BJTs Wrap-up, Solar Cells, LEDs

6.012 - Microelectronic Devices and Circuits Lecture 8 - BJTs Wrap-up, Solar Cells, LEDs - Outline

• Announcements Exam One - Tomorrow, Wednesday, October 7, 7:30 pm

• BJT Review Wrapping up BJTs (for now) History - 1948 to Today

• p-n Diode review Reverse biased junctions - photodiodes and solar cells

In the dark: no minority carriers, no current With illumination: superposition, iD(vAB, L); photodiodes The fourth quadrant: optical-to-electrical conversion; solar cells Video: "Solar cell electricity is better electricity - putting 6.012 to work

improving our world (a true story)"

Forward biased junctions - light emitting diodes, diode lasers Video: "The LEDs Around Us" Diode design for efficient light emission: materials, structure The LED renaissance: red, amber, yellow, green, blue, white

Clif Fonstad, 10/6/09 Lecture 8 - Slide 1

BJT Modeling: FAR models/characteristics

B

E

C

vBE

+

–

iF

IES

!FiF

iB"FiB

or

B

E

C

vBE

+

–

iB

IBS

!FiB

!

"F# $

iC

iE

=1$%

B( )1+ %

E( )&

1

1+ %E( )

!

"F#

iC

iB

=1$%

B( )%

E+ %

B( )&

1

%E

Defects

!

"E

=D

h

De

#N

AB

NDE

#w

B ,eff

wE ,eff

!

"B#

wB ,eff

2

2LeB

2

Design

Clif Fonstad, 10/6/09

!

Doping : npn with NDE

>> NAB

wB ,eff

: very small

LeB

: very large and >> wB ,eff Lecture 8 - Slide 2

BJT's, cont.: What about the collector doping, NDC? An effect we didn't put into our large signal model

• Base width modulation - the Early effect and Early voltage: The width of the depletion region at the B-C junction increases as vCE increases and the effective base width, wB,eff, gets smaller, thereby

E

Clif Fonstad, 10/6/09 reverse breakdown of the B-C junction. Lecture 8 - Slide 3

• Punch through - base width modulation taken to the limit When the depletion region at the B-C junction extends all the way

through the base to the emitter, IC increases uncontrolably. Punch through has a similar effect on the characteristics to that of

Punch through

p n

wB,eff

C Early effect

B

n+

increasing β and, in turn, iC.

• We will take this effect into account in our small signal LEC modeling.

To minimize the Early effect we make NDC < NAB

pnp BJT's: The other "flavor" of bipolar junction transistor

pnp Symbol and FAR model: Oriented with emitter down like npn:CStructure: i

C +

C iC

iB

B+ v

BE − Ev

CE

B

E

C

vEB

+

–

IES

!FIESe

iB "FiB

or

qVEB/kT

Oriented as found in circuits:

+

p+

NAE

n N

DB

p

NAC

iB

B

C

E

vEB

+

–IES

!FIESeiB

"FiB

or

qVEB/kT

B E

+ v

EB v

BE B

−

iB

-iC

C−− iEE


Metallic base contact

n-type base wafer

Active base region

Metallic emitter contact

(2.5 mm)

Recrystallized p-type regions

Metallic collector contact Approx. 0.001"

(25 µm)

Approx. 0.1"

Figure by MIT OpenCourseWare.

Alloy junction BJT - Early1950's

Early BipolarJunction

Transistors - the first 10 yrs.


Photograph of grown junction BJT (showing device width of 5 mm) removed due to copyright restrictions.

Grown junction BJT - mid-1950's

p-type emitter region Base contact

Base contact Emitter contact n-type base region

Approx. 0.0002" (0.25 mm) p-type collector region

Active base region

Collector contact

Approx. 0.01"

(5 µm)


Diffused junction BJT - late-1950's Lecture 8 - Slide 5

n

n

nn n

n n

p+ p+p

p

p

n++

n++n++

n+

n+

n+

n+

p-

n+

Junction isolated integrated BJT - 1960's onwards

Oxide isolated integrated BJT - a modern process

Integrated Bipolar Junction Transistors - integrated circuit processes.

Clif Fonstad, 10/6/09 Figure by MIT OpenCourseWare. Lecture 8 - Slide 6

Photodiodes - illuminated p-n junction diodes

Consider a p-n diode illuminated at x = xn + a(wn-xn), 0 ≤ a ≤ 1.

p n

Ohmic contact

Ohmic contact

A B iD

+ -vAB

qM

x -wp -xp 0 xn xL = wn

xn+a(wn-xn) What is iD(vAB, M)? Use superposition to find the answer:

!

iD(v

AB,M) = i

D(v

AB,0) + i

D(0,M)

We know iD(vAB,0) already…

!

iD(v

AB,0) = I

S(e

qvAB

kT"1)

The question is, "What is iD(0,M)."

!

iD(0,M) = ?


Photodiodes - cont.: the photocurrent, iD(0,M)

The excess minority carriers: n'p p'n

p'(xL) p'(wn) = 0 n'(-wp≤x≤-xp) = 0 p'(xn) = 0

x -wp -xp 0 xn xL = wn

xn+a(wn-xn)The minority carrier currents: Je Jh

xL-wp -xp wn0 xn

Je(-wp≤x≤-xp) = 0 aqM

x

-(1-a)qM qM

The photocurrent, iD(0,M):

!

iD(0,M) = " AqM 1" a( )


Photodiodes - cont.: The i-v characteristic and what it means.

The total current:

!

iD

(vAB

,M) = iD

(vAB

,0) + iD

(0,M)

= IS

eqv

ABkT"1( ) " AqM 1" a( )

The illumination shifts the ideal diode curve verticallydown. iD

vAB iD(0,M) = -AqM(1-a)

iD(vAB,0) iD(vAB,M)

Power conversion Light detection in this quadrant in this quadrant


Photovoltaic Energy Conversion: Solar cells and TPV, cont.

Efficiency issues: 1. hν:Eg mismatch 2. Voc and fill factor 3. Intensity (concentrator) effect

1.

2.

3. !

vOC

=kT

qln

q"iL

IS

#

$ %

&

' (

!

h"< E

gnot absorbed; energy lost

> Eg

excess energy, h" # Eg( ), lost

$ % &

!

Pout max

< "iSC

vOC

=#iL $ kT ln

q#iL

IS

%

& '

(

) *

!

L"#$"

Clif Fonstad, 10/6/09 Skyline Solar parabolic reflector/concentrator multi-junction cell Lecture 8 - Slide 14 installation photo-illustration from website.

Courtesy of Skyline Solar Inc. Used with permission.

Multi-junction cells - efficiency improvement with number

Clif Fonstad, 10/6/09 "Photovoltaics take a load off soldiers," Oct. 27, 2006, online at: Lecture 8 - Slide 15 http://www.solar.udel.edu/CSOctSOL-Darpa-reprint.pdf

Courtesy of Institute of Physics. Used with permission.

http://www.solar.udel.edu/CSOctSOL-Darpa-reprint.pdf

InGaP cellEg= 1.84 eV (0.67 µm)

GaAs cellEg= 1.43 eV (0.86 µm)

Ge cellEg= 0.7 eV (1.75 µm)

Multi-junction cells, cont. - 2 designs

InGaP cell Eg = 1.84 eV (0.67 µm)

GaAs cell Eg = 1.43 eV (0.86 µm)

Ge cell Eg = 0.7 eV (1.75 µm)

Tunnel junction

Tunnel junction

Substrate Ge or GaAs

A 3 -junction design (3 lattice-matched cells connected

in series by tunnel diodes)

A 6 -junction design* (3-tandem multi-junction

cells set side-by-side)

Clif Fonstad, 10/6/09 * "Photovoltaics take a load off soldiers," Oct. 27, 2006, online at: Lecture 8 - Slide 16 http://www.solar.udel.edu/CSOctSOL-Darpa-reprint.pdf

Courtesy of Institute of Physics. Used with permission.

http://www.solar.udel.edu/CSOctSOL-Darpa-reprint.pdf

Light emitting diodes: what they are all about The basic idea

In Si p-n diodes and BJTs we make heavy use of the very longminority carrier lifetimes in silicon, but in LEDs we want all theexcess carriers to recombination, and to do so creating aphoton of light. We want asymmetrical long-base operation:

n'p, p'n n' large

p'n very small

x -w -x 0 x wp p n n

Why have people cared so much about LEDs? a cool, efficient source of light rugged with extremely long lifetimes can be turned on and off very quickly,

and modulated at very high data rates Clif Fonstad, 10/6/09 Lecture 8 - Slide 18

P

1.0

0.9

Light emitting diodes - 0.8

0.7

600 620 640 660 680 700

lf = 20 mA TA = 25o C

GaAsP Red LED GaAsP

Red LED

Rel

ativ

e ph

oton

inte

nsity

typical spectra

• LED emission - typ. 20 nm wide

0.6

0.5

0.4

0.3

0.2 0.1

0

� - Wavelength-nm • Important spectra to compare

Figure by MIT OpenCourseWare.with LED emission spectra

Response of human eye

Rel

ativ

e sp

ectra

l cha

ract

eris

tics

Relative spectral response or output

Output of Tungsten source at 2870 K

Response of silicon phototransistors

Output of TIL23 TIL24 TIL25 GaAs LEDs

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0

0.2

0.4

0.6

0.8

1.0

1.2

� -Wavelength-µm

Figure by MIT OpenCourseWare. Clif Fonstad, 10/6/09 Lecture 8 - Slide 19

__________________________

Light emitting diodes: historical perspective

LEDs are a very old device, and were the first commercialcompound semiconductor devices in the marketplace. Red, amber, and green LEDs (but not blue) were sold in the 1960's, butthe main opto research focus was on laser diodes; little LED research in universities was done for many years.

Then…things changed dramatically in the mid-1990's:

In part because of new materials developed for red and blue lasers (AlInGaP/GaAs, GaInAlN/GaN)

In part because of packaging innovations (Improved heat sinking and advanced reflector designs)

In part due to advances in wafer bonding (Transparent substrates for improved light extraction)

In part due to the diligence of LED researchers (Taking advantage of advances in other fields)


Light emitting diodes - design issues

Significant challenges in making LEDs include:

1. Choosing the right semiconductor(s) - efficient radiative recombination of excess carriers - emission at the right wavelength (color)

2. Getting the light out of the semiconductor - overcoming total internal reflection and reabsorption

3. Packaging the diode - good light extraction and beam shaping - good heat sinking (for high intensity applications)


Compound Semiconductors: A wide variety of bandgaps. Diamond lattice (Si, Ge, C [diamond]) The majority are "direct gap"

Zinc blende lattice (GaAs shown)

Te52

Se34

S16

B5

Zn30

Cd48

Po84

P15

Hg80

As33

Ge32

Ga31

Bi83

Tl81

Pb82

In49

Sb51

Sn50

C6

N7

O8

Si14

Al13II

III IV V VI

must for efficient optical emission).


Ga

Ag

Lecture 8 - Slide 23 Figure by MIT OpenCourseWare.


Lecture 8 - Slide 24 Lecture 10 - Slide 20

Materials for Red LEDs: GaAsP and AlInGaP

Modern AlInGaP red LEDs grown

lattice-matched on GaAs, and then

transferred to GaP substrates

- Kish, et al, APL 64 (1994) 2838.

GaP red LEDs grown GaP and based on Zn-O pair transitions

Early GaAsP red LEDs grown on a linearly graded buffer on GaAs

- Holonyak and Bevacqua, APL 1 (1962) 82.


Materials for Amber LEDs: GaAsP, AlInGaP, and GaP

Modern AlInGaP amber LEDs

grown lattice-matched on

GaAs, and then transferred to

GaP substrates - Kish, et al, APL 64

(1994) 2838.

Early GaAsP amber LEDs grown on a

linearly graded buffer on GaAs

- Holonyak and Bevacqua, APL 1 (1962)

82.

Clif Fonstad, 10/6/09 Lecture 8 - Slide 25 Lecture 10 - Slide 21

Materials for Green LEDs: GaP, InGaAlP

The first green LEDs were GaP grown by liquid

phase epitaxy on GaP substrates and based on N doping. N is an "isoelectronic" donor, with a very small Ed.

InGaAlP grown by MOCVD on

GaAs substrates provide modern high brightness

LEDs


I

The III-V wurtzite quarternary: GaInAlN

Sze PSD Fig 2a

0.7

0.6

0.3

0.4

0.5

Lattice period, a (nm)

2.0

3.0

4.0

5.0

6.0

GaN

0.28 0.30 0.32 0.34 0.36 0.38

1.0

InN

AlN 0.2

0.25

Good for UV (unique)

Great for blue (the best)

Good for green Not so good for

red yet.

A material covering the spectrum:

nN

C

3

2

1

120o



Light emitting diodes: fighting total internal reflection

With an index of refraction ≈ 3.5, the angle for total internal *reflection is only 16˚. (Only 2% gets out the top!**)

Total internal reflection can be alleviated somewhat if the device is packaged in a domed shaped, high index plastic package:

(With ndome = 2.2, 10% gets out the top!**)

If the device is fabricated with a substrate that is transparent to the emitted radiation, then light can be extracted from the 4 sides and bottom of the device, as well as the top.

Increases the extraction efficiency by a factor of 6!

* Critical angle, θc = sin-1(nout/nin), ** Fraction ≈ (sinθc)2/4 =(nout/2nin)2 Clif Fonstad, 10/6/09 Lecture 8 - Slide 28

Left:

Light emitting diodes: the latest wrinkles Surface texturing, Super-thin (~ 5 µm) devices

p-pad (Cr/Au)

GaAs/AuGe Dot n-pad (Cr/Au)

ITO

Al/Al2O3/ITO

Roughened n-GaN

n-GaN p-GaNMQW active region

Metal anode and cathode contacts

Ceramic submount


©2005 IEEE. Used with permission.

Lee et al, "Increasing the extraction efficiency of AlGaInP LEDs via n-side surface roughening," IEEE Photonics Technology Letters 17 (2005) 2289.

Right: Shchekin et al, "High performance thin-film flip-chip InGaN-Gan light-emitting diodes," Applied Physics Letters 89 (2006) 071109. (Already in production by Philips LumiLeds.)

Courtesy of the American Institute of Physics. Used with permission.Clif Fonstad, 10/6/09 Lecture 8 - Slide 30

6.012 - Microelectronic Devices and Circuits

Lecture 10 - BJT Wrap-up, Solar Cells, LEDs - Summary

• Photodiodes and solar cells Characteristic: iD(vAB, L) = IS(eqvAB/kT -1) - IL

Reverse or zero bias: iD(vAB < 0) ≈ – IL (detects the presence of light)

In fourth quadrant: iD x vAB < 0 (power is being produced!!)

• Light emitting diodes; laser diodes Materials: red: GaAlAs, GaAsP, GaP amber: GaAsP

yellow: GaInN green: GaP, GaN blue: GaN white: GaN w. a phosphor

The LED renaissance: new materials (phosphides, nitrides) new applications (fibers, lighting, displays, etc)

Laser diodes: CD players, fiber optics, pointers Check out: http://www.britneyspears.ac/lasers.htm


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6.012 Microelectronic Devices and Circuits Fall 2009

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