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6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 2. Key questions, What is the topology of a small signal equivalent cir. cuit model of the MOSFET, What are the key dependencies of the leading model. elements in saturation, 6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 3. 1 Low frequency small signal equivalent cir,cuit model. Regimes of operation of MOSFET,VDSsat VGS VT,linear saturation.
ID nCox VGS VT VDS,Saturation,ID IDsat nCox VGS VT 2 1 VDS VDSsat. Effect of back bias,VT VBS VT o 2 p VBS 2 p, 6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 4. Small signal device modeling, In many applications interested in response of device to. a small signal applied on top of bias,Key points,Small signal is small. response of non linear components becomes linear,Can separate response of MOSFET to bias and small.
Since response is linear superposition can be used. effects of different small signals are independent. from each other, 6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 5. small signal,equivalent,circuit model,ID id ID id,VGS VBS VGS VBS. Mathematically,iD VGS vgs VDS vds VBS vbs,ID VGS VDS VBS vgs vds vbs. VGS Q VDS Q VBS Q,where Q bias point VGS VDS VBS,Small signal id. id gm vgs govds gmb vbs,gm transconductance S,go output or drain conductance S.
gmb backgate transconductance S,gm Q go Q gmb Q,VGS VDS VBS. 6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 6. 2 Transconductance,In saturation regime,ID nCox VGS VT 2 1 VDS VDSsat. Then neglecting channel length modulation,gm nCox VGS VT. Rewrite in terms of ID,gm 2 t nCox ID,saturation,saturation. 0 VT VGS 0 ID, 6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 7.
Transconductance of 3 m nMOSFET VDS 2 V,Equivalent circuit model representation of gm. 6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 8. 2 Output conductance,In saturation regime,ID nCox VGS VT 2 1 VDS VDSsat. go Q nCox VGS VT 2 ID, Output resistance is inverse of output conductance. saturation,saturation,0 VT VGS 0 ID, 6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 9. Output conductance of 3 m nMOSFET,Equivalent circuit model representation of go.
6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 10. 2 Backgate transconductance, In saturation regime neglect channel length modulation. ID nCox VGS VT 2,gmb nCox VGS VT,VBS Q L VBS Q,VT VBS VT o 2 p VBS 2 p. VBS Q 2 2 p VBS,All together,gmb inherits all dependencies of gm. 6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 11. Body of MOSFET is a true gate output characteristics. for different values of VBS VBS 0 3 V VBS,0 5 V VGS 2 V. Equivalent circuit model representation of gmb,vgs gmbvbs.
6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 12. Complete MOSFET small signal equivalent circuit model. for low frequency,vgs gmvgs gmbvbs ro, 6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 13. 2 High frequency small signal equivalent cir,cuit model. Need to add capacitances In saturation,Cfringe gate. source n drain,n Cov Cov n,Cjsw Csb i Cjsw,Cgs intrinsic gate capacitance. overlap capacitance Cov fringe,Cgd overlap capacitance Cov.
Cgb only parasitic capacitance,Csb source junction depletion capacitance. sidewall channel substrate capacitance,Cdb drain junction depletion capacitance. 6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 14. Complete MOSFET high frequency small signal equiva. lent circuit model,vgs Cgs gmvgs gmbvbs ro,Plan for development of capacitance model. Start with Cgs i,compute gate charge QG QN QB,compute how QG changes with VGS. Add pn junction capacitances, 6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 15.
Inversion layer charge in saturation,QN VGS W 0 Qn y dy W 0 Qn Vc dVc. dy W nQn Vc,W 2L n Z VGS VT 2,QN VGS 0 Qn Vc dVc,Qn Vc Cox VGS Vc VT. MOSFET Equivalent Circuit Models October 18 2005 Contents 1 Low frequency small signal equivalent circuit model 2 High frequency small signal equivalent circuit model Reading assignment Howe and Sodini Ch 4 4 5 4 6 6 012 Microelectronic Devices and Circuits Fall 2005 Lecture 11 2 Key questions What is the topology of a small signal equivalent cir cuit model of the MOSFET