VTU eNotes On Fundamentals of CMOS VLSI (Electronics and Communication)

VTU eNotes On Fundamentals of CMOS VLSI (Electronics and Communication)
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VTU eNotes On Fundamentals of CMOS VLSI (Electronics and Communication)

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Introduction A MOS transistor is a majority-carrier device, in which the current in a conducting channel between the source and the drain is modulated by a voltage applied to the gate. Symbols
Figure 1 symbols of various types of transistors.
NMOS n-type MOS transistor 1 Majority carrier electrons 2 A positive voltage applied on the gate with respect to the substrate enhances the number of electrons in the channel and hence increases the conductivity of the channel. 3 If gate voltage is less than a threshold voltage Vt , the channel is cut-off very low current between source drain . PMOS p-type MOS transistor 1 Majority carrier holes 2 Applied voltage is negative with respect to substrate.
1
Relationship between Vgs and Ids, for a fixed Vds
Figure 2 graph of Vgs vs Ids
Devices that are normally cut-off with zero gate bias are classified as "enhancementmode "devices. Devices that conduct with zero gate bias are called "depletion-mode"devices. Enhancement-mode devices are more popular in practical use. Threshold voltage Vt The voltage at which an MOS device begins to conduct "turn on" . The threshold voltage is a function of 1 Gate conductor material 2 Gate insulator material 3 Gate insulator thickness 4 Impurity at the silicon-insulator interface 5 Voltage between the source and the substrate Vsb 6 Temperature MOS equations Basic DC equations Three MOS operating regions are Cutoff or subthreshold region, linear region and saturation region. The following equation describes all these three regions
2
where is MOS transistor gain and it is given by tox W L again is the mobility of the charge carrier is the permittivity of the oxide layer. tox is the thickness of the oxide layer. W is the width of the transistor. shown in diagram L is the channel length of the transistor. shown in diagram
Diagram just to show the length and width of a MOSFET.