1. Reading: Finish Chapter 12
Lecture #27
OUTLINE
BJT small signal model
BJT cutoff frequency
BJT transient (switching) response
Reading: Finish Chapter 12
EE130 Lecture 27, Slide 1

2. Small-Signal Model “hybrid-pi” BJT small signal model:
Common-emitter configuration,
forward-active mode:
“hybrid-pi”
BJT small signal model:
Transconductance:
EE130 Lecture 27, 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 Lecture 27, Slide 3

4. Example: Small-Signal Model Parameters
A BJT is biased at IC = 1 mA and VCE = 3 V. bdc=90, tF=5 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)
EE130 Lecture 27, 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 Lecture 27, Slide 5

6. fT is commonly used as a metric for the speed of a BJT.
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
EE130 Lecture 27, Slide 6

7. Base Widening at High IC: the Kirk Effect
At very high current densities (>0.5mA/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.
EE130 Lecture 27, Slide 7

8. Summary: BJT Small Signal Model
Hybrid-pi model for the common-emitter configuration, forward-active mode:
EE130 Lecture 27, Slide 8

9. BJT Switching - Qualitative
EE130 Lecture 27, Slide 9

10. Turn-on transient We know: The general solution is:
Initial condition: QB(0)=0. since transistor is in cutoff
where IBB=VS/RS
EE130 Lecture 27, Slide 10

11. Turn-off transient We know: The general solution is:
Initial condition: QB(0)=IBBtB
EE130 Lecture 27, Slide 11

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, 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 Lecture 27, 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 Lecture 27, Slide 13