(S&S ) PMOS: holes flow from Source to Drain. from Source to Drain. W.-Y. Choi. Electronic Circuits 2 (09/1)

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1 (S&S ) NMOS: electrons flow from Source to Drain PMOS: holes flow from Source to Drain

2 In cut-off ( v < V ), i = 0 GS t D NMOS I-V Characteristics In triode, ( v > V but v v v ) GS t DS GS T W 1 i = k ( v V ) v v L ' D GS t DS DS In saturation ( v > V and v v v ) 1 i = k W ( v V ) L ' D GS t V t : threshold voltage GS t DS GS T k ' = μ C n ox μ : electron mobility C n ox : oxide capacitance

3 PMOS v < V : i = 0 SG t D v > V and v < v V (triode) : SG t SD SG t W 1 i = μ C ( v V ) v v L D p ox SG t SD SD v > V and v > v V (saturation): SG t SD SG t 1 W i = μ C ( v V ) L v SD D p ox SG t

4 Deviation from the ideal model 1) I D =0inCut-Off? Plot I D vs V GS for fixed V DS Non-zero sub-threshold currnet Leakage between S and D: more significant for smaller MOSFET Significant problem in modern digital circuits

5 ) In saturation, ' Lect. 3: MOSFET 1 id = k W ( vgs Vt) L Should have no dependence on v DS v DS increase causes reduction in actual channel length Channel length modulation. i 1 W ' (1 )( ) D = k + DS vgs Vt λ v L But I D increases with v DS even in saturation

6 3) Body effect: Voltage applied to B causes change in threshold voltage 1 i = k W ( v V ) L ' D GS t V = V + γ [ φ + V φ ] t to f SB f V = V when V = 0 φ t to SB f and γ process-dependent parameters If S and B can be tied, no body effect. In IC, B is shared among many transistors B is connected to - the most negative supply voltage (NMOS) - the most positive supply voltage (PMOS) V t depends on V S Very difficult to model analytically Simulation

7 Body effect: Voltage applied to B causes a change in threshold voltage.

8 4) Temperature effect: Many MOSFET parameters are temperature dependent Higher temperature causes reduction in I D

9 - Modern transistors are very complicated in their structure. - Many parameters are needed to model their characteristics accurately in SPICE - SPICE parameters for 0.5μm NMOS are shown - For detailed explanations, See MOSFET Users Manual at www-device.eecs.berkeley.edu/ ~bsim3/get.html Lect. 3: MOSFET MODEL orbitln NMOS ( LEVEL = 7 +TNOM = 7 TOX = 5.6E-9 +XJ = 1E-7 NCH =.3549E17 VTH0 = K1 = K = E-4 K3 = 1E-3 +K3B = W0 = 1E-7 NLX = E-7 7 +DVT0W = 0 DVT1W = 0 DVTW = 0 +DVT0 = DVT1 = DVT = U0 = UA = E-9 UB = E-18 +UC = E-11 VSAT = E5 A0 = AGS = B0 = E-8 B1 = -1E-7 +KETA = E-3 A1 = E-5 A = RDSW = 00 PRWG = PRWB = WR = 1 WINT = 0 LINT = E-10 +DWG = E-8 +DWB = E-10 VOFF = NFACTOR = CIT = 0 CDSC =.4E-4 4 CDSCD = 0 +CDSCB = 0 ETA0 = 5.073E-3 ETAB = E-5 +DSUB = PCLM = PDIBLC1 = PDIBLC =.9409E-3 PDIBLCB = DROUT = 1 +PSCBE1 = E8 PSCBE =.71109E-4 PVAG = E-3 +DELTA = 0.01 RSH = 4.6 MOBMOD = 1 +PRT = 0 UTE = KT1 = KT1L = 0 KT = 0.0 UA1 = 4.31E-9 +UB1 = -7.61E-18 UC1 = -5.6E-11 AT = 3.3E4 +WL = 0 WLN = 1 WW = 0 +WWN = 1 WWL = 0 LL = 0 +LLN = 1 LW = 0 LWN = 1 Although h complicated, they can +LWL = 0 CAPMOD = XPART = 0.5 +CGDO = 4.59E-10 CGSO = 4.59E-10 CGBO = 5E-10 precisely model the transistor +CJ = E-3 PB = 0.99 MJ = CJSW = E-10 PBSW = MJSW = characteristics and accurate circuit +CF = 0 PVTH0 = E-3 PRDSW = -10 +PK = E-3 WKETA = LKETA = E E-3 ) design is possible

10 Linearization of MOSFET: v D v GS Circuit model just for the linear relationship? Small-signal circuit

11 Small signal model for NMOS v GS =V GS + v gs, v DS =V DS + v ds i G = I G + i g, i D =I D + i d i g, i d as functions of v gs, d vs i g = 0 1 W L 1 W id id= μncox ( VGS VT) + V v... gs + GS L v GS From id = μn Cox ( vgs VT) with vgs = VGS + vgs W i = μ C ( V V ) v L d n ox GS T gs = gm vgs g

12 Various expressions for transconductance g m 1 W From i D = μ nc ox ( v GS V T ) L id W gm = V = μ ( ) ncox VGS V GS T v L GS I V = D V GS = μ W n C ox I D L T

13 Small-signal model including channel-length modulation i D v GS 1 W ' (1 )( ) id = k + λ vds vgs Vt L id id Δ i = Δ v + Δv v v = D GS DS GS DS g m i = g v + d m gs v ds i i 1 D W ' ( ) = k vgs Vt v L λ 1 V = Often, r r 0 = 0 r DS 0 I A D

14 Small-signal model including Body effect Practically, Body Effect Is not easy to model analytically. Simulation g g mb mb i χ g D υ BS υ = υ = m GS DS constant constant ( χ : ) In Tutorial on 3/9, TA will review how to use PSPICE