EE31 Electronics I Lecture 8: MOSET Threshold Voltage and Parasitic apacitances Payman Zarkesh-Ha Office: EE Bldg. 3B Office hours: Tuesday :-3:PM or by appointment E-mail: payman@ece.unm.edu Slide: 1 Review of Last Lecture evice Model for Linear Region evice Model for Saturation Region hannel Length Modulation Slide: 1
Threshold Voltage Equation ynamic Parameters of Long hannel MOSET MOSET Parasitic apacitances Overlap capacitances hannel capacitances Junction capacitances Today s Lecture Slide: 3 Threshold Voltage Equation MOSET is a four terminal device; Gate, Source, rain, and Bulk. The Bulk may not be always connected to the Source. Slide: 4
Threshold Voltage Equation We normally assume that the bulk of the MOSET is connected to source. However, sometimes the bulk and source are in different potentials (V SB ). V SB is called body bias. The applied V SB changes the threshold voltage as shown below: V T V T VSB In this equation, V T is the zero bias threshold voltage, ү is the body bias coefficient, and φ is: KT q N Ln ni A Where N A is the doping concentration in the substrate. Slide: 5 Example: Threshold Voltage & Body Bias Assume that V T =.8V, ү=.6 V 1/, φ =.4 V. ind V T if V SB =.5 V V T V T VSB V T.4.5.4.8.55 1. 35.8.6 Observations: 1) Body bias is normally reverse bias. (why?) ) More reverse body bias increases the threshold voltage. Slide: 6 3
MOSET Threshold Voltage Slide: 7 More etail on MOSET Threshold Voltage Zero body bias threshold voltage: V T ms qn A si Q Threshold voltage with body bias: Important acts: Where: V T V Where: KT q T VSB N Ln ni qn Body bias increases threshold voltage Threshold voltage is positive for normal NMOS Threshold voltage is negative for normal PMOS A A si and t Slide: 8 4
MOS apacitance elay of digital MOS circuits depends of capacitance of MOS device There is a trade off between parasitic capacitance and drive strength of MOS device Larger increases the drive strength (I S equation) However, larger increases the device parasitic capacitance MOS parasitic capacitance includes Overlap capacitances hannel capacitances Junction capacitances Between almost every two terminals of MOS device, there is a source of parasitic capacitance Slide: 9 MOS Parasitic apacitances rain G GB B Gate Bulk GS Source SB Slide: 1 5
Overlap apacitances Because of the lateral S/ diffusion, there is an overlap between gate and S/ junctions This overlap capacitance is a constant linear capacitance GSOV GOV W X d L eff Slide: 11 hannel apacitances hannel capacitance is a voltage dependent and non-linear capacitance G G G S G S G S G P-sub Bulk P-sub Bulk P-sub Bulk utoff Region Linear Region Saturation Region Operation Region GBH GSH GH utoff OX WL eff Linear Saturation 1 WL OX eff WL OX eff 3 1 WL OX eff Slide: 1 6
Junction apacitances Junction capacitance is the depletion region capacitance of S/ It is a voltage dependent capacitance (remember reverse biased diode) G S n+ n+ P-sub j 1 V SB j m j A siq NAN NA N 1 KT NAN Ln q ni Slide: 13 Junction apacitance omponents The Junction capacitance of bottom plate is treated separately from the three non-gate edges The gate edge is often ignored since it is part of the conducting channel The bottom plate is usually step graded with m=.5 The sidewall are step graded with m=.33 and face the channel-stop implant which has much higher doping than substrate hannel-stop implant N A+ Side wall W Source N Bottom x j L S Side wall Substrate N A hannel Slide: 14 7
Junction apacitance omponents iff Bottom SW WL Bottom SW J JSW s L W s Slide: 15 MOS Parasitic apacitances G rain GB B GS G GSH GH GSOV GOV Gate Bulk GB GBH SB Sdiff GS Source SB B diff Slide: 16 8