Mosfet Gate capacitance

Generally, a linear SPICE primitive capacitor is not the best way to model the nonlinear capacitance of a MOSFET. The capacitance of the MOSFET s gate is dependent on the gate-to-source voltage, and to a lesser extent, the drain-to-source voltage. However, the fixed capacitance used in this simulation is adequate for our purposes. 

Figure 4.14. Cross-section high-resolution transmission electron microscopy (HRTEM) images of gate oxides for MOSFETs. Provided is an illustration of film thicknesses that result in identical capacitance for Si02( = 3.8), relative to a high- c dielectric k = 23.9) gate oxide. Hence, increased gate capacitance will result from thinner films comprising high- c dielectric materials. Reproduced with permission from Intel Corporation (http //www.intel.com).

By examination of (6.33) it becomes clear that in the absence of device area constraints if 1// noise can be avoided, it is desirable to increase the MOSFET gate width W until C3 = 3. In this case Cj includes the capacitance introduced by the switch Sj. With C3 = 3 the minimum detectable charge will be... 

Gate The control electrode of a MOSFET. The voltage on the gate capacitively modulates the resistance of the connecting channel between the source and drain. 

From a simple Wentzel-Kramers-Brillouin (WKB) approach [2], the tunneling current increases exponentially as the thickness of the Si02 decreases. MOSFET is the capacitance-operated device. The source-drain current of the MOSFET depends on the gate capacitance ... 

These capacitances are specified in each power MOSFET datasheet and are very important. Cg s, or drain-to-source capacitance, is considered in the drain loads, but does not directly enter into the drive design. The Ci s and C ss have direct and calculable effects upon the switching performance of the MOSFET. Figure 3-36 shows the gate and drain waveforms of a typical N-channel MOSFET switching cycle. 

Power MOSFE B can be driven more efficiently than bipolar power transistors. The gate of a MOSFET has two equivalent capacitors connected to the terminal, the gate-to-source capacitor (Ciss) and the drain-to-source capacitance (Crss). The loss exhibited by the gate drive of the MOSFET is created by charg-... 

In Figure 5-6, on the left, we have the basic (simplified) model of the mosfet. In particular, we observe that it has three parasitic capacitances — between its drain, source, and gate. These small interelectrode capacitances are the key to maximizing switcher efficiency, especially at higher switching frequencies. Their role in the switching transition needs to be understood clearly. 

The MOSFET is an extremely flexible circuit element because there can be n- and p-channel devices with current that increases or decreases with gate voltage. The capacitance of MOSFET structures can also be used for charge storage. The stored charge can be shifted from one capacitor to an adjacent element by control of the gate voltages. 

In the standard metal-oxide semiconductor field-effect transistor (MOSFET) drain-current equations, is a proportionality factor that relates the drain current /p, to the gate and drain voltages and V[)s, respectively the threshold voltage V, the channel width W and length L and the gate dielectric capacitance per unit area Cox. The standard drain-current equation for an -channel device in the linear region of operation - V) is [4]... 

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