CMOS VLSI Design - Lecture 4: Nonideal Transistor Theory pdf

Lecture 4: Nonideal Transistor Theory CMOS VLSI DesignCMOS VLSI Design 4th Ed. 4: Nonideal Transistor Theory 2 Outline  Nonideal Transistor Behavior – High Field Effects • Mobility Degradation • Velocity Saturation – Channel Length Modulation – Threshold Voltage Effects • Body Effect • Drain-Induced Barrier Lowering • Short Channel Effect – Leakage • Subthreshold Leakage • Gate Leakage • Junction Leakage  Process and Environmental Variations CMOS VLSI DesignCMOS VLSI Design 4th Ed. 4: Nonideal Transistor Theory 3 Ideal Transistor I-V  Shockley long-channel transistor models ( ) 2 cutoff linear saturatio 0 2 2 n gs t ds ds gs t ds ds dsat gs t ds dsat VV V I VV VVV VV VV β β   <    = −− <      −>   CMOS VLSI DesignCMOS VLSI Design 4th Ed. 4: Nonideal Transistor Theory 4 Ideal vs. Simulated nMOS I-V Plot  65 nm IBM process, V DD = 1.0 V 0 0.2 0.4 0.6 0.8 1 0 200 400 600 800 1000 1200 V ds I ds (µA) V gs = 1.0 V gs = 1.0 V gs = 0.8 V gs = 0.6 V gs = 0.4 V gs = 0.8 V gs = 0.6 Channel length modulation: Saturation current increases with V ds I on = 747 mA @ V gs = V ds = V DD Simulated Ideal Velocity saturation & Mobility degradation: Saturation current increases less than quadratically with V gs Velocity saturation & Mobility degradation: I on lower than ideal model predicts CMOS VLSI DesignCMOS VLSI Design 4th Ed. 4: Nonideal Transistor Theory 5 ON and OFF Current  I on = I ds @ V gs = V ds = V DD – Saturation  I off = I ds @ V gs = 0, V ds = V DD – Cutoff 0 0.2 0.4 0.6 0.8 1 0 200 400 600 800 1000 V ds I ds (µA) V gs = 1.0 V gs = 0.4 V gs = 0.8 V gs = 0.6 I on = 747 mA @ V gs = V ds = V DD CMOS VLSI DesignCMOS VLSI Design 4th Ed. 4: Nonideal Transistor Theory 6 Electric Fields Effects  Vertical electric field: E vert = V gs / t ox – Attracts carriers into channel – Long channel: Q channel ∝ E vert  Lateral electric field: E lat = V ds / L – Accelerates carriers from drain to source – Long channel: v = µE lat CMOS VLSI DesignCMOS VLSI Design 4th Ed. 4: Nonideal Transistor Theory 7 Coffee Cart Analogy  Tired student runs from VLSI lab to coffee cart  Freshmen are pouring out of the physics lecture hall  V ds is how long you have been up – Your velocity = fatigue × mobility  V gs is a wind blowing you against the glass (SiO 2 ) wall  At high V gs , you are buffeted against the wall – Mobility degradation  At high V ds , you scatter off freshmen, fall down, get up – Velocity saturation • Don’t confuse this with the saturation region CMOS VLSI DesignCMOS VLSI Design 4th Ed. 4: Nonideal Transistor Theory 8 Mobility Degradation  High E vert effectively reduces mobility – Collisions with oxide interface CMOS VLSI DesignCMOS VLSI Design 4th Ed. 4: Nonideal Transistor Theory 9 Velocity Saturation  At high E lat , carrier velocity rolls off – Carriers scatter off atoms in silicon lattice – Velocity reaches v sat • Electrons: 10 7 cm/s • Holes: 8 x 10 6 cm/s – Better model CMOS VLSI DesignCMOS VLSI Design 4th Ed. 4: Nonideal Transistor Theory 10 Vel Sat I-V Effects  Ideal transistor ON current increases with V DD 2  Velocity-saturated ON current increases with V DD  Real transistors are partially velocity saturated – Approximate with α-power law model – I ds ∝ V DD α – 1 < α < 2 determined empirically (≈ 1.3 for 65 nm) ( ) ( ) 2 2 ox 22 gs t ds gs t VV W I C VV L β µ − = = − ( ) ox maxds gs t I CWV V v= − [...]... (tox ≈ 10.5 Å) From [Song01] 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 22 Junction Leakage  Reverse-biased p-n junctions have some leakage – Ordinary diode leakage – Band-to-band tunneling (BTBT) – Gate-induced drain leakage (GIDL) p+ n+ n+ p+ p+ n+ n well p substrate 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 23 Diode Leakage  Reverse-biased p-n junctions have some leakage... coefficient γ = tox 2qε si N A = ε ox 4: Nonideal Transistor Theory 2qε si N A Cox CMOS VLSI Design 4th Ed 15 Body Effect Cont  For small source-to-body voltage, treat as linear 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 16 DIBL  Electric field from drain affects channel  More pronounced in small transistors where the drain is closer to the channel  Drain-Induced Barrier Lowering VVVη – Drain... temperature Vgs 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 27 So What?  So what if transistors are not ideal? – They still behave like switches  But these effects matter for… – Supply voltage choice – Logical effort – Quiescent power consumption – Pass transistors – Temperature of operation 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 28 Parameter Variation fast  Transistors have... increases with Vds n n L + + L – Even in saturation p GND bulk Si DD DD d eff 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 12 Chan Length Mod I-V I ds = β (V 2 gs − Vt ) (1 + λVds ) 2  λ = channel length modulation coefficient – not feature size – Empirically fit to I-V characteristics 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 13 Threshold Voltage Effects  Vt is Vgs for which the channel... Reverse-biased PN junction diode current 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 20 Subthreshold Leakage  Subthreshold leakage exponential with Vgs Vgs −Vt 0 +ηVds − kγ Vsb I ds I ds 0 e nvT −Vds  1 − e vT        n is process dependent – typically 1. 3-1 .7  Rewrite relative to Ioff on log scale  S ≈ 100 mV/decade @ room temperature 4: Nonideal Transistor Theory CMOS VLSI Design. .. Eg: bandgap voltage – A, B: tech constants 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 25 Gate-Induced Drain Leakage  Occurs at overlap between gate and drain – Most pronounced when drain is at VDD, gate is at a negative voltage – Thwarts efforts to reduce subthreshold leakage using a negative gate voltage 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 26 Temperature Sensitivity ... pMOS FF pMOS SF TT slow slow 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed FS SS fast 29 Environmental Variation  VDD and T also vary in time and space  Fast: – VDD: high – T: low Corner Voltage Temperature F 1.98 0C T 1.8 70 C S 1.62 125 C 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 30 Process Corners  Process corners describe worst case variations – If a design works in all corners,... which Vt decreases with L 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 18 Leakage  What about current in cutoff?  Simulated results  What differs? – Current doesn’t go to 0 in cutoff 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 19 Leakage Sources  Subthreshold conduction – Transistors can’t abruptly turn ON or OFF – Dominant source in contemporary transistors  Gate leakage – Tunneling... with four letters (T, F, S) – nMOS speed – pMOS speed – Voltage – Temperature 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 31 Important Corners  Some critical simulation corners include Purpose nMOS pMOS VDD Temp Cycle time S S S S Power F F F F Subthreshold leakage F F F S 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 32 ...  VD  vT = I S  e − 1 ID      At any significant negative diode voltage, ID = -Is  Is depends on doping levels – And area and perimeter of diffusion regions – Typically < 1 fA/µm2 (negligible) 4: Nonideal Transistor Theory CMOS VLSI Design 4th Ed 24 Band-to-Band Tunneling  Tunneling across heavily doped p-n junctions – Especially sidewall between drain & channel when halo doping is used to . Lecture 4: Nonideal Transistor Theory CMOS VLSI DesignCMOS VLSI Design 4th Ed. 4: Nonideal Transistor Theory 2 Outline  Nonideal Transistor. Environmental Variations CMOS VLSI DesignCMOS VLSI Design 4th Ed. 4: Nonideal Transistor Theory 3 Ideal Transistor I-V  Shockley long-channel transistor models (