BJTs Wrap-up, Solar Cells, LEDs
Transcript of 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
Clif Fonstad, 10/6/09 Lecture 8 - Slide 4
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.
Clif Fonstad, 10/6/09
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)
Figure by MIT OpenCourseWare.
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) = ?
Clif Fonstad, 10/6/09 Lecture 8 - Slide 10
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( )
Clif Fonstad, 10/6/09 Lecture 8 - Slide 11
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
Clif Fonstad, 10/6/09 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.
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.
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.
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)
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: 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).
Figure by MIT OpenCourseWare.
Ga
Ag
Lecture 8 - Slide 23 Figure by MIT OpenCourseWare.
Clif Fonstad, 10/6/09
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.
Clif Fonstad, 10/6/09
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
Clif Fonstad, 10/6/09 Lecture 8 - Slide 26
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
Figure by MIT OpenCourseWare.
Clif Fonstad, 10/6/09 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!
* 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
Figure by MIT OpenCourseWare.
©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
Clif Fonstad, 10/6/09 Lecture 8 - Slide 32
MIT OpenCourseWarehttp://ocw.mit.edu
6.012 Microelectronic Devices and Circuits Fall 2009
For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.