Physical Operation of BJTs
•
pn­junction review
•
Voltage vs. position in an npn structure
●
cutoff
●
active
●
saturation
•
Designing for high β
•
The Early effect
•
Parasitic capacitance (C
π, Cµ)
Periodic Table
 This abridged table contains elements with three to five valence
electrons, with Si being the most important.
Silicon
 Si has four valence electrons. Therefore, it can form covalent bonds
with four of its neighbours
 When temperature goes up, electrons in the covalent bond can
become free.
Electron­Hole Pair Interaction
 With free electrons breaking off covalent bonds, holes are
generated.
 Holes can be filled by absorbing other free electrons, so
effectively there is a flow of charge carriers.
The pn junction review
p­type
n­type
anode
cathode
metal
silicon oxide
integrated circuit diode
doped silicon
wafer (chip)
Dopant distribution inside a pn junction
excess holes diffuse
to the n­type region
p>>n
­ +
­ +
­ +
n>>p
excess electrons diffuse
to the p­type region
DEPLETION REGION:
p~0, and acceptor ions are exposed ­
n~0, and donor ions are exposed +
Voltage in a pn junction
p>>n
­ +
­ +
­ +
charge, r(x)
+
­
n>>p
x
x
1
E  x = ∫ ρ  x  dx
ε0
electric field,
E(x)
x
x
voltage,
V(x)
V  x =−∫ E  x  dx
~0.7 volts
(for Si)
x
0
Zero Bias
p>>n
voltage,
V(x)
­ +
­ +
­ +
n>>p
~0.7 volts
(for Si)
x
At zero bias (vD=0), neither electrons nor holes can overcome this built­in voltage barrier of ~ 0.7 volts
 iD = 0
Forward Bias
p>>n
­ +
­ +
­ +
n>>p

ID =IS e
voltage,
V(x)
0.65 volts
0.50 volts
qV D
kT
−1

x
0.0 volts
vD
As the bias (vD), increases toward 0.7V, the electrons and holes can overcome the built­in voltage barrier .  iD > 0
Reverse Bias
p>>n
voltage,
V(x)
­ +
­ +
­ +
n>>p
1
0.0 volts
­5 volts
1
/2 I s
/2Is
x
Is
vD
As the bias (vD) becomes negative, the barrier becomes larger. Only electrons and holes due to broken bonds in the depletion region contribute to the diode current.
 iD = ­Is
The npn Bipolar Junction Transistor
n­type
p­type
n+­type
collector
emitter
base
E
npn
structure
B
C
integrated circuit BJT
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
EBJ=off
CBJ=off
At zero bias (vBE=0, vBC=0), neither electrons nor holes can overcome this built­in voltage barrier of ~ 0.7 volts
 iB =iC = 0 (cutoff)
Biasing BJT in active region
n
(C)
+
+
+
­
­
­
­ +
­ +
­ +
p
(B)
V(x)
n+
(E)
iB
~0.7 volts
(for Si)
­iE>>iB
x
EBJ=on
CBJ=off
When the BE junction is forward biased (vBE >0.65, vC>vB) electrons and holes overcome the built­in voltage barrier between the base and emitter iB > 0 and iE > iB (due to n+ emitter dohping)
Biasing BJT in active region (contd.)
n
(C)
+
+
+
­
­
­
V(x)
n+
(E)
iB
vC
~0.7 volts
(for Si)
­ +
­ +
­ +
p
(B)
­iC
CBJ=off
­iE>>iB
x
EBJ=on
If the base region is very thin, the electrons injected by the emitter are collected by the reverse biased CB junction.
iC ≈ iE>>iB (active region)
Biasing BJT in active region
n
(C)
+
+
+
­
­
­
V(x)
n+
(E)
iB
vC
~0.7 volts
(for Si)
­ +
­ +
­ +
p
(B)
iC
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:
i C < iE (active region with low α,
β)
Carrier Flow in active npn BJT
n(collector)
p­(base)
n+(emitter)
longitudinal
electron flow
Recombination
in the base
Holes injected
into the emitter
Recombination
in the emitter
Conceptual Description
• Normally, the emitter is heavily doped and the base and collector lightly doped
• For active region operation, the base­emitter junction Is forward biased and the collector­base junction is reverse biased.
• Since the emitter is forward biased, it injects minority carriers into the base. Most of the injected carriers make it to the collector­base junction where they are swept away by the reverse bias electric field.
Conceptual Description(contd.)
• Since the collector­base junction is reverse biased, there is no injection of majority carriers. The C­B current primarily constitutes the current injected at the B­E junction and hence it's almost independent of the C­B voltage.
• Fraction of the base current recombines in the base which constitutes part of the base current and is also dependent on the B­E voltage.
• The other fraction of base current is from minority carrier injection into the emitter.
Biasing BJT in saturation region
+
+
+
n
(C)
­
­
­
V(x)
vC
­ +
­ +
­ +
p
(B)
n+
(E)
iB
­iC
­iE>>iB
­iB
CBJ=on
x
EBJ=on
If the vC drops such that the CBJ is slightly 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
How do we achieve high β?
• 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.) ≈ β
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...integrated circuit) The Early Effect
• As VC increases, the depletion width of the B­C junction becomes wider.
• This make the base width more narrow
• This increases the collection efficiency
• Finally, iC/iB increases (higher β)
n
(C)
100uA
10mA
Ic
n
(C)
+ ­
+ ­
+ ­
p
(B)
IC
­ +
­ +
­ +
n+
(E)
+
+
+
8mA
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
V CE
V_Vce
3.5V
4.0V
4.5V
5.0V
5.5V
6.0V
 incr. VC
­
­ +
p
­
­ +
(B)
­
­ +
n+
(E)
Parasitic (unwanted) Capacitance
• Each junction forms a parasitic capacitor
semiconductor/depletion/semiconductor
n
(C)
+ ­
+ ­
+ ­
p
(B)
­ +
­ +
­ +
Cµ Cπ
n+
(E)
Cµ
Cπ
At high frequency, (1/jωC  0), these capacitors become
short circuits and prevent the BJT from proper operation.
This is a fundamental limit on high frequency circuits.
Accepted Sign Convention
• For forward­active NPN: IC(+ve),IE(­ve), IB(+ve), VBE(+ve),VBC(­ve),VCE(+ve)
• For forward­active PNP: IC(­ve),IE(+ve), IB(­ve), VBE(­ve), VBC(+ve), VCE(­ve)
Forward­active biasing (NPN)
• (a) VBE > 0.6V and VCB > ­0.4V
• (b) More practical biasing: VBE > 0.6V and VCE > 0.2 (VCB = VCE ­VBE)
Forward­active biasing (PNP)
• (a) VBE < ­0.6V and VCB < 0.4V
• (b) More practical biasing: VBE < ­0.6V and VCE < ­0.2 (VBE = VCE ­VCB)
Current Direction Convention followed in the Class (NPN)


V BE
VT
IC =IS e −1 ≈ IS e
V BE
VT
where V T =
early effect

V CE

 ∣ ∣
IC =IS 1
VA
V BE
VT
e −1

kT
and neglect Early Effect
q
where V A is the early voltage
IE= IC IB
Current Direction Convention followed in the Class (PNP)

∣V BE∣
IC =IS e
VT

∣V BE∣
VT
−1 ≈ IS e
where V T=
kT
and neglect Early Effect
q
early effect

V CE

 ∣ ∣
IC =IS 1
VA
∣V BE∣
e
VT
−1

where V A is the early voltage
IC =IE− IB
Frequently Used Relations
IC
=
IB
⇒
IC=⋅IB
⇒
IC
IB=

where  is the current amplification factor
IC

IE =
where =

1