EE105 - Spring 2007Microelectronic Devices and CircuitsLecture 5MOSFET2Metal-Oxide-Semiconductor (MOS) Capacitor The MOS structure can be thought of as a parallel-plate capacitor, with the top plate being the positive plate, oxide being the dielectric, and Si substrate being the negative plate. (We are assuming P-substrate.)3Structure and Symbol of MOSFET This device is symmetric, so either of the n+ regions can be source or drain. 4State of the Art MOSFET Structure The gate is formed by polysilicon, and the insulator by Silicon dioxide.5Formation 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”).6Voltage-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.7Voltage-Controlled Attenuator As the gate voltage decreases, the output drops because the channel resistance increases. This type of gain control finds application in cell phones to avoid saturation near base stations.8MOSFET Characteristics The MOS characteristics are measured by varying VG while keeping VD constant, and varying VD while keeping VG constant. (d) shows the voltage dependence of channel resistance.9L (Gate Length) and tox(Gate Oxide Thickness) Dependence Small gate length and oxide thickness yield low channel resistance, which will increase the drain current. 10Effect of Gate Width: 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.11Channel Potential Variation Since there’s a channel resistance between drain and source, and if drain is biased higher than the source, channel potentialincreases from source to drain, and the potential between gate and channel will decrease from source to drain.12Channel Pinch-Off As the potential difference between drain and gate becomes more positive, the inversion layer beneath the interface starts to pinch off around drain. When VD–VG= Vth, the channel at drain totally pinches off, and when VD–VG> Vth, the channel length starts to decrease.13Channel Charge Density The channel charge density is equal to the gate capacitance times the gate voltage in excess of the threshold voltage. )(THGSoxVVWCQ −=14Charge 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).[]THGSoxVxVVWCxQ −−= )()(15Charge 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⋅=16Drain Current[][]2)(221)()(DSDSTHGSoxnDnTHGSoxDnVVVVLWCIdxxdVVxVVWCIdxdVv−−=−−=+=μμμ17Parabolic ID-VDSRelationship By keeping VGconstant and varying VDS, we obtain a parabolic relationship. The maximum current occurs when VDSequals to VGS-VTH. 18ID-VDSfor Different Values of VGS()2max, THGSDVVI −∝19Linear 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.()THGSoxnonVVLWCR−=μ120Application 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.21Effects of On-Resistance To minimize signal attenuation, Ronof the switch has to be as small as possible. This means larger W/L aspect ratio and greater VGS. 22Different Regions of Operation23How to Determine ‘Region of Operation’ When the potential difference between gate and drain is greater than VTH, the MOSFET is in triode region. When the potential difference between gate and drain becomes equal to or less than VTH, the MOSFET enters saturation region. 24Triode 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.25Channel-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 VDincreases, increasing ID. Therefore, the current in the saturation region is a weak function of the drain voltage.()()DSTHGSoxnDVVVLWCIλμ+−= 121226λ 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.27Transconductance Transconductance is a measure of how strong the drain current changes when the gate voltage changes. It has three different expressions.()mnox GSTHWgCVVLμ=−2mnoxDWgCILμ=2DmGSTHIgVV=−28Doubling of gmDue to Doubling W/L If W/L is doubled, effectively two equivalent transistors are added in parallel, thus doubling the current (if VGS-VTHis constant) and hence gm.29Velocity 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. ()oxsatGSDmTHGSoxsatsatDWCvVIgVVWCvQvI=∂∂=−⋅=⋅=30()FSBFTHTHVVVφφρ220−++=Body Effect As the source potential departs from the bulk potential, the threshold voltage changes.31Large-Signal Models Based on the value of VDS,MOSFET can be represented with different large-signal models.32Example: Behavior of IDwith V1as a Function Since V1is connected at the source, as it increases, the current drops. ()2121THDDoxnDVVVLWCI −−=μ33Small-Signal Model When the bias point is not perturbed significantly, small-signal model can be used to facilitate calculations. To represent channel-length modulation, an output resistance is inserted into the model. DoIrλ1≈34PMOS 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
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