spring 2007ee130 lecture 26, slide 1 lecture #26 outline modern bjt structures –poly-si emitter...
Post on 15-Jan-2016
220 views
TRANSCRIPT
![Page 1: Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control](https://reader035.vdocuments.site/reader035/viewer/2022062423/56649d3a5503460f94a14916/html5/thumbnails/1.jpg)
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
![Page 2: Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control](https://reader035.vdocuments.site/reader035/viewer/2022062423/56649d3a5503460f94a14916/html5/thumbnails/2.jpg)
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
![Page 3: Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control](https://reader035.vdocuments.site/reader035/viewer/2022062423/56649d3a5503460f94a14916/html5/thumbnails/3.jpg)
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
![Page 4: Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control](https://reader035.vdocuments.site/reader035/viewer/2022062423/56649d3a5503460f94a14916/html5/thumbnails/4.jpg)
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
![Page 5: Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control](https://reader035.vdocuments.site/reader035/viewer/2022062423/56649d3a5503460f94a14916/html5/thumbnails/5.jpg)
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
![Page 6: Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control](https://reader035.vdocuments.site/reader035/viewer/2022062423/56649d3a5503460f94a14916/html5/thumbnails/6.jpg)
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)
![Page 7: Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control](https://reader035.vdocuments.site/reader035/viewer/2022062423/56649d3a5503460f94a14916/html5/thumbnails/7.jpg)
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
![Page 8: Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control](https://reader035.vdocuments.site/reader035/viewer/2022062423/56649d3a5503460f94a14916/html5/thumbnails/8.jpg)
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
![Page 9: Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control](https://reader035.vdocuments.site/reader035/viewer/2022062423/56649d3a5503460f94a14916/html5/thumbnails/9.jpg)
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(),(
![Page 10: Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control](https://reader035.vdocuments.site/reader035/viewer/2022062423/56649d3a5503460f94a14916/html5/thumbnails/10.jpg)
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.
![Page 11: Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control](https://reader035.vdocuments.site/reader035/viewer/2022062423/56649d3a5503460f94a14916/html5/thumbnails/11.jpg)
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
![Page 12: Spring 2007EE130 Lecture 26, Slide 1 Lecture #26 OUTLINE Modern BJT Structures –Poly-Si emitter –Heterojunction bipolar transistor (HBT) Charge control](https://reader035.vdocuments.site/reader035/viewer/2022062423/56649d3a5503460f94a14916/html5/thumbnails/12.jpg)
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