Lecture 6: HBT ac properties

1
Lecture 6 HBT ac properties

• Goal be able to understand and calculate fT,
  fmax
• (That will be HW 4)

2
Announcements

• HW 3 on website (there will be 6 total)
• IB,p is reverse injection current base-em
• IB,scr is current due to bulk recombination in
  base
• Presentations weeks 8,9,10
• If you are in week 8, two week delay on all HW
  allowed
• If you are in week 9, one week delay on all HW
  allowed.

3
Possible presentation topics

• Resonant tunnel diodes (RTD)
• GaN
• SiGe
• Quantum dots
• Landauer-Buttiker formalism for conductance and
  quantum point contact
• Fabrication techniques
• Optical properties of heterostructures
• Noise models
• S-parameter measurement techniques
• Circuit designs based on our developed device
  models (tools available such as SPICE, etc.)
• Growth techniques MBE, MOCVD, LPE
• High power applications and effects
• High speed digital applications
• Gunn diode
• IMPATT diode, P-I-N diode
• Carbon nanotubes
• Polymer transistors
• If you are involved with related research, I
  encourage presentation of your data.

4
HW 3 help
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Objectives

• Understand y, h parameters (HW4.1)
• Understand how to calculate intrinsic y
  parameters from geometry and doping profile
  (HW4.2)
• Understand how to calculate fT, fmax for simple
  circuits (HW4.3)
• Understand design tradeoffs (HW4, prs. 4-7)

6
Bipolar advantages

• Speed set by base thickness, which is easy to
  control during material growth(Dont need
  deep-submicron lithography)
• Large area contributes to current, good for
  POWER
• Low 1/f noise good for OSCILLATORS

7
Bipolar dis-advantages

• Need base current at dc
• No easy complementary digital logic
• Not so good at low-noise at high frequencies
  (HEMTs win here) due to generation-recombination
  noise

8
Normal active schematic
p type base
n type collector
n type emitter
electrons
Ic
electrons
Ie
recombine
a few holes
holes
(review all current components)
Ib
9
Normal active bias

• E-B forward bias(VbgtVe)
• C-B reverse bias(VcgtVb)
• Ice 100 IbebIbe

Ic
Ib
Ie
Like a diode.
10
Equivalent circuit 1
Ic
Ib
Ie
11
Equivalent circuit 2
Ic
Ib
Ie
12
Equivalent circuit 3
Ib
Ic
Ie
13
Equivalent circuit 4
Ib
Ic
Ie
14
npn spelled backwards?

• npn

15
Equivalent circuit 1
Ic
Ib
Ie
16
Ebers-Moll model
Ic
Note Npn spelled backwards is npN
Ib
Ie
Not just limited to forward-active regime. Not in
Liu, applies to all bias voltages. (Good for all
sorts of circuits.)
17
Ebers-Moll model AC
Ic
Cbc
Ib
Ceb
Ie
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E-M model AC with parasitics
Ic
j is for junction (discuss) Diffusion capacitance
not shown.
Cjc
Ib
Cej
Note at frequencies near fT, things are not
quite this simple.
Ie
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Global dc properties
Note Early effect.
(Discuss bias scheme.)
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Global dc properties
Note Early effect.
It is assumed you know this, so it is rare to see
on data sheets!
http//www.toshiba.com/taec/components/Datasheet/2
SA1244DS.pdf
21
S-parameters
This is what you see on data sheets. Related to
input impedance, output impedance, and gain vs.
frequency. gt Need to discuss ac performance.
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ac properties notation
Ic
Ib
Ie
We will use equivalent circuit 1 (implicitly).
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ac properties
common-emitter configuration
VCC
Rc
output
Note three terminal device has three-terminal
equivalent ac circuit.
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dc analysis
common-emitter configuration
VCC
Rc
IC
IB
0 for now
IE
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ac analysis
VCC
Rc
output
We will prove
Transconductance
Note three terminal device has three-terminal
equivalent ac circuit.
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Transconductance
VCC
Rc
But
Typical number is 40 mA/V.
gm
So
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Input impedance
VCC
Rc
But
So
So
What is typical input impedance?
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Gain
VCC
Rc
But
So
So
What is typical gain?
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Summary
VCC
Rc
input imp.
transcond.
In matrix form
Admittance (Y) matrix
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AC equivalent circuit
If we are only interested in ac components, life
can be simplified
VCC
Rc
Hybrid p model
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T-model
If we are only interested in ac components, life
can be simplified
VCC
Rc
T model
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Rules for ac analysis

• From complete circuit, calculate dc currents and
  voltages
• For ac analysis only
• dc voltage source -gt short circuit
• dc current source -gt open circuit
• Replace transistor with p or T-model
• Now solve (simplified) ac circuit

33
Next lecture

• Generalized y-parameters
• not just common emitters
• Capacitances
• y-parameters from doping profile
• Definition of fT

34
Time permitting

• Early effect in p model
• S-parameters