TAILIEUCHUNG - Analog Integrated Circuit Design P2

Before proceeding, it is worth discussing the terms weak, moderate, and strong inversion. As just discussed, a gate-source voltage greater than V,, results in an inverted channel, and drain-source current can flow. However, as the gate-source voltage is increased, the channel does not become inverted (i .e., n-region) suddenly, but rather gradually. | 24 Chapter 1 Integrated-Circuit Devices and Modelling Fig. The Iq versus VGg curve for an ideal MOS transistor. For Vos DS-sat Id s approximately constant. Before proceeding it is worth discussing the terms Wflk moderate and strong inversion. As just discussed a gate-source voltage greater than Vtn results in an inverted channel and drain-source current can flow. However as the gate-source voltage is increased the channel does not become inverted . n-region suddenly but rather gradually. Thus it is useful to define three regions of channel inversion with respect to the gate-source voltage. In most circuit applications noncutoff MOS-FET transistors are operated in strong inversion with Veif 100 mV many prudent circuit designers use a minimum value of 200 mV . As the name suggests strong inversion occurs when the channel is strongly inverted. It should be noted that all the equation models in this section assume strong inversion operation. Weak inversion occurs when VGS is approximately 100 mV or more below vtn and is discussed as subthreshold operation in Section . Finally moderate inversion is the region between weak and strong inversion. Large-Signal Modelling The triode region equation for a MOS transistor relates the drain current to the gatesource and drain-source voltages. It can be shown see Appendix that this relationship is given by . VGS Id - n ox VGS-Vtn VDS L As VDS increases ID increases until the drain end of the channel becomes pinched off and then ID no longer increases. This pinch-off occurs for VDG -Vtn or approximately Vds - VGS- Vtn - Veff Right at the edge of pinch-off the drain current resulting from and the drain current in the active region which to a first-order approximation is constant with MOS Transistors 25 respect to VDS must have the same value. Therefore the active region equation can be found by substituting into resulting in Id For VDS Veff the current stays constant at the value given by

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