Estrutura e Funcionamento de Transistores MOSFETs

Copyright 2004 by Oxford University Press Inc 1 MOS FieldEffect Transistors MOSFETs Copyright 2004 by Oxford University Press Inc Microelectronic Circuits Fifth Edition SedraSmith 2 Figure 41 Physical structure of the enhancementtype NMOS transistor a perspective view b crosssection Typically L 01 to 3 mm W 02 to 100 mm and the thickness of the oxide layer tox is in the range of 2 to 50 nm Copyright 2004 by Oxford University Press Inc Microelectronic Circuits Fifth Edition SedraSmith 3 Figure 410 a Circuit symbol for the nchannel enhancementtype MOSFET b Modified circuit symbol with an arrowhead on the source terminal to distinguish it from the drain and to indicate device polarity ie n channel c Simplified circuit symbol to be used when the source is connected to the body or when the effect of the body on device operation is unimportant Copyright 2004 by Oxford University Press Inc Copyright 2004 by Oxford University Press Inc Microelectronic Circuits Fifth Edition SedraSmith 5 Figure 42 The enhancementtype NMOS transistor with a positive voltage applied to the gate An n channel is induced at the top of the substrate beneath the gate Copyright 2004 by Oxford University Press Inc Microelectronic Circuits Fifth Edition SedraSmith 6 Figure 43 An NMOS transistor with vGS Vt and with a small vDS applied The device acts as a resistance whose value is determined by vGS Specifically the channel conductance is proportional to vGS Vt and thus iD is proportional to vGS Vt vDS Note that the depletion region is not shown for simplicity Copyright 2004 by Oxford University Press Inc Microelectronic Circuits Fifth Edition SedraSmith 7 Figure 44 The iDvDS characteristics of the MOSFET in Fig 43 when the voltage applied between drain and source vDS is kept small The device operates as a linear resistor whose value is controlled by vGS Copyright 2004 by Oxford University Press Inc Microelectronic Circuits Fifth Edition SedraSmith 8 Figure 45 Operation of the enhancement NMOS transistor as vDS is increased The induced channel acquires a tapered shape and its resistance increases as vDS is increased Here vGS is kept constant at a value Vt