ECE606: Solid State Devices Lecture 20 Heterojunction Bipolar Transistor Gerhard Klimeck [email protected] 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 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 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 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 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 EC 2 G 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 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 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 1.