模拟集成电路的分析与设计：Chapter 2-Basic MOS Device Physics.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Basic MOS Device Physics,1.In analog circuit,transistors are not considered as simple switches and many of their second-order effects(,二级效应）,directly impact the performance.,2.As new generation of IC technologies scales the devices,these effects become more significant.Designer must often decide which effects can be neglected in a given circuit.,Two reasons for studying the device physics:,Basic MOS Device Physics,MOSFET structure(1),M:metal(or high doping polysilicon);,O:silicon dioxide(sometimes just call it as oxide);,S:silicon,An n-type MOS device:,Figure 2.1 Simple view of a MOS device,High doping area for omic contact(,欧姆接触）,Channel area(,沟道区域）,Minimum channel length L,eff,=0.15,m,often represents the technology nodes,;,Oxide thickness t,ox,=5nm or 50,often affects the threshold voltage V,t,(,阈值电压）,and thus the power supply.,These two figures play most important role in technology nodes scaling down.,Basic MOS Device Physics,MOSFET structure(2),NMOS and PMOS devices:,When both NMOS and PMOS devices are needed to be placed on one chip,n-well(n,阱）,or p-well(p,阱,)is needed.,Basic MOS Device Physics,MOSFET structure(3),NMOS and PMOS symbols:,For digital circuit.,For analog circuit.,If bulk is always tied to ground or V,DD,;most used in this book.,If bulk is not tied to ground or V,DD,.,Basic MOS Device Physics,MOS I-V Characteristics(1),Threshold Voltage(V,th,):,The gate voltage that the inversion layer is formed in the channel,Full of holes,Holes are depleted,Electrons accumulated and,inversion layer formed,Q,dep,Basic MOS Device Physics,MOS I-V Characteristics(2),Threshold voltage calculation:,Where,Work functions(,功函数差）,between poly and silicon,Surface potential(,表面势）,Charges(,电荷）,in the depletion region,Gate oxide capacitance per unit area(,单位面积栅电容）,Adjust V,TH,through implantation:,Implantation of p,+,or n,+,can alter the substrate concentration N,sub,thus to change,F,and Q,dep,therefore V,TH,is adjusted,Basic MOS Device Physics,MOS I-V Characteristics(3),Formation of inversion layer in a PMOSFET:,Similar to NMOSFET,but inversion layer is full of holes and threshold voltage is negative.,Basic MOS Device Physics,MOS I-V Characteristics(4),1.,Triode region(,放大区）,Due to the complex of the I-V curves,I-V characteristics of a MOSFET are often divided into two regions:,Basic MOS Device Physics,MOS I-V Characteristics(5),If V,DS,2(V,GS,V,TH,),device is in deep triode region.,In this case,device actives as a controlled linear resistor.,So we also call this region as,linear region,（线性区）,.,Basic MOS Device Physics,MOS I-V Characteristics(6),2.,Saturation region,（饱和区）,!,The drain current I,DS,keeps constant with the increase of drain voltage V,DS,Basic MOS Device Physics,MOS I-V Characteristics(7),It is the pinch-off(,夹断）,of the channel that leads to saturation region of the device.,At pinch-off point,V,p,V,GS,-V,TH,.In the simplest model,the pinch-off point displace is neglected.In this case,the channel can be taken as a resistor with constant resistance R,sat,.,I,DS,is dominated by the channel current:V,p,/R,sat,and therefore is constant.,Basic MOS Device Physics,MOS I-V Characteristics(8),MOS device model modification-Second-order effect,Above model is the simplest model of MOS device.It can fit the experiment at old technology larger than 1,m.When dimension goes into sub micron(,亚微米）,or deep sub micron,（深亚微米）,second-order effects something like follows must be considered:,Body effect,Channel-Length modulation,Subthreshold conduction,Basic MOS Device Physics,MOS I-V Characteristics(9),Second-order effect:body effect,Normally,for NMOS,V,B,is equal to V,S,and tied to ground,that is V,BS,=0.,If sometimes V,B,is less than V,S,then V,BS,0.In this case,V,TH,will become larger,，,current is smaller,;in samll-signal analysis,g,mb,is considered.,Basic MOS Device Physics,MOS I-V Characteristics(10),Second-order effect:Channel-Length modulation,In saturation region of simplest model,the channel length change due to pinch-off point move is neglected,so that I,D,can be dealt with constant value.,If Channel-Length modulation is considered,the effective channel length is shorter,I,D,will increase slightly with the increase of V,DS,;,in small-signal analysis,output resistance r,o,need to be considered,.,Is Channel-Length modulation factor,Basic MOS Device Physics,MOS I-V Characteristics(11),Second-order effect:Subthreshold conduction,In above simplest model,there is no current when gate voltage is small than threshold voltage(V,GS,V,TH,).,In fact,the weak inversion layer exists even if V,GS,V,TH,and there exists a subthreshold current,shown as follow:,Basic MOS Device Physics,MOS device models(1),Device layout,Basic MOS Device Physics,MOS device models(2),Device layout,Basic MOS Device Physics,MOS device models(3),Device Capacitance,C,1,=WLC,ox,oxide capacitance between the gate and the channel,Depletion capacitance between channel and substrate,C,3,and C,4,Capacitance due to the overlap of the gate poly with the source and drain areas.,C,5,and C,6,Junction capacitance between the source/drain areas and the substrate,is usually decomposed into two components:bottom-plate capacitance C,j,per unit area,and sidewall capacitance C,jsw,per unit length.,Basic MOS Device Physics,MOS device models(4),Device Capacitance,The capacitance will change with the change of applied voltage.,Basic MOS Device Physics,MOS Small-Signal model(1),Above I-V Characteristics describe the large-signal model of MOS devices and often used in setting the bias of the devices in the designed circuit(usually related to DC analysis).,Input signals are often small signals(often milivolts,毫伏,),large-signal model cannot deal with these signals.Therefore,small-signal model is needed for analysis of the analog circuit(usually related to,AC analysis,).,Most of the devices in analog circuit design are set in,saturation region,in order to get enough small signal amplification.,Basic MOS Device Physics,MOS Small-Signal model(2),Take most popular connection of the transistor:common source transistor as an example.Suppose the input small signal v,gs,is a small AC voltage(for example,sine wave with peak to peak only 10 mV),it is added on the gate bias V,GS,=1.5 V.The drain bias V,DS,=4 V and V,TH,=1 V.Apparently,from large signal analysis we can assure that this device is set at saturation region.On the channel,there will exist a bias DC current I,DS,plus a small AC current i,ds,.,Basic MOS Device Physics,MOS Small-Signal model(3),The input small ac signal v,gs,is amplified by a factor of transconductance,（跨导）,g,m,.,While g,m,is decided by DC bias:,V,GS,I,DS,v,gs,i,ds,This means that DC bias will affect the small signal amplification!,Basic MOS Device Physics,MOS Small-Signal model(4),Basic MOS Device Physics,MOS Small-Signal model(5),Introducing the concept of dependent current source(,受控电流源）,the analysis of small signals can be performed by a equivalent small signal model shown as follow:,Basic MOS Device Physics,MOS Small-Signal model(6),Considering different cases,the low-frequency model can be simplified to several different forms:,Considering channel-length modulation effect,Considering body effect,Basic MOS Device Physics,MOS Small-Signal model(7),Where:,Output resistance(,输出阻抗）：,Body effect transconductance:,Basic MOS Device Physics,Level 1 SPICE models(1),Basic MOS Device Physics,Level 1 SPICE models(2),Basic MOS Device Physics,NMOS versus PMOS,Mobility of electrons is greatly higher than mobility of holes,therefore NMOS transistors have higher current drive and transconductance;,Moreover,NMOS transistors exhibit higher output resistance.,It is preferred to use NMOS rather than PMOS wherever possible!,Basic MOS Device Physics,Questions for chapter 2(1),(1).Why is studying device physics so important for analog IC design?,(2).Which two figures play most important role in technology nodes scaling down?Please describe in detail.,(3).When both NMOS and PMOS devices are needed to be placed on one chip,what is needed?,(4).Plot symbols of NMOS transistors for three cases:,Digital circuit,Analog circuit if bulk is always tied to ground,Analog circuit if bulk is not tied to ground,(5).What kinds of parameters will affect the threshold voltage V,th,?How does the implantation adjust the V,th,?,(6).What is the condition that leads to the device works in deep triode region?And in this case,what will the device active as?,Basic MOS Device Physics,Questions for chapter 2(2),(7).Describe the reason why the drain current I,DS,keeps constant with the increase of drain voltage V,DS,when the device works in saturation.,(8).In deep sub micron,which kinds of second-order-effects should be considered for MOS device model?,(9).Explain the body effect and its effects on the characteristics of the devices.,(10).Explain the channel-length modulation and its effects on the characteristics of the devices.,(11).Explain the subthreshold conduction and write the subthreshold current equation.,(12).What kinds of capacitances should be considered in the device model?,Basic MOS Device Physics,Questions for chapter 2(3),(13).Plot the MOS device small-signal model if the channel-length modulation and body effect are considered.,(14).Describe the relationship between the output resistance r,o,and the channel-length modulation coefficient,.,(15).Describe the relationship between the body effect transconductance g,mb,and the transconductance g,m,.,(16).What type of MOS device do we prefer to use wherever possible in the analog IC design?And why?,(17).Page 39:Ex.2.1,(18).Page 39:Ex.2.2,(19).Page 45:Ex.2.25,