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.
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
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.