Subthreshold current

Harris-Warrick, R.M., L. Coniglio, R.M. Levini, S. Gueron, and ). Guckenheimer (1995b). Dopamine modulation of two subthreshold currents produces phase shifts in activity of an identified motoneuron. /. Neurophysiol. 74,1404-1420. 

Doubtless, it will not be long before similar demonstrations of unit conductance can be made for other biological systems. There is already some evidence to show that, when subthreshold currents are passed through neural membranes, the potential adjustment proceeds by a series of quantal steps which probably corresponds to the opening of discrete channels [39]. 

Knowledge of the distribution of the electric field in the channel is very useful for a number of reasons. In many organic materials, the mobiUty is electric-field dependent, and an accurate model of the current-voltage characteristics must consider the influence of the electric field on the mobility. The numerically computed potential profile for L— 1.5 m channel length a-6T TFT is shown in Fig. 4. In this device the gate dielectric thickness is 300 nm, and the I-V characteristics exhibit pronounced short-channel effects. As noted in the previous sub-section, short-channel effects are a set of effects which become prominent when the electric field along the channel becomes comparable to the gate field (perpendicular to the channel). The main effects on the transistor characteristics are (1) the saturation characteristics are adversely affected, and, (2) the subthreshold current is increased. 

Figure 9.7a shows an example of a-Si H TFT transfer characteristics in linear regime with different W/L ratios and an example of a-Si H TFT characteristics in saturation regime with W/L = 170 pm/6 kpm. A threshold voltage (Vth) of 10 = 11 V, a field-effect mobility (p) of 0.2 0.3 cm2/(V s), a subthreshold swing slope of 0.8 dec/V, and a current ON/OFF ratio of larger than 106 for VGS from —10 to 30 V were obtained from these curves for a-Si H TFTs. These devices were used in 4-a-Si H TFTs AM-PLEDs. The electrical properties of a-Si H TFTs used in 3-a-Si H TFTs AM-PLEDs are described in Ref. [18]. 

FIGURE 9.8 (a) An example of the BTS data for driver a-Si H TFT. The inset shows the threshold voltage and subthreshold slope shift versus the stress time. (From Kim, J.-H., Lee, D., and Kanicki, J., Proc. 22nd Int. Display Res. Conf.,601,2002. With permission.) (b) The output current density changes versus threshold voltage shifts of a-Si H TFT driver for our pixel electrode circuit. (From Kim, J.-H., Lee, D., and Kanicki, J., Proc. 22ndInt. Display Res. Conf., 601, 2002. With permission.)... 

A small overlap in the voltage dependence of activation and inactivation leads to a persistent sodium current, also referred to as a window current. This current is very small compared to the peak current and may go unnoticed in typical whole cell current recordings. However, this current may play an important role in amplifying subthreshold depolarizations and thus in controlling excitability [7]. 

Slow and persistent Na+ channels represented by INa are responsible for regenerative events underlying subthreshold ramp depolarizations and action potential firing in spiny projection neurons. This current normally produces a depolarizing prepotential, just before the action potential. The prepotential is sensitive to the sodium-channel blocker, TTX but not to calcium channel blockers (Bargas et al., 1989). It is responsible for the... 

Nisenbaum ES, Wilson CJ (1995b) The role of potassium currents in the subthreshold responses of neostriatal spiny projection neurons. In Ariano MA, Surmeier DJ (Eds), Molecular and Cellular Mechanisms of Neostriatal Function, pp. 165-181. R.G. Landes Company, Austin. 

Perhaps the most crucial component of the transistor structure is the gate oxide, which serves as the insulator between the gate and channel. Ideally, the oxide layer should be as thin as possible to increase the channel conductivity when the transistor is on, and reduce subthreshold leakage when the transistor is off. Since the first MOSFETs were demonstrated, the gate length has steadily decreased to a current size of less than 30 nm (Figure4.13). For the current 65 nm technology node, ... 

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