C-305: Spring 2018 Final xam Review Pierret, Semiconductor Device Fundamentals (SDF) Chapters 10 and 11 (pp. 371-385, 389-403) Professor Peter Bermel lectrical and Computer ngineering Purdue University, West Lafayette, IN USA pbermel@purdue.edu 4/26/2018 Bermel C 305 S18 1
BJT Symbols and Conventions Symbols B N + P Poly emitter Low-doped base NPN Collector PNP Collector N Collector doping optimization Base Base C mitter mitter I C +I B +I =0 V B +V BC +V C =0 4/26/2018 Bermel C 305 S18 2
BJT Current Gain Common mitter current gain.. = DC I I C B V B (in) B I B P +N P I C C Common Base current gain.. a = DC I I I C C C DC = = I B I IC I a DC = - 1 - a DC V B (in) B I P + N P I C C V CB (out) B 4/26/2018 Bermel C 305 S18 3
Current Gain 4/26/2018 Bermel C 305 S18 4
essence of current gain N P V B V BC N + Input Response Input I B 2» qdp ni, q W N - ( V 1) B e I 2» qdn i, B - W B n N B ( qv 1) B e 4/26/2018 Bermel C 305 S18 Response 5
x= 0 mitter Current 2 2 dn qd n n i, B qd ni B qvbc kbt J n, = qdn = - - + - d x W N W N ( qv ), 1 ( 1) B kbt n e e B B B B J p, 2 dp Dp n qv = - qd = - i B p e - dx W N x= 0' n D ( 1) J = J + J p, n, V B V B Bermel C 305 S18 6
collector current: forward active region FB RB I I = I n + I p I n n+ emitter p base I p a T n collector I n n+ I C I C» I n 2 æ n I n = qa i è ç N AB 2 æ n I p = qa i è ç N D ö ø ö ø D n W B e qv B /k BT D p W e qv B /k BT I B = I p I C = a T I n I C = a T g F I I C = a F I F 0 e qv B k ( T B -1) a F = a T g F 4/26/2018 Bermel C 305 S18 7
bers-moll model FB RB I I n n+ emitter p base I Cn n collector n+ I C I p I Cp I B ( V B,V BC ) = I ( V B,V BC ) - I C ( V B,V BC ) I C I ( V B,V BC ) = a F I ( F0 e qv B k T B -1) - I ( R0 e qv BC k T B -1) ( V B,V BC ) = I ( F0 e qv B k T B -1) -a R I ( R0 e qv BC k T B -1) 4/26/2018 Bermel C 305 S18 8
bers-moll model I C I ( V B, V BC ) = a F I ( F0 e qv B k T B -1) - I ( R0 e qv BC k T B -1) ( V B,V BC ) = I ( F0 e qv B k T B -1) - a R I ( R0 e qv BC k T B -1) a F = a T g F 2 æ n I F 0 = qa i è ç N AB 2 æ n a R = a T g R I R0 = qa i è ç N AB ö ø ö ø D n W B + qa D n W B + qa æ è ç æ è ç 2 n i N D 2 n i N DC ö ø ö ø D p W D p W C a F I F0 = a R I R0 4/26/2018 9 Bermel C 305 S18
Gummel Plot and Output Characteristics 2 2 IC qd n n i, B qvb / kt qd n n i, B qvbc / kt = - ( e -1) + ( e -1) A W N W N B B B B 2 I qdp n B i, qvb / kt = ( e -1) A W N V B = DC DC I C I B Common emitter Current Gain 4/26/2018 Bermel C 305 S18 10
maximizing gains in BJTs» dc n 2 n i, B N 2 B p i, B D W W D n N mitter doping: As high as possible without band gap narrowing Base doping: As low as possible, without current crowding, arly effect N Collector doping: Lower than base doping without Kirk ffect N B N C N D + + Base Width: As thin as possible without punch through (~1 mm in 50s, 200 Å now) 4/26/2018 Bermel C 305 S18 11
How to make better Transistor» n 2 n i, B N 2 B p i, B D W W D n N Graded Base transport Polysilicon mitter Classical Shockley Transistor Heterojunction Bipolar Transistor 4/26/2018 Bermel C 305 S18 12
poly-silicon HBT emitter P- Base mitter N+ N+ N- N SiGe intrinsic base Collector P+ N+ Dielectric trench emitter N + P N Poly-silicon 4/26/2018 Bermel C 305 S18 13
SiGe HBT 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. 4/26/2018 14 Bermel C 305 S18
New equations on equation sheet Bipolar transistors: (assuming NPN, short emitter, base, and collector) bers-moll quations: 4/26/2018 Bermel C 305 S18 15
Practice Question 1 Region of Operation: A. Forward Active B. Inverse Active C. Pinchoff D. Saturation. Cutoff 4/26/2018 Bermel C 305 S18 16
Practice Question 2 Region of Operation: A. Forward Active B. Inverse Active C. Pinchoff D. Saturation. Cutoff 4/26/2018 Bermel C 305 S18 18
Practice Question 3 Region of Operation: A. Forward Active: B. Inverse Active: C. Pinchoff : D. Saturation :. Cutoff : 4/26/2018 Bermel C 305 S18 20
Practice Question 4 4/26/2018 Bermel C 305 S18 22
Practice Question 5 Which of the following would reduce base width modulation (i.e. the arly effect) and, therefore, increase the output resistance? a) Increasing the emitter doping. b) Increasing the collector doping. c) Increasing the base width. d) Increase the emitter thickness. e) Decrease the base doping. 4/26/2018 Bermel C 305 S18 24
Practice Question 6 Consider the transistor circuit below What is the region of operation for this transistor if the base voltage V A is positive? 4/26/2018 Bermel C 305 S18 26
Practice Question 7 Write down the bers-moll equations for the transistor circuit below: 4/26/2018 Bermel C 305 S18 28
Fall 2015: Multiple Choice 1 (8 points). Which of the following would increase the collector breakdown voltage in a BJT? a. Increasing the emitter doping b. Increasing the base doping c. Increasing the collector doping d. Decreasing the collector width e. Decreasing the collector doping 4/26/2018 Bermel C 305 S18 30
Fall 2015: Multiple Choice 2 (8 points). How are the PN junctions biased in the forward active region of an NPN BJT? a. Base-collector: forward biased mitter-base: forward biased b. Base-collector: forward biased mitter-base: reverse biased c. Base-collector: reverse biased mitter-base: forward biased d. Base-collector: reverse biased mitter-base: reverse biased e. Base-collector: forward biased mitter-base: biased in breakdown 4/26/2018 Bermel C 305 S18 32
Fall 2015: Multiple Choice 3 (8 points). What would be considered good values for and, respectively, in a BJT? a. 0.1 and 0.11 b. 0.5 and 1 c. 0.99 and 99 d. 3 and 1.5 e. 20 and 1.052 4/26/2018 Bermel C 305 S18 34
Fall 2015: Multiple Choice 4 (8 points). For large gain in a bipolar transistor, the emitter doping must be.? a. Larger than the base doping only b. Smaller than the base doping only c. Larger than the collector doping only d. Larger than both base and collector doping e. It does not matter 4/26/2018 Bermel C 305 S18 36
Fall 2015: Multiple Choice 5 (8 points). How can a heterojunction bipolar transistor be designed to improve? a. Low bandgap material in the base region b. Low bandgap material in the emitter region c. High electron affinity material in the base region d. Low breakdown voltage material in the collector region e. High bandgap material in the base region 4/26/2018 Bermel C 305 S18 38
Fall 2015: Part II Consider the NPN BJT made of silicon, depicted below. Assume that it is in the forward active region, with IC=10 ma, doping concentrations N D, =10 18 /cm 3, N A,B =10 17 /cm 3, and N D,C = 10 16 /cm 3 ; thicknesses W =0.5 mm, W B =0.25 mm, and W C =2 mm; and diffusion constants D p, =2 cm 2 /s, D n,b =20 cm 2 /s, and D p,c =12 cm 2 /s, and L b =1 mm. The cross-sectional area A=1 mm 2. Assume that recombination within the BJT is small (i.e., diffusion lengths are much larger than the thicknesses of each layer). a) What is the emitter injection efficiency γf? b) What is the base transport factor αt? c) What is the common base current gain αdc? d) What is the emitter gain βf (aka βdc)? e) What is the forward saturation current density IF0? 4/26/2018 Bermel C 305 S18 40
Fall 2015: Part III Consider the crystalline silicon-based bipolar junction transistor (BJT) depicted below. The shaded regions represent the depletion regions, while white regions may be treated as having zero electric field (in the depletion approximation). Assume that the doping concentrations N D, =10 18 /cm 3, N A,B =10 17 /cm 3, and N D,C = 10 16 /cm 3. Assume LD W and LD WC. a) Sketch the excess minority carrier concentrations (in all white regions) in forward active mode biasing. 4/26/2018 Bermel C 305 S18 46
Fall 2015: Part III Consider the crystalline silicon-based bipolar junction transistor (BJT) depicted below. The shaded regions represent the depletion regions, while white regions may be treated as having zero electric field (in the depletion approximation). Assume that the doping concentrations N D, =10 18 /cm 3, N A,B =10 17 /cm 3, and N D,C = 10 16 /cm 3. Assume LD W and LD WC. b) Sketch the excess minority carrier concentrations (in all white regions) in inverted mode biasing. 4/26/2018 Bermel C 305 S18 48
Fall 2015: Part III Consider the crystalline silicon-based bipolar junction transistor (BJT) depicted below. The shaded regions represent the depletion regions, while white regions may be treated as having zero electric field (in the depletion approximation). Assume that the doping concentrations N D, =10 18 /cm 3, N A,B =10 17 /cm 3, and N D,C = 10 16 /cm 3. Assume LD W and LD WC. c) Sketch the excess minority carrier concentrations (in all white regions) in cutoff mode biasing. 4/26/2018 Bermel C 305 S18 50
Fall 2015: Part III (Questions d and e are based on the following information) The minority carrier concentrations in the quasineutral regions of a pnp BJT are given in the diagram below. d) stimate the sign of the collector-base and emitter-base biases, and deduce the operating mode of this BJT. e) Calculate the magnitude of the collector-base bias V CB. 4/26/2018 Bermel C 305 S18 52
Thank you for taking C 305 Good luck on your Final xams!