1
Heterojunction Bipolar Transistors
for
High

Frequency Operation
D.L. Pulfrey
Department of Electrical and Computer Engineering
University of British Columbia
Vancouver, B.C. V6T1Z4, Canada
pulfrey@ece.ubc.ca
http://nano.ece.ubc.ca
Day 3A, May 29, 2008, Pisa
2
Outline
•
What are the important features of HBTs?
•
What are the useful attributes of HBTs?
•
What are the determining factors for I
C
and I
B
?
•
Why are HBTs suited to high

frequency operation?
•
How are the capacitances reduced?
3
Schematic of InGaP/GaAs HBT
•
Epitaxial structure
•
Dissimilar emitter and base materials
•
Highly doped base
•
Dual B and C contacts
•
Identify W
B
and R
B
4
HETEROJUNCTION BIPOLAR TRANSISTORS
•
The major development in bipolar transistors (since 1990)
•
HBTs break the link between N
B
and
•
Do this by making different barrier heights for electrons and
holes
•
N
B
can reach 1E20cm

3
e

h
+
•
Key feature is the wide

bandgap emitter

this improves f
T
and f
max

this allows reduction of both W
B
and R
B
SHBT
An example of Bandgap Engineering
5
Selecting an emitter for a GaAs base
AlGaAs / GaAs
InGaP / GaAs
6
InGaP/GaAs and AlGaAs/GaAs
Draw band diagrams for different
emitter
7
Preparing to compute I
C
•
Why do we show asymmetrical hemi

Maxwellians?
8
Current in a hemi

Maxwellian
Full Maxwellian distribution
Counter

propagating hemi

M's for
n
0
=1E19/cm
3
/
1E20
What is the current?
9
Density of states
Recall:
In 1

D, a state occupies how much k

space?
What is the volume in 3

D?
If k
x
and k
y
(and k
z
in 3

D) are large enough, k

space
is approximately spherical
Divide by V (volume) to get states/m
3
Use parabolic E

k (involves m*) to get
dE/dk
Divide by dE to get states/m
3
/eV
10
Velocities
Turn n(E) from previous slide into n(v) dv using
v
R
= 1E7 cm/s for GaAs
Currents associated with hemi

M's and M's
= 1E7 A/cm
2
for n
0
=6E18 /cm
3
*
What is J
e,total
?
11
Collector current: boundary conditions
12
Reduce our equation

set for the electron current
in the base
What about the recombination term?
13
Diffusion and Recombination in the base
In modern HBTs
W
B
/L
e
<< 1
and
is constant
Here, we need:
14
Collector current: controlling velocities
Diffusion (and no recombination) in the base:

1
Note:

the reciprocal velocities

inclusion of v
R
necessary in modern
HBTs
*
*
Gives limit to validity of SLJ
15
Comparing results
•
What are the reasons for the difference?
16
Base current: components
•
Which I
B
components do we need to consider?
(iv)
17
Base current components and Gummel plot
•
What is the DC gain?
I
C
(A/cm
2
)
V
BE
(V)
I
B
(injection)
I
B
(recombination)
I
C
18
Preparing for the high

frequency analysis
•
Make all these functions of time and solve!
•
Or, use the quasi

static approximation
19
The Quasi

Static Approximation
q(x, y, z, t' ) = f( V
Terminals
, t')
q(x, y, z, t' )
f( V
Terminals
, t < t')
20
Small

signal circuit components
g
m
= transconductance
g
o
= output conductance
g
= input conductance
g
12
= reverse feedback conductance
21
Recall
g
12
=dI
b
/dV
ce
next
22
Small

signal hybrid

equivalent circuit
What are the parasitics?
23
HBT Parasitics
•
C
EB
and R
B2
need explanation
24
y
Base

spreading resistance
25
Capacitance
V
+
+


Generally:
Specifically:
1
2
26
E
B
C
QNE
QNC
QNB
V
BE
+
•
Q
E,j
is the change in charge entering the device
through the emitter and creating the new width of the
depletion layer (narrowing it in this example),
•
in response to a change in V
BE
(with E & C at AC
ground).
•
It can be regarded as a parallel

plate cap.
W
B2
W
B1
What is the voltage dependence of this cap?
Emitter

base junction

storage capacitance
27
E
B
C
QNE
QNC
QNB
V
BE
+
•
Q
E,b
is the change in charge entering the device
through the emitter and resting in the base (the black
electrons),
•
in response to a change in V
BE
(with E & C at AC
ground).
•
It’s not a parallel

plate cap, and we only count one
carrier.
Emitter

base base

storage capacitance: concept
28
B
QNB
n(x)
x
n(W
B
)
W
B
For the case of no recombination
in the base:
What is the voltage dependence of C
EB,b
?
Emitter

base base

storage capacitance: evaluation
29
Base

emitter transit capacitance: evaluation
Q = 3q
q
e
=

2q
•
What are
q
0
and q
d
?
•
Where do
they come
from ?
30
f
T
from hybrid

pi equivalent circuit
•
g
0
and g
set to 0
•
f
T
is measured under
AC
short

circuit
conditions.
•
We seek a solution for ic/ib
2
that has a single

pole roll

off with
frequency.
•
Why?
•
Because we wish to extrapolate
at

20 dB/decade to unity gain.
31
Extrapolated f
T
•
Assumption:
•
Conditions:
•
Current gain:
•
Extrapolated
f
T
:
32
Improving f
T
•
III

V for high g
m
•
Implant isolation to reduce C
•
Highly doped sub

collector and supra

emitter to reduce R
ec
•
Dual contacts to reduce R
c
•
Lateral shrinking to reduce C's
33
Designing for high f
T
values
Why do collector delays
dominate ?
34
How does Si get

in on the act?
35
Developing an expression for f
max
Assumption and
conditions:
36
Improving f
max
•
Pay even more
attention to R
b
and C
Final HF question:
How far behind are
Si MOSFETs?
37
HF MOS
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