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EE130 Lecture 26, Slide 1Spring 2007
Lecture #26
OUTLINE
• Modern BJT Structures– Poly-Si emitter– Heterojunction bipolar transistor (HBT)
• Charge control model
• Base transit time
Reading: Finish Chapter 11, 12.2
EE130 Lecture 26, Slide 2Spring 2007
• Narrow base • n+ poly-Si emitter• Self-aligned p+ poly-Si base contacts• Lightly-doped collector• Heavily-doped epitaxial subcollector• Shallow trenches and deep trenches filled with SiO2 for electrical isolation
B E C
p+ p+ P base
N collector
N+ subcollector
P substrate
N+ polySi
N+
Deeptrench
Deep trench
Shallowtrench
P+polySiP+polySi
Modern BJT Structure
EE130 Lecture 26, Slide 3Spring 2007
• dc is larger for a poly-Si emitter BJT as compared with an all-crystalline emitter BJT, due to reduced dpE(x)/dx at the edge of the emitter depletion region
Polycrystalline-Silicon (Poly-Si) Emitter
dx
pd
dx
pd
D
D
dx
pddx
pdqD
dx
pdqD
E
E
EE
E
EE
EE
EE
2
1
22
1
21
22
11
Si)2 Si;-poly(1
Continuity of hole current in emitter
EE130 Lecture 26, Slide 4Spring 2007
Emitter Gummel Number w/ Poly-Si Emitter
pEEi
EEi
E
EW
Ei
i
polyE
EEEi
EEi
E
EW
Ei
iE
SWn
WNndx
D
N
n
n
WDWn
WNnxd
D
N
n
nG
E
E
)(
)(
)(
)(
2
2
0 2
2
,2
2
0 2
2
For a uniformly doped emitter,
pE
E
iE
iEE SD
W
n
nNG
12
2
1/2
kTqV
E
iB
EBeG
AqnI
where Sp DEpoly/WEpoly is the surface recombination velocity
EE130 Lecture 26, Slide 5Spring 2007
Emitter Band Gap Narrowing
BiE
EiBdc
Nn
Nn2
2
To achieve large dc, NE is typically very large, so that band gap narrowing (Lecture 8, Slide 5) is significant.
/)( /2 kTEEvc
kTEvciE
GEGGE eNNeNNn
/22 kTEiiE
GEenn EGE is negligible for NE < 1E18/cm3
N = 1018 cm-3: EG = 35 meV
N = 1019 cm-3: EG = 75 meV
EE130 Lecture 26, Slide 6Spring 2007
Narrow Band Gap (SiGe) Base
BiE
EiBdc
Nn
Nn2
2
To improve dc, we can increase niB by using a base material (Si1-xGex) that has a smaller band gap
• for x = 0.2, EGB is 0.1eV
Note that this allows a large dc to be achieved with large NB (even >NE), which is advantageous for
• reducing base resistance• increasing Early voltage (VA)
EE130 Lecture 26, Slide 7Spring 2007
If DB = 3DE , WE = 3WB , NB = 1018 cm-3, and niB2 = ni
2, find dc for
(a) NE = 1019 cm-3, (b) NE = 1020 cm-3, and (c) NE = 1019 cm-3 and a Si1-xGex base with EgB = 60 meV
(a) At NE = 1019 cm-3, EgE 35 meV
(b) At NE = 1020cm-3, EgE meV:
(c)
226/352/22 8.3 imeVmeV
ikTE
iiE nenenn gE
6.238.310
109
218
219
2
2
i
i
iEB
iE
BE
EBdc n
n
nN
nN
WD
WD
226/16022 470 imeVmeV
iiE nenn
226/602/22 10 imeVmeV
ikTE
iiB nenenn gB 236F
EXAMPLE: Emitter Band Gap Narrowing
9.147010
109
218
220
2
2
i
i
iEB
iE
BE
EBdc n
n
nN
nN
WD
WD
EE130 Lecture 26, Slide 8Spring 2007
Charge Control Model
B
BB
B Qi
dt
dQ
Wx
BB tptxp 1),0(),(
W
BBB
tpqAWdxtxpqAQ
0 2
),0(),(
A PNP BJT biased in the forward-active mode has excessminority-carrier charge QB stored in the quasi-neutral base:
In steady state,B
BB
B Qi
dt
dQ
0
EE130 Lecture 26, Slide 9Spring 2007
2
),0( tpqAWQ B
B
Bt D
W
2
2
• time required for minority carriers to diffuse across the base • sets the switching speed limit of the transistor
Base Transit Time, t
t
BBBC
BB
Wx
BBC
Q
W
QDi
W
tpqAD
x
txpqADi
2
2
),0(),(
EE130 Lecture 26, Slide 10Spring 2007
Relationship between B and t
tdcB
t
BC
Qi
B
BB
Qi
• The time required for one minority carrier to recombine in the base is much longer than the time it takes for a minority carrier to cross the quasi-neutral base region.
EE130 Lecture 26, Slide 11Spring 2007
The base transit time can be reduced by building into the base an electric field that aids the flow of minority carriers.
• Fixed EgB , NB decreases from emitter end to collector end.
• Fixed NB , EgB decreases from emitter end to collector end.-E B C
-E B C
Ec
dx
dE
qC1E
Ec
Ev
Ev
Ef
Ef
Drift Transistor: Built-in Base Field
EE130 Lecture 26, Slide 12Spring 2007
EXAMPLE: Drift Transistor
• Given an npn BJT with W=0.1m and NB=1017cm-3 (n=800cm2/Vs), find t and estimate the base electric field required to reduce t
cmkVssVcm
cmW
tW
driftv
W
tn
n
/6102/800
10122
5
ps
sVcmV
cm
D
W
Bt 2
/800026.02
10
2 2
252
