OUTLINE

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Lecture 27

• OUTLINE
• BJT small signal model
• BJT cutoff frequency
• BJT transient (switching) response
• Reading Finish Chapter 12

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Small-Signal Model
Common-emitter configuration, forward-active mode
hybrid-pi BJT small signal model
Transconductance
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Small-Signal Model (cont.)
where QF is the magnitude of minority-carrier
charge stored in the base and emitter regions
forward transit time
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Example Small-Signal Model Parameters
A BJT is biased at IC 1 mA and VCE 3 V.
bdc90, tF5 ps, and T 300 K. Find (a) gm ,
(b) rp , (c) Cp .Solution (a) (b) rp
bdc / gm 90/0.039 2.3 kW c)
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Cutoff Frequency, fT
The cutoff frequency is defined to be the
frequency (f w/2p) at which the short-circuit
a.c. current gain equals 1
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For the full BJT equivalent circuit
fT is commonly used as a metric for the speed
of a BJT.
SiGe HBT by IBM

• To maximize fT
• increase IC
• minimize CJ,BE, CJ,BC
• minimize re, rc
• minimize tF

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Base Widening at High IC the Kirk Effect

• At very high current densities (gt0.5mA/mm2), base
  widening occurs, so QB increases.
• tF increases, fT decreases.

Consider an npn BJT At high current levels, the
density of electrons (n ? IC/qAvsat) in the
collector depletion region is significant,
resulting in widening of the quasi-neutral base
region. As W increases, the depletion width in
the collector also increases, since the charge
density decreases At very high current
densities, the excess hole concentration in the
collector is so high that it effectively extends
the p-type base.
Top to bottom VCE 0.5V, 0.8V, 1.5V, 3V.
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Summary BJT Small Signal Model
Hybrid-pi model for the common-emitter
configuration, forward-active mode
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BJT Switching - Qualitative
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Turn-on transient

• We know
• The general solution is
• Initial condition QB(0)0. since transistor is
  in cutoff

where IBBVS/RS
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Turn-off transient

• We know
• The general solution is
• Initial condition QB(0)IBBtB

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Reducing tB for Faster Turn-Off

• The speed at which a BJT is turned off is
  dependent on the amount of excess
  minority-carrier charge stored in the base, and
  also the recombination lifetime tB
• By reducing tB, the carrier removal rate is
  increased
• Example Add recombination centers (Au atoms) in
  the base

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Schottky-Clamped BJT

• When the BJT enters the saturation mode, the
  Schottky diode begins to conduct and clamps the
  C-B junction voltage at a relatively low positive
  value.
• ? reduced stored charge in quasi-neutral base