ECE606: Solid State Devices Lecture 20. Heterojunction Bipolar Transistor

Transcription

1 C606: Solid State Devices Lecture 0 Heterojuctio ipolar Trasistor Gerhard Klimeck gekco@purdue.edu 1 Outlie 1. Itroductio. quilibrium solutio for heterojuctio 3. Types of heterojuctios 4. Itermediate Summary 5. Abrupt juctio HTs 6. Graded juctio HTs 7. Graded base HTs 8. Double heterojuctio HTs 9. Coclusios Heterostructure Fudametals, by Mark Ludstrom, Purdue Uiversity, Herbert Kroemer, Heterostructure bipolar trasistors ad itegrated circuits, Proc. I, 70, pp. 13-5, 198.

2 How to make a better Trasistor β i, N poly, ballistic i, N υth υ s Graded ase trasport Polysilico mitter Heterojuctio bipolar trasistor g, NC, NV, e ( g, g, ) = e g, β N N e i, i,,, mitter badgap > ase adgap β β 3 Heterojuctio ipolar Trasistors i) Wide gap mitter HT emitter G1 > G p+ base G collector + G ii) Double Heterojuctio ipolar Trasistor emitter G1 > G p+ base G collector + G3 > G 4

3 Mesa HTs emitter G1 > G p+ base G collector + G3 > G Mesa HT p+ base -collector + semi-isulatig substrate Itetioal traps, Fermi level pied Low coductace Low capacitace High speed 5 Applicatios 1) Optical fiber commuicatios -40Gb/s.160Gb/s ) Widebad, high-resolutio DA/AD coverters ad digital frequecy sythesizers -military radar ad commuicatios 3) Moolithic, millimeter-wave IC s (MMIC s) -frot eds for receivers ad trasmitters future eed for trasistors with 1 THz power-gai cutoff freq. 6

4 ackgroud A heterojuctio bipolar trasistor Kroemer Schokley realized that HT is possible, but Kroemer really provided the foudatio of the field ad worked out the details. 7 Outlie 1. Itroductio. quilibrium solutio for heterojuctio 3. Types of heterojuctios 4. Itermediate Summary 5. Abrupt juctio HTs 6. Graded juctio HTs 7. Graded base HTs 8. Double heterojuctio HTs 9. Coclusios Heterostructure Fudametals, by Mark Ludstrom, Purdue Uiversity, Herbert Kroemer, Heterostructure bipolar trasistors ad itegrated circuits, Proc. I, 70, pp. 13-5,

5 Topic Map Diode quilibrium DC Small sigal Large Sigal Circuits Schottky JT/HT MOS 9 adgaps ad Lattice Matchig 10

6 ad Diagram at quilibrium + ( D A ) D = q p + N N 1 = J r + g t q p 1 = J r + g t q J N N N J = qµ + qd N N N P P P = qpµ qd p P P P quilibrium DC d/dt=0 Small sigal d/dt ~ jωt Trasiet --- Charge cotrol model 11 N-Al 0.3 Ga 0.7 As: p-gaas (Type-I Heterojuctio) N D N A Vacuum Level χ F G 1.80 ev χ 1 G 1.4 ev Abrupt juctio HT 1

7 uilt-i Potetial: oudary + χ + qv = + χ 1 1 bi g, qv bi χ χ 1 F 1 g, qv = + χ χ bi g, 1 1 N N = k T l + N N e A D g, / kt V, C, 1 ( χ χ ) 1 13 Iterface oudary Coditios -field 1 D κ ε x x p + ( 0 ) = D( 0 ) + ( 0 ) = κε ( 0 ) 0 0 dv κ1ε 0 dx 0 dv = κε 0 dx + 0 Positio Displacemet is cotiuous D is ot cotiuous if there is a surface charge 14

8 Aalytical Solutio for Heterojuctios Charg e -field x x p x x ( 0 ) + ( 0 ) qn Dx = k ε s, 0 qn Axp = k ε s, 0 NDx = N Axp Charge cotiuity (idepedet of k) Potetial x s, 0 + ( 0 ) ( 0 ) x x Vbi = + qndx qn Axp = + k ε k ε s, 0 p 15 ase mitter Depletio Regio N D x N A x p V N x bi, p, qn x, qnx = + κ ε κ ε s, = N x p, 0 s, 0 x x = p = ε 0 s, s, κ κ N ( κ s, + κ s, ) q N N N ε 0 s, s, κ κ N V ( κ s, + κ s, ) q N N N bi V bi 16

9 Outlie 1. Itroductio. quilibrium solutio for heterojuctio 3. Types of heterojuctios 4. Itermediate Summary 5. Abrupt juctio HTs 6. Graded juctio HTs 7. Graded base HTs 8. Double heterojuctio HTs 9. Coclusios Heterostructure Fudametals, by Mark Ludstrom, Purdue Uiversity, Herbert Kroemer, Heterostructure bipolar trasistors ad itegrated circuits, Proc. I, 70, pp. 13-5, N-Al 0.3 Ga 0.7 As: p-gaas (Type-I Heterojuctio) N D N A Vacuum Level χ F G 1.80 ev χ 1 G 1.4 ev Abrupt juctio HT 18

10 P-Al 0.3 Ga 0.7 As : -GaAs (Type I juctios) 0 Vacuum level χ 1 qv (x) qv I l V jp G 1.80 ev χ V j F G 1.4 ev Depletio layer Depletio layer Alam C-606 S09 19 N-p vs P- of Type I Heterostructure 0

11 (AlIAs/IP) Type II Juctios N D N A Vacuum level χ χ 1 F 1 N-Al 0.3 Ga 0.7 As : -GaAs Juctios Isotype Heterojuctio V G 1.4 ev G 1.80 ev Depletio Layer Accumulatio Layer Metal-Metal juctios have similar features differet workfuctios => differet bad lieups thermoelectric coolers, electros travelig from oe side to the other ca gai ad lose eergy

12 P-GaSb : -IAs (Type III) 0 Field-free vacuum level χ 1 G 0.7 ev χ FP F G 0.36 ev 3 P-GaSb : -IAs (Type III) = 0.87 ev G 0.7 ev F G 0.36 ev Accumulatio Layer! Accumulatio Layer! 4

13 Coclusio 1. Heterojuctio trasistors offer a solutio to the limitatios of poly-si bipolar trasistors.. quilibrium solutios for HTs are very similar to those of ormal JTs. 3. Depedig o the aligmet, there could be differet types of heterojuctios. ach has differet usage. 4. We will discuss curret trasport i HTs i the ext sectio. 5 Outlie 1. Itroductio. quilibrium solutio for heterojuctio 3. Types of heterojuctios 4. Itermediate Summary 5. Abrupt juctio HTs 6. Graded juctio HTs 7. Graded base HTs 8. Double heterojuctio HTs 9. Coclusios Heterostructure Fudametals, by Mark Ludstrom, Purdue Uiversity, Herbert Kroemer, Heterostructure bipolar trasistors ad itegrated circuits, Proc. I, 70, pp. 13-5,

14 Abrupt Juctio HTs i C / kt J, = q υrpe = J ( V = 0) N J F J J p i = q υ e N D i = q N W ( ) C / kt qv / kt Rp p qv / k T N υrp i C / kt β = e N D p W i e e J p Gai i abrupt p JT defied oly by valece bad discotiuity! g, NC, NV, e ( g, g, ) = e g, β N N e i, i,,, β = N D N A β υ Rp /kt ( D p W ) e V 7 β but we are hopig for eve better gai N N β N D W N D W i, υth υth ~ i, p / p / ( g ) e β β = N υ N D W R, p / k T ( ) p e V Abrupt juctio HT For full gai, we eed graded juctio HT 8

15 Outlie 1. Itroductio. quilibrium solutio for heterojuctio 3. Types of heterojuctios 4. Itermediate Summary 5. Abrupt juctio HTs 6. Graded juctio HTs 7. Graded base HTs 8. Double heterojuctio HTs 9. Coclusios Heterostructure Fudametals, by Mark Ludstrom, Purdue Uiversity, Herbert Kroemer, Heterostructure bipolar trasistors ad itegrated circuits, Proc. I, 70, pp. 13-5, Abrupt Juctio V (x) x N x P x V jn V jp G 1.4 ev G 1.80 ev χ(x) (x) = 0 χ(x) qv ( x) (x) = (x) G (x) x N x P x 30

16 Graded ase-mitter Juctio V (x) x N x P x G 1.4 ev G 1.80 ev χ(x) (x) = 0 χ(x) qv ( x) (x) = (x) G (x) x N x P x 31 Curret Gai No expoetial Suppressio! J G > G F J i qv / kt = q N A W e D J q D i p = ND W e p qv / k T J p β = N D N A D D p W i W i x i = N C N V e G /k T β N D N A D D p W W e G /k T 3

17 Advatages of HT: Iverted ase Dopig β DC N N D W e D W D A p G / kt 1) Thi ase for high speed ) Very heavily doped ase to prevet Puch Through, reduce arly effect, ad to lower R ex 3) Moderately doped mitter (lower C j, ) iverted base dopig N A >> N D 33 Outlie 1. Itroductio. quilibrium solutio for heterojuctio 3. Types of heterojuctios 4. Itermediate Summary 5. Abrupt juctio HTs 6. Graded juctio HTs 7. Graded base HTs 8. Double heterojuctio HTs 9. Coclusios Heterostructure Fudametals, by Mark Ludstrom, Purdue Uiversity, Herbert Kroemer, Heterostructure bipolar trasistors ad itegrated circuits, Proc. I, 70, pp. 13-5,

18 How to make a better Trasistor β N D W i, poly, ballistic i, N υs Heterojuctio bipolar trasistor Graded ase trasport Polysilico mitter g, i, NC, NV, e ( g, g, ) = e g, β N N e i,,, β β 35 Graded ases AC χ(x) G (x) F G (x) Itrisic compositioally graded Uiformly p-doped compositioally graded 36

19 Graded ase HTs J i D J = q e N W qv / kt D = e W F i J p q N p qv / k T N D W β DC = N i Dp W i J p x W W τ b = << µ D eff eff G q = W 37 Outlie 1. Itroductio. quilibrium solutio for heterojuctio 3. Types of heterojuctios 4. Itermediate Summary 5. Abrupt juctio HTs 6. Graded juctio HTs 7. Graded base HTs 8. Double heterojuctio HTs 9. Coclusios Heterostructure Fudametals, by Mark Ludstrom, Purdue Uiversity, Herbert Kroemer, Heterostructure bipolar trasistors ad itegrated circuits, Proc. I, 70, pp. 13-5,

20 Double HJT Symmetrical operatio (simplify circuiots) J G > G F GC > G No charge storage whe the b-c juctio is forward biased (i is smaller) J p x Reduced collector offset voltage Higher collector breakdow voltage (higher gap) 39 Offset Voltage I C C I C I I V V C does I C = 0 at V C = 0? 40

21 Offset Voltage J 1 J i D ( )/ J1 = q e F N A W i D ( )/ J = q e N A W D ic p ( )/ J3 = q e N DC WC q V V kt q V VC kt q V VC kt J 3 x JC = J J J 1 3 set J C = 0, assume V = 0, solve for V C =V OS 41 Offset Voltage Result V OS = k T q l ( 1+ 1 γ R) J ( i N A )( D W ) γ R = = (Reverse mitter ijectio efficiecy) J3 ( ic NDC )( Dp WC ) Wat a large γ R for small V os. Wide badgap collector helps. 4

22 Outlie 1. Itroductio. quilibrium solutio for heterojuctio 3. Types of heterojuctios 4. Itermediate Summary 5. Abrupt juctio HTs 6. Graded juctio HTs 7. Graded base HTs 8. Double heterojuctio HTs 9. Coclusios moder desig Heterostructure Fudametals, by Mark Ludstrom, Purdue Uiversity, Herbert Kroemer, Heterostructure bipolar trasistors ad itegrated circuits, Proc. I, 70, pp. 13-5, Puttig the Terms Together log10 f T Kirk Curret ase trasit time Collector trasit time 1 W W C = + + π ft D υ sat I K log10 I C kt C j, C C j, qi + C juctio chargig time Quatios: Why does HTs have such high performace? 44

23 pitaxial Layer Desig (II) DHT: Abrupt IP emitter, IGaAs base, IAlGaAs C/ grades 45 pitaxial Layer Desig (III) 46

24 Summary 1) The use of a wide badgap emitter has two beefits: -allows heavy base dopig -allows moderate emitter dopig ) The use of a wide badgap collector has beefits: -symmetrical device -reduced charge storage i saturatio -reduced collector offset voltage -higher collector breakdow voltage 3) adgap egieerig has potetial beefits: -heterojuctio lauchig ramps -compositioally graded bases -elimiatio of bad spikes 4) HTs have the potetial for THz cutoff frequecies. However, it has yield issues ad heatig ad cotact R problems. 4 7 Outlie from previous lecture 1) Curret gai i JTs ) Cosideratios for base dopig 3) Cosideratios for collector dopig 4) Itermediate Summary 5) Problems of classical trasistor 6) Poly-Si emitter 7) Short base trasport 8) High frequecy respose 9) Coclusios RF: SDF, Chapter 10 48

25 Topic Map Diode quilibrium DC Small sigal Large Sigal Circuits Schottky JT/HT MOS 49 Small Sigal Respose log 10 β C I V (i) P+ N P IC VC (out) 1 ω β( ω) β β DC ω log 10 f f β f T 1 W W C kt = + + C j, C + C j, π ft D sat qi υ C Desire high f T High IC Low capacitaces Low widths 50

26 Frequecy Respose The gai of a amplifier is affected by the capacitace associated with its circuit. This capacitace reduces the gai i both the low ad high frequecy rages of operatio. The reductio of gai i the low frequecy bad is due to the couplig ad bypass capacitors selected. They are essetially short circuits i the mid ad high bads. The reductio of gai i the high frequecy bad is due to the iteral capacitace of the amplifyig device, e.g., JT, FT, etc. This capacitace is represeted by capacitors i the small sigal equivalet circuit for these devices. They are essetially ope circuits i the low ad mid bads. 51 Small Sigal Respose (Commo mitter) From bers Moll Model DC Circuit C µ => AC small sigal Circuit C I reverse bias, I R =0 I V (i) P+ N P IC C V C (out) I ( α ) 1 F IF C π IR α I I α I F F R R r π C π C µ g m V I C F qv / kt ( ) I = I e F 0 1 ( 1 α ) 1 di d F IF qi = = = r dv dv k T π g δ m d( α F IF ) qic = = dv k T ( α I ) F F m m = g δv = g υ = β 1 qic DC k T I 5

27 Short Circuit Curret Gai C µ C i gmυ + jωc C µ υc β ( f ) = = i 1 υ + jωcπ υ + jωcµ υ rπ r π C π gmυ gm jωt Cµ gm β ( ft ) 1 = 1 jωt C + C + jωt Cπ + jωcµ rπ ( π µ ) 1 1 Cπ + Cµ kt k T = = ω π f g qi qi T T ( C j, C C j, ) ( Cd, C Cd, ) m C C k T C dq qi di dv di d, C Cd, C = = C C 53 Short Circuit Curret Gai C µ C r π C π gmυ gm jωt Cµ gm β ( ft ) 1 = 1 jωt C + C + jωt Cπ + jωcµ rπ ( π µ ) 1 1 Cπ + Cµ kt k T = = ω π f g qi qi T T ( C j, C C j, ) ( Cd, C Cd, ) m C C k T C dq qi di dv di d, C Cd, C = = C C 54

28 Short Circuit Curret Gai C µ C r π C π gmυ 1 1 Cπ + Cµ kt k T = = ω π f g qi qi T T ( C j, C C j, ) ( Cd, C Cd, ) m C C k T C dq qi di dv di d, C Cd, C = = C C 55 ase Trasit Time 1 Ref. Charge cotrol model N + P dq di C Q = I C 1 q W = 1 = 1 q W W D ase trasit time 56

29 Collector Trasit Time lectros ijected ito collector depletio regio very high fields more tha diffusio => drift => acceleratio of carriers Charge imaged i collector N + P τ = W υ C sat? i τ t τ q τ W = = = C eff, C i υ sat 1 i τ = q 57 Puttig the Terms Together Collector trasit time log10 f T Kirk Curret ase trasit time 1 W W C = + + π ft D υ sat I K log10 I C kt C j, C C j, qi + C Juctio chargig time Do you see the motivatio to reduce W ad W C as much as possible? What problem would you face if you push this too far? Icreasig I C too high reduces W C ad icreases the overall capacitace => frequecy rolls off. 58

30 High Frequecy Metrics (curret-gai cutoff frequecy, f T ) 1 τ = π f = D + υ + I + W WC kt q C C Rex c C cb T sat C (power-gai cutoff frequecy, f max ) ( j, + j, C ) + ( R ) f max = f T 8π R bb C cbi 59 Summary We have discussed various modificatios of the classical JTs ad explaied why improvemet of performace has become so difficult i recet years. The small sigal aalysis illustrates the importace of reduced juctio capacitace, resistaces, ad trasit times. Classical homojuctios JTs ca oly go so far, further improvemet is possible with heterojuctio bipolar trasistors. 60