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
C
!FiF
or "FiB
iF
IES
B iB
+
vBE
C
iB
B+
"F # $
iC
=
iE
Defects
!
Design
!
Clif Fonstad, 10/6/09
"E
IBS
vBE
–
E
(1$ %B )
(1+ %E )
!FiB
&
1
(1+ %E )
Dh N AB w B,eff
=
#
#
De N DE !w E,eff
"F #
–
E
iC
(1$ %B ) & 1
=
iB
(%E + %B ) %E
"B #
w B2 ,eff
2L2eB
Doping : npn with N DE >> N AB
w B,eff : very !
small
LeB : very large and >> w B,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
increasing β and, in turn, iC.
B
E
n+
p
n
C
Early effect
Punch through
wB,eff
To minimize the Early effect we make NDC < NAB
• We will take this effect into account in our small signal LEC modeling.
• 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
Clif Fonstad, 10/6/09 reverse breakdown of the B-C junction.
Lecture 8 - Slide 3
pnp BJT's: The other "flavor" of bipolar junction transistor
pnp
C
iC
+
Structure:
Symbol and FAR model:
Oriented with emitter down like npn:
C
iC
iB
iB
vCE
+
p+
NAE
E
–
IES
vEB
− E
+
vEB
B
−
−
"FiB
E
Oriented as found in circuits:
+ E
E
n
NDB
vBE
Clif Fonstad, 10/6/09
vBE
+
B
or
B iB
B
+
p
NAC
C
qVEB/kT
!FIESe
B
iB
iE
−
C
-iC
vEB
–
iB
IES
!FIESe
qVEB/kT
or
"FiB
C
Lecture 8 - Slide 4
Metallic base contact
n-type base wafer
Active base
region
Metallic emitter
contact
Approx. 0.1"
(2.5 mm)
Photograph of grown junction BJT (showing device width of 5 mm)
removed due to copyright restrictions.
Recrystallized
p-type regions
Metallic collector contact
Approx. 0.001"
(25 µm)
Grown junction BJT - mid-1950's
Figure by MIT OpenCourseWare.
p-type emitter region
Alloy junction BJT
- Early1950's
Early Bipolar
Junction
Transistors
- the first 10 yrs.
Base contact
Base contact Emitter contact
n-type base region
Active base region
Approx. 0.0002"
(5 µm)
p-type collector region
Approx. 0.01"
(0.25 mm)
Collector contact
Figure by MIT OpenCourseWare.
Clif Fonstad, 10/6/09
Diffused junction BJT - late-1950's
Lecture 8 - Slide 5
Integrated Bipolar Junction Transistors
- integrated circuit processes.
p+
n
n
n++
n+
n+
p
p+
n
n
n+
p
Junction isolated integrated BJT - 1960's onwards
n
n++
n+
n++
p
n
n+
n
p-
Oxide isolated integrated BJT - a modern process
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.
Ohmic
contact
A
+
qM
iD
p
Ohmic
contact
B
n
-
vAB
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) = iD (v AB ,0) + iD (0, M)
We know iD(vAB,0) already…
iD (v AB ,0) = IS (e qv AB
kT
"1)
!
The question is, "What is iD(0,M)."
Clif Fonstad, 10/6/09
!
iD (0, M) = ?
Lecture 8 - Slide 10
Photodiodes - cont.: the photocurrent, iD(0,M)
The excess minority carriers:
n'p p'n
p'(xL)
n'(-wp≤x≤-xp) = 0
p'(wn) = 0
p'(xn) = 0
-wp
-xp 0 xn
The minority carrier currents:
Je Jh
Je(-wp≤x≤-xp) = 0
x
xL =
wn
xn+a(wn-xn)
aqM
-wp
-xp 0 xn
xL
x
wn
qM
-(1-a)qM
The photocurrent, iD(0,M):
Clif Fonstad, 10/6/09
iD (0, M) = " AqM (1" a)
Lecture 8 - Slide 11
Photodiodes - cont.: The i-v characteristic and what it means.
The total current:
iD (v AB , M) = iD (v AB ,0) + iD (0, M)
= IS (e qv AB
kT
"1) " AqM (1" a)
The illumination shifts the ideal diode curve vertically
down.
iD
!
iD(vAB,0)
iD(0,M) = -AqM(1-a)
Light detection
in this quadrant
Clif Fonstad, 10/6/09
iD(vAB,M)
vAB
Power conversion
in this quadrant
Lecture 8 - Slide 12
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.
h"
vOC
!
!
3.
$< E g
%
&> E g
not absorbed; energy lost
excess energy, ( h" # E g ), lost
kT # q"i L &
=
ln%
(
q $ IS '
L"# $"
Pout max < "iSC vOC
% q# i L (
= #i L $ kT ln'
*
& IS )
!
!
Clif Fonstad, 10/6/09
Skyline Solar parabolic reflector/concentrator multi-junction cell
installation photo-illustration from website.
Courtesy of Skyline Solar Inc. Used with permission.
Lecture 8 - Slide 14
Multi-junction cells - efficiency improvement with number
Clif Fonstad, 10/6/09
"Photovoltaics take a load off soldiers," Oct. 27, 2006, online at:
http://www.solar.udel.edu/CSOctSOL-Darpa-reprint.pdf
Courtesy of Institute of Physics. Used with permission.
Lecture 8 - Slide 15
Multi-junction cells,
cont. - 2 designs
InGaP cell
InGaP cell
Eg= 1.84 eV (0.67 µm)
Eg= 1.84 eV (0.67 µm)
Tunnel junction
GaAs
GaAs cell
cell
E
Egg=
= 1.43
1.43 eV
eV (0.86
(0.86 µm)
µm)
Tunnel junction
Ge cell
Eg= 0.7 eV (1.75 µm)
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:
http://www.solar.udel.edu/CSOctSOL-Darpa-reprint.pdf
Courtesy of Institute of Physics. Used with permission.
Lecture 8 - Slide 16
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 long
minority carrier lifetimes in silicon, but in LEDs we want all the
excess carriers to recombination, and to do so creating a
photon of light. We want asymmetrical long-base operation:
n'p, p'n
n' large
p'n very small
-wp
-xp0 xn
wn
x
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
1.0
Relative photon intensity
0.9
Light emitting diodes typical spectra
•
LED emission - typ. 20 nm wide
lf = 20 mA
TA = 25o C
0.8
0.7
0.6
GaAsP
Red
LED
0.5
0.4
GaAsP
Red
LED
0.3
0.2
0.1
•
0
600
P
Important spectra to compare
with LED emission spectra
620
640
660
680
� - Wavelength-nm
700
Figure by MIT OpenCourseWare.
Relative spectral response or output
Relative spectral characteristics
1.2
Output of Tungsten
source at 2870 K
1.0
0.8
Response of
human eye
Output of
TIL23
TIL24
TIL25
GaAs
LEDs
0.6
0.4
0.2
0
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Response of silicon
phototransistors
1.0
1.1
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 commercial
compound semiconductor devices in the marketplace. Red,
amber, and green LEDs (but not blue) were sold in the 1960's, but
the 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)
Clif Fonstad, 10/6/09
Lecture 8 - Slide 20
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)
Clif Fonstad, 10/6/09
Lecture 8 - Slide 22
Compound Semiconductors:
Diamond lattice (Si, Ge, C [diamond])
A wide variety of bandgaps.
The majority are "direct gap"
must for efficient optical emission).
III
IV
V
VI
B
C
N
O
5
6
7
8
Al
Si
P
S
13
14
15
16
Zn
Ga
Ge
As
Se
30
31
32
33
34
Cd
In
Sn
Sb
Te
48
49
50
51
52
Hg
Tl
Pb
Bi
Po
80
81
82
83
84
II
Figure by MIT OpenCourseWare.
Zinc blende lattice (GaAs shown)
Ga
Ag
Figure by MIT OpenCourseWare.
Clif Fonstad, 10/6/09
Lecture 8 - Slide 23
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.
Clif Fonstad, 10/6/09
Lecture 10
8 - -Slide
Slide2420
Materials for Amber LEDs: GaAsP, AlInGaP, and GaP
Modern AlInGaP
amber LEDs
grown latticematched 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 10
8 - -Slide
Slide2521
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
Clif Fonstad, 10/6/09
Lecture 8 - Slide 26
A material covering the spectrum:
1
3
2
C
The III-V wurtzite quarternary: GaInAlN
120o
AlN
0.2
6.0
Figure by MIT OpenCourseWare.
5.0
0.25
Sze PSD Fig 2a
0.3
4.0
Good for UV
(unique)
GaN
0.4
3.0
0.5
2.0
0.6
0.7
InN
Great for blue
(the best)
Good for green
Not so good for
red yet.
1.0
0.28
Clif Fonstad, 10/6/09
0.30
0.34
0.32
Lattice period, a (nm)
0.36
0.38
InN
Lecture 8 - Slide 27
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!
Clif Fonstad, 10/6/09
* Critical angle, θc = sin-1(nout/nin), ** Fraction ≈ (sinθc)2/4 =(nout/2nin)2 Lecture 8 - Slide 28
Light emitting diodes: the latest wrinkles
Surface texturing, Super-thin (~ 5 µm) devices
n-pad (Cr/Au)
Roughened n-GaN
GaAs/AuGe Dot
ITO
MQW active region
n-GaN
p-GaN
Metal anode and
cathode contacts
p-pad (Cr/Au)
Ceramic submount
Figure by MIT OpenCourseWare.
Al/Al2O3/ITO
©2005 IEEE. Used with permission.
Left:
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
In fourth quadrant: iD x vAB < 0
(detects the presence of light)
(power is being produced!!)
• Light emitting diodes; laser diodes
Materials: red: GaAlAs, GaAsP, GaP amber: GaAsP
yellow: GaInN
blue: GaN
green: GaP, 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
Clif Fonstad, 10/6/09
Lecture 8 - Slide 32
MIT OpenCourseWare
http://ocw.mit.edu
6.012 Microelectronic Devices and Circuits
Fall 2009
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