Oxide capacitance

Fig. 5. NMOS capacitance voltage characteristics where C is the oxide capacitance, A shows low frequency characteristics, and B shows high frequency characteristics. At low frequencies C approaches C for negative voltages (accumulation) and positive voltages (inversion). In the flat-band (FB) condition there is no voltage difference between the semiconductor s surface and bulk. The threshold voltage, Dp for channel formation is the point where the...

When Uq3 > Up the MOSEET conducts. The conduction current is deterrnined by 1 where Q is the amount of charge in the inversion layer and t is the transit time for electrons to travel from source to drain. Q = C LW (U g — Up) where C = is the gate oxide capacitance per unit area... 

Figure 19 Schematic Bode plots from EIS measurements and equivalent circuits that could be used to fit them for various possible corrosion product deposit structures (A) nonporous deposit (passive film) (B) deposit with minor narrow faults such as grain boundaries or minor fractures (C) deposit with discrete narrow pores (D) deposit with discrete pores wide enough to support a diffusive response (to the a.c. perturbation) within the deposit (E) deposit with partial pore blockage by a hydrated deposit (1) oxide capacitance (2) oxide resistance (3) bulk solution resistance (4) interfacial capacitance (5) polarization resistance (6) pore resistance (7) Warburg impedance (8) capacitance of a hydrated deposit.

AC Impedance of Contaminated Specimens. The ACIS of the contaminated sample under DC bias at 100% RH is consistent with a corroding system (15) in which a fixed number of aqueous pathways have formed, resulting in a constant area of metallization exposed to the electrolyte. In this case, the parallel capacitance corresponds to an electrical double layer of ions on the metallization. The capacitance of the contaminated sample is > 100 times larger than that of the clean sample at 100% RH due to the relatively larger concentrations of ions and water at the IC surface, which overwhelms the oxide capacitance described earlier. The reduction in the parallel resistance with increasing bias arises from the voltage dependent charge transfer process (i.e. electrochemical reaction). 

Here W is the channel width, L is the channel length, is the gate-oxide capacitance per unit area, Vq is the gate voltage, and additionally Fp is the drain voltage. Mobility /j and threshold voltage Vj were determined from the linear regression of the measured data plotted as vs.. The in-... 

Figures 8.5 and 8.6 show the voltage dependence of the corrected capacitance Q and corrected conductance G,. characteristics at 1 MHz before and after 50 MeV Li + ion irradiation for different fluences at room temperature. The peaks seen in the Figure 8.7 correspond to the depletion area of the device. The value of interface traps density (Dj is determined from this peak value. This peak was observed for samples. From C-V and G-V measurements in the accumulation region, the oxide capacitance Cox was calculated using Equation 8.5 (Nicollian and Brews 1982) ...

Fig. 7 Comparison of different electrochemical methods of estimating the partial free surface ex situ oxide capacitance, in situ electrochemical admittance, and corrosion current (determined from Rp). Differences between these values may be accounted for by the variation of surface morphology as shown in the schematic diagram. (From Ref [42].)...

The impedance of a FET with the gate immersed in solution and potential applied to a reference electrode in solution may be represented by the equivalent circuit shown in Fig. 6.5. The circuit consists of the silicon resistance Rsi, space-charge capacitance Csc, oxide capacitance Cox of the FET, and the Randles equivalent circuit for the double layer, where Zw has been omitted since there are no redox molecules in solution. In the absence of redox molecules. Ret is large and Zj ag can be considered to result from the series combination of the three capacitances. 

Cq thus increases as the well fills up and W decreases. Before the well fills up, if (2ee5N ) /Co <(KG), then the oxide capacitance will be much larger than the depletion layer capacitance and (f) will track Vq. For this reason lightly doped material and thin gate oxide layers ( 1000 A) are needed. Examination of (6.1) also shows that barriers can be permanently built into the device by varying and/or by changing the oxide thickness (which changes C ). These techniques are used to form channel stops to define the CCD channel and can also be used to provide the built-in barriers necessary for two-phase CCD operation. 

A typical CCD well of dimension 10 pm x 20 pm thus has a storage capacity of 4 X 10 electrons. Buried channel CCDs have a storage capacity a factor of two to three smaller than this [6.3]. For d = 1000A the oxide capacitance of the 10 pm X 20 pm well would be 0.07 pF. In materials other than silicon where the impurity dopant density is larger (leading to smaller depletion widths) and the bandgap is smaller the limit on Vq can be set by avalanche breakdown in the semiconductor itself or tunneling. 

The depletion layer capacitance is usually smaller than both the gate oxide capacitance and the external capacitance so that approximately... 

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