Threshold Voltage -4 In practice,the "native"threshold value may not be suited for circuit design,e.g.,VTH may be zero and the device may be on for any positive gate voltage. Typically threshold voltage is adjusted by ion implantation into the channel surface (doping P-type material will increase VTH of NMOS devices). p-substrate 。 When Vos is zero,there is no horizontal electric field present in the channel,and therefore no current between the source to the drain. When Vos is more than zero,there is some horizontal electric field which causes a flow of electrons from source to drain. 2西/
2020/3/7 11 SM 20 EECE 488 ± Set 1: Introduction and Background Threshold Voltage - 4 In practice, the ³naWiYe´ threshold value may not be suited for circuit design, e.g., VTH may be zero and the device may be on for any positive gate voltage. Typically threshold voltage is adjusted by ion implantation into the channel surface (doping P-type material will increase VTH of NMOS devices). When VDS is zero, there is no horizontal electric field present in the channel, and therefore no current between the source to the drain. When VDS is more than zero, there is some horizontal electric field which causes a flow of electrons from source to drain
Long Channel Current Equations -1 。 The voltage of the surface under the gate,V(x),depends on the voltages of Source and Drain. If Vos is zero,VD=Vs=V(x).The charge density Q(unit C/m)is uniform. Q,=9=C-P--Cm)小s-'m) LL L n n 2a=-WCax(Wcs-Vm） p-substrate (a) ● If Vos is not zero,the channel is tapered,and V(x)is not constant.The charge density depends on x. Va VG Qa(x)=-WCox(VGs-V(x)-VIH) p-substrate 0 b 2西/
2020/3/7 12 SM 21 EECE 488 ± Set 1: Introduction and Background Long Channel Current Equations - 1 The voltage of the surface under the gate, V(x), depends on the voltages of Source and Drain. If VDS is zero, VD= VS=V(x). The charge density Qd (unit C/m) is uniform. ( ) Qd WCox VGS VTH ( ) ( ( ) ) d ox GS VTH Q x WC V V x L C WL V V L C V L Q Q ox GS TH d If VDS is not zero, the channel is tapered, and V(x) is not constant. The charge density depends on x.
Long Channel Current Equations-3 。 Current:ddeveloci I= Gate dt dx dt Oxide >Velocity in terms of V(x): dv Capacitor of value velocity=·E,E= CaW dx dt -dv(x)) Source Channel d Drain →velo=(u Velocity d Charge dg >Qd in terms of V(x): di(x) E- dx Qa(x)=-WCos(VGs-V(x)-VTH) Lv(T) Voltage Current in terms of V(x): 0 L dV(x) Microelectronic Circuits,2004 Oxford University Press 1p =WC.lVas-V(x)-Vm l,d VDS Ipdx=JWCoxH,VGs -V(x)-Vmldv x=0 V=0 Long-channel current equation: 1o-H.CwHWVo-Vm)a-Visl 22西/d
2020/3/7 13 SM 22 EECE 488 ± Set 1: Introduction and Background Long Channel Current Equations - 3 Q velocity dt dx dx dQ dt dQ I d u dt dV velocity P E , E ) ( ) ( dx dV x velocity o P ( ) ( ( ) ) d ox GS VTH Q x WC V V x ³ ³ DS V V ox n GS TH L x I D dx WC V V x V dV 0 0 P [ ( ) ] ] 2 1 [( ) 2 D n ox VGS VTH VDS VDS L W I P C Current : ¾ Velocity in terms of V(x): ¾ Qd in terms of V(x): dx dV x I D WCox VGS V x VTH n ( ) [ ( ) ]P Current in terms of V(x): Long-channel current equation: © Microelectronic Circuits, 2004 Oxford University Press
Long Channel Current Equations-4 If Voss Ves-VTH we say the device is operating in triode(or linear)region. Current in Triode Region: 。=%c亚[。-)s号g】 Triode Region Vas3 VGs2 Vos 色 ·Terminology: W Aspect Ratio Overdrive Vohtage =Effective Voltage=Vas-Vm =Vor 2西7 14
2020/3/7 14 SM 23 EECE 488 ± Set 1: Introduction and Background Long Channel Current Equations - 4 » ¼ º « ¬ ª 2 2 1 D n ox VGS VTH VDS VDS L W I P C Terminology: Overdrive Voltage Effective Voltage VGS VTH Veff L W Aspect Ratio Current in Triode Region: If VDS VGS-VTH we say the device is operating in triode (or linear) region.
Long Channel Current Equations -5 For very small Vos(deep Triode Region): Ip can be approximated to be a linear function of Vps. The device resistance will be independent of Vos and will only depend on Ver. The device will behave like a variable resistor G Vas f'ps<2Vs-VH)月 1。=么C是e小 RON ID -c0s-r） VGS1 Vos 2西/
2020/3/7 15 SM 24 EECE 488 ± Set 1: Introduction and Background Long Channel Current Equations - 5 n ox GS TH D DS ON D n ox GS TH DS DS GS TH V V L W C I V R V V V L W I C If V V V P P 1 2 : For very small VDS (deep Triode Region): ID can be approximated to be a linear function of VDS. The device resistance will be independent of VDS and will only depend on Veff. The device will behave like a variable resistor