Krishna Jayant

Ferroelectric-Assisted Dual-Switching Speed DRAM–Flash Hybrid Memory

This paper presents a novel one-transistor low-voltage DRAM-Flash hybrid memory. The proposed device integrates ferroelectric thin film and nonvolatile charge injection, and demonstrates two modes of operations: 1) a fast (10-100 ns) DRAM mode with ~ 103 s of retention, associated with ferroelectric switching, and 2) a slower (0.1-1 ms) Flash mode with long retention time, from charge tunneling into the floating nodes. The time evolution of the electric field in the ferroelectric and the tunnel oxide is shown to naturally establish the two-step mechanism during the program operation. The complementary characteristics of ferroelectric switching and gate-charge injection enable low-voltage program/erase (±8 V), reasonable memory window (0.8 V), and long retention time. Devices were fabricated with the lead zirconatetitanate thin film as the ferroelectric layer and Au nanocrystals for gate-injected electron storage. Pulsed programming measurements were also performed to distinguish the memory window obtained from the two mechanisms in DRAM and Flash operations.

Label-free electronic detection of growth factor induced cellular chatter on chemoreceptive neuron MOS (CvMOS) transistors

In this paper we present a novel label-free detection scheme to sense growth factor over-expression in tumor cells at the single cell level using a chemoreceptive neuron MOSFET (C?MOS) [1]. Tumor cell lines including A431, HeLa, TSE and NIH-3T3s were dispensed onto poly-Llysine-coated sensing gates with a limited dilution and their responses to EGF (epidermal growth factor) were recorded. A change in cell shape after stimulation is observed and is validated through SEM images on witness samples. Stimulation of downstream pathways upon EGF activation is detected through an abrupt change in threshold voltage and subthreshold slope indicating cell effects from both charge exchange and capacitance.

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.

Dynamic Modeling of Dual Speed Ferroelectric and Charge Hybrid Memory

This paper presents a physical model for program and retention transients in ferroelectric (FE) and charge hybrid nonvolatile memory. A region-by-region statistical model for domain switching in polycrystalline FEs was incorporated with the tunneling current simulations to predict the memory window (AVTH) evolution during program and retention operations. The simulations validated the two-step program mechanism experimentally observed in such memories: rapid initial domain switching on account of high fields in the FE layer followed by field enhancement in the tunneling dielectric which initiates electron injection into the storage nodes. Further, these simulations were shown to accurately account for individual ΔVTH from the two additive memory mechanisms at all program times. The depolarization effect was shown to be dominant for ΔVTH loss at short and moderate retention time scales (<;100 s). This model was further used to provide realistic estimates in achieving dual speed program and the corresponding dual mode retention characteristics akin to a DRAM and flash hybrid operation.

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.

Towards DRAM-Flash hybrid: Dual-speed low-voltage ferroelectric and charge memory

We present a one-transistor low-voltage hybrid ferroelectric and charge nonvolatile memory with dual switching speed. The device can be operated in two modes: 1) fast (10 ns-1 μs) DRAM-like mode with hours of retention, benefiting from ferroelectric (FE) switching, and 2) slow Flash-like mode (100 μs-1 ms) with long retention, resulting from charge injection. The combined FE and charge mechanisms offer additive memory window (MW) and cancelling retention fields. The hybrid memory was fabricated with PZT as the FE and Au nanocrystals (NCs) as charge storage nodes. Simulations incorporating FE switching dynamics were performed to corroborate the two-step program process as well as provide useful insight into hybrid design.

The DNA transistor interface: The interplay between pH, electric field and membrane screening dictates sensitivity

When DNA binds to a transistor, the surface potential (ψo) shifts in response to charges located within a Debye length. The native surface charge and screening capacitance are often described by the Gouy-Chapman (GC) double-layer model, which uses the Poisson-Boltzmann (PB) distribution for ions. GC model neglects screening within the DNA layer and is often insufficient to explain experimentally observed Δψo (40-80mV). We show that surface buffering capacity, E-field in the underlying oxide and ion screening in the DNA layer strongly influence sensitivity and lead to Δψo values larger than the GC model prediction. We present a formulation based on the Born charge dielectric interaction and find that a lowering in permittivity within the DNA lattice leads to ion exclusion and lower screening. We find that sensitivity to DNA charge is highest when the surface is closest to the point of zero charge (PZC).

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.