OUTLINE

1
Lecture 28

• OUTLINE
• The BJT (contd)
• Small-signal model
• Cutoff frequency
• Transient (switching) response
• Reading Pierret 12 Hu 8.8-8.9

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Small-Signal Model
Common-emitter configuration, forward-active mode
R. F. Pierret, Semiconductor Device Fundamentals,
Fig.12.1(a)
hybrid pi BJT small signal model
Transconductance
EE130/230A Fall 2013
Lecture 28, Slide 2
3
Small-Signal Model (cont.)
where QF is the magnitude of minority-carrier
charge stored in the base and emitter regions
forward transit time
EE130/230A Fall 2013
Lecture 28, Slide 3
4
Example
A BJT is biased at IC 1 mA and VCE 3V. bdc
90, tF 5ps, T 300K. Find (a) gm , (b) rp ,
(c) Cp .Solution (a) (b) rp bdc /
gm 90/0.039 2.3 kW (c)
EE130/230A Fall 2013
Lecture 28, Slide 4
5
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
EE130/230A Fall 2013
Lecture 28, Slide 5
6
For the full BJT equivalent circuit
fT is commonly used as a metric for the speed
of a BJT.
Si/SiGe HBT by IBM

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

EE130/230A Fall 2013
Lecture 28, Slide 6
7
Base Widening at High IC Kirk Effect
For a NPN BJT

• At very high current densities (gt0.5mA/mm2), the
  density of mobile charge passing through the
  collector depletion region exceeds the ionized
  dopant charge density

increasing IC

• ? The base width (W) is effectively increased
  (referred to as base push out)
• ? tF increases and hence fT decreases.
• This effect can be avoided by increasing NC ?
  increased CJ,BC , decreased VCE0

EE130/230A Fall 2013
Lecture 28, Slide 7
C. C. Hu, Modern Semiconductor Devices for
Integrated Circuits, Figure 8-18
8
Summary BJT Small Signal Model
Hybrid pi model for the common-emitter
configuration, forward-active mode
EE130/230A Fall 2013
Lecture 28, Slide 8
9
BJT Switching - Qualitative
R. F. Pierret, Semiconductor Device Fundamentals,
Figs. 12.3-12.4
EE130/230A Fall 2013
Lecture 28, Slide 9
10
Turn-on Transient Response

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

where IBBVS/RS
EE130/230A Fall 2013
Lecture 28, Slide 10
R. F. Pierret, Semiconductor Device Fundamentals,
Fig. 12.5
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Turn-off Transient Response

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

EE130/230A Fall 2013
Lecture 28, Slide 11
R. F. Pierret, Semiconductor Device Fundamentals,
Fig. 12.5
12
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, QB,
  and also the recombination lifetime, tB.
• By reducing tB, the carrier removal rate is
  increased
• Example Add recombination centers (Au atoms) in
  the base

EE130/230A Fall 2013
Lecture 28, Slide 12
13
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

EE130/230A Fall 2013
Lecture 28, Slide 13
R. F. Pierret, Semiconductor Device Fundamentals,
Fig. 12.7