Subthreshold Current Research Articles

Reptitive Firing In Neurons - Analysing The Interaction Between Channel Density And Kinetics In Membrane Models

More than sixty years after Alan Hodgkin presented his classification of firing patterns in the axons of the crab Carcinus maenas, the underlying mechanisms of the firing patterns are still only fragmentarily understood. Two main types have been discerned in neurons and dynamical membranes models. Type 1 shows a continuous frequency-stimulation current (f-I) relationship and thus an arbitrarily low frequency at threshold current, while Type 2 shows a discontinuous f-I relationship and a minimum frequency. Type 1 obtains rhythmicity via a saddle-node bifurcation, thus requiring three stationary potentials at subthreshold stimulation current. Type 2 obtains rhythmicity via a Hopf or double-orbit bifurcation. In a previous investigation of a hippocampal neuron model we showed that the membrane density of critical ion channels could regulate the bifurcation type and consequently the threshold dynamics. In the present study we extend our previous analysis to other quantitatively well-described excitable membranes. These studies show that not merely the channel density, but the overall structure of the phase space around the stationary potentials determine the onset frequency. We show, by means of techniques from nonlinear dynamical system theory, that this phase space is altered both by changes in channel density and channel kinetics. Understanding these interactions is an important step towards understanding global oscillatory activity in brain networks.

Evidence for a Critical Period in the Development of Excitability and Potassium Currents in Mouse Lumbar Superficial Dorsal Horn Neurons

The output of superficial dorsal horn (SDH; laminae I-II) neurons is critical for processing nociceptive, thermal, and tactile information. Like other neurons, the combined effects of synaptic inputs and intrinsic membrane properties determine their output. It is well established that peripheral synaptic inputs to SDH neurons undergo extensive reorganization during pre- and postnatal development. It is unclear, however, how membrane properties or the subthreshold whole cell currents that shape SDH neuron output change during this period. Here we assess the intrinsic membrane properties and whole cell currents in mouse SDH neurons during late embryonic and early postnatal development (E15-P25). Transverse slices were prepared from lumbar spinal cord and whole cell recordings were obtained at 32 degrees C. During this developmental period resting membrane potential (RMP) became more hyperpolarized (by approximately 10 mV, E15-E17 vs. P21-P25) and input resistance decreased (1,074 +/- 78 vs. 420 +/- 27 MOmega). In addition, action potential (AP) amplitude and AP afterhyperpolarization increased, whereas AP half-width decreased. Before and after birth (E15-P10), AP discharge evoked by intracellular current injection was limited to a single AP at depolarization onset in many neurons (>41%). In older animals (P11-P25) this changed, with AP discharge consisting of brief bursts at current onset ( approximately 46% of neurons). Investigation of major subthreshold whole cell currents showed the rapid A-type potassium current (I(Ar)) dominated at all ages examined (90% of neurons at E15-E17, decreasing to >50% after P10). I(Ar) expression levels, based on peak current amplitude, increased during development. Steady-state inactivation and activation for I(Ar) were slightly less potent in E15-E17 versus P21-P25 neurons at potentials near RMP (-55 mV). Together, our data indicate that intrinsic properties and I(Ar) expression change dramatically in SDH neurons during development, with the greatest alterations occurring on either side of a critical period, P6-P10.

Analytical modeling for the estimation of leakage current and subthreshold swing factor of nanoscale double gate FinFET device

PurposeThe aim of this paper is to formulate the effect of the process variation on various leakage currents and subthreshold swing factor in FinFET devices. These variations cause a large spread in leakage power, since it is extremely sensitive to process variations, which in turn results in larger temperature variations across different dies.Design/methodology/approachOwing to large temperature variation within the die, the authors investigate the variation of various leakage currents with absolute die temperature.FindingsThe results obtained on the basis of the model are compared and contrasted with reported numerical and experimental results. A close match was found which validates the analytical approach.Originality/valueThe analytical modeling of subthreshold leakage current, subthreshold swing, gate leakage current and its variation with process parameters are carried out in this paper.

Reliable Bottom Gate Amorphous Indium–Gallium–Zinc Oxide Thin-Film Transistors with TiO[sub x] Passivation Layer

Titanium oxide (TiO x ) passivation layer was employed and optimized to stabilize the performance of the bottom gate amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). A molybdenum/titanium (Mo/Ti) source-drain electrode was deposited on an a-IGZO layer, and the TiO, passivation layer was formed by oxidizing the Ti layer using oxygen plasma after etching the Mo layer. By increasing the oxygen plasma treatment time, the subthreshold slope and leakage current of the a-IGZO TFTs were improved to 0.78 V decade -1 and 0.3 pA, respectively, and the degradation of the TFT performance was not observed, even after thermal treatment at 280°C for 1 h.

A Conditional Isolation Technique for Low-Energy and High-Performance Wide Domino Gates

A new conditional isolation technique (CI-Domino) in domino logic is proposed for wide domino gates. This technique can not only reduce the subthreshold and gate oxide leakage currents simultaneously without sacrificing circuit performance, but also it can be utilized to speed up the evaluation time of domino gate. Simulations on high fan-in domino OR gates with 0.18µm process technology show that the proposed technique achieves reduction on total static power by 36%, dynamic power by 49.14%, and delay time by 60.27% compared to the conventional domino gate. Meanwhile, the proposed technique also gains about 48.14% improvement on leakage tolerance.

From tonic firing to bursting in a model for thermosensitive neurons

Event Abstract Back to Event From tonic firing to bursting in a model for thermosensitive neurons Christian Finke1*, Epaminondas Rosa2, Jan A. Freund1, Hans A. Braun3 and Ulrike Feudel1 1 University of Oldenburg, ICBM, Germany 2 Illinois State University, United States 3 University of Marburg, Germany We study the dynamics of the model for thermosensitive neurons originally developed by Huber and Braun [1] which mimics the response of a neuron to temperature changes. The mechanism of spike generation in this model is based on the introduction of slow repolarizing and depolarizing currents leading to subthreshold oscillations. Depending on several control parameters such as temperature, external current and the impact of subthreshold oscillations, the model exhibits a variety of different dynamical behaviours. The most prominent ones are tonic firing as well as regular and chaotic bursting. Variations of the control parameters lead to characteristic changes in the dynamics of the neuron, the most important one being the transition from tonic firing to bursting. We show that regardless of the control parameter considered, a homoclinic bifurcation which is related to extremely large interspike intervals appears to be connected with this transition. Our emphasis lies on the role of subthreshold oscillations leading to different dynamical characteristics of the tonic firing and bursting regimes, respectively. Moreover, the impact of the subthreshold oscillations can be measured in terms of their amplitude. In the absence of external currents, a decreasing amplitude finally yields a disappearance of spikes. Along this route, the neuron dynamics undergoes a series of systematic changes underlining that variations in the amplitude of subthreshold currents are of high physiological importance. We compare this scenario to the dynamics emerging from variations in temperature and an applied external current as control parameters. Furthermore, we discuss the rationale behind the consideration of different control parameters for coupled neurons in networks and the response of a neuron to external currents which have certain temporal characteristics.

Punchthrough-Voltage Enhancement of AlGaN/GaN HEMTs Using AlGaN Double-Heterojunction Confinement

In this paper, we present an enhancement of punchthrough voltage in AlGaN/GaN high-electron-mobility-transistor devices by increasing the electron confinement in the transistor channel using an AlGaN buffer-layer structure. An optimized electron confinement results in a scaling of punchthrough voltage with device geometry and a significantly reduced subthreshold drain leakage current. These beneficial properties are pronounced even further if gate-recess technology is applied for device fabrication. Physical-based device simulations give insight in the respective electronic mechanisms.

The Effects of Proton Irradiation on the Performance of High-Voltage n-MOSFETs Implemented in a Low-Voltage SiGe BiCMOS Platform

This paper presents the first comprehensive investigation of the impact of proton irradiation on the performance of high-voltage (HV) nMOS transistors implemented in a low-voltage (LV) SiGe BiCMOS technology. The effects of irradiation gate bias, irradiation substrate bias, and operating substrate bias on the radiation response of these transistors are examined. Experimental results show that the radiation-induced subthreshold leakage current under different irradiation biasing conditions remains negligible after exposure to a total dose of 600 krad(Si). We find that there are differences in the radiation response of LV and HV MOSFETs, suggesting that the mechanisms involved in causing degradation in LV and HV transistors could be of fundamentally different origins.

Modeling of the subthreshold current and subthreshold swing of fully depleted short-channel Si–SOI-MESFETs

A model for the subthreshold current and subthreshold swing of fully depleted short-channel Si–SOI-MESFETs is presented in the paper. The subthreshold current of the device is modeled empirically by an exponential function of gate–source and drain–source voltages. The short-channel effects have been included in terms of some empirical constants in the model which have been finally derived from the two-dimensional potential distribution function of the device. The subthreshold swing characteristics of the device are obtained using the subthreshold current model. The validity of the proposed model is shown by comparing the theoretical data with the simulation results obtained by using the ATLAS™ device simulation software.

Semi‐analytical modelling of short channel effects in Si double gate, tri‐gate and gate all‐around MOSFETs

AbstractAn analytical expression for the three‐dimensional potential distribution along the channel of short channel silicon multi‐gate MOSFETs is described in weak inversion. The analytical solutions cover three different cases of multi‐gate devices: Symmetrical double gate (DG), tri‐gate (TG) and gate‐all‐around (GAA) MOSFETs. Using the three‐dimensional potential distribution in the silicon film, the subthreshold drain current is calculated from which the threshold voltage and the subthreshold swing were extracted. The threshold voltage roll‐off, the subthreshold swing and the drain‐induced barrier lowering of the multi‐gate devices are compared, showing the critical advantage of GAA structures concerning the short channel effects. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamic, Nonlinear Feedback Regulation of Slow Pacemaking by A-Type Potassium Current in Ventral Tegmental Area Neurons

We analyzed ionic currents that regulate pacemaking in dopaminergic neurons of the mouse ventral tegmental area by comparing voltage trajectories during spontaneous firing with ramp-evoked currents in voltage clamp. Most recordings were made in brain slice, with key experiments repeated using acutely dissociated neurons, which gave identical results. During spontaneous firing, net ionic current flowing between spikes was calculated from the time derivative of voltage multiplied by cell capacitance, signal-averaged over many firing cycles to enhance resolution. Net inward interspike current had a distinctive nonmonotonic shape, reaching a minimum (generally <1 pA) between -60 and -55 mV. Under voltage clamp, ramps over subthreshold voltages elicited a time- and voltage-dependent outward current that peaked near -55 mV. This current was undetectable with 5 mV/s ramps and increased steeply with depolarization rate over the range (10-50 mV/s) typical of natural pacemaking. Ramp-evoked subthreshold current was resistant to alpha-dendrotoxin, paxilline, apamin, and tetraethylammonium but sensitive to 4-aminopyridine and 0.5 mM Ba2+, consistent with A-type potassium current (I(A)). Same-cell comparison of currents elicited by various ramp speeds with natural spontaneous depolarization showed how the steep dependence of I(A) on depolarization rate results in small net inward currents during pacemaking. These results reveal a mechanism in which subthreshold I(A) is near zero at steady state, but is engaged at depolarization rates >10 mV/s to act as a powerful, supralinear feedback element. This feedback mechanism explains how net ionic current can be constrained to <1-2 pA but reliably inward, thus enabling slow, regular firing.

Biophysical Basis for Three Distinct Dynamical Mechanisms of Action Potential Initiation

Transduction of graded synaptic input into trains of all-or-none action potentials (spikes) is a crucial step in neural coding. Hodgkin identified three classes of neurons with qualitatively different analog-to-digital transduction properties. Despite widespread use of this classification scheme, a generalizable explanation of its biophysical basis has not been described. We recorded from spinal sensory neurons representing each class and reproduced their transduction properties in a minimal model. With phase plane and bifurcation analysis, each class of excitability was shown to derive from distinct spike initiating dynamics. Excitability could be converted between all three classes by varying single parameters; moreover, several parameters, when varied one at a time, had functionally equivalent effects on excitability. From this, we conclude that the spike-initiating dynamics associated with each of Hodgkin's classes represent different outcomes in a nonlinear competition between oppositely directed, kinetically mismatched currents. Class 1 excitability occurs through a saddle node on invariant circle bifurcation when net current at perithreshold potentials is inward (depolarizing) at steady state. Class 2 excitability occurs through a Hopf bifurcation when, despite net current being outward (hyperpolarizing) at steady state, spike initiation occurs because inward current activates faster than outward current. Class 3 excitability occurs through a quasi-separatrix crossing when fast-activating inward current overpowers slow-activating outward current during a stimulus transient, although slow-activating outward current dominates during constant stimulation. Experiments confirmed that different classes of spinal lamina I neurons express the subthreshold currents predicted by our simulations and, further, that those currents are necessary for the excitability in each cell class. Thus, our results demonstrate that all three classes of excitability arise from a continuum in the direction and magnitude of subthreshold currents. Through detailed analysis of the spike-initiating process, we have explained a fundamental link between biophysical properties and qualitative differences in how neurons encode sensory input.

Semianalytical Modeling of Short-Channel Effects in Lightly Doped Silicon Trigate MOSFETs

A simple analytical expression of the 3-D potential distribution along the channel of lightly doped silicon trigate MOSFETs in weak inversion is derived, based on a perimeter-weighted approach of symmetric and asymmetric double-gate MOSFETs. The analytical solution is compared with the numerical solution of the 3-D Poisson's equation in the cases where the ratios of channel length/silicon thickness and channel length/channel width are ges 2. Good agreement is achieved at different positions within the channel. The perimeter-weighted approach fails at the corner regions of the silicon body; however, by using corner rounding and undoped channel to avoid corner effects in simulations, the agreement between model and simulation results is improved. By using the extra potential induced in the silicon film due to short-channel effects, the subthreshold drain current is determined in a semianalytical way, from which the subthreshold slope, the drain-induced barrier lowering, and the threshold voltage are extracted.

Persistent Sodium Current Contributes to Induced Voltage Oscillations in Locomotor-Related Hb9 Interneurons in the Mouse Spinal Cord

Neurochemically induced membrane voltage oscillations and firing episodes in spinal excitatory interneurons expressing the HB9 protein (Hb9 INs) are synchronous with locomotor-like rhythmic motor outputs, suggesting that they contribute to the excitatory drive of motoneurons during locomotion. Similar to central pattern generator neurons in other systems, Hb9 INs are interconnected via electrical coupling, and their rhythmic activity does not depend on fast glutamatergic synaptic transmission. The primary objective of this study was to determine the contribution of fast excitatory and inhibitory synaptic transmission and subthreshold voltage-dependent currents to the induced membrane oscillations in Hb9 INs in the postnatal mouse spinal cord. The non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) reduced the amplitude of voltage oscillations but did not alter their frequency. CNQX suppressed rhythmic motor activity. Blocking glycine and GABAA receptor-mediated inhibitory synapses as well as cholinergic transmission did not change the properties of CNQX-resistant membrane oscillations. However, disinhibition triggered new episodes of slow motor bursting that were not correlated with induced locomotor-like rhythms in Hb9 INs. Our observations indicated that fast excitatory and inhibitory synaptic inputs did not control the frequency of induced rhythmic activity in Hb9 INs. We next examined the contribution of persistent sodium current (INaP) to subthreshold membrane oscillations in the absence of primary glutamatergic, GABAergic and glycinergic synaptic drive to Hb9 INs. Low concentrations of riluzole that blocked the slow-inactivating component of sodium current gradually suppressed the amplitude and reduced the frequency of voltage oscillations. Our finding that INaP regulates locomotor-related rhythmic activity in Hb9 INs independently of primary synaptic transmission supports the concept that these neurons constitute an integral component of the rhythmogenic locomotor network in the mouse spinal cord.

Spike-triggered averages for passive and resonant neurons receiving filtered excitatory and inhibitory synaptic drive

A path-integral approach is developed for the analysis of spike-triggered average quantities in neurons with voltage-gated subthreshold currents. Using a linearization procedure to reduce the models to the generalized integrate-and-fire form, analytical expressions are obtained in an experimentally relevant limit of fluctuation-driven firing. The influences of voltage-gated channels as well as excitatory and inhibitory synaptic filtering are shown to affect significantly the neuronal dynamics prior to the spike. Analytical forms are given for all relevant physiological quantities, such as the mean voltage triggered to the spike, mean current flowing through voltage-gated channels, and the mean excitatory and inhibitory conductance waveforms prior to a spike. The mathematical results are shown to be in good agreement with numerical simulations of the underlying nonlinear conductance-based models. The method promises to provide a useful analytical tool for the prediction and interpretation of the temporal structure of spike-triggered averages measured experimentally.

Investigation of Thermal Crosstalk Between SOI FETs by the Subthreshold Sensing Technique

Experimental-modeling investigation of the transient thermal crosstalk between the field-effect transistors implemented on a silicon-on-insulator substrate is reported. The measurements were performed using a high-speed electrical pulse-probe sampling technique, which allowed detection of thermally modulated subthreshold currents. The technique achieved a temperature resolution of ~50 mK, a time resolution of 5 ns, and a temperature sensitivity of ~0.6 muA/K. The finite-element method was used to solve the heat diffusion equation and to obtain the temperature profiles for the given device structures. The combined high-resolution experimental-simulation approach allowed the study of the thermal crosstalk between two adjacent devices and probe the local temperature at different locations of the structure. The effects of the interface quality, layer thickness, material selection, and interdevice spacing on the heat diffusion and device performance were investigated in detail.

Physiological types of substantia gelatinosa neurons in the rat spinal cord

Electrophysiological properties of neurons in the substantia gelatinosa (SG, or lamina II) were studied in vitro in spinal cord slices from 3-to 5-week-old rats. Based on the type of action potentials (APs) firing in response to long depolarization (0.5 to 0.8 sec), neurons were categorized into three types: tonic (APs were generated over the whole duration of the stimulus, n = 26, or 41.2%), adapting (a few APs occurred only at the beginning of stimulation, n = 8, 12.7%), and delayed-firing neurons, DFNs (APs occurred at the end of stimulation, n = 22, 35.1%); 11% of the cells had intermediate properties. Neurons of each type expressed distinct ion currents that were subthreshold for AP generation (< −40 mV). Tonic and adapting neurons either had no subthreshold currents (n = 21, or 61.3%) or expressed T-type calcium currents (n = 13, or 38.7%). All DFNs had outward A-type potassium currents. Statistical analysis confirmed this classification scheme: neurons of each type were differentially distributed in a 3-D parametric space of the main cellular properties. Distributions of tonic and adapting neurons partially overlapped, while that of DFNs differed significantly from both the above groups. It is suggested that DFNs perform a special function in the processing of sensory information; the functions of tonic and adapting neurons might be rather similar to each other.

Improvement of n-channel metal-oxide-semiconductor transistors by tensile stress despite increase in both on and subthreshold off currents

We found that tensile stress actually increases both the on current and the subthreshold off current for n-channel metal-oxide-semiconductor transistors because of an increase in mobility. Our theory is that stress engineering works because the increase in the subthreshold off current can be easily offset by a slight increase in the saturation threshold voltage, while the increase in the on current can be offset only by a much larger increase in the saturation threshold voltage. In this paper, experimental variation in the saturation threshold voltage is achieved by the statistical variation in gate length and short-channel effect. Thus, the overall effect is an improvement.

An Adaptive Body-Bias Generator for Low Voltage CMOS VLSI Circuits

A CMOS body-bias generating circuit has been designed for generating adaptive body-biases for MOSFETs in CMOS circuits for low voltage operation. The circuit compares the frequency of an internal ring oscillator with an external reference clock. When the reference clock is “high,” forward body-bias is generated. When the reference clock is “low,” a reverse body-bias is generated. The forward body bias is limited to no more than 0.4 V to avoid CMOS latchup. The reverse body bias is limited to 0.4 V and is very effective in suppressing the subthreshold current. The frequency adaptive body-bias generator circuit has been implemented in standard 1.5 μm n-well CMOS technology and simulated using SPICE. Excellent agreement is obtained between the simulated output characteristics and the corresponding experimentally measured behavior. It is also demonstrated that up to 90% leakage current in CMOS circuits can be reduced by applying the adaptive bias generator to lower threshold voltage CMOS circuits. The design is simple and can be embedded in low power CMOS designs such as the physical nodes of wireless sensor networks.

Temperature-adaptive voltage tuning for enhanced energy efficiency in ultra-low-voltage circuits

Circuits optimized for minimum energy consumption operate typically in the subthreshold regime with ultra-low power-supply voltages. Speed of a subthreshold logic circuit is enhanced with an increase in the die temperature. The excessive timing slack observed in the clock period of subthreshold logic circuits at elevated temperatures provides opportunities to lower the active-mode energy consumption. A temperature-adaptive dynamic-supply voltage-tuning technique is proposed in this paper to reduce the high-temperature energy consumption without degrading the clock frequency in ultra-low-voltage subthreshold logic circuits. Results indicate that the energy consumption can be lowered by up to 40% by dynamically scaling the supply voltage at elevated temperatures. An alternative technique based on temperature-adaptive reverse body bias to exponentially reduce the subthreshold leakage currents at elevated temperatures is also investigated. The active-mode energy consumption with two temperature-adaptive voltage-tuning techniques is compared. The impact of the process parameter and supply voltage variations on the proposed temperature-adaptive voltage scaling techniques is evaluated.