CS 152 Computer Architecture and Engineering

Transcription

1 CS 152 Computr Architctur and Enginring Lctur 11 VLSI I John Lazzaro ( TAs: Td Hong and David Marquardt www-inst.cs.brkly.du/~cs152/

2 Last Tim: Piplin Hazard Rsolution ADD R4,R3,R2 OR R2,R3,R1 AND R2,R2,R1 Which do w forward from? ID (Dcod) EX IR IR IR MEM IR WE, MmToRg WB Mux,Logic rs1 rs2 ws wd RgFil WE rd1 rd2 From WB A M op A L U 32 Y M Data Mmory Addr Dout Din WE MmToRg R Ext B

3 Today: Stat Storag Tools on Silicon ICs Capacitanc: Holds stat as charg Transistors: How to mov charg Layout: How to fabricat your IC VLSI == Vry Larg Scal Intgration Th tall thin dsignr, with ft on th ground and had in th sky. Th ground: Physics and IC Fabrication Th sky: Architctur and Applications Carvr Mad

4 Capacitanc

5 Rcall: Building a capacitor Dilctric Top Plat Conducts lctricity wll. (mtal, dopd polysilicon) An insulator. Dos not conducts lctricity at all. (air, glass (silicon dioxid)) Bottom Plat Conducts lctricity wll (mtal, dopd polysilicon)

6 Rcall: Capacitors in action Bcaus th dilctric is an insulator, and dos not conduct. I = Aftr circuit sttls... Q = C V = C * 1.5 Volts (D cll) Q: Charg stord on capacitor C: Th capacitanc of th dvic: function of dvic shap and typ of dilctric. Aftr battry is rmovd: Still, Q = C * 1.5 Volts Capacitor rmmbrs charg 1.5V

7 Capacitors and currnt... Q = C V I V Diffrntiat with rspct to tim... if C!= C(t)... dq/dt = C dv/dt I is dfind as dq/dt... I = C dv/dt Obsrvation: If a voltag chang dv occurs in zro tim (dt = 0), th currnt I is infinit (impossibl). Th voltag across a capacitor cannot chang instantanously. And by Q = C V, th charg stord on a capacitor cannot chang instantanously.

8 Storing computational stat as charg Stat is codd as th amount of nrgy stord by a dvic V Stat is rad by snsing th amount of nrgy Problms: nois changs Q (up or down), parasitics lak or sourc Q. Fortunatly, Q cannot chang instantanously, but that only gts us in th ballpark.

9 How do w fight nois and win? Stor mor nrgy than w xpct from th nois. Q = CV. To stor mor charg, us a biggr V or mak a biggr C. Cost: Powr, chip siz. Exampl: 1 bit pr capacitor. Writ 1.5 volts on C. To rad C, masur V. V > 0.75 volts is a 1. V < 0.75 volts is a 0. Cost: Could hav stord many bits on that capacitor. Rprsnt stat as charg in ways that ar robust to nois. Corrct small stat rrors that ar introducd by nois. Cost: Complxity. Ex: rad C vry 1 ms Is V > 0.75 volts? Writ back 1.5V (ys) or 0V (no).

10 MOS Transistors Two diods and a capacitor in an intrsting arrangmnt. So, w bgin with a diod rviw...

11 Diods in action... Rsistor Light mitting diod (LED) Light on? Ys! Light on? No!

12 Diods: Currnt vs Voltag Anod + Diod is off I - Io Diod is on I Io xp(v/vo) I V - Cathod I = Io [xp(v/vo) - 1] Io rang: 1fA to 1nA Vo rang: 25mV to 60 mv

13 Making a diod on a silicon wafr

14 A pur ( intrinsic ) silicon crystal... Conducts lctricity bttr than an insulator, wors than a conductor. Why? Most lctrons (dots) ar in a full valnc band. Moving in th band is difficult. Espcially nar 0 dgrs K. Lots of room, but fw lctrons. Forbiddn band gap Conduction band Valnc band Many lctrons, but packd too tight to mov. l c t r o n n r g y

15 Intrinsic silicon crystal as T riss... Som valnc band lctrons diffus into th conduction band. Ths lctrons lav bhind hols in th valnc band, allowing rmaining lctrons to mov asir. Mor lctrons, bttr conduction Conduction band Valnc band W think of hols as positiv carrirs... l c t r o n n r g y

16 W nginr crystal with impuritis...

17 N-typ silicon: add donor atoms Us diffusion or ion implantation to rplac som of th Si atoms with As Arsnsic has an xtra lctron that is donats to th conduction band. n+ : havy doping. n- : light doping. Elctrons from donor atoms. Improvs conductivy. No chang in th numbr of hols Conduction band Donor nrgy Valnc band l c t r o n n r g y

18 P-typ silicon: add accptor atoms Us diffusion or ion implantation to rplac som of th Si atoms with Boron Boron has on fwr lctron than Si. It can accpt valnc band lctrons, crating hols. p+ : havy doping. p- : light doping. No chang in conduction band lctron count Conduction band Accptor nrgy Valnc band Numbr of hols incrasd, conductivity improvs l c t r o n n r g y

19 How to mak a silicon diod V Cathod: - Anod: + n+ p- Wafr cross-sction Wafr dopd p-typ p- rgion dpltion At V = 0, hill too high for rgion lctrons to diffus up. n+ rgion For hols, going downhill is hard. no carrirs V controls hill. dpltion rgion l c t r o n n r g y

20 Diods: Currnt vs Voltag Anod + Diod is off I - Io Diod is on I Io xp(v/vo) I V - Cathod I = Io [xp(v/vo) - 1] Io rang: 1fA to 1nA Vo rang: 25mV to 60 mv

21 Not: Diods ar biasd off! V1 V2 V1 n+ n+ V2 p- 0 V - ground V1, V2 > 0V. Diods off, only currnt is Io lakag. I = Io [xp(v/vo) - 1] Anods of all diods on wafr connctd to ground.

22 Admin: Tsting and Intrfacs on Friday Homwork 1: du Friday 3/4 Midtrm 1: Thurs 3/17, 6PM to 9PM

23 MOS Transistors Two diods and a capacitor in an intrsting arrangmnt...

24 What w want: th prfct switch. V1 n+ V1 Switch is off. V1 is not connctd to V2. p- Switch is on. V1 is connctd to V2. n+ p- V2 n+ V2 W want to turn a p-typ rgion into an n-typ rgion undr voltag control. W nd lctrons to fill valnc hols and add conduction band lctrons

25 An n-channl MOS transistor (nfet) Vd = 1V Vd = 1V I na n+ I µa n+ Vg = 0V dilctric p- Vg = 1V dilctric p- n+ Vs = 0V n+ Vs = 0V Polysilicon gat, dilctric, and substrat form a capacitor. nft is off (I is lakag ) Vg = 1V, small rgion nar th surfac turns from p-typ to n-typ. nft is on.

26 Drawing an nfet Mask drawings snt to th fabrication facility to mak th chips.

27 Mask st for an n-ft (circa 1986) Vg = 0V Vd = 1V I na n+ dilctric p- Vs = 0V Masks #1: n+ diffusion n+ #2: poly (gat) #3: diff contact #4: mtal Top-down viw: Layrs to do p-ft not shown. Modrn procsss hav mor 6 to 10 mtal layrs (in 1986: 2)

28 Dsign ruls for masks, Poly ovrhang. So that if masks ar misalignd, w still gt --- in channl. Minimum gat lngth. So that th sourc and drain dpltion rgions do not mt! lngth Mtal ruls: Contact sparation from channl, on fixd contact siz, ovrlap ruls with mtal, tc... #1: n+ diffusion #3: diff contact #2: poly (gat) #4: mtal

29 Lssons larnd Capacitors hold stat Smiconductor physics Drawing transistors

30 Lcturs: Coming up nxt... Transistor quations, basic mmory circuits. Mmory array structurs and intrfacs. Th mmory hirarchy