6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-1
Lecture 38 - Bipolar Junction Transistor
(cont.)
May 9, 2007
Contents:
1. Non-ideal effects in BJT in FAR
Reading material:
del Alamo, Ch. 11, §11.5 (§§11.5.1, 11.5.3, 11.5.4, 11.5.5
but only qualitatively)
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-2
Key questions
• Why are the output characteristics of a BJT in FAR not perfectly
flat?
• What is the maximum voltage that the collector of a BJT can
sustain in FAR? What are the key design issues for the break­
down voltage?
• Why does the performance of a BJT degrade at high collector
current?
• How does one model the base resistance in a BJT?
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
Lecture 38-3
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
1. Non-ideal effects in BJT
� Base-width modulation
VBE and VBC affect xBE and xBC , respectively ⇒ WB = f (VBE , VBC ).
log N
log N
NE
NE
NB
NB
NC
xBE xBC
NC
x
thermal equilibrium (VBE=VBC=0)
xBE
xBC
x
forward active (VBE>0, VBC<0)
• Early effect: impact of VBC on WB
|VBC | ↑⇒ xBC ↑⇒ WB ↓⇒ IC ↑, IB unchanged
• Reverse Early effect: impact of VBE on WB
VBE ↑⇒ xBE ↓⇒ WB ↑⇒ smaller IC than ideal ⇒ βF ↓, IB unchanged
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
Lecture 38-4
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
� Early effect: impact of VBC on WB ⇒ IC = f (VBC )
n
nB(0)
nB(x)
IC
nB(WB)=0
ni2
NB
WB(VBC)
0
x
VBC more negative ⇒ WB (VBC ) ↓⇒ IC (VBC ) ↑
To capture first-order impact, linearize WB (VBC ):
WB (VBC ) WB (VBC = 0)(1 +
VBC
)
VA
VA is Early voltage:
VA =
qNB WBo
Cjco
Impact on IC :
IC (VBC ) IC (VBC = 0)
VBC
I
(V
=
0)(1
−
)
C
BC
VA
1 + VVBC
A
Also, since IB unchanged:
βF (VBC ) =
βF (VBC = 0)
VBC
β
(V
=
0)(1
−
)
F
BC
VA
1 + VVBC
A
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
Lecture 38-5
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Manifestation of Early effect in output characteristics:
IC
IB
IB=0
-VA
0
VCE
Main consequence of Early effect: finite slope in output characteris­
tics in FAR: output conductance.
Cjc
B
C
+
vbe
Cπ+Cje
gmvbe
gπ
go
-
E
with go given by:
go =
IC
VA
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
Lecture 38-6
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
� Avalanche breakdown
Sudden rise in IC for large reverse VCB .
Key issue: breakdown voltage depends on terminal configuration:
IC
VCB
VCE
IE=0
IB=0
0
0
BVCEO
BVCBO
VCE
Experimental observation:
BVCEO < BVCBO
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
Lecture 38-7
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
• Common-base configuration: breakdown as in isolated p-n junction
n-Emitter
p-Base
n-Collector
IC
IE=0
VBC < 0
⇒ breakdown when M → ∞
• Common-emitter configuration: BE and BC junctions interact in
a positive feedback loop
n-Emitter
p-Base
n-Collector
IC
IB=0
VCE
With IB = 0 (or fixed), holes generated in B-C junction must be
injected into E ⇒ B-E goes into forward bias ⇒ IC ↑.
Breakdown occurs at finite M and enhanced by high βF :
Mcrit 1 +
1
βF
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-8
Key dependencies:
• NC ↑⇒ BVCEO ↓, BVCBO ↓
• βF ↑⇒ BVCEO ↓ but BVCBO unchanged ⇒
don’t want more βF than necessary!.
Yamaguchi, T., et. al. "Process and Device Characterization for a 30-GHz fT Submicrometer Double Poly-Si Bipolar Technology
Using BF2 -implanted Base with Rapid Thermal Process." IEEE Transactions on Electron Devices 40, no. 8 (1993): 1484-1495.
Copyright 1993 IEEE. Used with permission.
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
Lecture 38-9
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
� High collector current effects
As IC ↑, electron velocity in collector ↑. But, there is a limit: vsat.
Then, as IC approaches:
ICK = qAE NC vsat
the electrostatics of the collector are profoundly modified ⇒ transis­
tor performance degrades:
log fT
βF
ideal
βFo
ideal
1
2πτF
fTpk
actual
actual
1
IC2
log IC
ICpk
log IC
To first order:
IC2 ICpk ICK
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-10
Main origin: formation of current-induced base inside collector SCR
⇒ effective quasi-neutral base width ↑ ⇒ βF ↓ ⇒ new delay com­
ponent ⇒ fT pk ↓
Key design issue: NC
NC ↑ ⇒ ICK ↑ ⇒ fT pk ↑
Experiments [Crabbé IEDM 1993]:
Si base
SiGe base
Key trade-off:
Crabbe, E.F., et. al. "Vertical Profile Optimization of Very High Frequency Epitaxial Si and SiGe-base Bipolar Transistors."
Technical Digest of the International Electron Devices Meeting, Washington, DC, December 5-8, 1993. Piscataway, NJ: Institute
of Electrical and Electronics Engineers, 1993, pp. 83-86. ISBN: 9780780314504. Copyright 1993 IEEE. Used with permission.
NC ↑ ⇒ BV ↓
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-11
� Base resistance
Very important parasitic for digital and analog applications.
SE
B
E
LE
B
n+
RBext
p
RBint
n
C
Two components to RB :
• external: associated with ohmic contact to base and lateral flow
through extrinsic base
• internal: associated with intrinsic base
RB = RBext + RBint
Important issues:
1. RBint is distributed: how does it scale?
2. Non-linear behavior of RBint for high IB
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-12
1. Low-current resistance (in absence of debiasing)
Define lateral resistance of intrinsic base:
Rblat = Rshb
SE
LE
With:
Rshb =
1
qμB NB WB
Clearly,
RBint < Rblat
because not all IB flows across entire intrinsic base.
But, what is proportionality constant?
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
Lecture 38-13
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
• BJT with one base contact:
SE
B
LE
E
n+
p
IB
JhE(y)
IB
AE
0
0
SE
y
0
SE
y
iB(y)
IB
0
Assume small ohmic drop along base ⇒ iB (x) uniformly distributed:
iB (x) = IB (1 −
x
)
SE
RBint obtained by examining power dissipation in base:
pB =
IB2 RBint
=
� S
E
0
i2B (x)
1
SE
Rshb
dx = IB2 Rshb
LE
3
LE
Then:
1
1
SE
= Rblat
RBint = Rshb
3
LE 3
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
Lecture 38-14
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
• BJT with two base contacts:
SE
B
LE
E
n+
p
IB
2
IB
2
JhE(y)
IB
AE
0
0
SE
y
SE
y
iB(y)
IB
2
0
0
SE
2
Now:
IB
2x
(1 −
)
2
SE
IB 2x
iB (x) =
(
− 1)
2 SE
iB (x) =
for 0 ≤ x ≤
for
SE
2
SE
≤ x ≤ SE
2
Intrinsic base resistance:
RBint =
1
Rblat
12
This is a quarter of the design with one-base contact.
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-15
Total base resistance is:
RB = RBext + RBint
To get small RB :
• minimize RBext ⇒ self-aligned process
• minimize RBint ⇒
– use two base contacts ⇒ takes more space
– increase WB ⇒ degrades frequency response
– increase NB ⇒ hard, costly, Cje ↑, BVEBO ↓
– decrease SE (scaling!) ⇒ costly
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-16
Useful observation: trade-off between RB and IC
Remember:
qVBE qAE n2i DB
qVBE
IC = IS exp
=
exp
kT
NB WB
kT
Then:
JC
qVBE
= q 2n2i DB μB exp
Rshb
kT
rather independent of NB or doping profile.
[from Tang TED 27, 563 (1980)]
Tang, D. D. "Heavy Doping Effects in p-n-p Bipolar Transistors." IEEE Transactions on
Electron Devices 27, no. 3 (1980): 563-570. Copyright 1980 IEEE. Used with permission.
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
Lecture 38-17
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
2) Non-linear behavior of RB
More accurate equivalent circuit model of distributed RB :
SE
B
E
B
n+
RBext
LE
p
RBint
jB(x)
IB
AE
0
0
SE
x
SE
x
iB(x)
IB
2
0
0
SE
2
Non-linear B-E diode current ⇒ non-uniform current distribution
across base:
• base resistance ”debiases” center of base
• current ”crowds” at edges of base
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
Lecture 38-18
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Analytical model possible (but not pretty).
General behavior of RBint:
RBint
1 R
blat
12
0
log IB
For negligible debiasing to occur:
IB Rblat < 0.2
kT
q
RBin drops by 50% at:
IB Rblat 6
kT
q
Overall behavior of RB :
RB
1 R +R
blat Bext
12
RBext
0
log IB
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-19
Consequences of base debiasing:
• RB depends on IB ⇒ RB depends on VBE and IC
Weng, J., J. Holz, and T. F. Meister. "New Method to Determine the Base Resistance of Bipolar Transistors." IEEE Electron
Device Letters 13, no. 3 (1992): 158-160. Copyright 1992 IEEE. Used with permission.
• For high current,
RBint → 0 ⇒ RB RBext and independent of SE .
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-20
• Non-linear behavior of RBint:
small-signal resistance = large-signal resistance
Ohmic drop in RB :
VB = RB IB
Then, small-signal base resistance:
rB =
dVB
dRB
= RB +
IB
dIB
dIB
Since IB ↑⇒ RB ↓:
rB ≤ RB
RBint, rBint
1 R
blat
12
rBint
RBint
0
log IB
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-21
Consequences of RB :
• RB has no direct impact on fT , but has indirect impact through
constraint in base design.
Actually, fT,pk does not appear correlated to RB
(fT,pk dominated by NC ):
• rB crucially important to noise
• rB Cjc important time constant in high-frequency operation of
BJT
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-22
Key conclusions
• Early effect: modulation of WB by VBC ⇒ output conductance
in output characteristics.
go =
IC
VA
• Base-width modulation effects affect IS and βF , but not IB .
• In avalanche breakdown in a BJT:
BVCEO < BVCBO
because of transistor current gain.
• βF ↑⇒ BVCEO ↓ w/o affecting BVCBO.
• Key device design issue in avalanche breakdown:
NC ↑⇒ BV s ↓
• At high collector current, velocity saturation of electrons in col­
lector ⇒ performance degrades: βF ↓, fT ↓
• Maximum current:
ICpk qAE NC vsat
• Two components in RB :
– RBext: associated with ohmic contact and transport through
extrinsic base
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].
6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007
Lecture 38-23
– RBint: associated with intrinsic base
• RBint is a distributed resistance ⇒
– RBint < Rblat
– RBint is non linear in IB .
• Consequence of non-linearity of RBint at high IB : small-signal
resistance different from large-signal resistance.
• Three technological approaches to reduce RB :
– use two base contacts
– use self-aligned process
– decrease SE
• Fundamental trade-off between IC and Rshb.
Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].