ECE606: Solid State Devices Lecture 20 Heterojunction Bipolar Transistor

ECE606: Solid State Devices Lecture 20

Heterojunction Bipolar Transistor

1 Klimeck – ECE606 Fall 2012 – notes adopted from Alam

Outline

1. Introduction 2. Equilibrium solution for heterojunction 3. Types of heterojunctions 4. Intermediate Summary 5. Abrupt junction HBTs 6. Graded junction HBTs 7. Graded base HBTs 8. Double heterojunction HBTs 9. Conclusions

“Heterostructure Fundamentals,” by Mark Lundstrom, Purdue University, 1995. Herbert Kroemer, “Heterostructure bipolar transistors and integrated circuits,” Proc. IEEE , 70 , pp. 13-25, 1982.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 2 How to make a better Transistor

n2 N υ β →i, B ×E × th Graded Base transport poly, ballist ic 2 υ ni,E N B s

Polysilicon Emitter

Heterojunction bipolar transistor

− β 2 Eg, B n N N e ()− β i, B =CB, VB , ≈ Eg, E Eg ,B − β e 2 Eg, E ni,E NC,E N V , E e

Emitter bandgap > Base Bandgap

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 3

Heterojunction Bipolar Transistors

i) Wide gap Emitter HBT

p+ n n base emitter collector n+

EG1 >E G2 EG2 EG2 ii) Double Heterojunction Bipolar Transistor

p+ n n base emitter collector n+

EG1 >E G2 EG2 EG3 >E G2

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 4 Mesa HBTs

p+ n n base emitter collector n+

EG1 >E G2 EG2 EG3 >E G2

Mesa HBT n p+ base Intentional traps, n-collector Fermi level pinned n+ Low conductance Low capacitance semi-insulating substrate High speed

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 5

Applications

1) Optical fiber communications -40Gb/s…….160Gb/s 2) Wideband, high-resolution DA/AD converters and digital frequency synthesizers -military radar and communications 3) Monolithic, millimeter-wave IC’s (MMIC’s) -front ends for receivers and transmitters

future need for transistors with 1 THz power-gain cutoff freq.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 6 Background

A heterojunction bipolar transistor

Kroemer

Schokley realized that HBT is possible, but Kroemer really provided the foundation of the field and worked out the details.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 7

Outline

“Heterostructure Fundamentals,” by Mark Lundstrom, Purdue University, 1995. Herbert Kroemer, “Heterostructure bipolar transistors and integrated circuits,” Proc. IEEE , 70 , pp. 13-25, 1982.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 8 Topic Map

Equi libr ium DC Small Large Circuits signal Signal Diode

Schottky

BJT /HBT

MOS

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 9

Bandgaps and Lattice Matching

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 10 Band Diagram at Equilibrium

∇•=( −++ − − ) D qpnND N A Equilibrium ∂n 1 = ∇•J −r + g ∂t q N N N =µ + ∇ J Nqn N E qD N n DC dn/dt=0 Small signal dn/dt ~ jωtn ∂p 1 Transient --- Charge control model = ∇•J −r + g ∂t q P P P =µ − ∇ J Pqp P E qD P p

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 11

N-Al 0.3 Ga 0.7 As: p-GaAs (Type-I Heterojunction)

ND NA

Vacuum Level

χ 2 χ 1 EC ≈ EG 1.42 eV EF EV E ≈ 1.80 eV G Abrupt junction HBT

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 12 Built-in Potential: Boundary Condition @Infinity ∆ ++χ = − ∆ + χ 1 1 qV bi Eg ,22 2

qV bi χ2 χ 1 EC Eg,2 EF ∆ 1 E ∆2 V

= −∆ −+− ∆ χ χ qVbi E g, 2 2 1 2 1

=NA N D +χ − χ k T ln − () B Eg ,2 /kT B 2 1 NV,2 N C, 1 e

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 13

Interface Boundary Conditions

E-field

Position xn xp

D(0−) = D ( 0 + ) Displacement is continuous κ ε E(0−) = κ ε E ( 0 + ) 1 02 0 D is not continuous if κ ε dV= κ ε dV 1 0 2 0 there is a − + dx0 dx 0 surface charge

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 14 Analytical Solution for Heterojunctions

Charg − qN x E ()0 = D n e k ε x x s, E 0 n p x qN x E ()0+ = A p k ε E-field s, B 0 ⇒ = ND xn N A x p x Charge continuity (independent of k)

Potential − + E(0) xE( 0 ) x =n + p Vbi x 2 2 qN x 2 qN x 2 =D n + A p ε ε 2ks,E 0 2 ks,B 0 Klimeck – ECE606 Fall 2012 – notes adopted from Alam 15

Base Emitter Depletion Region

ND NA

xn xp

2ε κ κ N Nx= Nx x= 0 s, E s,B B V E nBE, B pBE , n κ + κ bi q NE()s,E N B s,B N E

2 2 qNE x n, BE qNB x p, BE 2ε κ κ N V = + = 0 s, E s,B E bi κ ε κ ε xp V bi 2s, E 0 2 s, B 0 κ + κ q NB()s,E N B s,B N E

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 16 Outline

“Heterostructure Fundamentals,” by Mark Lundstrom, Purdue University, 1995. Herbert Kroemer, “Heterostructure bipolar transistors and integrated circuits,” Proc. IEEE , 70 , pp. 13-25, 1982.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 17

N-Al 0.3 Ga 0.7 As: p-GaAs (Type-I Heterojunction)

ND NA

Vacuum Level

χ 2 χ 1 EC ≈ EG 1.42 eV EF EV E ≈ 1.80 eV G Abrupt junction HBT

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 18 P-Al 0.3 Ga 0.7 As : n-GaAs (Type I junctions)

E Vacuum level 0 χ qV (x) qV 1 BI

E l EC V jP χ E ≈ 1.80 eV ∆E 2 G C

V jn E E F C E V E ≈ 1.42 eV G ∆E E V V Depletion layer Depletion layer Klimeck – ECE606 Fall 2012Alam – notes ECE-606adopted from Alam S09 19

N-p vs P-n of Type I Heterostructure

20 Klimeck – ECE606 Fall 2012 – notes adopted from Alam (AlInAs/InP) Type II Junctions

ND NA

Vacuum level

χ2 χ 1 EC

EF EV

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 21

N-Al 0.3 Ga 0.7 As : n-GaAs Junctions

‘Isotype Heterojunction’

EC E C

E ≈ 1.42E eVV G E ≈ 1.80 eV E G V

E V Depletion Layer Accumulation Layer Metal-Metal junctions have similar features … different workfunctions => different band lineups thermoelectric coolers , electrons traveling from one side to the other can gain and lose energy Klimeck – ECE606 Fall 2012 – notes adopted from Alam 22 P-GaSb : n-InAs (Type III)

E Field-free vacuum level 0

χ 1

E C χ 2 E ≈ 0.72 eV G E E FP V E E C Fn E ≈ 0.36 eV G E V

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 23

P-GaSb : n-InAs (Type III)

E ∆E = 0.87 eV C C E ≈ 0.72 eV G E E F C E ≈ 0.36 eV EV G E V

Accumulation Layer! Accumulation Layer!

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 24 Conclusion

1. Heterojunction transistors offer a solution to the limitations of poly-Si bipolar transistors.

2. Equilibrium solutions for HBTs are very similar to those of normal BJTs.

3. Depending on the alignment, there could be different types of heterojuctions. Each has different usage.

4. We will discuss current transport in HBTs in the next section.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 25

Outline

“Heterostructure Fundamentals,” by Mark Lundstrom, Purdue University, 1995. Herbert Kroemer, “Heterostructure bipolar transistors and integrated circuits,” Proc. IEEE , 70 , pp. 13-25, 1982.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 26 Abrupt Junction HBTs

2 n  −∆ =iB υ EC/ k B T = = Jn, B→ E q  Rp e J n (VBE 0) 2 N n  −∆ B  = iB υ EC// k B T qVBE kT B J n q  Rp e e NB  J n n2  D ∆E iE p qVBE/ k B T C = J p q  e NE  W E

E EF υ 2 C N Rp n −∆  β = E iB EC/ k B T 2 e  ∆E NB ()Dp W E niE  V

− β 2 Eg, B n N N e ()− β i, B =CB, VB , ≈ Eg, E Eg ,B − β e EV J 2 Eg, E p ni,E NC,E N V , E e Gain in abrupt npn BJT defined υ N Rp ∆ only by valence band β = DE e EV /kBT discontinuity! N AE ()Dp WE Klimeck – ECE606 Fall 2012 – notes adopted from Alam 27

… but we are hoping for even better gain

2 n Nυ N υ ()∆E β β →××i, B E th E × th g 2 ~ e niE, N B DW pE/ N B DW pE /

υ N R, p ∆E/ k T β = E e V B Abrupt junction HBT N B ()Dp W E

For full gain, we need graded junction HBT

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 28 Outline

“Heterostructure Fundamentals,” by Mark Lundstrom, Purdue University, 1995. Herbert Kroemer, “Heterostructure bipolar transistors and integrated circuits,” Proc. IEEE , 70 , pp. 13-25, 1982.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 29

Abrupt Junction

V (x) −x x x N P

E V C V jp jN E ≈ 1.42 eV E G C E V E ≈ 1.80 eV ∆E G V χ (x) E V = − χ − ( ) EC(x) E0 (x) qV x −x x x N P E (x) = E (x)− E (x) V C G

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 30 Graded Base-Emitter Junction

V (x) −x x x N P E C E ≈ 1.42 eV E G C E V E ≈ 1.80 eV ∆E G V χ (x) E V = − χ − ( ) EC(x) E0 (x) qV x −x x x N P E (x) = E (x)− E (x) V C G

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 31

Current Gain No exponential Suppression!

Jn n2  D = iB n qVBE/ k B T J n q  e NAB  W B EF E 2 C n  Dp J = qiE  e qVBE/ k B T p N W > DE  E EGE EGB N D W n2 E β = DE n E iB V Jp 2 NAE Dp WB niE

x − = EG /2kBT ni NC NV e

N D W ∆ β ≈ DE n E e EG /kBT N AE Dp WB

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 32 Advantages of HBT: Inverted Base Doping

N D W ∆ β ≈ DE n E EG/ k B T DC e NAB D p W B

1) Thin Base for high speed

2) Very heavily doped Base to prevent Punch Through,

reduce Early effect, and to lower Rex

3) Moderately doped Emitter (lower Cj,BE ) >> “inverted base doping” N AB N DE

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 33

Outline

“Heterostructure Fundamentals,” by Mark Lundstrom, Purdue University, 1995. Herbert Kroemer, “Heterostructure bipolar transistors and integrated circuits,” Proc. IEEE , 70 , pp. 13-25, 1982.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 34 How to make a better Transistor

n2 N D W β →i, B ×E × n B Graded Base transport poly, ballistic 2 υ niE, N B s

Polysilicon Emitter Heterojunction bipolar transistor

− β 2 Eg, B n NNe ()− β iB,= CB , VB , ≈ EgE, E gB , − β e 2 Eg, E ni, E NCE, N VE , e

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 35

Graded Bases

EVAC

χ(x) EC EC

EG(x) E (x) G EF E EV V

Intrinsic Uniformly p-doped compositionally graded compositionally graded

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 36 Graded Base HBTs

n 2  D = iB n qVBE/ kT B Jn J n q  e N B  WB

2 n  Dp E E = iE qVBE/ k B T C F J p q  e NE  W E

N D W n 2 β = E n E iB DC 2 NB Dp W B n iE EV J p W W 2 x τ =B << B b µ E neff 2D n ∆E q E = G eff WB Klimeck – ECE606 Fall 2012 – notes adopted from Alam 37

Outline

“Heterostructure Fundamentals,” by Mark Lundstrom, Purdue University, 1995. Herbert Kroemer, “Heterostructure bipolar transistors and integrated circuits,” Proc. IEEE , 70 , pp. 13-25, 1982.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 38 Double HBJT

• Symmetrical operation (simplify circuiots) Jn • No charge storage when EF E the b-c junction is forward C biased > > EGC EGE (ni is smaller) EGE EGB • Reduced collector offset EV Jp voltage x • Higher collector breakdown voltage (higher gap)

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 39

Offset Voltage

C B I C I VB B E I B

VCE

does I C = 0 at VCE = 0?

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 40 Offset Voltage

J J1 2 2 n  D ()− = iB n qVB V E/ kT B J1 q  e EF NAB  W B E 2 C n  D ()− = iB n qVB V C/ kT B J 2 q  e NAB  W B 2 n  D ()− = iC p qVB V C/ kT B J3 q  e N  W EV DC C J3 J =J −J − J x C 1 2 3

set JC = 0, assume VE = 0, solve for VC=VOS

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 41

Offset Voltage Result

k T V = B ln() 1+ 1 γ OS q R

2 ( ) J (niB N AB) D nW B γ =2 = (Reverse Emitter injection efficiency) R 2 J3 ()niC N DC()D p W C

γ Want a large R for small Vos . Wide bandgap collector helps.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 42 Outline

“Heterostructure Fundamentals,” by Mark Lundstrom, Purdue University, 1995. Herbert Kroemer, “Heterostructure bipolar transistors and integrated circuits,” Proc. IEEE , 70 , pp. 13-25, 1982.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 43

Putting the Terms Together

Collector transit time log f Kirk Current 10 T Base transit time

1 W2 W  =B + BC + π υ  2fT 2 D n 2 sat  k T B +  CjBC, C jBE ,  qI C

I K log 10 IC junction charging time …

Quations: Why does HBTs have such high performance ?

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 44 Epitaxial Layer Design (II)

DHBT: Abrupt InP emitter, InGaAs base, InAlGaAs C/B grades

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 45

Epitaxial Layer Design (III)

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 46 Summary

1) The use of a wide bandgap emitter has two benefits: -allows heavy base doping -allows moderate emitter doping 2) The use of a wide bandgap collector has benefits: -symmetrical device -reduced charge storage in saturation -reduced collector offset voltage -higher collector breakdown voltage 3) Bandgap engineering has potential benefits: -heterojunction launching ramps -compositionally graded bases -elimination of band spikes 4) HBTs have the potential for THz cutoff frequencies. However, it has yield issues and heating and contact R problems.

4 Klimeck – ECE606 Fall 2012 – notes adopted from Alam 7

Outline from previous lecture

1) Current gain in BJTs 2) Considerations for base doping 3) Considerations for collector doping 4) Intermediate Summary 5) Problems of classical transistor 6) Poly-Si emitter 7) Short base transport 8) High frequency response 9) Conclusions REF: SDF, Chapter 10

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 48 Topic Map

Equilibrium DC Small Large Circuits signal Signal Diode

Schottky

BJT/HBT

MOS

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 49

Small Signal Response

β log10

P+ IC B IB ω β() ω ≈ β β N VEC DC (out) ω VE P B (in)

E E 1 log10 f

fβ fT

Desire high f 1 W2 W  kT T =++B BC  B C + C  ⇒High IC π υ j, BC j , BE  ⇒ 2fT 2 D n 2 sat  qI C Low capacitances ⇒Low widths

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 50 Frequency Response

The gain of an amplifier is affected by the capacitance associated with its circuit. This capacitance reduces the gain in both the low and high frequency ranges of operation. The reduction of gain in the low frequency band is due to the coupling and bypass capacitors selected. They are essentially short circuits in the mid and high bands. The reduction of gain in the high frequency band is due to the internal capacitance of the amplifying device, e.g., BJT, FET, etc. This capacitance is represented by capacitors in the small signal equivalent circuit for these devices. They are essentially open circuits in the low and mid bands. Klimeck – ECE606 Fall 2012 – notes adopted from Alam 51

Small Signal Response (Common Emitter) From Ebers Moll Model DC Circuit => AC small signal Circuit Cµ BC In reverse bias, I R=0

Cµ C I IC B IB IR α− α P+ IC FFI RR I B IB N VEC Cπ r Cπ πππ g V VEB P (out) (1−α ) I m BE (in) F F

E E IE ( −α )  1 dI d 1 FI F q I 1 qI = B =  = B = C β rπ dVBE dVBE k B T DCk B T I= I( e qVBE / kT − 1) d(α I ) qI F F 0 =F F = C gm dVBEk B T δ(α ) = δ = υ FI F gm V B E gm BE

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 52 Short Circuit Current Gain

υ + ω υ i g j Cµ β (f ) =C = m BE CB C i   B µ C B 1 υ+ ωυ + ω υ BEj Cπ BE  j C µ BC rπ  rπ Cπ υ g g − jω Cµ g m BE β (f ) ≡1 =m T ≈ m T   jω () C+ C 1 +ω + ω T π µ jT Cπ  j C µ E rπ 

1 1 Cπ+ C µ kT kT ≡= =B ()CC ++B () CC + ω π jBC,, j BE dB ,,C d BE T 2 fT gmqI C qI C k TC dQ B =d, BC = B Cd, BC qIC dIdV C BE dI C

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 53

Short Circuit Current Gain

B Cµ C

rπ Cπ υ g g − jω Cµ g m BE β (f ) ≡1 =m T ≈ m T   jω () C+ C 1 +ω + ω T π µ jT Cπ  j C µ E rπ 

1 1 Cπ+ C µ kT kT ≡= =B ()CC ++B () CC + ω π jBC,, j BE dB ,,C d BE T 2 fT gmqI C qI C k TC dQ B =d, BC = B Cd, BC qIC dIdV C BE dI C

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 54 Short Circuit Current Gain

B Cµ C

rπ Cπ υ gm BE

1 1 Cπ+ C µ kT kT ≡= =B ()CC ++B () CC + ω π jBC,, j BE dB ,,C d BE T 2 fT gmqI C qI C k TC dQ B =d, BC = B Cd, BC qIC dIdV C BE dI C

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 55

Base Transit Time

Ref. Charge control model ∆n n1 + P N

1 q nW 2 dQ Q 1 B W Base B= B =2 = B dI I n 2D transit time C C q 1 n WB

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 56 Collector Transit Time

Electrons injected into collector depletion region – very high fields more than diffusion => drift => acceleration of carriers Charge imaged in collector + P N W τ = BC ? υ sat

i τ t

q τ W 1 τ = = = BC ×i ×τ = q eff, BC υ 2 i 2 2 sat Klimeck – ECE606 Fall 2012 – notes adopted from Alam 57

Putting the Terms Together

Collector transit time Kirk Current log 10 fT Base transit time

1 W2 W  =B + BC + π υ  2fT 2 D n 2 sat  k T B +  Cj, BC C j , BE  qI C log I I K 10 C Junction charging time

Do you see the motivation to reduce WB and W BC as much as possible? What problem would you face if you push this too far ?

Increasing I C too high reduces W BC and increases the overall capacitance => frequency rolls off…. Klimeck – ECE606 Fall 2012 – notes adopted from Alam 58 High Frequency Metrics

(current-gain cutoff frequency, fT) 1 W2 W kTq τ = =B + BC + B ()C+ C + ()R + R C π υ jBE, jBC , ex c cb 2fT 2 D n 2 satI C

(power-gain cutoff frequency, fmax ) f f = T max π 8 RbbCcbi

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 59

Summary

We have discussed various modifications of the classical BJTs and explained why improvement of performance has become so difficult in recent years.

The small signal analysis illustrates the importance of reduced junction capacitance, resistances, and transit times.

Classical homojunctions BJTs can only go so far, further improvement is possible with heterojunction bipolar transistors.

Klimeck – ECE606 Fall 2012 – notes adopted from Alam 60