimulation action potentials and/or catecholamine release events.