CS 152 Computer Architecture and Engineering Lecture 11 VLSI I 2005 2 22 John Lazzaro www cs berkeley edu lazzaro TAs Ted Hong and David Marquardt www inst eecs berkeley edu cs152 CS 152 L11 VLSI I UC Regents Spring 2005 UCB Last Time Pipeline Hazard Resolution OR R2 R3 R1 ADD R4 R3 R2 AND R2 R2 R1 Which do we forward from ID Decode EX IR WB MEM IR IR IR WE MemToReg From WB Mux Logic op 32 A 32 A L U 32 Y Dout rs1 ws wd Din rd1 WE M rd2 MemToReg M WE Ext CS 152 L11 VLSI I R Addr RegFile rs2 Data Memory B UC Regents Spring 2005 UCB Today State Storage Tools on Silicon ICs Capacitance Holds state as charge Transistors How to move charge Layout How to fabricate your IC VLSI Very Large Scale Integration The tall thin designer with feet on the ground and head in the sky The ground Physics and IC Fabrication The sky Architecture and Applications Carver Mead CS 152 L11 VLSI I UC Regents Spring 2005 UCB Capacitance CS 152 L11 VLSI I UC Regents Spring 2005 UCB Recall Building a capacitor Top Plate Dielectric Bottom Plate CS 152 L11 VLSI I Conducts electricity well metal doped polysilicon An insulator Does not conducts electricity at all air glass silicon dioxide Conducts electricity well metal doped polysilicon UC Regents Spring 2005 UCB Recall Capacitors in action Because the dielectric is an insulator and does not conduct I 0 After circuit settles Q C V C 1 5 Volts D cell Q Charge stored on capacitor C The capacitance of the device function of device shape and type of dielectric 1 5V After battery is removed Still Q C 1 5 Volts Capacitor remembers charge CS 152 L11 VLSI I UC Regents Spring 2005 UCB Capacitors and current Q CV I V Differentiate with respect to time if C C t dQ dt C dV dt I is defined as dQ dt I 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 CS 152 L11 VLSI I UC Regents Spring 2005 UCB Storing computational state as charge State is coded as the amount of energy stored by a device 1 5V State is read by sensing the amount of energy 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 CS 152 L11 VLSI I UC Regents Spring 2005 UCB How 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 Represent state Write 1 5 volts on C as charge in ways that To read C measure V V 0 75 volts is a 1 are robust to noise V 0 75 volts is a 0 Cost Could have stored many bits on that capacitor Ex read C every 1 ms Correct small state errors Is V 0 75 volts that are introduced by noise Write back 1 5V Cost Complexity CS 152 L11 VLSI I yes or 0V no UC Regents Spring 2005 UCB MOS Transistors Two diodes and a capacitor in an interesting arrangement So we begin with a diode review CS 152 L11 VLSI I UC Regents Spring 2005 UCB Diodes in action Resistor Light emitting diode LED Light on Yes Light on No CS 152 L11 VLSI I UC Regents Spring 2005 UCB Diodes Current vs Voltage Diode is off I Io Anode I Diode is on I Io exp V Vo V Cathode I Io exp V Vo 1 Io range 1fA to 1nA CS 152 L11 VLSI I Vo range 25mV to 60 mV UC Regents Spring 2005 UCB Making a diode on a silicon wafer CS 152 L11 VLSI I UC Regents Spring 2005 UCB A 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 Lots of room but few electrons Forbidden band gap CS 152 L11 VLSI I Conduction band Valence band Many electrons but packed too tight to move UC Regents Spring 2005 UCB e l e c t r o n e n e r g y Intrinsic 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 More electrons better conduction Conduction band Valence band CS 152 L11 VLSI I We think of holes as positive carriers UC Regents Spring 2005 UCB e l e c t r o n e n e r g y We engineer crystal with impurities CS 152 L11 VLSI I UC Regents Spring 2005 UCB N type silicon add donor atoms Use diffusion or ion implantation to replace some of the Si atoms with As Arsensic has an extra electron that is donates to the conduction band n heavy doping n light doping Electrons from donor atoms Improves conductivy Conduction band Donor energy Valence band CS 152 L11 VLSI I No change in the number of holes UC Regents Spring 2005 UCB e l e c t r o n e n e r g y P type silicon add acceptor atoms Use diffusion or ion implantation to replace some of the Si atoms with Boron Boron has one fewer electron than Si It can accept valence band electrons creating holes p heavy doping p light doping No change in conduction band electron count Conduction band Acceptor energy Valence band Number of holes increased conductivity improves CS 152 L11 VLSI I UC Regents Spring 2005 UCB e l e c t r o n e n e r g y How to make a silicon diode Wafer cross section Cathode n V p Anode p region depletion region depletion region Wafer doped p type At V 0 hill too high for electrons to diffuse up e l n region e c t r o n For holes going downhill is hard CS 152 L11 VLSI I no carriers V controls hill e n e r g y UC Regents Spring 2005 UCB Diodes Current vs Voltage Diode is off I Io Anode I Diode is on I Io exp V Vo V Cathode I Io exp V Vo 1 Io range 1fA to 1nA CS 152 L11 VLSI I Vo range 25mV to 60 mV UC Regents Spring 2005 UCB Note Diodes are biased off V1 V2 V1 V2 n n p0 V ground V1 V2 0V Diodes off only current is Io leakage I Io exp V Vo 1 Anodes of all diodes on wafer connected to ground CS 152 L11 VLSI I UC Regents Spring 2005 UCB Admin Testing and Interfaces on Friday …
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