COPYRIGHTED MATERIAL. Chapter 1. Bipolar Transistors John D. Cressler and Katsuyoshi Washio. 1.1 Motivation

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1 Chapter 1 Bipolar Trasistors Joh D. Cressler ad Katsuyoshi Washio 1.1 Motivatio I terms of its ifluece o the developmet of moder techology ad hece, global civilizatio, the ivetio of the poit cotact trasistor o December 23, 1947 at Bell Labs i New Jersey by Bardee ad Brattai was by ay reckoig a watershed momet i huma history [1]. The device we kow today as a bipolar juctio trasistor was demostrated four years later i 1951 by Shockley ad co-workers [2] settig the stage for the trasistor revolutio. Our world has chaged profoudly as a result [3]. Iterestigly, there are actually seve major families of semicoductor devices (oly oe of which icludes trasistors!), 74 basic classes of devices withi those seve families, ad aother 130 derivative types of devices from those 74 basic classes (Figure 1.1) [4]. Here we focus oly o three basic devices: (1) the p homojuctio juctio diode (or p juctio or diode), (2) the homojuctio bipolar juctio trasistor (or BJT), ad (3) the special variat of the BJT called the silico-germaium heterojuctio bipolar trasistor (or SiGe HBT). As we will see, diodes are useful i their ow right, but also are the fuctioal buildig block of all trasistors. Surprisigly, all semicoductor devices ca be built from a remarkably small set of materials buildig blocks (Figure 1.2), icludig [4]: the metal semicoductor iterface (e.g., Pt/Si; a Schottky barrier ) the dopig trasitio (e.g., a Si p-type to -type dopig trasitio; the p juctio) the heterojuctio (e.g., -AlGaAs/p-GaAs) the semicoductor/isulator iterface (e.g., Si/SiO 2 ) the isulator/metal iterface (e.g., SiO 2 /Al). COPYRIGHTED MATERIAL Guide to State-of-the-Art Electro Devices, First Editio. Edited by Joachim N. Burghartz Joh Wiley & Sos, Ltd. Published 2013 by Joh Wiley & Sos, Ltd

2 4 Guide to State-of-the-Art Electro Devices The Trasistor Food Chai Diodes Trasistors No-Volatile Memories Thyristors / Power Devices Rectifiers Negative R (N-shaped) Negative R (S-Shaped) Negative R (Trasit Time) Field Effect Trasistors Potetial Effect Trasistors Juctio Diodes p-i- Diode Schottky Barrier Diode Plaar Doped Diode Isotype Heterojuctio Isulated Gate FETs JFET MESFET MODFET Permeable Base Trasistor SIT RST Plaar-Doped FET Surface Tuel Trasistor LRTFET Stark Effect VMT p Juctio Diode Zeer Diode Step-Recovery Diode Fast Recovery Diode Sap-back Sap-off Diode Varactor Diode Esaki Diode MOSFET Straied Si MOSFET DMOS LDMOS HEXFET VMOS UMOS TFT MISFET PRESSFET Photoic Devices Resistace ad Capacitace Devices Sesors Bipolar Trasistors THETA Metal Base Trasistor BiCFET TETRAN PDB HHET Iduced Base Trasistor RTBT QWBRTT Spi-Valve Trasistor Poit Cotact Trasistor BJT HBT DHBT Darligto Amplifier Tuelig-Emitter Trasistor Figure 1.1 The trasistor food chai showig all major families of semicoductor devices. Reproduced with permissio from Cressler, J. D.; Silico Earth: Itroductio to the Microelectroics ad Naotechology Revolutio; 2009, Cambridge Uiversity Press Why do we actually eed trasistors i the first place? Basically, because ature atteuates all electrical sigals. By this we mea that the magitude of all electrical sigals (thik 1s ad 0s iside a computer, or a EM radio sigal from a cell phoe) ecessarily decreases as it moves from poit A to poit B, somethig we call loss. Whe we preset a (atteuated) iput sigal to the trasistor, the trasistor is capable of creatig a output sigal of larger magitude (i.e., gai ), ad hece the trasistor serves as a gai block to regeerate (recover) the atteuated sigal i questio, a essetial cocept for electroics. I the electroics world, whe the trasistor is used as a source of sigal gai, we refer to it as a amplifier. Amplifiers are ubiquitous to all electroic systems. Namig of the Trasistor The ame trasistor was actually coied by J.R. Pierce of Bell Labs, followig a office bettig pool which he wo. He started with a literal descriptio of what the device actually does electroically, a trasresistace amplifier, which he first shorteed to tras-resistor, ad the fially trasistor [3] E. vo Kleist ad P. va Musschebroek ivet the capacitor (Leyde Bottle)

3 Bipolar Trasistors 5 Ohmic Cotact Plaar- Doped Barrier Quatum Well Isulator E C Metal Semicoductor E F E V Metal- Semicoductor Cotact Dopig Trasitio Heterojuctio Semicoductor- Isulator Trasitio Isulator- Metal Trasitio Figure 1.2 The essetial buildig blocks of all semicoductor devices. Reproduced with permissio from Cressler, J. D.; Silico Earth: Itroductio to the Microelectroics ad Naotechology Revolutio; 2009, Cambridge Uiversity Press Not oly ca the trasistor serve as a woderful aoscale sized amplifier, but importatly it ca also be used as a tiy regeerative switch ; meaig, a o/off switch that does NOT have loss associated with it. Why is this so importat? Well, imagie that the computatioal path through a microprocessor requires biary switches (thik light switch o the wall o/off, o/off) to implemet the complex digital biary logic of a give computatio. If each of those switches eve cotributes a tiy amout of loss (which it ievitably will), multiplyig that tiy loss by adds up to uacceptably large system loss. That is, if we push a logical 1 or 0 i, it rapidly will get so small durig the computatio that it gets lost i the backgroud oise. If, however, we implemet our biary switches with gai-eabled trasistors, the each switch is effectively regeerative, ad we ca ow propagate the sigals through the millios of requisite logic gates without excessive loss, maitaiig their magitude above the backgroud oise level. I short, the trasistor ca serve i oe of two fudametal capacities: (1) a amplifier or (2) a regeerative switch. Amplifiers ad regeerative switches work well oly because the trasistor has the ability to produce gai. So a logical questio becomes, where does trasistor gai come from? To aswer this, first we eed to uderstad p juctios. 1.2 The p Juctio ad its Electroic Applicatios Virtually all semicoductor devices (both electroic ad photoic) rely o p juctios (a.k.a., diodes, a ame which harkes back to a vacuum tube legacy) for their fuctioality. The simplest embodimet of a p juctio is the p homojuctio, meaig that withi a sigle piece of semicoductor (e.g., silico Si) we have a trasitio betwee p-type dopig ad -type dopig (e.g., p-si/-si). The opposite would be Alesadro Volta develops the codeser

4 6 Guide to State-of-the-Art Electro Devices B A p A' A p A' B p B' B' Dopig cocetratio N A 0 N D Metallurgical Juctio Dopig cocetratio x 0 0 N D N A Step Juctio Approximatio Depth x 0 Depth Figure 1.3 Cartoos of a p juctio, showig dopig trasitio from -type to p-type. Reproduced with permissio from Cressler, J. D.; Silico Earth: Itroductio to the Microelectroics ad Naotechology Revolutio; 2009, Cambridge Uiversity Press a p heterojuctio, i which the p-type dopig is withi oe type of semicoductor (e.g., p-gaas), ad the -type dopig is withi aother type of semicoductor (e.g., -AlGaAs). As show i Figure 1.3, to build a p juctio we might, for istace, io implat ad the diffuse -type dopig ito a p-type wafer. The importat thig is the resultat dopig profile as oe moves through the juctio (N D (x) N A (x), which is just the et dopig cocetratio). At some poit i the dopig trasitio, N D = N A, ad we thus have a trasitio betwee et -type ad et p-type dopig. This poit is called the metallurgical juctio (x 0 i Figure 1.3) ad all of the importat electrical actio of the juctio is cetered here. To make the physics easier, two simplificatios are typically made: (1) Let us assume a step juctio approximatio to the real p juctio dopig profile, which is just what it says, a abrupt chage (a step) i dopig occurrig at the metallurgical juctio (Figure 1.3). (2) Let us assume that all of the dopat impurities are ioized (oe door atom equals oe electro, etc., a excellet approximatio for commo dopats i silico at 300 K). So, how does a p juctio actually work? The operatio of ALL semicoductor devices is best uderstood at a ituitive level by cosiderig the eergy bad diagram, which plots electro ad hole eergy as a fuctio of positio as we move physically through a device. A -type semicoductor is electro rich (i.e., majority carriers), ad hole poor (i.e., miority carriers). Coversely, a p-type semicoductor is hole-rich ad electro-poor. If we imagie brigig a -type ad p-type semicoductor ito itimate electrical cotact where they ca freely exchage electros ad/or holes from to p ad p to, thefial equilibrium bad diagram show i Figure 1.4 will result. Note, that uder equilibrium coditios, there is o NET curret flow across the juctio. We might logically woder what actually happeed iside the juctio to establish this equilibrium coditio. Whe brought ito cotact, the -type side of the juctio is electro rich, while the p-type side is electro poor. That is, there is a large drivig force for electros to diffuse from the regio to the p regio. Recall, that there are i fact two ways to move charge i a semicoductor: (1) drift, whose drivig force is the electric field (voltage/legth), ad (2) diffusio, whose drivig force is the carrier desity gradiet (chage i carrier desity per uit distace). The latter process is what is operative here The first battery was demostrated by Alessadro Volta

5 Bipolar Trasistors 7 p Juctio Eergy Bad Diagram E vac E qf bi χ Φ g g p Φ p χ p Eergy Electros qf bi E C, p E C, E F, Door ios Acceptor ios E g E i, p EF, p E i, E V, E g qf bi Holes E V, p Neutral regio -type silico Neutral regio p-type silico Juctio width W Positio Cathode p Aode Figure 1.4 Eergy bad diagram of a p juctio at equilibrium. Reproduced with permissio from Cressler, J. D.; Silico Earth: Itroductio to the Microelectroics ad Naotechology Revolutio; 2009, Cambridge Uiversity Press Oce i electrical cotact a electro moves from the -side to the p-side, leavig behid a positively charged door impurity (N D + ). Note, that far away from the juctio, for each charged door impurity there is a matchig doated electro, hece the semicoductor is charge eutral. Oce the electro leaves the -side, however, there is o balacig charge, ad a regio of space charge results. The same thig happes o the p-side. Hole moves from p to, leavig behid a ucompesated acceptor impurity (N A ) behid. This resultat charge dipole produces a electric field, poitig from + to (to the right i this case). How does that iduced field affect the diffusio-iitiated side-to-side trasfer of charge just described? It opposes the diffusive motio of both electro ad holes via Coulomb s law. Therefore, i a p juctio the diffusio gradiet moves electros from to p ad holes from p to, but as this happes a dipole of space charge is created betwee the ucompesated ioized dopats, ad a iduced electric Georg Ohm formulates ìohm s Law I = V / R

6 8 Guide to State-of-the-Art Electro Devices field opposes the further diffusio of charge. Whe does equilibrium i the p juctio result? Whe the diffusio ad the drift processes are perfectly balaced ad the et curret desity is zero. The p juctio i equilibrium cosists of a eutral regio ad a eutral p regio, separated by a space charge regio of width W. This structure forms a capacitor (coductor/isulator/coductor), ad p juctios have built-i capacitace which will partially dictate their switchig speed. The electric field i the space charge regio (for a step juctio) is characteristically triagular shaped, with some peak value of electric field preset. There is a built-i voltage drop across the juctio, ad, thus, from the eergy bad diagram we see that there is a potetial barrier for ay further movemet of electros ad holes from side-to-side. This barrier to carrier trasport maitais a et curret desity of zero, ad the juctio is by defiitio i equilibrium. If oe wated to get curret flowig agai across the juctio, how would this be doe? Well, we must ubalace the drift ad diffusio mechaisms by lowerig the potetial barrier to the electro ad hole trasport, ad we ca do this trivially by applyig a exteral voltage to the ad p regios such that the p regio (aode) is more positively biased tha the regio (cathode). As show i Figure 1.5, this effectively lowers the side-to-side barrier, drift o loger balaces diffusio, ad the carriers will oce agai start diffusig from side-to-side, geeratig useful curret flow. This is called forward bias. What happes if we apply a voltage to the juctio of opposite sig? (i.e., p regio more egatively biased tha the regio). Well, the barrier the carriers experiece grows, effectively prevetig ay curret flow, a coditio called reverse bias (Figure 1.5). The p juctio thus forms a solid-state switch (a.k.a. the diode ). Cosider: Apply a voltage of oe polarity ad curret flows. Apply a voltage of the opposite polarity ad o curret flows; a o/off switch. Shockley shared the Nobel Prize with Bardee ad Brattai largely for explaiig this pheomeo, ad of course by wrappig predictive theory aroud it which led to the demostratio of the BJT. The result of that particularly elegat derivatio is the celebrated Shockley equatio which govers the curret flow i a p juctio I = qa { D 2 i L N A + D p 2 i L p N D } ( ) ( ) e qv/kt 1 = I S e qv/kt 1 where A is the juctio area, V is the applied voltage, D,p is the electro/hole diffusivity (D,p = μ,p kt), L,p is the electro/hole diffusio legth, ad I S is the juctio saturatio curret which collapses all of these factors ito a sigle (measurable) parameter. Observe, that all of the parameters i the Shockley equatio refer to the miority carriers. If we build our juctio with the ad p dopig the same, the the relative cotributios of the electro ad hole miority carrier currets to the total curret flowig will be comparable (to first order). Let us look closer at the operatio of the juctio. Uder forward bias, electros diffuse from the -side to the p-side, where they become miority carriers. Those miority electros are ow free to recombie ad will do so, o a legth scale determied by L, ad thus as we move from the ceter of the juctio out ito the eutral p-regio, the miority electro populatio decreases due to recombiatio, iducig a cocetratio gradiet as we move to the p-side, which drives a miority electro diffusio curret. The same thig is happeig with holes o the opposite side of the juctio, ad these two miority carrier diffusio currets add to produce the total forward bias curret flow. What is the actual drivig force behid the forward bias curret i a p juctio? Recombiatio i the eutral regios, sice recombiatio iduces the miority diffusio currets. Alas, simple theory ad reality are ever coicidet, ad there a fiite limits to the voltages that (1.1) Michael Farady ad Joseph Hery formulate 1831 Faraday's law of 1833 iductio K.F. Gauss ad W.Weber devise electromagetic telegraph

7 Bipolar Trasistors 9 p Juctio uder Bias Reverse bias e h + E C (reverse bias) E V (reverse bias) Equilibrium Tuelig curret e Geeratio Recombiatio h + E C (equilibrium) E V (equilibrium) e Multiplicatio curret h + e Forward bias Excess electro cocetratio Equilibrium electro cocetratio E C (forward bias) Curret voltage I Equilibrium hole cocetratio Recombiatio curret Excess hole cocetratio h + E V (forward bias) Reverse Bias Tuelig ad multiplicatio curret Forward Bias V Figure 1.5 The p juctio uder both forward ad reverse bias, showig the resultat curret voltage characteristics. Reproduced with permissio from Cressler, J. D.; Silico Earth: Itroductio to the Microelectroics ad Naotechology Revolutio; 2009, Cambridge Uiversity Press ca be applied to the diode, ad how much curret ca be passed through it ad how much voltage ca be applied across it [3]. So, what makes the juctio so useful? Well, as stated, it makes a ice o/off switch with low loss whe forward biased, ad it ca provide very good electrical isolatio whe reverse biased. I power electroics the diode would be said to provide a blockig voltage, ot allowig curret flow i reverse bias up to some fiite, ad ofte huge, applied reverse voltage (hudreds to eve thousads of volts). This is very useful. The diode ca also fuctio as a woderful solid-state rectifier. Rectifiers are ubiquitous i power geeratio, coversio, ad trasmissio, (e.g., to tur AC voltage ito DC voltage). Fially, the diode ca also emit ad detect light, which is also extremely useful as a trasducer for covertig optical to electrical eergy, ad vice versa (see Chapters 16 ad 20). All of this said, however, the diode does NOT possess gai, ad, thus, is isufficiet for realizig complex electroic systems. From a trasistor perspective, however, the p juctio ca be used to make a tuable miority carrier ijector, which, if cleverly employed, ca ideed produce gai whe carefully implemeted 1836 Iductor coil is iveted by Nicholas Calla

8 10 Guide to State-of-the-Art Electro Devices withi a trasistor. Importatly, oe ca trivially skew the relative magitudes of the miority carrier ijectio from side-to-side i a p juctio by makig the dopig levels o oe side of the juctio much more heavily doped tha o the other side. Let us imagie that the -dopig is far larger tha the p-dopig. Fittigly, this is referred to as a oe-sided juctio. I this sceario, it ca be easily show that electros make up most of the total curret flow i forward bias i such a juctio. If we wated to use a p juctio uder forward bias to ehace the forward-ijectio of electros ito the p-regio, ad suppress the back-ijectio of holes ito the -regio, we could simply use a ++ p juctio as a electro ijector! This will lead us directly to the BJT, a trasistor with gai. 1.3 The Bipolar Juctio Trasistor ad its Electroic Applicatios The p juctio, as a two-termial object, ca be made to serve as a efficiet miority carrier ijector, but it does NOT possess iheret gai. This is the fudametal reaso why we do ot build microprocessors from diode-resistor logic. Diodes make excellet biary switches, but without a gai mechaism to overcome Nature s preferece for atteuatio, complex fuctios are ot goig to be achievable i practice. Let us imagie, however, that we add a additioal third termial to the device which somehow cotrols the curret flow betwee the origial two termials. Let termial 1 = the iput cotrol termial, ad termials 2 ad 3 have high curret flow betwee them whe biased appropriately by the cotrol termial. The, uder the right bias coditios, with large curret flow betwee 2 ad 3, if we could somehow maage to suppress the curret flow to/from 1, we d be i busiess. That is, small iput curret (1) geerates large output curret (from 2 to 3), ad hece we have gai! How do we do this i practice? Let us use two p juctios, placed back-to-back, such that the cotrol termial (our #1; which we will call the Base termial B) is i the cetral p regio, ad the two high curret flow path output termials (our #2 ad #3, which we will call the Emitter ad Collector termials E, ad C), are the two outside regios (see Figure 1.6). Sice the two cetral p regios are shared by both diodes, those ca be coicidet. That is, a regio separated from aother regio by a itermediate p regio actually cotais two p juctios. BJT versus FET At a deep level, the BJT ad the FET are closely related devices. Both have two p juctios which are itegral to their fuctioality. I a FET, a gate electrode is capacitively coupled (through the gate oxide) to the charge coductio path, alterig the curret flow from source to drai. I the BJT, the base electrode is directly tied to the charge coductio path, alterig the curret flow from emitter to collector. Thus, the differeces betwee BJTs ad FETs lie with the how the cotrol termial is electrically tied to the charge coductio path Wheatstoe ad Cooke file patet o electric telegraph This telegraph oly trasmitted 20 of the 26 letters of the eglish alphabet leavig out C, J, Q, V, X ad Z

9 Bipolar Trasistors 11 Emitter Base p Collector E C B p E C B (a) p BJT C C pp BJT I C + I C B V CE B V CE I B I B + I E I E V BE E V BE + E + (b) Figure 1.6 (a) Schematic of the two back-to-back p juctios that form a bipolar juctio trasistor; (b) the circuit symbol of both dopig polarity types are also show. Reproduced with permissio from Cressler, J. D.; Silico Earth: Itroductio to the Microelectroics ad Naotechology Revolutio; 2009, Cambridge Uiversity Press Let us imagie forward biasig the emitter base juctio, ad reverse biasig the collector base juctio, ad the addig two more puzzle pieces: (1) We must dope the emitter very heavily with respect to the base, such that whe we forward bias the emitter base juctio we have large electro flow from E to B ad simultaeously suppress the hole flow from B to E (this is our tuable miority carrier ijector!). (2) We must make the cetral base regio VERY thi. Why? Well, if we do t, the the electros ijected from E to B will simply recombie i the base before they ca reach the collector (to be collected ad 1839 Photovoltaic effect discovered by Alexadre-Edmud Bequerel, the foudatio of moder solar cell techology

10 12 Guide to State-of-the-Art Electro Devices to geerate the required large output curret flow from E to C). Recall that the rough distace a miority carrier ca travel before it recombies is give by the diffusio legth (L,p ). Clearly, we eed the width of the p-type base regio to be much, much less tha this umber; i practice, a few hudred m is required for a moder BJT. The fial result? We have created the p BJT! (Oe could of course swap the dopig polarities to p ad p to ad achieve the same result a pp BJT. We thus have two flavors of BJT, ad this is ofte VERY hady i electroic circuit desig. Cosider ow how the BJT actually works: (1) The reverse-biased CB juctio has egligible curret flow. (2) The forward-biased EB juctio ijects (emits) lots of electros from E to B, that diffuse across the base without recombiig (because it is thi) ad are collected at C, geeratig large electro flow from E to C (curret). BUT, due to the dopig asymmetry i the EB juctio, while a large umber of electros get ijected from E to B, very few holes flow from B to E. Forward electro curret is large, but reverse hole curret is small. That is: small iput base curret; large output collector. Gai! This is otherwise kow i electroics as curret gai (or β). How do we make the BJT? Well, as might be imagied it is more complex tha a p juctio, but eve so, the effort is worth it. Figure 1.7 shows the simplest possible variat. Figure 1.7 also superposes both the equilibrium ad forward-active bias eergy bad diagrams, with the carrier miority ad majority carrier distributios, to help coect the p juctio physics to the BJT operatio. Withi the bad diagram cotext, here is ituitively how the BJT works. I equilibrium, there is a large barrier for ijectig electros from the emitter ito the base. Forward bias the EB juctio ad reverse bias the CB juctio, ad ow the EB barrier is lowered, ad large umbers of electros are ijected from E to B. Sice B is very thi, ad the CB juctio is reverse biased, these ijected electros will diffuse across the base, slide dow the potetial hill of the CB juctio, ad be collected at C, where they geerate a large electro curret flow from E to C. Meawhile, due to the dopig asymmetry of the EB juctio, oly a small desity of holes is ijected from B to E to support the forward bias EB juctio curret flow. Hece, I C is large, ad I B is small. Gai! A differet visualizatio of the magitudes of the various curret cotributios i a well-made, high gai, BJT, are illustrated i Figure 1.8. Shockley s theory to obtai a expressio for β is fairly straightforward from basic p juctio physics (although you have two differet oes to coted with obviously), provided you make some reasoable assumptios o the thickess of the base (base width W b L b ). For the output ad iput currets uder forward-active (amplifier) bias, we obtai: } I C = qa { Db 2 i W b N Ab I B = qa { Dpe 2 i L pe N De e qv BE /kt = I CS e qv BE /kt (1.2) } e qv BE /kt = I BS e qv BE /kt (1.3) where the b ad e, or B ad E, subscripts stad for base ad emitter, respectively. Iterestigly, the curret gai does ot to first-order deped o bias voltage, the size of the juctio, or eve the badgap! We fially obtai, { } β = I C = I CS Db L pe N De = (1.4) I B I BS D pe W b N Ab Morse's telegraph first 1844 used for trasmissio from 1850 Baltimore to Washigto Mica used as isulator i capacitors

11 Bipolar Trasistors 13 Operatio of the BJT E B C p + P + p + I B + + F p + P substrate W E W B W C E + emitter p base collector + collector C I E x = 0 I B x I C B b0 Equilibrium E0 = N DE C0 = N DC e h + C0 E F, E C P E0 P C0 P B0 = N AB Forward Bias I E I C e E V I E I rec h + E F, E C I pe I' pc I C I" pc E V Figure 1.7 Basic structure ad operatioal priciples of the bipolar trasistor. Reproduced with permissio from Cressler, J. D.; Silico Earth: Itroductio to the Microelectroics ad Naotechology Revolutio; 2009, Cambridge Uiversity Press 1855 David E. Hughes ivets the pritig telegraph

12 14 Guide to State-of-the-Art Electro Devices Clearly, the curret gai is a tuable parameter, givig us great flexibility i desig. A commo way to plot the BJT curret voltage characteristics is show i Figure 1.8, where liear I C is plotted versus liear V CE, as a further fuctio of I B.SiceI C is larger tha I B, the gai is implicit here. This plot is kow as the output family or output characteristics. We use the output family to defie the three regios of operatio of the BJT: (1) forward-active (EB juctio forward-biased; CB juctio reverse-biased); (2) saturatio (both EB ad CB juctios forward-biased), ad (3) cut-off (both EB ad CB juctios reverse biased). As idicated, forward-active bias is typically for amplifiers, ad as we will see, switchig betwee cutoff ad saturatio will make a excellet regeerative digital switch! e h + I E I C I E I C I pe I rec I pc I B (a) Switch closed Saturatio Forward active I B,4 Collector Curret (A) I B,3 I B,2 I B,1 I B,0 Collector-Emitter Voltage (V) (b) Switch ope Figure 1.8 Sketch of (a) the relative curret cotributios of the bipolar trasistor ad (b) the resultat curret voltage characteristics. Reproduced with permissio from Cressler, J. D.; Silico Earth: Itroductio to the Microelectroics ad Naotechology Revolutio; 2009, Cambridge Uiversity Press 1860 Philipp Reis builds the first telephoe

13 Bipolar Trasistors 15 How fast ca trasistors switch states (o to off)? The curret speed record for a bipolar trasistor digital switch is less tha 10 picosecods ( secods 10 trillioths of a secod!). What limits that speed? Ituitively, the speed is limited by the time it takes the electros to be ijected from the emitter, trasit (diffuse across) the base, ad the be collected by the collector. I other words, a trasistor ca t be faster tha it takes the charge to move through it. I most trasistors, step two is the limitig oe, ad the so-called base trasit time (τ b ) sets the fudametal speed limit o how fast the BJT ca switch. A first-order base trasit time expressio ca be easily derived, 2 τ W b b = (1.5) 2D b Hece, the smaller τ b is, the faster the BJT ca switch. Clearly, makig W b as small as possible gives us a double beefit. It helps icrease the curret gai, yes, but eve more importatly, it makes the trasistor faster quadratically! So what does the BJT do for us? Let s restate some poits for clarity. This beautiful three-termial semicoductor device, if costructed correctly, will exhibit a (tuable) gai. Gai is the key to success i buildig ay electroic system; hece the deserved fame of the BJT. This itrisic gai will allow us to create a wide variety of amplifiers for use i a myriad of electroics applicatios. Amplifiers that take: (1) A small iput curret ad tur it ito a large output curret (a.k.a., a curret amplifier ); (2) a small iput voltage ad tur it ito a large output voltage (a.k.a., a voltage amplifier ); (3) a small iput curret ad tur it ito a large output voltage (a.k.a., a trascoductace amplifier ); ad (4) a small iput voltage ad tur it ito a large output curret (a.k.a., a trasimpedace amplifier ). Trascoductace (g m ) i the electroics world just meas the icremetal chage i curret divided by the icremetal chage i voltage. As a real-world example of amplifiers-i-actio, at the iput of your cell phoe you have a had-crafted voltage amplifier that takes the tiy little RF sigals ad boosts them to a level sufficiet to maipulate ad decode them (see Chapter 14). I a receiver for a fiber optic lik, you have a had-crafted trasimpedace amplifier that iterfaces with the iput photodetector, to chage the i-comig photoic sigals ito electroic sigals for processig (see Chapter 20). I additio to buildig amplifiers, gai also allows us to costruct ice regeerative biary switches. As ca be see i Figure 1.8, if the iput base curret I B (or iput voltage V BE ) is zero, the output curret I C is zero, the o/off switch is ow ope ad the output voltage V CE is thus high. Let us call that state a logical 1. Coversely, if the iput curret I B (or iput voltage V BE ) is large eough to tur o the trasistor, the output curret I C is large, output voltage V CE drops to a low value, ad the o/off switch is ow closed. Let s call that state a logical 0. A regeerative biary switch! 1.4 Optimizatio of Bipolar Trasistors There are two typical performace metrics (or figures-of-merit: FoM), which idicate how fast or how high a frequecy a bipolar trasistor ca operate. The first is the so-called cutoff frequecy (f T ), the frequecy at which the AC (alteratig curret) curret gai becomes uity. The f T is simply give by theiverseoftotaltrasittime(τ ec ) from the emitter to the collector (f T = 1/2πτ ec ) ad, thus, gives a estimate of the speed-limit of the BJT switch ad is a good FoM for digital circuits. As described above, to improve f T (make it larger) major attetio must be paid to make the base width as arrow as possible. Alfred Nobel 1866 ivets dyamite 1869 Opeig of Suez Caal

14 16 Guide to State-of-the-Art Electro Devices Here, heavily doped polysilico is itroduced to form the emitter regio. This idea is widely used eve i the moder BJTs ad is called a poly-emitter. The poly-emitter is utilized to form a shallow out-diffusio for the emitter impurities, ad thereby allows both a thi base ad emitter desig. The poly-emitter has a additioal advatage; amely, that the very thi ative iterface oxide which aturally occurs betwee the polysilico ad the sigle-crystal silico acts as a effective barrier to prevet the miority carrier (hole) back-ijectio from B to E. It is ecessary to icrease the base dopig cocetratio for a arrower base (to avoid the disappearace of the eutral base, so-called device puchthrough ) but the emitter dopig cocetratio already reaches its maximum value (limited by solid solubility), so the curret gai i a scaled BJT aturally decreases due to the low emitter ijectio efficiecy (the ratio of the ijectig electros from E to B to the ijectig holes from B to E). Therefore, the poly-emitter iterfacial oxide helps to icrease the curret gai ad is a very useful secodary by-product. However, as the emitter scalig progresses, the iterfacial oxide causes the problem of the high emitter resistace ad, thus, must be carefully optimized. The secod importat BJT FoM is the so-called maximum oscillatio frequecy (f max ), the frequecy at which the uilateral power gai becomes uity. The uilateral power gai is the forward power gai i a feedback amplifier, so it is a suitable idex for may aalog ad RF circuits. The f max is approximately give by, f max = f T/ 8πCc r b, where C c is collector capacitace ad r b is base resistace. The critical differece betwee f T ad f max is as follows. The f T is a FoM determied from the oe-dimesioal (vertical) structure, but the f max is a FoM which icludes the two-dimesioal (plaar) structure of the device, because the parasitic C c ad r b appear i the equatio. This meas, to improve f max, it is essetial to miimize the parasitic capacitaces ad resistaces of the plaar structure. As ca be see i Figure 1.7, the itrisic regio for BJT is a oe-dimesioal structure just uder the emitter. The other areas of the trasistor structure are provided maily to lead the base ad collector curret to their electrodes, so they are oessetially the operatio of the device. To improve lateral parasitics, several importat trasistor structures ad process sequeces (e.g., the so-called self-aliged trasistor structure or self-aliged fabricatio process ) have bee developed. Figure 1.9 shows a typical self-aliged BJT structure formed by usig a self-aliged fabricatio process. To reduce C c, it is very importat to reduce the juctio area betwee the base ad collector. Therefore, i this self-aliged trasistor, the base electrode costructed by a polysilico film is placed o a thick oxide layer, miimizig C c i the extrisic base regio. To reduce r b, the arrow (typically 100 m wide or less) space betwee the polysilico emitter ad the polysilico base is defied by the thickess of the isulator which is formed o the side of emitter or base polysilico. This is the origi of the usage of the term self-aliged, that is, the edge of the emitter ad base is automatically defied by base emitter isulator impurity cocetratio (log scale) SiO 2 poly-si base poly-si emitter collector iterfacial oxide + collector depth Figure 1.9 Self-aliged bipolar trasistor structure ad impurity profile uder the emitter 1874 Ferdiad Brau observes rectifyig characteristics of metal poit cotacts i vacuum

15 Bipolar Trasistors 17 the structure, idepedet of the lithography used. I the case of a o-self-aliged plaar cofiguratio defied by lithography, as show i Figure 1.7, the base curret must flow alog a log (about 1 μm log or more) path, so it is difficult to decrease r b. O the other had, i a BJT formed usig a self-aliged process, the distace separatig from the emitter ad base is very small, so this ca be used to effectively reduce r b. Fially, the breakdow voltages (which set the maximum useful operatig voltage of the BJT) are key trasistor parameters for improvig the high-speed ad high-frequecy characteristics of BJT. There is a fudametal trade-off betwee the speed (f T ) ad the breakdow voltage (BV CEO, the breakdow voltage betwee the collector ad emitter whe the base is ope-circuited), ofte termed the Johso limit [6]. The Johso limit is derived oly from cosiderig fudametal issues associated with carrier trasport, ad predicts a achievable f T BV CEO product of 200 GHzV, though i practice this value is sigificatly higher. The cocept of a costat f T BV CEO product i a BJT is useful for desigig the collector regio of the BJT, sice it captures the tradeoff betwee achievable speed ad operatig voltage. 1.5 Silico-Germaium Heterojuctio Bipolar Trasistors The basic cocept of the heterojuctio bipolar trasistor (HBT) was proposed by Shockley i the origial BJT patet (refer to the history i [3]), ad the basic theory of the HBT was published by Kroemer i 1957 [7]. Figure 1.10 shows the equilibrium eergy bad diagram, with the miority ad majority carrier distributios, of the wide badgap emitter HBT. The wide badgap emitter creates a large barrier for ijectig holes from the base ito the emitter, thus icreasig the curret gai. May III-V compoud semicoductors (e.g., GaAs or IP) have bee successfully applied i HBTs by virtue of their compositioally-adjustable growth techology which ca tailor the badgap for a specific eed (called badgap egieerig ). III-V HBTs beefit from this approach ad ca provide a large advace i performace over BJTs (see Chapter 14). However, badgap egieerig did ot exted ito the world of Si-based techologies for may decades, eve though the basic idea was evisioed early-o for HBTs based o silico-compatible silico-germaium (SiGe) alloys. The lattice costats betwee Si ad Ge differ by roughly 4.2%, so the SiGe films grow o Si are compressively straied. The criterio givig the stability of such pseudomorphically grow straied SiGe films o Si idicates a maximum critical thickess Heterojuctio BJT Barrier to electio E C E F A1 x Ga 1 x As E V ΔE C e ΔEV E Bp E B h + p GaAs Barrier to holes GaAs Figure 1.10 Basic idea behid the wide badgap emitter heterojuctio bipolar trasistor. Reproduced with permissio from Cressler, J. D.; Silico Earth: Itroductio to the Microelectroics ad Naotechology Revolutio; 2009, Cambridge Uiversity Press Paper becomes used as isulator i capacitors

16 18 Guide to State-of-the-Art Electro Devices of SiGe film for a give Ge cotet [8]. SiGe films which are device quality, meaig the SiGe films remai stable after thermal processig, were first epitaxially grow i the mid-1980s, ad shortly thereafter the first SiGe HBTs were demostrated [5]. The badgap of Ge (0.66 ev) is smaller tha that of Si (1.12 ev), so the SiGe HBT has a arrow badgap base, differig from the wide badgap emitter HBT. The compressive strai associated with sadwiched SiGe base layer betwee Si emitter ad collector layers produces a additioal badgap shrikage. As a result, a badgap reductio of about mev for each 10% of Ge cotet ca be utilized i device egieerig. Figure 1.11 shows the basic structure ad forward-active bias eergy bad diagram of a SiGe Base Emitter Metal poly-si p+ + p+ Collector Shallow Trech p-sige Oxide + Deep Trech + p (a) ΔE g,ge (x= 0) ΔE g,ge (x = W b ) E C + Si emitter e h + p SiGe base E V p Si Ge Si collector (b) Figure 1.11 Sketch of (a) the basic structure ad (b) the bad structure ad dopig profile of the silico-germaium heterojuctio bipolar trasistor (SiGe HBT). Reproduced with permissio from Cressler, J. D.; Silico Earth: Itroductio to the Microelectroics ad Naotechology Revolutio; 2009, Cambridge Uiversity Press Alexader Graham Bell ivets the first practical telephoe 1877

17 Bipolar Trasistors 19 maximum oscillatio frequecy (GHz) circle:'03 triagle:'01 '02 square:'99 '00 '01 '02 '99 ' cutoff frequecy (GHz) '03 Figure 1.12 SiGe HBTs Evolutioary improvemet i cutoff frequecy ad maximum oscillatio frequecy from for HBT. Similar to the wide badgap emitter HBT, the emitter ijectio efficiecy effectively icreases due to the Ge-iduced bad offset occurred i the valece bad. After the first demostratio of fuctioal SiGe HBT i 1987 [9], the developmet of SiGe HBTs evolved rapidly ad their performace has dramatically improved from the mid-1990s to preset. For Si BJTs, the peak f T is limited to approximately 50 GHz. However, usig a SiGe HBT, both f T ad f max go rise above 300 GHz, as show i Figure I the early stages of evolutio, the SiGe HBT had a o-self-aliged structure, so oly f T was improved by the shrikage of the base width ad badgap egieerig. However, SiGe HBTs soo icorporated self-aliged trasistor structures, with rapid improvemet i trasistor f max. The schemes to fabricate self-aliged SiGe HBTs are roughly categorized ito two types, depedig o the SiGe epitaxial growth techologies used: selective or blaket epitaxial growth [10]. Recetly, attetio has bee placed o achievig ultra-high f max due to the emergig applicatios such as terahertz wireless systems. Oe of the most importat aspects of SiGe HBTs is that it ca be easily combied with Si CMOS o the same wafer to eable highly-itegrated systems. So-called SiGe BiCMOS (SiGe HBT + Si CMOS) techologies ca be costructed usig well-established Si-based processes ad are 100% silico maufacturig compatible. This represets a fudametal differece betwee SiGe HBTs techology ad III-V HBTs (see also Chapter 14). The wide-spread applicatio of SiGe HBTs i high-speed digital ad RF/aalog itegrated circuits offer ample evidece to this crucial advatage ejoyed by SiGe BiCMOS techology (see examples i [10]). Refereces [1] J. Bardee ad W. H. Brattai, The trasistor, a semicoductor triode, Physical Review, vol. 74, pp , [2] W. Shockley, M. Sparks, ad G. K. Teal, p- juctio trasistors, Physical Review, vol. 83, p. 151, [3] J. D. Cressler, Silico Earth: Itroductio to the Microelectroics ad Naotechology Revolutio, New York, NY, Cambridge Uiversity Press, [4] K. K. Ng, Complete Guide to Semicoductor Devices, 2d Ed, New York, NY, Joh Wiley & Sos, Ic., Thomas Alva Ediso ivets the phoograph

18 20 Guide to State-of-the-Art Electro Devices [5] J. D. Cressler (ed.), Silico Heterostructure Hadbook: Materials, Fabricatio, Devices, Circuits, ad Applicatios of SiGe ad Si Straied-Layer Epitaxy, Boca Rato, FL, CRC Press, [6] E.O. Johso, Physical limitatios o frequecy ad power parameters of trasistors, RCA Rev., vol. 26, pp , [7] H. Kroemer, Theory of a wide-gap emitter for trasistors, Proc. IRE, vol. 45, pp , [8] J. W. Matthews ad A.E. Blakeslee, Defects i epitaxial multilayers I:misfit dislocatios i layers, J. Cryst. Growth, vol. 27, pp , [9] S. S. Iyer, G. L. Patto, J. M. C. Stork, et al., Silico-germaium base heterojuctio bipolar trasistors by molecular beam epitaxy, Tech. Dig. IEEE It. Elect. Dev. Meetig, pp , [10] K. Washio, Silico-germaium (SiGe) heterojuctio bipolar trasistor (HBT) ad bipolar complemetary metal oxide semicoductor (BiCMOS) techologies, Chapter 18 i, Silico-Germaium Naostructures (eds Y. Shiraki ad N. Usami), Cambridge, Woodhead Publishig, Ediso ivets 1879 the electric light bulb 1880 Jaques ad Pierre Curie discover the piezo electric effect i crystals