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
Spring 2007
EE130 Lecture 26, Slide 1
Modern BJT Structure
B
P+polySi
E
N+ polySi
p+ P base
N collector
Deep
trench
C
P+polySi
+
p
Shallow
trench
+
N subcollector
P substrate
• 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
Spring 2007
EE130 Lecture 26, Slide 2
N+
Deep
trench
Polycrystalline-Silicon (Poly-Si) Emitter
• bdc 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
Continuity of hole current in emitter
dpE1
dpE 2
 qDE1
 qDE 2
dx
dx
dpE1 DE 2 dpE 2  E 2 dpE 2


dx
DE1 dx
 E1 dx
(1  poly - Si; 2  Si)
Spring 2007
EE130 Lecture 26, Slide 3
Emitter Gummel Number w/ Poly-Si Emitter
2
2
ni N E
ni N E (WE )
dx  2
WE , poly
2
ni E DE
ni E (WE ) DE
WE
GE  
0
WE

0
ni N E
ni N E (WE )
dx  2
2
ni E DE
ni E (WE ) S p
2
2
where Sp  DEpoly/WEpoly is the surface recombination velocity
2
ni  WE 1 

For a uniformly doped emitter, GE  N E 2 
niE  DE S p 
qni A qVEB / kT

IB 
e
 1
GE
2
Spring 2007
EE130 Lecture 26, Slide 4
Emitter Band Gap Narrowing
2
niB N E
b dc  2
niE N B
To achieve large bdc, NE is typically very large, so that
band gap narrowing (Lecture 8, Slide 5) is significant.
n N c N v e
2
iE
 EGE / kT
niE2 ni2 e EGE / kT
Spring 2007
 Nc Nve
 ( EG  EGE ) / kT
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 5
Narrow Band Gap (SiGe) Base
2
niB N E
b dc  2
niE N B
To improve bdc, 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 bdc to be achieved with
large NB (even >NE), which is advantageous for
• reducing base resistance
• increasing Early voltage (VA)
Spring 2007
EE130 Lecture 26, Slide 6
EXAMPLE: Emitter Band Gap Narrowing
If DB = 3DE , WE = 3WB , NB = 1018 cm-3, and niB2 = ni2, find bdc 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
niE2  ni2e
EgE / kT
 ni2e35meV / 26meV  3.8ni2
1019  ni2
DBWE N E ni2
b dc 

 9  18
 23.6
2
2
DEWB N B niE
10  3.8ni
(b) At NE = 1020cm-3, EgE  160 meV:
niE2  ni2 e160meV / 26meV  470ni2
10 20  ni2
DBWE N E ni2
b dc 

 9  18
 1.9
DEWB N B niE2
10  470ni2
(c) niB2  ni2e
Spring 2007
EgB / kT
 ni2e60meV / 26meV  10ni2
EE130 Lecture 26, Slide 7
b F  236
Charge Control Model
A PNP BJT biased in the forward-active mode has excess
minority-carrier charge QB stored in the quasi-neutral base:
pB ( x, t )  pB (0, t )1  Wx 
qAWpB (0, t )
QB  qA  pB ( x, t )dx 
2
0
W
dQB
In steady state,
0
dt
Spring 2007

iB 
EE130 Lecture 26, Slide 8
dQB
QB
 iB 
dt
B
QB
B
Base Transit Time, t
p B ( x, t )
iC   qADB
x
x W
qADB p B (0, t )

W
qAWpB (0, t )
QB 
2
2 DB QB QB
iC 

2
W
t
W2
t 
2 DB
• time required for minority carriers to diffuse across the base
• sets the switching speed limit of the transistor
Spring 2007
EE130 Lecture 26, Slide 9
Relationship between B and t
iB 
iC 
QB
B
QB
  B  b dc t
t
• 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.
Spring 2007
EE130 Lecture 26, Slide 10
Drift Transistor: Built-in Base Field
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.
E
B
-
C
Ec
Ef
Ev
• Fixed NB , EgB decreases from emitter end to collector end.
E
-
B
C
Ec
Ef
Ev
Spring 2007
EE130 Lecture 26, Slide 11
1 dEC
E
q dx
EXAMPLE: Drift Transistor
• Given an npn BJT with W=0.1m and NB=1017cm-3
(n=800cm2/Vs), find t and estimate the base
electric field required to reduce t

2
5

2
W
10 cm
t 

 2 ps
2
2 DB 20.026V  800cm / V  s
W
v
drift

Spring 2007

W
 n



t
10 5 cm


 6kV / cm
2
12
 n t 800cm / V  s 2 10 s
W


EE130 Lecture 26, Slide 12
