Effect gating currents

Field-effect transistors (FETs) have dominated the semiconductor industry, largely displacing the earlier bipolar junction transistor (BJT) because of its negligible gate current and convenience in the design of integrated circuits. Figure 29 sketches how an FET works. 

The saturation current is denoted /sd,sat> the effective mobihty and the capacitance of the gate oxide Cox- The effective geometrical parameters of the transistor include its overall gate width, W g, and the effective gate length, Leff. 

The rates of modiflcation of an introduced cysteine are obtained by measuring the effect of successive low applications of MTS on agonist-gated current (O Figure 21-3). The optimal MTS concentration to use... 

Figure 15-28 Operation of a field effect transistor, (a) Nearly random distribution of holes and electrons in the base in the absence of gate potential. ( >) Positive gate potential attracts electrons that form a conductive channel beneath the gate. Current can flow through this channel between source and drain.

Chen, F. S. P., Steele, D., and Fedida, D. (1997). Allosteric effects of permeating cations on gating currents during K+ channel deactivation./ Gen. Physiol. 110, 87-100. 

If the effective gate length decreases, subthreshold drain current (/p) may increase, as shown by Equation 15.1 " ... 

These experiments directly reveal the change in effectiveness of ligand-gated current caused by voltage-dependent dendritic channels. Activation of inward... 

Nonner W 1979 Effects of Ceiurus scorpion venom on the gating current in myelinated nerve. Adv Cytopharmacol 3 345-352... 

In an ideal FET with no gate leakage Iq = 0) and Id = Is-The gate current can be measured while other I-V measurements are taken. The simplest measure of leakage is taken while Vds = 0 over a range of Vgs, but the gate current can be measured over a range of bias conditions and accounted for in circuits where it can have an effect. 

In each experiment before the gating current runs, we measured the / a y curve in 1 /4 Na-SW, and after correcting for the effects of the uncompensated fraction of the measured resistance in series (for further details, see Keynes and Rojas [38]), we measured the reversal potential for the sodium currents, and estimated the limiting value of gNa,n,ax-... 

Gating currents in sodium channels are measured in the presence of TTX or STX, indicating that the gating portion of the channel is distinct and probably in series with the selectivity Filter which allows only sodium (and other similarly-sized ions) through [78,79]. As TTX is effective only from the outside of excitable membranes, the selectivity filter of the channel is probably to the outside of the membrane. 

In the squid axon, channel density has been calculated from the rate of onset and offset of the effect of TTX on sodium conductance. A value of 522 sites/ jum was obtained which is almost identical to that derived from TTX binding [120]. An alternative approach has been to calculate sodium channel density from the maximal size of the gating currents in squid nerve. The result, 483/ jum, is again close to that obtained by direct measurement [121]. Of further interest are the calculations given by Hodgkin [122] who found that the optimal density of sodium channels in an umnyelinated nerve which are required to give a maximal conductance velocity was around 500/jum. ... 

While the modeling efforts of Barnes and Hu, Cain, Pickard, and Rosenbaum are not likely to reveal the precise nature of athermal coupling between field and membrane, they are useful in that correlations can conceivably be made, in the case of excitable membranes, between possible changes in membrane permeability to various ionic species, effects on gating currents, displacement of the resting potential, etc., to the frequency and intensity of the applied radiation, as well as to the duration of exposure. 

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