1 heterojunction bipolar transistors heterojunction bipolar transistorsfor high-frequency operation...
TRANSCRIPT
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Heterojunction Bipolar TransistorsHeterojunction Bipolar Transistors
forfor
High-Frequency OperationHigh-Frequency Operation
D.L. Pulfrey
Department of Electrical and Computer EngineeringUniversity of British ColumbiaVancouver, B.C. V6T1Z4, Canada
http://nano.ece.ubc.ca
Day 3A, May 29, 2008, Pisa
2
OutlineOutline
• What are the important features of HBTs?
• What are the useful attributes of HBTs?
• What are the determining factors for IC and IB?
• Why are HBTs suited to high-frequency operation?
• How are the capacitances reduced?
3
Schematic of InGaP/GaAs HBTSchematic of InGaP/GaAs HBT
• Epitaxial structure
• Dissimilar emitter and base materials
• Highly doped base
• Dual B and C contacts
• Identify WB and RB
4
HETEROJUNCTION BIPOLAR TRANSISTORS
• The major development in bipolar transistors (since 1990)
• HBTs break the link between NB and
• Do this by making different barrier heights for electrons and holes
• NB can reach 1E20cm-3
e-
h+• Key feature is the wide-bandgap emitter
- this improves fT and fmax
- this allows reduction of both WB and RB SHBT
An example of Bandgap Engineering
5
2.5
2
1.5
1
0.5
05.4 5.5 5.6 5.7 5.8 5.9 6 6.1
Lattice Constant, A
Ban
dg
ap, e
V
Si
Ge
GaAs
GaP
AlP
AlAs
InAs
InP90
8070
6050
4030
2010
a = 5.6533 Amatched to GaAs
°
°
a = 5.8688 Amatched to InP
°
Selecting an emitter for a GaAs baseSelecting an emitter for a GaAs base
AlGaAs / GaAs
InGaP / GaAs
6
InGaP/GaAs and AlGaAs/GaAsInGaP/GaAs and AlGaAs/GaAs
Draw band diagrams for different emitter
7
Preparing to compute IPreparing to compute ICC
• Why do we show asymmetrical hemi-Maxwellians?
8
Current in a hemi-MaxwellianCurrent in a hemi-Maxwellian
Full Maxwellian distribution
Counter-propagating hemi-M's for n0=1E19/cm3
/1E20What is the current?
9
Density of statesDensity of states
Recall:
In 1-D, a state occupies how much k-space?What is the volume in 3-D?
If kx and ky (and kz in 3-D) are large enough, k-space is approximately spherical
Divide by V (volume) to get states/m3
Use parabolic E-k (involves m*) to get dE/dk
Divide by dE to get states/m3/eV
10
VelocitiesVelocities
Turn n(E) from previous slide into n(v) dv using
vR = 1E7 cm/s for
GaAs
Currents associated with hemi-M's and M's
= 1E7 A/cm2 for n0=6E18 /cm3
*
What is Je,total ?
11
Collector current: boundary conditionsCollector current: boundary conditions
12
Reduce our equation-set for the electron Reduce our equation-set for the electron current in the basecurrent in the base
What about the recombination term?
13
Diffusion and Recombination in the baseDiffusion and Recombination in the base
In modern HBTs WB/Le << 1
and is constant
Here, we need:
1016
1017
1018
1019
1020
10-6
10-5
10-4
10-3
10-2
10-1
Doping (cm-3)
Diff
usio
n le
ngth
(cm
)
Le
Lh
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Collector current: controlling velocitiesCollector current: controlling velocities
Diffusion (and no recombination) in the base:
-1
Note:
- the reciprocal velocities
- inclusion of vR necessary in modern HBTs
*
* Gives limit to validity of SLJ10
110
210
30
1
2
3
4
5
6
7
8
9
10x 10
4
WB (nm)
J C
(Acm
-2)
vR
=infinityv
R =1e7 cm/s
15
Comparing resultsComparing results
• What are the reasons for the difference?
16
Base current: componentsBase current: components
• Which IB components do we need to consider?
(iv)
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Base current components and Gummel Base current components and Gummel plotplot
• What is the DC gain?
I C (
A/c
m2 )
VBE (V)
IB (injection)
IB (recombination)
IC
18
Preparing for the high-frequency analysisPreparing for the high-frequency analysis
• Make all these functions of time and solve!
• Or, use the quasi-static approximation
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The Quasi-Static ApproximationThe Quasi-Static Approximation
q(x, y, z, t' ) = f( VTerminals, t')
q(x, y, z, t' ) f( VTerminals, t < t')
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Small-signal circuit components
gm = transconductance
go = output conductance
cebeb vgvgi 12
g = input conductance
g12 = reverse feedback conductance
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Recallg12=dIb/dVce
next
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Small-signal hybrid-Small-signal hybrid- equivalent equivalent circuitcircuit
What are the parasitics?
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HBT ParasiticsHBT Parasitics
• CEB and RB2 need explanation
24
y
Base-spreading resistanceBase-spreading resistance
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Capacitance Capacitance
V
QC
V
+ +
- -
Generally:
Specifically:
1
2
26
E B C
QNE
QNC
QNB
VBE+
BE
jEjEB V
QC
,
,
• QE,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 VBE (with E & C at AC ground).
• It can be regarded as a parallel-plate cap.B
jEB W
AC
,
WB2
WB1
What is the voltage dependence of this cap?
Emitter-base junction-storage capacitanceEmitter-base junction-storage capacitance
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E B C
QNE
QNC
QNB
VBE+
BE
bEbEB V
QC
,,
• QE,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 VBE (with E & C at AC ground).
• It’s not a parallel-plate cap, and we only count one carrier.
Emitter-base base-storage capacitance: Emitter-base base-storage capacitance: conceptconcept
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B
QNB
n(x)
xn(WB)
WB
)/exp(),0(
)/exp(),0(
202,
101
thBEpBE
thBEpBE
VVnVn
VVnVn
bEB
BE
bE
BBBth
BEpBBEbE
C
dV
dQ
WAnqWWnV
VnAWqVQ
,
,
BE
bE,
0,
Hence
V
Q Take
)()()exp(2
1)(
For the case of no recombination in the base:
What is the voltage dependence of CEB,b ?
Emitter-base base-storage capacitance: Emitter-base base-storage capacitance: evaluationevaluation
29
Base-emitter transit capacitance: evaluation Base-emitter transit capacitance: evaluation
Q = 3q
qe = -2q
• What are q0 and qd ?
• Where do they come from ?
30fT from hybrid-pi equivalent circuit
• g0 and g set to 0
• fT 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.
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Extrapolated fExtrapolated fTT
• Assumption:
• Conditions:
• Current gain:
• Extrapolated fT:
32 Improving fImproving fTT
• III-V for high gm
• Implant isolation to reduce C
• Highly doped sub-collector and supra-emitter to reduce Rec
• Dual contacts to reduce Rc
• Lateral shrinking to reduce C's
33
Designing for high fDesigning for high fTT values values
Why do collector delays dominate ?
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How does Si get-in on the act?How does Si get-in on the act?2.5
2
1.5
1
0.5
05.4 5.5 5.6 5.7 5.8 5.9 6 6.1
Lattice Constant, A
Ban
dg
ap, e
V
Si
Ge
GaAs
GaP
AlP
AlAs
InAs
InP90
8070
6050
4030
2010
a = 5.6533 Amatched to GaAs
°
°
a = 5.8688 Amatched to InP
°
35
Developing an expression for Developing an expression for ffmaxmax
Assumption and conditions:
36
Improving fImproving fmaxmax
• Pay even more attention to Rb and C
Final HF question:
How far behind are Si MOSFETs?
37
HF MOS HF MOS
What is this?