Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16S. RossEECS 40 Spring 2003 Lecture 16NMOS (N-Channel Metal Oxide Semiconductor)Transistorn-typemetalmetaloxide insulatormetalp-typemetalgatesourcedrainn-typeeeeeeehhhhhhS. RossEECS 40 Spring 2003 Lecture 16NMOS Transistor in Equilibriumn-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.hhhhS. RossEECS 40 Spring 2003 Lecture 16NMOS Transistor in Cutoffn-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 > 0S. RossEECS 40 Spring 2003 Lecture 16NMOS Transistor Channeln-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.Thus the applied VGS creates an induced n-type channel under the gate (an area with free electrons).hhh h-+_ _ _hVGS > VTH(N)hhh hheeeeeS. RossEECS 40 Spring 2003 Lecture 16NMOS Transistor Drain Currentn-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__ _ _eeeeeS. RossEECS 40 Spring 2003 Lecture 16NMOS Transistorn-typemetalmetaloxide insulatormetalp-typemetalgatesourcedrainn-type-+VGS-+VDSIDIGGDSIDIG- VDS ++VGS_S. RossEECS 40 Spring 2003 Lecture 16GDSIDIG- VDS ++VGS_NMOS I-V CHARACTERISTIC•Since 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.S. RossEECS 40 Spring 2003 Lecture 16MODES 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.S. RossEECS 40 Spring 2003 Lecture 16NMOS I-V CHARACTERISTICCutoff Mode• Occurs when VGS ≤ VTH(N) ID = 0 Triode Mode• Occurs when VGS > VTH(N) and VDS < VGS - VTH(N) Saturation Mode• Occurs when VGS > VTH(N) and VDS ≥ VGS - VTH(N) DSDSTH(N)GSOXnDV/2VVVCμLWI DSn2TH(N)GSOXnDVλ1VV21CμLWI S. RossEECS 40 Spring 2003 Lecture 16triode modecutoff modesaturation modeVDSIDVGS = 3 VVGS = 2 VVGS = 1 VVDS = VGS - VTH(N)NMOS I-V CHARACTERISTICSS. RossEECS 40 Spring 2003 Lecture 16drainsourcep-typep-typemetalmetaloxide insulatormetaln-typemetalgateSame as NMOS, only p-type and n-type switchedPMOS (P-Channel Metal Oxide Semiconductor)TransistorS. RossEECS 40 Spring 2003 Lecture 16PMOS Transistor Channelmetalmetaloxide 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_+ ++++S. RossEECS 40 Spring 2003 Lecture 16PMOS Transistor Drain Currentp-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+ ++++S. RossEECS 40 Spring 2003 Lecture 16GDSIDIG- VDS ++VGS_PMOS TRANSISTORSymbol 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.S. RossEECS 40 Spring 2003 Lecture 16PMOS I-V CHARACTERISTICCutoff Mode• Occurs when VGS ≥ VTH(P) ID = 0 Triode Mode• Occurs when VGS < VTH(P) and VDS > VGS - VTH(P) Saturation Mode• Occurs when VGS < VTH(P) and VDS ≤ VGS - VTH(P) DSDSTH(P)GSOXpDV/2VVVCμLWI DSp2TH(P)GSOXpDVλ1VV21CμLWI S. RossEECS 40 Spring 2003 Lecture 16triode modecutoff modesaturation modeVDSIDVGS = -3 VVGS = -2 VVGS = -1 VVDS = VGS - VTH(P)PMOS I-V
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