Lecture 5: Bipolar junction transistors

1
Lecture 5 Bipolar junction transistors

• Bipolar means both electrons, holes participate
  in the device.
• (In contrast, FET is unipolar.)

2
Outline for today

• n-p-n homojunction band diagrams
• n-p-n DC I-V curves, etc.
• DC circuit models
• Why are we doing this? YOU, the designer will
  need to know how different transistor geometries
  translate into different effective circuit
  elements.

3
n-p-n geometry
emitter
ohmic
emitter
n
ohmic
base
base
p
n
collector
ohmic
circuit only if base LONG
collector
4
n-p-n junction at zero bias
p
n
n
Ec
EFermi
Ev
5
Bias conditions
6
Normal active bias

• E-B forward bias(VbgtVe)
• C-B reverse bias(VcgtVb)
• Ibe 100 IcebIce

Ic
Ib
Ie
All of chapter 3 is about calculating b.
7
Many current components

• Electron drift E-B
• Electron diffusion E-B
• Hole drift E-B
• Hole diffusion E-B
• Electron drift C-B
• Electron diffusion C-B
• Hole drift C-B
• Hole diffusion C-B

concentrate on these components, since they are
the largest (Discuss why others small.)
8
Normal active band diagram
Base
Emitter
Collector
electrons
electrons
Ec
VEB
EFermi
Ev
VCB
If no recombination in base, all current goes
emitter-collector. No current into/out of base.
9
Normal active band diagram
Base
Emitter
Collector
electrons
electrons
Ec
recombination
VEB
EFermi
Ev
VCB
holes

• What fraction of electrons makes it,
• and what fraction recombines?
• Want Wlt Ln (diffusion length)
• Holes from B-E cause base current.

10
Normal active schematic
p type base
n type collector
n type emitter
electrons
Ic
electrons
Ie
recombine
holes
Ib
11
Normal active current gain
Ic
Ib
Ie
That is simplest circuit model. It just gets more
complicated from here!
12
Normal active schematic
p type base
n type collector
n type emitter
electrons
Ic
electrons
Ie
recombine
a few holes
holes
Ib
Goal Find relationship between Ib, Ic
13
Normal active schematic
p type base
n type collector
n type emitter
electrons
Ic
electrons
Ie
recombine
a few holes
holes
Ib
(Bad!)

• Want low p-type doping.
• Can show need ngtp
• But ngtp bad for base resistance (speed)
• Solution later heterojunctions block p
  injenction into emitter
• HBTs can have pgtn good for speed

14
Normal active schematic
p type base
n type collector
n type emitter
electrons
Ic
electrons
Ie
Xb
n (no recombination in base)
carrier concentration
(explain) (consider V0)
(Liu 3.2)
x
Discuss J vs. I. Discuss line vs. tanh.
15
Normal active schematic
p type base
n type collector
n type emitter
electrons
Ic
electrons
Ie
a few holes
(explain like HW1) Liu equation 3.1
16
Normal active schematic
p type base
n type collector
n type emitter
electrons
Ic
electrons
Ie
a few holes
Bad for base resistance. Bad for E-B capacitance.
So bad for speed.
17
Normal active HBT
p type base
n type collector
n type emitter
electrons
Ic
electrons
Ie
a few holes
Holes exponentially suppressed if emitter is
wider gap. (Graded.)
Good for speed. Claims of 1 THz fT in literature.
18
Normal active schematic
p type base
n type collector
n type emitter
electrons
electrons
Ic
Ie
a few holes
Ib

• Depends on recombination
• processes in
• Surface (not contact)
• Surface (at base contact)
• Bulk
• Space charge region
• Try to minimize 1,2,4.

?
19
Normal active schematic
p type base
n type collector
n type emitter
electrons
electrons
Ic
Ie
a few holes
Ib
How to calculate Dont use line for n(x) but
tanh. dn/dx em. dn/dx coll base curr. Explain
on board. Will be HW3.
20
Normal active schematic
p type base
n type collector
n type emitter
electrons
Ic
electrons
Ie
recombine
a few holes
holes
(A different notation not in Liu.)
Ib
21
Complications

• High power
• Spreading resistance (draw on board)
• We dont have time to discuss.

22
Equivalent circuit 1
Ic
Ib
Ie
23
Equivalent circuit 2
Ic
Ib
Ie
24
Equivalent circuit 3
Ib
Ic
Ie
25
Equivalent circuit 4
Ib
Ic
Ie
26
Early effect
Ic
1 mA
Ie1 mA
2 mA
2 mA
3mA
3mA
4 mA
4 mA
Vcb (volts)
5
10
27
Early effect
Ic
1 mA
Ie1 mA
2 mA
2 mA
3mA
3mA
4 mA
4 mA
Vcb (volts)
5
10
W changes with Vcb.
28
Bipolar advantages

• Speed set by base width, which is easy to control
• Large area contributes to current, good for power

29
Bipolar dis-advantages

• Need base current at dc
• No easy complementary digital logic