Lecture 17NMOS (N-Channel Metal Oxide Semiconductor) TransistorNMOS Transistor in EquilibriumNMOS Transistor in CutoffNMOS Transistor ChannelNMOS Transistor Drain CurrentNMOS Transistor Circuit SymbolNMOS I-V CharacteristicNMOS I-V CurvesModes of OperationFaucet AnalogySlide 12NMOS EquationsPMOS (P-Channel Metal Oxide Semiconductor) TransistorPMOS Transistor ChannelPMOS Transistor Drain CurrentPMOS Transistor Circuit SymbolPMOS I-V CurvesPMOS EquationsSolving Transistor CircuitsLecture 17Today we will discussMetal Oxide Semiconductor (MOS) TransistorsPhysical structurePhysical operationCircuit symbol and current/voltage designationsModes of operationI-V RelationshipSolution of MOS circuitsn-typemetalmetaloxide insulatormetalp-typemetalgatesourcedrainn-typeeeeeeehhhhhhNMOS (N-Channel Metal Oxide Semiconductor)Transistorn-typemetalmetaloxide insulatormetalp-typemetalgatesourcedrainn-type+ + + + + +______When the transistor is left alone, some electrons from the n-type wells diffuse into the p-type material to fill holes.This creates negative ions in the p-type material and positive ions are left behind in the n-type material.hhhhNMOS Transistor in Equilibriumn-typemetalmetaloxide insulatormetalp-typemetalgatesourcedrainn-type+ + + + + +______When a small, positive VGS is applied, holes “move away” from the gate.Electrons from complete atoms elsewhere in the p-type material move to fill holes near the gate instead.hhh h-+_ _ _hVGS > 0NMOS Transistor in Cutoffn-typemetalmetaloxide insulatormetalp-typemetalgatesourcedrainn-type+ + + + + +______When VGS is larger than a threshold voltage VTH(n), the attraction to the gate is so great that free electrons collect there.The applied VGS creates an induced n-type channel under the gate (an area with free electrons).hhh h-+_ _ _hVGS > VTH(n)hhh hheeeeeNMOS Transistor Channeln-typemetalmetaloxide insulatormetalp-typemetalgatesourcedrainn-type+ + + + + +_____When a positive VDS is applied, the free electrons flow from the source to the drain. (Positive current flows from drain to source).The amount of current depends on VDS, as well as the number of electrons in the channel, channel dimensions, and material.hhh h-+hVGS > VTH(n)hhh hh-+VDS > 0__ _ _eeeeeNMOS Transistor Drain Currentn-typemetalmetaloxide insulatormetalp-typemetalgatesourcedrainn-type-+VGS-+VDSIDIGGDSIDIG- VDS ++VGS_NMOS Transistor Circuit SymbolGDSIDIG- VDS ++VGS_NMOS I-V CharacteristicSince the transistor is a 3-terminal device, there is no single I-V characteristic.Note that because of the insulator, IG = 0 A.We typically define the MOS I-V characteristic asID vs. VDS for a fixed VGS. The I-V characteristic changes as VGS changes.triode modecutoff mode (when VGS < VTH(N))saturation modeVDSIDVGS = 3 VVGS = 2 VVGS = 1 VVDS = VGS - VTH(n)NMOS I-V CurvesModes of OperationFor small values of VGS, VGS ≤ VTH(n), the n-type channel is not formed. No current flows. This is cutoff mode.When VGS > VTH(n), current ID may flow from drain to source, and the following modes of current flow are possible.The mode of current flow depends on the propelling voltage, VDS, and the channel-inducing voltage, VGS – VTH(n).When VDS < VGS – VTH(n), current is starting to flow. ID increases rapidly with increased VDS. This is triode mode.When VDS ≥ VGS – VTH(n), current is reaching its maximum value. ID does not increase much with increased VDS. This is called saturation mode.Faucet AnalogyImagine the faucet on your kitchen sink.To make water flow, the water supply has to be connected to the faucet. This establishes a path for water to flow.Setting VGS above the threshold voltage is like connecting the water supply.Cutoff = water supply disconnected (no path for current flow)Setting VGS to a larger value is like connecting a high-pressure water supply—more flow can potentially occur.Faucet AnalogyThe faucet itself is used to adjust water flow. You can turn the flow up and down.VDS is like the faucet. It controls the amount of flow.There is, of course, a saturation point. If you keep turning the faucet control, eventually you won’t get any more flow.Triode = faucet in “normal range”, controls flowSaturation = faucet turned up to (or past) point for maximum flowCutoff ModeOccurs when VGS ≤ VTH(n) ID = 0 Triode ModeOccurs when VGS > VTH(n) and VDS < VGS - VTH(n) Saturation ModeOccurs when VGS > VTH(n) and VDS ≥ VGS - VTH(n) DSV/2DSVTH(n)VGSVOXCnμLWDI DSVnλ12TH(n)VGSV21OXCnμLWDI NMOS Equationsdrainsourcep-typep-typemetalmetaloxide insulatormetaln-typemetalgateSame as NMOS, only p-type and n-type switchedPMOS (P-Channel Metal Oxide Semiconductor)Transistormetalmetaloxide insulatormetalmetalgatesourcedrainWhen VGS is more negative than a threshold voltage VTH(p), the gate attracts many positive ions and holes (repels electrons)Thus the applied VGS creates an induced p-type channel under the gate (an area with positive ions).-+VGS < VTH(p) < 0p-typen-typep-type+ + ++ + +_____eee eeeee eehhhhh_+ ++++PMOS Transistor Channelp-typemetalmetaloxide insulatormetaln-typemetalgatesourcedrainp-type+ + ++ + +_____When a negative VDS is applied, the positive ions flow from the source to the drain. (Positive current flows from source to drain).The amount of current depends on VDS, as well as the number of ions in the channel, channel dimensions, and material.eee e-+eVGS < VTH(P) < 0eee ee-+VDS < 0_hhhhh+ ++++PMOS Transistor Drain CurrentGDSIDIG- VDS ++VGS_PMOS Transistor Circuit SymbolSymbol has “dot” at gate. NMOS does not.ID, VGS, VDS, and VTH(p) are all negative.These values are positive for NMOS.Channel formed when VGS < VTH(p). Opposite for NMOS.Saturation occurs when VDS ≤ VGS – VTH(p). Opposite for NMOS.triode modecutoff modesaturation modeVDSIDVGS = -3 VVGS = -2 VVGS = -1 VVDS = VGS - VTH(p)PMOS I-V CurvesCutoff ModeOccurs when VGS ≥ VTH(p) ID = 0 Triode ModeOccurs when VGS < VTH(p) and VDS > VGS - VTH(p) Saturation ModeOccurs when VGS < VTH(p) and VDS ≤ VGS - VTH(p) DSV/2DSVTH(p)VGSVOXCpμLWDI DSVpλ12TH(p)VGSV21OXCpμLWDI PMOS EquationsSolving Transistor CircuitsGuess the transistor mode (for each transistor).Sometimes you can make educated guessWrite down the I-V relationships that go with those modes: 1 equation, 3 unknowns (ID,
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