Metal Oxide Semiconductor MOS Capacitor EE105 Spring 2007 Microelectronic Devices and Circuits Lecture 5 MOSFET 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 2 Structure and Symbol of MOSFET This device is symmetric so either of the n regions can be source or drain State of the Art MOSFET Structure The gate is formed by polysilicon and the insulator by Silicon dioxide 3 4 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 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 5 Voltage 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 6 MOSFET 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 7 8 L Gate Length and tox Gate Oxide Thickness Dependence Effect 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 Small gate length and oxide thickness yield low channel resistance which will increase the drain current 9 Channel Potential Variation An increase in W can be seen as two devices in parallel 10 Channel Pinch Off 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 11 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 12 Channel Charge Density Charge Density at a Point Q WCox VGS VTH Q x WCox VGS V x VTH The channel charge density is equal to the gate capacitance times the gate voltage in excess of the threshold voltage 13 Charge Density and Current 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 14 Drain Current v n dV dx ID WCox VGS V x VTH n dV x dx 1 W ID nCox 2 VGS VTH VDS VDS2 2 L I Q v The current that flows from source to drain electrons is related to the charge density in the channel by the charge velocity 15 16 Parabolic ID VDS Relationship ID VDS for Different Values of VGS I D max VGS VTH 2 By keeping VG constant and varying VDS we obtain a parabolic relationship The maximum current occurs when VDS equals to VGSVTH 17 Linear Resistance Ron 18 Application of Electronic Switches 1 n Cox W VGS VTH L 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 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 19 20 Effects of On Resistance Different Regions of Operation 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 21 Triode or Saturation How 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 22 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 23 24 and L Channel Length Modulation 1 W 2 I D n Cox VGS VTH 1 VDS 2 L 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 25 Transconductance 2I W gm D gm 2 nCox ID VGS VTH L Transconductance is a measure of how strong the drain current changes when the gate voltage changes It has three different expressions g m nCox 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 26 Doubling of gm Due to Doubling W L W VGS VTH L If W L is doubled effectively two equivalent transistors are added in parallel thus doubling the current if VGSVTH is constant and hence gm 27 28 Velocity Saturation I D v sat Q v sat WC ox V GS VTH gm Body Effect I D v sat WC ox V GS 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 V TH V TH 0 2 F V SB 2 F As the source potential departs from the bulk potential the threshold voltage changes 29 Large Signal Models 30 Example Behavior of ID with V1 as a Function W 1 2 ID nCox VDD V1 VTH 2 L Based on the value of VDS MOSFET can be represented with different large signal models 31 Since V1 is connected at the source as it increases the current drops 32 Small Signal Model ro PMOS Transistor 1 I D 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 33 PMOS Equations 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 34 Small Signal Model of PMOS Device 1 W 2 I D sat p Cox VGS VTH 1 VDS 2 L 1 W I D tri p Cox 2 VGS VTH VDS VDS2 2 L 1 W 2 I D sat p Cox VGS VTH 1 VDS 2 L 1 W I D tri p Cox 2 VGS VTH VDS VDS2 2 L 35 The small
View Full Document
Unlocking...