Physics of MOS Transistors - Analog Circuits - Lecture Slides, Slides of Computer Science

Download Physics of MOS Transistors - Analog Circuits - Lecture Slides and more Slides Computer Science in PDF only on Docsity! Chapter 6 Physics of MOS Transistors • 6.1 Structure of MOSFET • 6.2 Operation of MOSFET • 6.3 MOS Device Models • 6.4 PMOS Transistor • 6.5 CMOS Technology • 6.6 Comparison of Bipolar and CMOS Devices Docsity.com Chapter Outline Operation of MOSFETs MOS Device Models PMOS Devices ® MOS Structure © Large-Signal Model ® Structure © Operation in Triode Region = ® Small-Signal Model ® Models ® Operation in Saturation ® I/V Characteristics ® Docsity.com State of the Art MOSFET Structure  The gate is formed by polysilicon, and the insulator by Silicon dioxide. Docsity.com Formation of Channel  First, the holes are repelled by the positive gate voltage, leaving behind negative ions and forming a depletion region. Next, electrons are attracted to the interface, creating a channel (“inversion layer”). Docsity.com Voltage-Dependent Resistor  The inversion channel of a MOSFET can be seen as a resistor.  Since the charge density inside the channel depends on the gate voltage, this resistance is also voltage-dependent. Docsity.com L and tox Dependence  Small gate length and oxide thickness yield low channel resistance, which will increase the drain current. Docsity.com Effect of W  As the gate width increases, the current increases due to a decrease in resistance. However, gate capacitance also increases thus, limiting the speed of the circuit.  An increase in W can be seen as two devices in parallel. Docsity.com Channel Potential Variation  Since there‟s a channel resistance between drain and source, and if drain is biased higher than the source, channel potential increases from source to drain, and the potential between gate and channel will decrease from source to drain. Docsity.com Charge Density at a Point  Let x be a point along the channel from source to drain, and V(x) its potential; the expression above gives the charge density (per unit length).   THGSox VxVVWCxQ  )()( Docsity.com Charge Density and Current  The current that flows from source to drain (electrons) is related to the charge density in the channel by the charge velocity. vQI  Docsity.com Drain Current    2)(2 2 1 )( )( DSDSTHGSoxnD nTHGSoxD n VVVV L W CI dx xdV VxVVWCI dx dV v       Docsity.com Linear Resistance  At small VDS, the transistor can be viewed as a resistor, with the resistance depending on the gate voltage.  It finds application as an electronic switch.   THGSoxn on VV L W C R    1 Docsity.com Application of Electronic Switches  In a cordless telephone system in which a single antenna is used for both transmission and reception, a switch is used to connect either the receiver or transmitter to the antenna. Docsity.com Effects of On-Resistance  To minimize signal attenuation, Ron of the switch has to be as small as possible. This means larger W/L aspect ratio and greater VGS. Docsity.com Triode or Saturation?  When the region of operation is not known, a region is assumed (with an intelligent guess). Then, the final answer is checked against the assumption. Docsity.com Channel-Length Modulation  The original observation that the current is constant in the saturation region is not quite correct. The end point of the channel actually moves toward the source as VD increases, increasing ID. Therefore, the current in the saturation region is a weak function of the drain voltage.     DSTHGSoxnD VVV L W CI   1 2 1 2 Docsity.com  and L  Unlike the Early voltage in BJT, the channel- length modulation factor can be controlled by the circuit designer.  For long L, the channel-length modulation effect is less than that of short L. Docsity.com Velocity Saturation  Since the channel is very short, it does not take a very large drain voltage to velocity saturate the charge particles.  In velocity saturation, the drain current becomes a linear function of gate voltage, and gm becomes a function of W.   oxsat GS D m THGSoxsatsatD WCv V I g VVWCvQvI      Docsity.com Body Effect  As the source potential departs from the bulk potential, the threshold voltage changes.   FSBFTHTH VVV  22 0  Docsity.com Large-Signal Models  Based on the value of VDS, MOSFET can be represented with different large-signal models. Docsity.com PMOS Transistor  Just like the PNP transistor in bipolar technology, it is possible to create a MOS device where holes are the dominant carriers. It is called the PMOS transistor.  It behaves like an NMOS device with all the polarities reversed. Docsity.com PMOS Equations            2 , 2 , 2 , 2 , 2 2 1 1 2 1 2 2 1 )1( 2 1 DSDSTHGSoxptriD DSTHGSoxpsatD DSDSTHGSoxptriD DSTHGSoxpsatD VVVV L W CI VVV L W CI VVVV L W CI VVV L W CI         Docsity.com Small-Signal Model of PMOS Device  The small-signal model of PMOS device is identical to that of NMOS transistor; therefore, RX equals RY and hence (1/gm)||ro. Docsity.com