Drain-to-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-... 

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 6.4.4 The gate-source capacitance and the gate-drain capacitance Cqj, are each composed of two components, one due to the overlap between gate and source/drain and the other due to the capacitance between gate and channel, which is partitioned between source and drain. 

A similar situation exists in an insulated gate structure the difference between the WF of the semiconductor and the gate metal adjacent to the insulator causes a deficiency or an excess of electrons in the semiconductor at its interface with the insulator. In the semiconductor/insulator/metal diodes the amount of the excess charge at the semiconductor plate is determined from the capacitance-voltage curves while in a field-effect transistor the excess charge at the semiconductor/insulator interface is related to the magnitude of the drain-to-source current [13]. In either case the observed WF difference contains the contribution from both the bulk and from the surface (or interface). 

The metal-oxide-semiconductor field-effect transistor (MOSFET) is a transistor that uses a control electrode, the gate, to capacitively modulate the conductance of a surface channel joining two end contacts, the source and the drain. The gate is separated from the semiconductor body underlying the gate by a thin gate insulator, usually silicon dioxide. The surface channel is formed at the interface between the semiconductor body and the gate insulator, see Fig. 7.25. 

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... 

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