Introduction to BJTs

ECE-305: Spring 2015
Introduction to BJTs:
active region
Professor Mark Lundstrom
Electrical and Computer Engineering
Purdue University, West Lafayette, IN USA
[email protected]
Pierret, Semiconductor Device Fundamentals (SDF)
pp. 371-385
Lundstrom ECE 305 S15
4/20/15
1
SiGe HBTs
Martin Claus
TU-Dresden
2
Lundstrom ECE 305 S15
MOS transistors
ID
D
saturation
ID
VGS1
G
IG ≈ 0
S
IS
linear region
VDS
n-channel
enhancement
mode MOSFET
3
Lundstrom ECE 305 S15
bipolar transistors
C: collector
IC
IC
(forward) active region
B: base
VBE1 , I B1
IB
E: emitter
IE
VCE
NPN BJT
saturation region
4
Lundstrom ECE 305 S15
bipolar transistors
C: collector
IC
IC
(forward) active region
EB: FB, BC: RB
B: base
VBE1 , I B1
IB
saturation region
EB: FB, BC: FB
IE
E: emitter
VCE
cut-off region
EB: RB, BC: RB
NPN BJT
5
Lundstrom ECE 305 S15
NPN bipolar transistor
IC
KCL:
I B + IC = I E
VCB
IB
VCE
KVL:
VBE
VBE + VCB = VCE
IE
Lundstrom ECE 305 S15
6
common base (active region)
VCB
IB
I C = α dc I E
I E = IC + I B
IC
N
N
P
N
VEB
P
VCB
IB
VCE
N
VBE
BE: FB VEB < 0
BC: RB VCB ≥ 0
IE
Lundstrom ECE 305 S15
7
common emitter (active region)
I C = β dc I B
IC
VCB
IB
VCB
N
P
VCE
IB
N
VCE
P
N
VBE
VBE
N
IE
IE
BE: FB VBE > 0
BC: RB VCB = VCE − VBE > 0
Lundstrom ECE 305 S15
8
alpha and beta
I C = α dc I E
IC
VCB
IB
N
P
VCE
N
I C = β dc I B
α dc ≡
IC
IC
=
<1
I E IC + I B
α dc =
IC
β dc
=
<1
I C + I C β dc β dc + 1
VBE
β dc
β dc + 1
α dc
β dc =
1− α dc
α dc =
I E = IC + I B
Lundstrom ECE 305 S15
9
outline
1)  Introduction
2)  E-Band view of BJT active region
Lundstrom ECE 305 S15
10
understanding MOSFETs
VGS > VT
0
VD
n-Si
n-Si
x
p-Si
y
To understand this device, we should first draw an
Energy Band Diagram.
Lundstrom ECE 305 S15
11
how MOS transistors work
VCE = 0.6 V
2007 N-MOSFET
EC
VVGS
BE
VVGS
BE
VCE = 0.05 V
(Courtesy, Shuji Ikeda, ATDF, Dec. 2007)
EC
12
Lundstrom ECE 305 S15
BJT operation: active region
FB
n+
emitter
RB
p
base
n
collector
n+
y
To understand this device, we should first draw an
Energy Band Diagram.
Lundstrom ECE 305 S15
13
13
equilibrium
E
EC
qVbi
EF
EV
y
emitter
base
collector
14
Lundstrom ECE 305 S15
VBE = 0, VCE > 0
E
qVbi
EC
Fn
EC
EV
Fn
“off”
“cut-off”
EV
y
emitter
base
collector
15
Lundstrom ECE 305 S15
VBE > 0, VCE > 0
E
EC
q (Vbi − VBE )
I Cn = α T I En = I C
I En
IC = I0e
Fn
EV
qVBE k BT
I B = I C β dc << I C
I Ep
EC
Fn
“active”
EV
I B = I Ep
y
emitter
base
collector
16
Lundstrom ECE 305 S15