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Mextram 504 BJT model

Mextram 504 BJT model. F. Yuan Advisor ： Prof. C. W. Liu Graduate Institute of Electronics Engineering and Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan. Outline. Charge modeling Collector current Base current series resistance, epilayer resistance

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Mextram 504 BJT model

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  1. Mextram 504 BJT model F. Yuan Advisor： Prof. C. W. Liu Graduate Institute of Electronics Engineering and Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan

  2. Outline • Charge modeling • Collector current • Base current • series resistance, epilayer resistance • Avalanche multiplication • Extrinsic region • AC small-signal model • Noise and temperature effect

  3. Depletion charge (Qte,Qtc) • Set Q=0 at V=0 • Change function to a smooth one to prevent the value become infinite at V=Vd

  4. Base diffusion charge (QBE,QBC) • Injected , so we caculate injected e- • Define base charge at zero bias • QBE+QBC=all diffusion charge in base Assumed linear

  5. Main current (IN) q1=1 means no early effect

  6. Total base charge (qB) • Early effect (base width modulation) Qte,Qtc • High level injection QBE,QBC

  7. Base current (IB1,IB2) • IB1 is ideal forward base current • IB2 is non-ideal forward base current (2kT current at low bias) • S means sidewall

  8. SiGe HBT • qB is modified by the bandgap difference of the base region • Only considered the linear graded Ge profile • If there are a lot of defects in SiGe base, there is neutral base recombination current (1kT current)

  9. Diffusion charge (QE, Qepi) • Emitter diffusion charge QE • Collector epilayer diffusion charge Qepi

  10. Base capacitance • Base current is injected from side, the voltage on B1 and B2 may be different • We must compensate the charge

  11. Base resistance • DC crowding effect B2 B B1 RBv RBc

  12. Collector resistance • Buried layer to collector electrode resistance is constant RCC • Epilayer resistance is a variable

  13. Collector resistance • When IC large, RC ：small to high to small

  14. Collector resistance • Kull, TED vol.32, no.6, p1103, 1985

  15. Collector resistance • Jeroen, SSC vol.36, no.9, p1390, 2001 • Also considered the high current base push-out (Kirk effect) • Velocity saturation • Final equation is

  16. RF performance • fT roll-off at high IC, IC1C2 is the key • When IC get large enough, base push-out occurs, increase and makes fT roll-off • Mextram model based on more physical parameters

  17. Avalanche multiplication • Weak avalanche effect • Valid only for IC1C2 < Ihc • Kloosterman, p172, BCTM 2000

  18. Extrinsic region • Base-SIC：intrinsic • Base-epilayer-buried layer：extrinsic • Base-(p-poly)-buried layer：external

  19. Reverse base current (Iex,IB3) • Iex is ideal reverse base current • IB3 is non-ideal reverse base current (2kT current at low bias) • Xext is partitioning factor

  20. Extrinsic region • External reverse base current, XIex • Extrinsic depletion charge, Qtex • External depletion charge, XQtex • Extrinsic diffusion charge, Qex • External diffusion charge, XQex

  21. Parasitic PNP • Base-Collector-Substrate：parasitic PNP • Only for it’s main current

  22. Others • Collector-Substrate depletion capacitance • Reverse substrate current • Constant B-E, B-C overlap capacitance

  23. Small-signal equivalent circuit

  24. Small-signal equivalent circuit • x：VB2E1 • y：VB2C2 • z：VB2C1

  25. Small-signal equivalent circuit • x：VB2E1 • y：VB2C2 • z：VB2C1

  26. Small-signal equivalent circuit • x：VB2E1 • y：VB2C2 • z：VB2C1

  27. Small-signal equivalent circuit • x：VB2E1 • y：VB2C2 • z：VB2C1

  28. Small-signal equivalent circuit • x：VB2E1 • y：VB2C2 • z：VB2C1 • Can get more precise parameters • Extrinsic added

  29. Hybrid-π model • Let the equivalent circuit has only One current source B2-E1-(C1-E1)=B2-C1

  30. Cutoff frequency fT

  31. Cutoff frequency fT

  32. Noise (for AC) • Thermal noise -- consider variable resistance • Shot noise • Flicker noise (1/f noise) -- non-ideal base current use KfN

  33. Temperature • Temperature rules are applied to various parameter • Self-Heating is considered

  34. Comparison to GP • fT-IC is more accurate • Mextram parameters are base on more physical way • Noise is considered more accurate because the variable resistance • Linear graded SiGe HBT model in Mextram 504 • Weak avalanche breakdown

  35. Still unconsidered • B-E junction breakdown • High injection current breakdown

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