Institute of Solid State PhysicsTechnische Universität Graz

12. Memories /Bipolar transistors

Jan. 9, 2019

Institute of Solid State Physics

ExamsTechnische Universität Graz

January 31March 8May 17June 19October ...

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, Schottkydiode, Heterojunction bipolar transistor, ...)

U-MOSFET and D-MOSFET

Power transistors

Fransila

CMOS inverter

2dd ddE QV CV

Dissipates little power except when it is switching

CMOS inverter

out

p

p+ p+p+n

n+n+n+

Vdd

in

inverter

CMOS inverter

out

p

p+ p+p+n

n+n+n+

Vdd

in

Gate delay

Gate delay is limited by CgateVdd/I.

Ring oscillator

SRAM Static random access memory

No refresh circuitry needed.

DRAMDynamic random access memory

DRAM Read and refresh DRAM with a SRAM cell

http://electroiq.com/chipworks_real_chips_blog/

DRAM

75:1

Silicon oxynitride SiOxNy dielectric

Flash memory

Charge is stored on a floating gate

nonvolatile

http://www.dongyitech.com/en/newsdetail.asp?newsid=128

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

http://lamp.tu-graz.ac.at/~hadley/ss1/appendix/tunnel/tunneltrans.php

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

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

http://web.stanford.edu/group/cui_group/research.htm

High Bandwidth Memory

AMD to launch its HBM graphics cards on 16 June 2015.

Through-Silicon Via (TSV)

A vertical electrical connection (via) passing completely through a silicon wafer.

Used in 3D integration.

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Bosch process

http://en.wikipedia.org/wiki/Deep_reactive-ion_etching

Repeat 2 processes over and over

1. Etch Si with SF6 (nearly isotropic)

2. Deposit passivation layer C4F8

Directional etching at the bottom breaks through the passivation layer.

Short cycles: smooth walls

Long cycles: fast etching

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.

Ferroelectric RAM

http://baldengineering.blogspot.co.at/2013_12_01_archive.html

Magnetic memory In MRAM the resistance depends on whether the magnetic layers are parallel or antiparallel.

nonvolatile

Orientaion of the magnetic free layer is set by sending a current through the bit and word lines.

Institute of Solid State Physics

bipolar transistorsTechnische Universität Graz

collector base emitter

np

n+

lightly doped p substrate

npn transistor

Used in front-end high-frequency receivers (mobile telephones).

p+n+

bipolar transistors

p+

abrupt junction

x

E-xp xn

Ap

eNE x x

Dn

eNE x x

x

-xp

xn

+

-

eND

eNA

2

2

02

02

Ap p

Dn n

eN xV xx x x

eN xV xx x x

0

0

p

n

x x

x x

2ln D Abi B

i

N NeV k Tn

dEdx

dV Edx

Forward bias, V > 0

Electrons and holes are driven towards the junction.The depletion region becomes narrower

Minority electrons are injected into the p-regionMinority holes are injected into the n-region

np(xp) pn(xn)

+

( ) exp

( ) exp

bip p D

B

bin n A

B

e V Vn x N

k T

e V Vp x N

k T

Reverse bias, V < 0

Electrons and holes are driven away from the junction.The depletion region becomes wider

Minority electrons are extracted from the p-region by the electric fieldMinority holes are extracted from the n-region by the electric field

+

np(xp) pn(xn)

( ) exp

( ) exp

bip p D

B

bin n A

B

e V Vn x N

k T

e V Vp x N

k T

pnp transistor, no bias

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

Long/Short diode

n-type

,diff p pdpJ eDdx

0,

( )n n ndiff p p p

n

p x pdpJ eD eDdx d

pn(xn)

pn0

dn x

pn(x)Short diode

dn << Lp

Metal contact is much closer to the depletion region than the diffusion length

Long diode dn >> Lppn(xn)

Minority carrier concentration

emitter (n+) base (p) collector (n)contact contact

pe0

nb0 pc0

/0 1be BeV k T

eEp be p

e e

p eI eA D

W x

xe We Web WbcxcWc

/ /0

be B bc BeV k T eV k Tb

En be nbc be

n e eI eA D

W W

0 exp beb

B

eVnk T

0 exp bcb

B

eVnk T

0 exp bee

B

eVpk T

0 exp bcc

B

eVpk T

Emitter current

0 / /0 01 1be B bc Bbe p e eV k T eV k Tbe n b be n bE En Ep

eb e bc be bc be

eA D p eA D n eA D nI I I e eW x W W W W

/ /1 1be B bc BeV k T eV k TE ES R CSI I e I e

pe0

nb0 pc0

xe We Web WbcxcWc

0 exp bcb

B

eVnk T

0 exp bee

B

eVpk T

0 exp beb

B

eVnk T

Collector current

emitter (n+) base (p) collector (n)contact contact

/0 1bc BeV k T

ccp bc p

c c

p eI eA D

x W

/ /0

be B bc BeV k T eV k Tb

cn bc nbc eb

n e eI eA D

W W

pe0

nb0 pc0

xe We Web WbcxcWc

0 exp beb

B

eVnk T

0 exp bcb

B

eVnk T

0 exp bee

B

eVpk T

0 exp bcc

B

eVpk T

Collector current

0/ /0 01 1be B bc Bbc p ceV k T eV k Tbc n b bc n bc cp cn

bc be c c bc be

eA D peA D n eA D nI I I e eW W x W W W

/ /1 1be B bc BeV k T eV k Tc cp cn F ES CSI I I I e I e

pe0

nb0 pc0

xe We Web WbcxcWc

0 exp bcb

B

eVnk T

0 exp bee

B

eVpk T

0 exp beb

B

eVnk T

Minority carrier concentration

pe0

pc0

xe xc

0 exp beb

B

eVnk T

0 exp bcb

B

eVnk T

0 exp bee

B

eVpk T

0 exp bcc

B

eVpk T

xn1 xp1 xn2xp2

emitter (n+) base (p) collector (n)contact contact

pe0

nb0 pc0

xe We Web WbcxcWc

0 exp beb

B

eVnk T

0 exp bcb

B

eVnk T

0 exp bee

B

eVpk T

0 exp bcc

B

eVpk T

Not an npn transistor

Ebers-Moll model

/ /1 1be B bc BeV k T eV k TC F ES CSI I e I e

/ /1 1be B bc BeV k T eV k TE ES R CSI I e I e

IE

IB

IC

/ 1be BeV k TF ESI I e / 1bc BeV k T

R CSI I e

R RI F FI

B E CI I I

Emitter efficiency

/0 1be BeV k T

eEp be p

eb e

p eI eA D

W x

/ /0

be B bc BeV k T eV k Tb

En be nbc be

n e eI eA D

W W

For e ~ 1, Wbc - Wbe << Lb, Web - xe and nb0 >> pe0

Small base width and heavy emitter doping

for npn

2i

De

nN

2i

Ab

nN

11 /

Ene

En Ep Ep En

II I I I

neutral base width

Base transport factor

c

En

IBI

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

Current transfer ratio

Ce

E

I BI

~ 1 for a good BJT

Transistor modes

1. Forward active: emitter-base forward, base-collector reverse2. Saturation: emitter-base forward, base-collector forward3. Reverse active: emitter-base reverse, base-collector forward4. Cut-off: emitter-base reverse, base-collector reverse

Common base configuration

/ /1 1be B bc BeV k T eV k Tc F ES CSI I e I e

/ /1 1be B bc BeV k T eV k TE ES R CSI I e I e

solve for Vbe

saturation

active

cutoff IE < 0

http://lamp.tu-graz.ac.at/~hadley/psd/L13/commonbase/pnp_current.html