12. Memories / Bipolar transistors

Transcription

1 Technische Universität Graz Institute of Solid State Physics 12. Memories / Bipolar transistors Jan. 9, 2019

2 Technische Universität Graz Institute of Solid State Physics Exams January 31 March 8 May 17 June 19 October...

3 Exam Calculator is ok. One A4 of handwritten notes. Explain some concept: (tunnel contact, indirect band gap, thermionic emission, inversion, threshold voltage,...) Perform a calculation: (concentration of minority carriers, integrate charge density to find electric field,...) Explain how a device works: (JFET, MESFET, MOSFET, laser diode, bipolar transistor, LED, Schottky diode, Heterojunction bipolar transistor,...)

4 U-MOSFET and D-MOSFET Fransila Power transistors

5 CMOS inverter Dissipates little power except when it is switching E QV CV dd 2 dd

6 CMOS inverter in out Vdd p+ n+ n+ p+ p+ n p n+ inverter

7 CMOS inverter in out Vdd p+ n+ n+ p+ p+ n p n+

8 Gate delay Gate delay is limited by C gate V dd /I. Ring oscillator

9 SRAM Static random access memory No refresh circuitry needed.

10 DRAM Dynamic random access memory

11 DRAM Read and refresh DRAM with a SRAM cell

12 DRAM 75:1 Silicon oxynitride SiO x N y dielectric

13 Flash memory Charge is stored on a floating gate nonvolatile

14 Intel Micron Flash Technologies (IMFT) Shallow Trench Isolation (STI) Control Gate (CG) Floating Gate (FG) Self-Aligned Doubled Patterning (SADP)

15

16 Phase change memory Phase-change memory (PRAM) uses chalcogenide materials. These can be switched between a low resistance crystalline state and a high resistance amorphous state. GeSbTe is melted by a laser in rewritable DVDs and by a current in PRAM. nonvolatile

17 Phase change material Electron diffraction in a TEM of a GeSbTe alloy.

18 High Bandwidth Memory AMD to launch its HBM graphics cards on 16 June 2015.

19 Through-Silicon Via (TSV) A vertical electrical connection (via) passing completely through a silicon wafer. Used in 3D integration.

20 Bosch process Repeat 2 processes over and over 1. Etch Si with SF 6 (nearly isotropic) 2. Deposit passivation layer C 4 F 8 Directional etching at the bottom breaks through the passivation layer. Short cycles: smooth walls Long cycles: fast etching

21 Ferroelectric RAM FeRAM uses a Ferroelectric material like PZT to store information. Sometimes used in smart cards. nonvolatile To read, try to write a 0, if a current flows, it was a 1.

22 Ferroelectric RAM

23 Magnetic memory In MRAM the resistance depends on whether the magnetic layers are parallel or antiparallel. Orientaion of the magnetic free layer is set by sending a current through the bit and word lines. nonvolatile

24 Technische Universität Graz Institute of Solid State Physics bipolar transistors npn transistor collector base emitter n+ n p+ p n + lightly doped p substrate Used in front-end high-frequency receivers (mobile telephones).

25 bipolar transistors p+

26 abrupt junction de dx E dv dx E en A + -x p - x n en D x A E xxp -x p x n x ev bi D A kbt ln 2 ni en en D E xxn 2 en A x V xxp xp x0 2 2 en D x V xxn 0 x x 2 N N x x0 p 0 x x n n

27 Forward bias, V > 0 Electrons and holes are driven towards the junction. The depletion region becomes narrower n p (x p ) p n (x n ) evbi V np( xp) NDexp kt B evbi V pn( xn) NAexp kt B + Minority electrons are injected into the p-region Minority holes are injected into the n-region

28 Reverse bias, V < 0 Electrons and holes are driven away from the junction. The depletion region becomes wider n p (x p ) p n (x n ) evbi V np( xp) NDexp kt B evbi V pn( xn) NAexp kt B + Minority electrons are extracted from the p-region by the electric field Minority holes are extracted from the n-region by the electric field

29 pnp transistor, no bias

30 pnp transistor, forward active bias The base-emitter voltage controls the minority carriers injected from the emitter to the base. These diffuse to the base-collector junction and are swept into the collector. Always dissipate power due to the forward bias

31 Long/Short diode p n (x n ) Long diode d n >> L p p n (x n ) p n0 p n (x) d n n-type dp J ed ed dx diff, p p p x Short diode d n << L p Metal contact is much closer to the depletion region than the diffusion length J diff, p p ( x ) p n n n0 d n dp edp dx

32 Minority carrier concentration contact p emitter (n+) base (p) collector (n) exp ev be nb0 ev kt B kt B e0 exp be contact n b0 p c0 p exp ev bc nb0 e0 kt B x e We W eb W bc W x c c evbe / kbt pe0 e 1 exp ev bc pc0 kt B IEp eabedp W x I ea D En be n b0 n e e e e ev / k T ev / k T be B bc B W bc W be

33 Emitter current I I I ea D p ea D n e ea D n e be B bc B 1 1 be p e 0 be n b0 ev / k T be n b0 ev / k T E En Ep Web xe Wbc Wbe Wbc Wbe evbe / kbt / 1 evbc kbt 1 I I e I e E ES R CS p ev kt B e0 exp be n ev kt B b0 exp be p e0 n b0 exp ev bc nb0 kt B x e We W eb W x bc W c c p c0

34 Collector current contact p emitter (n+) base (p) collector (n) exp ev be nb0 ev kt B kt B e0 exp be contact n b0 p c0 p exp ev bc nb0 e0 kt B x e We W eb W bc W x c c evbc / kbt pc0 e 1 exp ev bc pc0 kt B Icp eabcdp x W I ea D cn bc n b0 n e e c ev / k T ev / k T c be B bc B W bc W eb

35 Collector current ea D n ea D p ea D n evbe / kbt bc p c0 evbc 1 / kbt 1 bc n b0 bc n b0 Ic Icp Icn e e Wbc Wbe xc Wc Wbc Wbe evbe / kt B / 1 evbc kt B 1 I I I I e I e c cp cn F ES CS p ev kt B e0 exp be n ev kt B b0 exp be p e0 n b0 exp ev bc nb0 kt B x e We W eb W x bc W c c p c0

36 Minority carrier concentration n ev kt B b0 exp be p ev kt B e0 exp be n ev kt B b0 exp bc p ev kt B c0 exp bc p c0 p e0 x e x n1 x p1 x p2 x n2 x c

37 Not an npn transistor contact p e0 exp be emitter (n+) base (p) collector (n) exp ev be nb0 ev kt B kt B contact n b0 p c0 p exp ev bc nb0 e0 kt B x e We W eb W x bc W c c exp ev bc pc0 kt B

38 Ebers-Moll model evbe / kbt / 1 evbc kbt 1 I I e I e E ES R CS evbe / kbt / 1 evbc kbt 1 I I e I e C F ES CS I I I B E C I B I E I RI F F R I C F evbe / kbt evbc / kbt ES 1 IR ICS e 1 I I e

39 Emitter efficiency e IEn 1 I I 1 I / I En Ep Ep En for npn I I ea D Ep be p ea D En be n b0 p evbe / kbt e0 e 1 W eb x n e e e ev / k T ev / k T be B bc B W bc W be For e ~ 1, W bc -W be << L b, W eb -x e and n b0 >> p e0 neutral base width Small base width and heavy emitter doping n N 2 i Ab n N 2 i De

40 Base transport factor B I I c En ratio of the injected current to the collected current recombination in the base would reduce the base transport factor A thin base with low doping results in a base transport factor ~ 1

41 Current transfer ratio I I C E B e ~ 1 for a good BJT

42 Transistor modes 1. Forward active: emitter-base forward, base-collector reverse 2. Saturation: emitter-base forward, base-collector forward 3. Reverse active: emitter-base reverse, base-collector forward 4. Cut-off: emitter-base reverse, base-collector reverse

43 Common base configuration evbe / kbt / 1 evbc kbt 1 I I e I e E ES R CS solve for V be I I e I e evbe / kt B / 1 evbc kt B 1 c F ES CS saturation active cutoff I E < 0