Lecture 38 - Bipolar Junction Transistor (cont.) May 9, 2007

Transcription

1 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture 38-1 Lecture 38 - Bipolar Junction Transistor (cont.) May 9, 27 Contents: 1. Non-ideal effects in BJT in FAR Reading material: del Alamo, Ch. 11, 11.5 ( , , , but only qualitatively)

2 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture 38-2 Key questions Why are the output characteristics of a BJT in FAR not perfectly flat? What is the maximum voltage that the collector of a BJT can sustain in FAR? What are the key design issues for the breakdown voltage? Why does the performance of a BJT degrade at high collector current? How does one model the base resistance in a BJT?

3 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture Non-ideal effects in BJT Base-width modulation V BE and V BC affect x BE and x BC, respectively W B = f(v BE,V BC ). log N log N N E N E N B N B N C N C x BE x BC x x BE x BC x thermal equilibrium (V BE =V BC =) forward active (V BE >, V BC <) Early effect: impact of V BC on W B V BC x BC W B I C,I B unchanged Reverse Early effect: impact of V BE on W B V BE x BE W B smaller I C than ideal β F,I B unchanged

4 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture 38-4 Early effect: impact of V BC on W B I C = f(v BC ) n n B () n B (x) I C n n B (W B )= i 2 N B W B (V BC ) x V BC more negative W B (V BC ) I C (V BC ) To capture first-order impact, linearize W B (V BC ): V A is Early voltage: V BC W B (V BC ) W B (V BC = )(1 + ) V A Impact on I C : V A = qn B W Bo C jco I C (V BC =) V BC I C (V BC ) I C (V BC = )(1 ) 1+ V BC V V A A Also, since I B unchanged: β F (V BC =) V BC β F (V BC )= β F (V BC = )(1 ) 1+ V BC V V A A

5 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture 38-5 Manifestation of Early effect in output characteristics: I C I B I B = -V A V CE Main consequence of Early effect: finite slope in output characteristics in FAR: output conductance. B + C jc C v be C π +C je gπ g m v be g o - E with g o given by: I C g o = VA

6 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture 38-6 Avalanche breakdown Sudden rise in I C for large reverse V CB. Key issue: breakdown voltage depends on terminal configuration: I C V CE V CB I B = I E = BV CEO BV CBO V CE Experimental observation: BV CEO <BV CBO

7 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture 38-7 Common-base configuration: breakdown as in isolated p-n junction n-emitter p-base n-collector I E = I C breakdown when M V BC < Common-emitter configuration: BE and BC junctions interact in a positive feedback loop n-emitter p-base n-collector I C I B = With I B = (or fixed), holes generated in B-C junction must be injected into E B-E goes into forward bias I C. Breakdown occurs at finite M and enhanced by high β F : V CE 1 M crit 1+ βf

8 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture 38-8 Key dependencies: N C BV CEO,BV CBO β F BV CEO but BV CBO unchanged don t want more β F than necessary!. Yamaguchi, T., et. al. "Process and Device Characterization for a 3-GHz ft Submicrometer Double Poly-Si Bipolar Technology Using BF2 -implanted Base with Rapid Thermal Process." IEEE Transactions on Electron Devices 4, no. 8 (1993): Copyright 1993 IEEE. Used with permission.

9 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture 38-9 High collector current effects As I C, electron velocity in collector. But, there is a limit: v sat. Then, as I C approaches: I CK = qa E N C v sat the electrostatics of the collector are profoundly modified transistor performance degrades: βf log f T ideal 1 β Fo 2πτF ideal actual f Tpk actual 1 I C2 log I C I Cpk log I C To first order: I C2 I Cpk I CK

10 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture 38-1 Main origin: formation of current-induced base inside collector SCR effective quasi-neutral base width β F new delay component f Tpk Key design issue: N C N C I CK f Tpk Experiments [Crabbé IEDM 1993]: SiGe base Si base Key trade-off: Crabbe, E.F., et. al. "Vertical Profile Optimization of Very High Frequency Epitaxial Si and SiGe-base Bipolar Transistors." Technical Digest of the International Electron Devices Meeting, Washington, DC, December 5-8, Piscataway, NJ: Institute of Electrical and Electronics Engineers, 1993, pp ISBN: Copyright 1993 IEEE. Used with permission. N C BV

11 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture Base resistance Very important parasitic for digital and analog applications. S E B E B L E R Bext R Bint n+ p n Two components to R B : C external: associated with ohmic contact to base and lateral flow through extrinsic base internal: associated with intrinsic base R B = R Bext + R Bint Important issues: 1. R Bint is distributed: how does it scale? 2. Non-linear behavior of R Bint for high I B

12 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture Low-current resistance (in absence of debiasing) Define lateral resistance of intrinsic base: S E R blat = R shb L E With: 1 R shb = qμb N B W B Clearly, R Bint <R blat because not all I B flows across entire intrinsic base. But, what is proportionality constant?

13 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture BJT with one base contact: B S E E L E p n+ I B J he (y) I B A E S E y i B (y) I B S E y Assume small ohmic drop along base i B (x) uniformly distributed: x i B (x) = I B (1 ) S E R Bint obtained by examining power dissipation in base: Then: p B = I B 2 R Bint = S E i 2 B(x) Rshb 2 dx = I 1 S E B R shb L E 3 L E 1 S E 1 R Bint = R shb = R blat 3 L E 3

14 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture BJT with two base contacts: B S E E L E p n+ I B 2 I B 2 J he (y) I B A E S E y i B (y) I B 2 S E 2 S E y Now: I B 2x S E i B (x) = (1 ) for x 2 S E 2 I B 2x S E i B (x) = ( 1) for x S E 2 S E 2 Intrinsic base resistance: R Bint = 1 12 R blat This is a quarter of the design with one-base contact.

15 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture Total base resistance is: R B = R Bext + R Bint To get small R B : minimize R Bext self-aligned process minimize R Bint use two base contacts takes more space increase W B degrades frequency response increase N B hard, costly, C je,bv EBO decrease S E (scaling!) costly

16 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture Useful observation: trade-off between R B and I C Remember: Then: qv BE qa E n 2 i D B qv BE I C = I S exp = exp kt N B W B kt J C 2 2 qv BE = q n i D B μ B exp kt R shb rather independent of N B or doping profile. [from Tang TED 27, 563 (198)] Tang, D. D. "Heavy Doping Effects in p-n-p Bipolar Transistors." IEEE Transactions on Electron Devices 27, no. 3 (198): Copyright 198 IEEE. Used with permission.

17 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture ) Non-linear behavior of R B More accurate equivalent circuit model of distributed R B : S E B E B L E R Bext n+ p R Bint j B (x) I B A E S E x i B (x) I B 2 S E S E x 2 Non-linear B-E diode current non-uniform current distribution across base: base resistance debiases center of base current crowds at edges of base

18 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture Analytical model possible (but not pretty). General behavior of R Bint : R Bint 1 Rblat 12 For negligible debiasing to occur: kt I B R blat <.2 q log I B R Bin drops by 5% at: kt I B R blat 6 q Overall behavior of R B : R B 1 Rblat +R Bext 12 R Bext log I B

19 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture Consequences of base debiasing: R B depends on I B R B depends on V BE and I C Weng, J., J. Holz, and T. F. Meister. "New Method to Determine the Base Resistance of Bipolar Transistors." IEEE Electron Device Letters 13, no. 3 (1992): Copyright 1992 IEEE. Used with permission. For high current, R Bint R B R Bext and independent of S E.

20 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture 38-2 Non-linear behavior of R Bint : small-signal resistance large-signal resistance Ohmic drop in R B : V B = R B I B Then, small-signal base resistance: dv B dr B r B = = R B + I B di B di B Since I B R B : r B R B R Bint,r Bint 1 12 R blat r Bint R Bint log I B

21 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture Consequences of R B : R B has no direct impact on f T, but has indirect impact through constraint in base design. Actually, f T,pk does not appear correlated to R B (f T,pk dominated by N C ): r B crucially important to noise r B C jc important time constant in high-frequency operation of BJT

22 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture Key conclusions Early effect: modulation of W B by V BC output conductance in output characteristics. I C g o = VA Base-width modulation effects affect I S and β F, but not I B. In avalanche breakdown in a BJT: BV CEO <BV CBO because of transistor current gain. β F BV CEO w/o affecting BV CBO. Key device design issue in avalanche breakdown: N C BV s At high collector current, velocity saturation of electrons in collector performance degrades: β F,f T Maximum current: Two components in R B : I Cpk qa E N C v sat R Bext : associated with ohmic contact and transport through extrinsic base

23 6.72J/3.43J - Integrated Microelectronic Devices - Spring 27 Lecture R Bint : associated with intrinsic base R Bint is a distributed resistance R Bint <R blat R Bint is non linear in I B. Consequence of non-linearity of R Bint at high I B : small-signal resistance different from large-signal resistance. Three technological approaches to reduce R B : use two base contacts use self-aligned process decrease S E Fundamental trade-off between I C and R shb.