C H A P T E R 5 MOS Field-Effect Transistors (MOSFETs)
advertisement
CHAPTE R 5 MOS Field-Effect Transistors (MOSFETs) Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Device Structure Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Operation with Zero Gate Voltage Two back-to-back diodes, High Resistance (Giga Ohms), No Current Flow Copyright © 2010 by Oxford University Press, Inc. Sedra/Smith Microelectronic Circuits, Sixth Edition Creating a Channel for Current Flow -vGS (gate to source voltage) -Vt: threshold voltage (vGS which channel starts conducting) -Current flowing when vDS applied -Effective voltage (or overdrive voltage): vGS − Vt ≡ vOV ≡ veff -Charge in the channel: Q = Cox (WL)vOV -Oxide capacitance: -Gate to source capacitance: Microelectronic Circuits, Sixth Edition Cox = ε ox tox C = CoxWL Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Applying a small vDS Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Applying a small vDS W iD = [( µ nCox )( )vOV ]vDS L W iD = [( µ nCox )( )(vGS − Vt ]vDS L Conductance gDS of the channel: g DS W = ( µ nCox )( )(vGS − Vt ) L Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Applying a small vDS Process transconductance parameter k 'n = µ nCox MOSFET transconductance parameter W k n = µ nCox ( MOSFET behaves as a linear resistance L ) rDS = rDS Microelectronic Circuits, Sixth Edition 1 g DS 1 = W ( µ nCox )( )(vGS − Vt ) L Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Operation as vDS is increased Channel becomes more tapered and its resistance increases Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Operation for vDS>>VOV Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. P-Channel MOSFET Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. P-Channel MOSFET Threshold voltage vGS ≤ Vtp Use absolute value vGS ≥ Vtp P-Channel transistor process transconductance parameter k 'n = µ nCox P-Channel transistor transconductance parameter “Formulae are the same, switch sign of voltages” W k n = µ n Cox ( ) L Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Complementary MOS or CMOS Figure 5.10 Cross-section of a CMOS integrated circuit. Note that the PMOS transistor is formed in a separate n-type region, known as an n well. Another arrangement is also possible in which an n-type body is used and the n device is formed in a p well. Not shown are the connections made to the p-type body and to the n well; the latter functions as the body terminal for the p-channel Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Microelectronic Circuits, Sixth Edition device. Figure 5.11 (a) Circuit symbol for the n-channel enhancement-type MOSFET. (b) Modified circuit symbol with an arrowhead on the source terminal to distinguish it from the drain and to indicate device polarity (i.e., n channel). (c) Simplified circuit symbol to be used when the source is connected to the body Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Microelectronic Circuits, Sixth Edition or when the effect of the body on device operation is unimportant. Table 5.1 Regions of Operation of the Enhancement NMOS Transistor Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Figure 5.12 The relative levels of the terminal voltages of the enhancement NMOS transistor for operation in the triode region and in the saturation region. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Figure 5.15 Large-signal equivalent-circuit model of an nchannel MOSFET operating in the saturation Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Figure 5.19 (a) Circuit symbol for the p-channel enhancement-type MOSFET. (b) Modified symbol with an arrowhead on the source lead. (c) Simplified circuit symbol for the case where the source is connected to the body. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Table 5.2 Regions of Operation of the Enhancement PMOS Transistor Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Figure 5.20 The relative levels of the terminal voltages of the enhancement-type PMOS transistor for operation in the triode region and in the saturation region. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Figure E5.7 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Example 5.3. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Example 5.4 Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Example 5.6.. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc. Example 5.7. Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.
