Heterojunction Bipolar Transistors

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Heterojunction Bipolar Transistors for High-Frequ
ency Operation
Day 3A, May 29, 2008, Pisa
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Outline

• 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?

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Schematic of InGaP/GaAs HBT

• Epitaxial structure
• Dissimilar emitter and base materials
• Highly doped base
• Dual B and C contacts

• Identify WB and RB

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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

- this allows reduction of both WB and RB
SHBT
- this improves fT and fmax

• Key feature is the wide-bandgap emitter

An example of Bandgap Engineering
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Selecting an emitter for a GaAs base
AlGaAs / GaAs
InGaP / GaAs
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InGaP/GaAs and AlGaAs/GaAs
Draw band diagrams for different ?emitter
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Preparing to compute IC

• Why do we show asymmetrical hemi-Maxwellians?

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Current in a hemi-Maxwellian
Full Maxwellian distribution
Counter-propagating hemi-M's for n01E19/cm3
What is the current?
/1E20
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Density 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
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Velocities
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
What is Je,total ?
1E7 A/cm2 for n06E18 /cm3
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Collector current boundary conditions
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Reduce our equation-set for the electron current
in the base
What about the recombination term?
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Diffusion and Recombination in the base
Here, we need
In modern HBTs WB/Le ltlt 1 ?
and
is constant
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Collector 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 SLJ
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Comparing results

• What are the reasons for the difference?

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Base current components
(iv)

• Which IB components do we need to consider?

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Base current components and Gummel plot
IC
IC (A/cm2)
IB (recombination)
IB (injection)
VBE (V)

• What is the DC gain?

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Preparing 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 Approximation
q(x, y, z, t' ) f( VTerminals, t') q(x, y, z,
t' ) ? f( VTerminals, t lt t')
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Small-signal circuit components
gm transconductance go output conductance
g? input conductance g12 reverse feedback
conductance
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Recall g12dIb/dVce
next
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Small-signal hybrid-? equivalent circuit
What are the parasitics?
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HBT Parasitics

• CEB and RB2 need explanation

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Base-spreading resistance
y
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Capacitance
Generally
1
Specifically
2
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Emitter-base junction-storage capacitance
E
B
C
WB2
QNB
QNE
QNC
WB1

?VBE

• ?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.

What is the voltage dependence of this cap?
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Emitter-base base-storage capacitance concept
E
B
C
QNB
QNE
QNC

?VBE

• ?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).
• Its not a parallel-plate cap, and we only count
  one carrier.

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Emitter-base base-storage capacitance evaluation
B
QNB
For the case of no recombination in the base
n(x)
n(WB)
x
WB
What is the voltage dependence of CEB,b ?
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Base-emitter transit capacitance evaluation
Q 3q qe -2q

• What are q0 and qd ?
• Where do they come from ?

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fT 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/ib2 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 fT

• Assumption

• Conditions

• Current gain

• Extrapolated fT

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• 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

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Designing for high fT values
Why do collector delays dominate ?
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How does Si get-in on the act?
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Developing an expression for fmax
Assumption and conditions
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Improving fmax

• Pay even more attention to Rb and C?

Final HF question How far behind are Si MOSFETs?
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HF MOS
What is this?