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

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1 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 to Today p-n Diode review Reverse biased junctions - photodiodes and solar cells In the dark: no minority carriers, no current With illumination: superposition, i D (v AB, L); photodiodes The fourth quadrant: optical-to-electrical conversion; solar cells Video: "Solar cell electricity is better electricity - putting 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

2 BJT Modeling: FAR models/characteristics B + ib vbe C!FiF or "FiB if E " F # $ i C = 1$% B i E 1+ % E IES ( ) ( ) & 1 ( ) 1+ % E B ib + vbe IBS " F # i C i B = Defects " E = D h D e # N AB N DE # w B,eff w E,eff " B # C E!FiB ( 1$% B ) ( ) & 1 % E + % B 2 w B,eff 2 2L eb % E Design Doping : npn with N DE >> N AB w B,eff : very small Clif Fonstad, 10/6/09 L eb : very large and >> w B,eff Lecture 8 - Slide 2

3 BJT's, cont.: What about the collector doping, N DC? 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 v CE increases and the effective base width, w B,eff, gets smaller, thereby increasing β and, in turn, i C. B E n+ p n C Early effect Punch through w B,eff To minimize the Early effect we make N DC < N AB 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, I C 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

4 pnp BJT's: The other "flavor" of bipolar junction transistor pnp Structure: B i B + v BE i C p C N AC n N DB p + N AE E i E + Symbol and FAR model: Oriented with emitter down like npn: v CE i B C B + v BE E i C B ib veb Oriented as found in circuits: B v EB i B + C E -i C B veb ib + + C E E C qveb/kt!fiese or "FiB IES IES qveb/kt!fiese or "FiB Clif Fonstad, 10/6/09 Lecture 8 - Slide 4

5 Metallic base contact n-type base wafer Metallic emitter contact Active base region Recrystallized p-type regions Approx " (25 µm) Approx. 0.1" (2.5 mm) Metallic collector contact Photograph of grown junction BJT (showing device width of 5 mm) removed due to copyright restrictions. Figure by MIT OpenCourseWare. Grown junction BJT - mid-1950's Alloy junction BJT - Early1950's p-type emitter region Base contact Base contact Emitter contact n-type base region Early Bipolar Junction Transistors - the first 10 yrs. Approx " (5 µm) Active base region p-type collector region Collector contact Approx. 0.01" (0.25 mm) Figure by MIT OpenCourseWare. Clif Fonstad, 10/6/09 Diffused junction BJT - late-1950's Lecture 8 - Slide 5

6 Integrated Bipolar Junction Transistors - integrated circuit processes. n p+ n++ p n+ n n+ n p+ n+ p n Junction isolated integrated BJT 's onwards n n++ n+ p n+ n n++ n p- Clif Fonstad, 10/6/09 Oxide isolated integrated BJT - a modern process Figure by MIT OpenCourseWare. Lecture 8 - Slide 6

7 Photodiodes - illuminated p-n junction diodes Consider a p-n diode illuminated at x = x n + a(w n -x n ), 0 a 1. Ohmic contact qm Ohmic contact i D p A + - v AB -w p -x p 0 x n x L = w n x n +a(w n -x n ) What is i D (v AB, M)? Use superposition to find the answer: i D (v AB,M) = i D (v AB,0) + i D (0,M) n B x We know i D (v AB,0) already i D (v AB,0) = I S (e qv AB kt "1) The question is, "What is i D (0,M)." i D (0,M) =? Clif Fonstad, 10/6/09 Lecture 8 - Slide 10

8 Photodiodes - cont.: the photocurrent, i D (0,M) The excess minority carriers: n' p p' n p'(x L ) n'(-w p x -x p ) = 0 p'(x n ) = 0 -w p -x p 0 x n x L = w n The minority carrier currents: x n +a(w n -x n ) J e J h J e (-w p x -x p ) = 0 aqm -w p -x p 0 x n x L w n -(1-a)qM qm p'(w n ) = 0 x x The photocurrent, i D (0,M): i D (0, M) = " AqM ( 1" a) Clif Fonstad, 10/6/09 Lecture 8 - Slide 11

9 Photodiodes - cont.: The i-v characteristic and what it means. The total current: i D (v AB,M) = i D (v AB,0) + i D (0,M) ( ) " AqM 1" a = I S e qv AB kt "1 ( ) The illumination shifts the ideal diode curve vertically down. i D i D (v AB,0) i D (v AB,M) i D (0,M) = -AqM(1-a) v AB Light detection in this quadrant Power conversion in this quadrant Clif Fonstad, 10/6/09 Lecture 8 - Slide 12

10 Photovoltaic Energy Conversion: Solar cells and TPV, cont. 1. Efficiency issues: 1. hν:e g mismatch 2. V oc and fill factor 3. Intensity (concentrator) effect h" $ < E g not absorbed; energy lost % & > E g excess energy, ( h" # E g ), lost 2. v OC = kt q ln # q" L & i % ( $ ' I S P out max < "i SC v OC % = # i L $ kt ln q# L ( i ' * & ) I S 3. 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.

11 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 Courtesy of Institute of Physics. Used with permission.

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

12 Multi-junction cells, cont. - 2 designs InGaP cell InGaP cell E g = 1.84 ev (0.67 µm) g 1.84 ev (0.67 µm) Tunnel junction GaAs cell E g = 1.43 ev (0.86 µm) Tunnel junction Ge cell E g = 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: Lecture 8 - Slide 16 Courtesy of Institute of Physics. Used with permission.

13 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' large n' p, p' n p' n very small -w -x 0 x w p p n n 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

14 1.0 Light emitting diodes - typical spectra LED emission - typ. 20 nm wide Relative photon intensity GaAsP Red LED l f = 20 ma T A = 25 o C GaAsP Red LED Important spectra to compare with LED emission spectra P λ - Wavelength-nm Figure by MIT OpenCourseWare. 1.2 Relative spectral response or output Relative spectral characteristics Response of human eye Output of TIL23 TIL24 TIL25 GaAs LEDs Output of Tungsten source at 2870 K Response of silicon phototransistors λ -Wavelength-µm Figure by MIT OpenCourseWare. Clif Fonstad, 10/6/09 Lecture 8 - Slide 19

15 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

16 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).

17 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 5 C 6 N 7 O 8 II Al 13 Si 14 P 15 S 16 Figure by MIT OpenCourseWare. Zinc blende lattice (GaAs shown) Zn 30 Ga 31 Ge 32 As 33 Se 34 Ag Ga Cd 48 Hg 80 In 49 Tl 81 Sn 50 Pb 82 Sb 51 Bi 83 Te 52 Po 84 Figure by MIT OpenCourseWare. Lecture 8 - Slide 23 Clif Fonstad, 10/6/09

18 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) 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 Slide 2420

19 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) 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 Slide 2521

20 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 E d. InGaAlP grown by MOCVD on GaAs substrates provide modern high brightness LEDs Clif Fonstad, 10/6/09 Lecture 8 - Slide 26

21 A material covering the spectrum: 1 3 The III-V wurtzite quarternary: GaInAlN 2 C 6.0 AlN o Figure by MIT OpenCourseWare. Sze PSD Fig 2a GaN Good for UV (unique) Great for blue (the best) InN Lattice period, a (nm) InN Good for green Not so good for red yet. Clif Fonstad, 10/6/09 Lecture 8 - Slide 27

22 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 n dome = 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, θ c = sin -1 (n out /n in ), ** Fraction (sinθ c ) 2 /4 =(n out /2n in ) 2 Clif Fonstad, 10/6/09 Lecture 8 - Slide 28

ITO Light emitting diodes: the latest wrinkles Surface texturing, Super-thin (~ 5 µm) devices n-pad (Cr/Au) GaAs/AuGe Dot p-pad (Cr/Au) MQW active region Roughened n-gan n-gan p-gan Metal anode and

23 ITO Light emitting diodes: the latest wrinkles Surface texturing, Super-thin (~ 5 µm) devices n-pad (Cr/Au) GaAs/AuGe Dot p-pad (Cr/Au) MQW active region Roughened n-gan n-gan p-gan Metal anode and cathode contacts Ceramic submount Al/Al 2 O 3 /ITO Figure by MIT OpenCourseWare 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) Right: Shchekin et al, "High performance thin-film flip-chip InGaN-Gan light-emitting diodes," Applied Physics Letters 89 (2006) (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

24 Microelectronic Devices and Circuits Lecture 10 - BJT Wrap-up, Solar Cells, LEDs - Summary Photodiodes and solar cells Characteristic: i D (v AB, L) = I S (e qv AB/kT -1) - I L Reverse or zero bias: i D (v AB < 0) I L In fourth quadrant: i D x v AB < 0 Light emitting diodes; laser diodes (detects the presence of light) (power is being produced!!) 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: Clif Fonstad, 10/6/09 Lecture 8 - Slide 32

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