UC Regents Spring 2005 © UCBCS 152 L11: VLSI I2005-2-22John Lazzaro (www.cs.berkeley.edu/~lazzaro)CS 152 Computer Architecture and EngineeringLecture 11 – VLSI Iwww-inst.eecs.berkeley.edu/~cs152/TAs: Ted Hong and David MarquardtUC Regents Spring 2005 © UCBCS 152 L11: VLSI IMux,LogicLast Time: Pipeline Hazard Resolutionrd1RegFilerd2WEwdrs1rs2wsExtIRIRBAM32ALU3232opIRYMIRDoutData MemoryWEDinAddrMemToRegRWE, MemToRegID (Decode) EXMEMWBFrom WBADD R4,R3,R2OR R2,R3,R1 AND R2,R2,R1Which do we forward from?UC Regents Spring 2005 © UCBCS 152 L11: VLSI IToday: State Storage Tools on Silicon ICsCapacitance: Holds state as chargeTransistors: How to move chargeVLSI == “Very Large Scale Integration”The tall thin designer, with feet on the ground and head in the sky.Carver Mead The ground: Physics and IC FabricationThe sky: Architecture and ApplicationsLayout: How to fabricate your ICUC Regents Spring 2005 © UCBCS 152 L11: VLSI ICapacitanceUC Regents Spring 2005 © UCBCS 152 L11: VLSI IRecall: Building a capacitorConducts electricity well.(metal, doped polysilicon)TopPlateBottomPlateConducts electricity well(metal, doped polysilicon)DielectricAn insulator. Does not conducts electricity at all.(air, glass (silicon dioxide))UC Regents Spring 2005 © UCBCS 152 L11: VLSI IRecall: Capacitors in actionI = 0Because the dielectric is an insulator, and does not conduct.After circuit “settles” ...Q = C V = C * 1.5 Volts (D cell)Q: Charge stored on capacitorC: The capacitance of the device: function of device shape and type of dielectric. +++ +++--- ---After battery is removed:+++ +++--- ---Still, Q = C * 1.5 VoltsCapacitor “remembers” charge1.5VUC Regents Spring 2005 © UCBCS 152 L11: VLSI ICapacitors and current ...Q = C V +++ +++--- ---IVDifferentiate with respect to time ... if C != C(t) ...dQ/dt = C dV/dt Observation: If a voltage change dV occurs in zero time (dt = 0), the current I is infinite (impossible).The voltage across a capacitor cannot change instantaneously. And by Q = C V, the charge stored on a capacitor cannot change instantaneously.I is defined as dQ/dt ...I = C dV/dtUC Regents Spring 2005 © UCBCS 152 L11: VLSI IState is coded as the amount of energy stored by a device.+++ +++--- ---Storing computational state as chargeState is read by sensing the amount of energy+++ +++--- ---1.5V Problems: noise changes Q (up or down), parasitics leak or source Q. Fortunately, Q cannot change instantaneously, but that only gets us in the ballpark.UC Regents Spring 2005 © UCBCS 152 L11: VLSI IHow do we fight noise and win?Store more energy than we expect from the noise.Q = CV. To store more charge, use a bigger V or make a bigger C.Cost: Power, chip size.Example: 1 bit per capacitor.Write 1.5 volts on C.To read C, measure V.V > 0.75 volts is a “1”.V < 0.75 volts is a “0”.Cost: Could have stored many bits on that capacitor.Represent stateas charge in ways that are robust to noise.Correct small state errors that are introduced by noise.Ex: read C every 1 msIs V > 0.75 volts?Write back 1.5V (yes) or 0V (no).Cost: Complexity.UC Regents Spring 2005 © UCBCS 152 L11: VLSI IMOS TransistorsTwo diodes and a capacitor in an interesting arrangement. So, we begin with a diode review ...UC Regents Spring 2005 © UCBCS 152 L11: VLSI IDiodes in action ...Light emitting diode (LED)ResistorLight on?Yes!No!Light on?UC Regents Spring 2005 © UCBCS 152 L11: VLSI IDiodes: Current vs VoltageAnode+-CathodeI VDiode is onI ⋲ Io exp(V/Vo)I = Io [exp(V/Vo) - 1]Io range: 1fA to 1nAVo range: 25mV to 60 mVDiode is offI ⋲ - IoUC Regents Spring 2005 © UCBCS 152 L11: VLSI IMaking a diode on a silicon waferUC Regents Spring 2005 © UCBCS 152 L11: VLSI IA pure (”intrinsic”) silicon crystal ...Conducts electricity better than an insulator, worse than a conductor.Why? Most electrons (dots) are in a full “valence” band. Moving in the band is difficult.Especially near 0 degrees K.Many electrons, but packed too tight to move.Lots of room, but few electrons.electron energyValence bandConduction bandForbidden “band gap”UC Regents Spring 2005 © UCBCS 152 L11: VLSI IIntrinsic silicon crystal as T rises ...Some valence band electrons diffuse into the conduction band.These electrons leave behind “holes” in the valence band, allowing remaining electrons to move easier.We think of “holes” as positive carriers ...More electrons,better conductionValence bandConduction bandelectron energyUC Regents Spring 2005 © UCBCS 152 L11: VLSI IWe “engineer” crystal with impurities ...UC Regents Spring 2005 © UCBCS 152 L11: VLSI IN-type silicon: add donor atomsUse diffusion or ion implantation to replace some of the Si atoms with AsArsensic has an extra electron that is “donates” to the conduction band.Valence bandConduction bandelectron energyElectronsfrom donor atoms.Improves conductivy.Donor energyNo change in the number of holesn+ : heavy doping. n- : light doping.UC Regents Spring 2005 © UCBCS 152 L11: VLSI IP-type silicon: add acceptor atomsUse diffusion or ion implantation to replace some of the Si atoms with BoronBoron has one fewer electron than Si. It can accept valence band electrons, creating holes.No change in conduction band electron countAcceptor energyNumber of holes increased, conductivity improvesValence bandConduction bandelectron energyp+ : heavy doping. p- : light doping.UC Regents Spring 2005 © UCBCS 152 L11: VLSI IHow to make a silicon diode ...Wafer cross-sectionn+p-Wafer doped p-typen+ regionp- regionAt V = 0, “hill” too high for electrons to diffuse up. VCathode: -+-Anode: +no carriersdepletion regionFor holes, going “downhill” is hard.V controls hill.electron energydepletion regionUC Regents Spring 2005 © UCBCS 152 L11: VLSI IDiodes: Current vs VoltageAnode+-CathodeI VDiode is onI ⋲ Io exp(V/Vo)I = Io [exp(V/Vo) - 1]Io range: 1fA to 1nAVo range: 25mV to 60 mVDiode is offI ⋲ - IoUC Regents Spring 2005 © UCBCS 152 L11: VLSI INote: Diodes are biased “off”!p-n+V10 V - “ground”n+V2V1 V2V1, V2 > 0V. Diodes “off”, only current is Io “leakage”.I = Io [exp(V/Vo) - 1]Anodes of all diodes on wafer connected to ground.UC Regents Spring 2005 © UCBCS 152 L11: VLSI IAdmin: Testing and Interfaces on FridayHomework 1: due Friday 3/4Midterm 1: Thurs 3/17, 6PM to 9PMUC Regents Spring 2005 © UCBCS 152 L11: VLSI IMOS TransistorsTwo diodes and a capacitor in an interesting arrangement ...UC Regents Spring 2005 © UCBCS 152 L11: VLSI
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