Download Physical Operation of Diodes

Physical Operation of BJTs
•
•
Structure review
Voltage vs. position in an npn
structure
– cutoff
– active
– saturation
• Designing for high b
• The Early effect
• Parasitic capacitance (Cp, Cm)
From Prof. J. Hopwood
The npn Bipolar Junction Transistor
n-type
p-type
n+-type
collector
emitter
base
E
npn
structure
B
integrated circuit BJT
C
metal
silicon oxide
doped silicon
wafer (chip)
Voltage inside the npn structure
n
C
+ + + -
p
B
- +
- +
- +
n+
E
V(x)
~0.7 volts
(for Si)
x
CBJ=off
EBJ=off
At zero bias (vBE=0, vBC=0), currents
balanced, no net current flow iB =iC =
0 (cutoff)
Voltage inside the npn structure
n
(C)
+ + + -
V(x)
- +
- +
- +
p
(B)
n+
(E)
iB
~0.7 volts
(for Si)
-iE>>iB
CBJ=off
EBJ=on
When (vBE=0.65, vC>vB) electrons and holes
overcome the built-in voltage barrier between
the base and emitter
iB > 0 and iE > iB
(due to n+ emitter doping)
x
Voltage inside the npn structure
n
(C)
+ + + -
- +
- +
- +
p
(B)
n+
(E)
V(x)
iB
vC
~0.7 volts
(for Si)
-iC
CBJ=off
-iE>>iB
EBJ=on
If the base region is very thin, the electrons
injected by the emitter are collected by the
positive voltage applied at vC
iC  iE>>iB
(active region)
x
Voltage inside the npn structure
n
(C)
+ + + -
- +
- +
- +
p
(B)
n+
(E)
V(x)
iB
vC
iC
~0.7 volts
(for Si)
iE>>iB
CBJ=off
x
EBJ=on
If the base region is too thick, the electrons
injected by the emitter are lost by recombining
with holes in the base before the voltage applied
at vC can collect them (another component of
base current):
iC < iE
(active region with low a, b)
Voltage inside the npn structure
+ + + -
n
(C)
- +
- +
- +
p
(B)
n+
(E)
V(x)
iB
vC
-iC
-iE>>iB
-iB
CBJ=on
EBJ=on
If vC drops such that the CBJ is forward biased,
the collector no longer is able to gather the
injected electrons!
These uncollected electrons exit through the
base! iB is large and iC is small Saturation
x
How do we achieve high b?
• make the base region thin (typ. <1 micron)
– this makes the collection efficiency of injected
electrons high and decreases the chance of these
electrons recombining in the base region
• make the emitter heavily doped
– iE/iB  n(emitter)/p(base)
 (emitter doping concentration)/(base doping conc.)  b
These two quantities are difficult to control precisely!
Therefore, the current gain is not uniform among BJTs
(except when the BJTs are all made on the same chip...
an integrated circuit)
The Early Effect
• As VC increases, the depletion width of the B-C
junction becomes wider.
• This make the base width more narrow
• This increases the collection efficiency
• Finally, iC/iB increases (higher b)
n
(C)
+ + + -
p
(B)
- +
10mA
- +
Ic
- +
n
(C)
n+
(E)
100uA
8mA
IC
6mA
4mA
30uA
2mA
20uA
10uA
Ib=0
uA
0A
0V
0.5V
IC(Q2)
1.0V
1.5V
2.0V
2.5V
3.0V
VCE
V_Vce
3.5V
4.0V
4.5V
5.0V
5.5V
6.0V
+
+
+
-
- +
p
- (B) - +
- +
n+
(E)
Parasitic (unwanted) Capacitance
• Each junction forms a parasitic capacitor:
semiconductor/depletion/semiconductor
n
(C)
+ + + -
p
(B)
- +
- +
n+
(E)
Cm
- +
Cm Cp
Cp
At high frequency, (1/jwC  0), these capacitors become
short circuits and prevent the BJT from proper operation.
This is a fundamental limit on high frequency circuits.