Transcript L41a

ECE 340 Lecture 41
Other Modes of BJT Operation; Common-Emitter Gain
• So far we’ve studied BJT only in “normal active” mode (for PNP,
VEB >0 forward biased, VCB < 0 reverse biased). Here, well-designed
devices have high gain (β = 50-300); most useful.
• What about operation in other bias conditions?
• “Inverted” mode: CB forward-biased,
EB reverse-biased.
• “Cut-off” mode: both junctions
reverse-biased. Negligible currents
(~1 pA), nearly an open circuit  useful.
© 2012 Eric Pop, UIUC
ECE 340: Semiconductor Electronics
1
• “Saturation” mode: Both EB and CB junctions are forward-biased.
Base is flooded with minority carriers, i.e. _________ for a PNP.
 No isolation, slow recovery, in practice  avoid.
 But, interesting to study because it is a superposition of “forward” and
“inverted” modes of operation
• We can still write down the minority carrier
concentration in the Base, at the E and C side:
ΔpE =
ΔpC =
Where pn = __________.
© 2012 Eric Pop, UIUC
ECE 340: Semiconductor Electronics
2
• So I can write the hole diffusion current in the Base:
dp
I p  qAD p

dx
• And the Base current due to recombination:
I B , rec 
QB
p

• Key observation: minority carrier diffusion equation is linear,
so all BJT operation modes can be written as a
superposition of the “normal active” and “inverted” modes.
I E  I Ep  I En
I C  I Cp  I Cn
IB 
© 2012 Eric Pop, UIUC
Qp
p
 I En  I Cn   I B , rec  I B ,inj
ECE 340: Semiconductor Electronics
(note that IC and IE can
have either sign depending
on voltages.)
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• Ex: Common-Emitter configuration. Draw circuit:
• Draw current-voltage diagram:
IB
IC
active region
saturation
region
0
VCE
 At low VEC both CB and EB diodes are _________________.
Consequently the transistor is in ________________.
 At higher VEC, CB diode goes into deeper and deeper reverse bias,
and EB stays forward-biased with roughly constant VEB ~ _________.
© 2012 Eric Pop, UIUC
ECE 340: Semiconductor Electronics
4
• As the CB diode is pushed into deeper reverse bias, the CB
depletion region _____________.
• This effectively modulates (decreases) the ___________.
• Which also modulates (increases) the _____________.
Experimental
Data
© 2012 Eric Pop, UIUC
ECE 340: Semiconductor Electronics
5
• Ideally, we do not want the gain to change due to Base width
modulation. During design, we have three options:
1) Increase the Base width, WB
2) Increase the Base doping, NB
3) Decrease the Collector doping, NC
• Which one is the most acceptable in practice?
• What if we really push
VEC until the transistor breaks?
 Punch-through (WB  0)
 Avalanche breakdown
© 2012 Eric Pop, UIUC
ECE 340: Semiconductor Electronics