© Digital Integrated Circuits 2nd Devices Digital Integrated Circuits A Design Perspective The Devices Jan M. Rabaey Anantha Chandrakasan Borivoje Nikolic July 30, 2002

© Digital Integrated Circuits 2nd Devices Goal of this chapter  Present intuitive understanding of device operation  Introduction of basic device equations  Introduction of models for manual analysis  Introduction of models for SPICE simulation  Analysis of secondary and deep-sub- micron effects  Future trends

© Digital Integrated Circuits 2nd Devices The Diode Mostly occurring as parasitic element in Digital ICs

© Digital Integrated Circuits 2nd Devices Depletion Region

© Digital Integrated Circuits 2nd Devices Diode Current

© Digital Integrated Circuits 2nd Devices Models for Manual Analysis

© Digital Integrated Circuits 2nd Devices Diode Model

© Digital Integrated Circuits 2nd Devices SPICE Parameters

© Digital Integrated Circuits 2nd Devices What is a Transistor? A Switch! |V GS | An MOS Transistor

© Digital Integrated Circuits 2nd Devices The MOS Transistor Polysilicon Aluminum

© Digital Integrated Circuits 2nd Devices MOS Transistors - Types and Symbols D S G D S G G S DD S G NMOS Enhancement NMOS PMOS Depletion Enhancement B NMOS with Bulk Contact

© Digital Integrated Circuits 2nd Devices Threshold Voltage: Concept

© Digital Integrated Circuits 2nd Devices The Threshold Voltage

© Digital Integrated Circuits 2nd Devices The Body Effect

© Digital Integrated Circuits 2nd Devices Current-Voltage Relations A good ol’ transistor Quadratic Relationship x V DS (V) I D (A) VGS= 2.5 V VGS= 2.0 V VGS= 1.5 V VGS= 1.0 V ResistiveSaturation V DS = V GS - V T

© Digital Integrated Circuits 2nd Devices Transistor in Linear

© Digital Integrated Circuits 2nd Devices Transistor in Saturation Pinch-off

© Digital Integrated Circuits 2nd Devices Current-Voltage Relations Long-Channel Device

© Digital Integrated Circuits 2nd Devices A model for manual analysis

© Digital Integrated Circuits 2nd Devices Current-Voltage Relations The Deep-Submicron Era Linear Relationship -4 V DS (V) x 10 I D (A) VGS= 2.5 V VGS= 2.0 V VGS= 1.5 V VGS= 1.0 V Early Saturation

© Digital Integrated Circuits 2nd Devices Velocity Saturation  (V/µm)  c = 1.5  n ( m / s )  sat = 10 5 Constant mobility (slope = µ) Constant velocity

© Digital Integrated Circuits 2nd Devices Perspective I D Long-channel device Short-channel device V DS V DSAT V GS - V T V GS = V DD

© Digital Integrated Circuits 2nd Devices I D versus V GS x V GS (V) I D (A) x V GS (V) I D (A) quadratic linear Long Channel Short Channel

© Digital Integrated Circuits 2nd Devices I D versus V DS -4 V DS (V) x 10 I D (A) VGS= 2.5 V VGS= 2.0 V VGS= 1.5 V VGS= 1.0 V x V DS (V) I D (A) VGS= 2.5 V VGS= 2.0 V VGS= 1.5 V VGS= 1.0 V ResistiveSaturation V DS = V GS - V T Long ChannelShort Channel

© Digital Integrated Circuits 2nd Devices A unified model for manual analysis S D G B NB For PMOS, use Vmax=max(…) instead of Vmin

© Digital Integrated Circuits 2nd Devices Simple Model versus SPICE V DS (V) I D (A) Velocity Saturated Linear Saturated V DSAT =V GT V DS =V DSAT V DS =V GT

© Digital Integrated Circuits 2nd Devices A PMOS Transistor x V DS (V) I D (A) Assume all variables negative! VGS = -1.0V VGS = -1.5V VGS = -2.0V VGS = -2.5V

© Digital Integrated Circuits 2nd Devices Transistor Model for Manual Analysis

© Digital Integrated Circuits 2nd Devices The Transistor as a Switch

© Digital Integrated Circuits 2nd Devices The Transistor as a Switch

© Digital Integrated Circuits 2nd Devices The Transistor as a Switch

© Digital Integrated Circuits 2nd Devices Unified Model Examples

© Digital Integrated Circuits 2nd Devices MOS Capacitances Dynamic Behavior

© Digital Integrated Circuits 2nd Devices Dynamic Behavior of MOS Transistor

© Digital Integrated Circuits 2nd Devices The Gate Capacitance t ox n + n + Cross section L Gate oxide x d x d L d Polysilicon gate Top view Gate-bulk overlap Source n + Drain n + W

© Digital Integrated Circuits 2nd Devices Gate Capacitance Cut-off ResistiveSaturation Most important regions in digital design: saturation and cut-off

© Digital Integrated Circuits 2nd Devices Gate Capacitance Capacitance as a function of VGS (with VDS = 0) Capacitance as a function of the degree of saturation

© Digital Integrated Circuits 2nd Devices Measuring the Gate Cap

© Digital Integrated Circuits 2nd Devices Diffusion Capacitance Bottom Side wall Channel Source N D Channel-stop implant N A 1 SubstrateN A W x j L S

© Digital Integrated Circuits 2nd Devices Junction Capacitance

© Digital Integrated Circuits 2nd Devices Linearizing the Junction Capacitance Replace non-linear capacitance by large-signal equivalent linear capacitance which displaces equal charge over voltage swing of interest

© Digital Integrated Circuits 2nd Devices Capacitances in 0.25  m CMOS process

© Digital Integrated Circuits 2nd Devices Sub-Threshold Conduction V GS (V) I D (A) VTVT Linear Exponential Quadratic Typical values for S: mV/decade The Slope Factor S is  V GS for I D 2 / I D 1 =10

© Digital Integrated Circuits 2nd Devices Sub-Threshold I D vs V GS V DS from 0 to 0.5V

© Digital Integrated Circuits 2nd Devices Sub-Threshold I D vs V DS V GS from 0 to 0.3V

© Digital Integrated Circuits 2nd Devices Summary of MOSFET Operating Regions  Strong Inversion V GS > V T  Linear (Resistive) V DS < V DSAT  Saturated (Constant Current) V DS  V DSAT  Weak Inversion (Sub-Threshold) V GS  V T  Exponential in V GS with linear V DS dependence

© Digital Integrated Circuits 2nd Devices Parasitic Resistances