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1、Constant Field Scaling The frequency response of MOSFETs increases as the channel length decreases.Channel lengths of 0.25m to 3 nm is now the standard.One question that must be considered:what other device parameters must be scaled as the channel length is scaled down.MOSFET ScalingConstant Field S
2、caling Constant-Field：The principle of constant-field scaling is that device dimensions and device voltages be scaled such that electric fields(both horizontal and vertical)remain essentially constant.To ensure that the reliability of the scaled device is not compromised,the electric fields in the s
3、caled device must not increase.The Scaling Parameter k：Typically,k=0.7 for a given technology.Dimension Scaling：the channel length is scaled from L to kL.Voltage Scaling：To maintain a constant horizontal electric field,the drain voltage:from VD to kVD.The maximum gate voltage:from VG to kVG so that
4、the gate and drain voltages remain compatible.To maintain a constant vertical electric field,the oxide thickness then must also be scaled from tox to ktox.kVDoriginal NMOS transistorscaled NMOS transistorConstant Field Scaling The maximum depletion width at the drain terminal,for a one-sided pn junc
5、tion,is The Depletion Widths：since the channel length is being reduced,the depletion widths also need to be reduced.If the substrate doping concentration is increased by the factor(1/k),then the depletion width is reduced by approximately the factor k since VDis reduced by.(12.19)Constant Field Scal
6、ing The drift current per channel：The drift current per channel width remains essentially a constant,so if the channel width is reduced by k,then the drain current is also reduced by k.The power density：The area of the device,AWL,is then reduced by k2 and the power,P=IV,is also reduced by k2.The pow
7、er density in the chip remains unchanged.The drain current:The drain current per channel width,for the transistor biased in the saturation region:Constant Field Scaling Keep in mind that the width and length of interconnect lines are also assumed to be reduced by the same scaling factor.Table 12.1 S
8、ummary of constant-field device scalingConstant Field ScalingIn constant-field scaling,the applied voltages are scaled with the same scaling factor k as the device dimensions.However,in actual technology evolution,voltages have not been reduced with the same scaling factor.Other factors that do not
9、scale,such as threshold voltage and subthreshold currents,have made the reduction in applied voltages less desirable.Generalized ScalingElectric fields in MOS devices have tended to increase as device dimensions shrinked,1.Consequences of increased electric fields are reduced reliability and increas
10、ed power density;and then the device temperature may increase,which may affect the device reliability.2.As the oxide thickness is reduced and the electric field is increased,gate oxides are closer to breakdown and oxide integrity may be more difficult to maintain.Direct tunneling of carriers through
11、 the oxide may be more likely to occur.3.Increased electric fields may also increase the chances of hot-electron effects.Generalized Scaling In constant-field scaling,the device voltages are reduced by the scaling factor k.It would seem appropriate that the threshold voltage should also be scaled by
12、 the same factor.The threshold voltage,for a uniformly doped substrate,can be written asThe first two terms in Equation(12.21)are functions of material parameters that do not scale and are only very slight functions of doping concentration.The last term is approximately proportional to k,so the threshold voltage does not scale directly with the scaling factor k.Threshold Voltage:First Approximation(12.21)