Monolithically Coupled Photo Diode - HBT or a Photo - HBT : A Modeled Comparison

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Monolithically Coupled Photo Diode - HBT or a
Photo - HBT A Modeled Comparison

• BENNY SHEINMAN, DAN RITTER
• MICROELECTRONIC RESEARCH CENTER
• ELECTRICAL ENGINEERING DEPARTMENT
• TECHNION ISRAEL INSTITUTE OF TECHNOLOGY

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Workshop outline

• Introduction.
• Phototransistor electrical configurations.
• General bandwidth / efficiency limitations in a
  photo-detector.
• Additional limitation in a top illuminated PIN
  diode / phototransistor.
• Electrical modeling of the top illuminatedPIN
  diode / phototransistor.

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Phototransistor structure
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Photo-diode, HBT structure
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Photo-diode and HBT or PhotoHBT ?
Photo HBT HBTphotodiode same layers HBTphotodiode different layers
Transistor performance Compromised 1. Miller effect 2. Collector transit time Compromised 1. Collector transit time Optimal
Responsivity Compromised Compromised Maximal
Optical window Small ? Large ? Large
Technology Standard Standard Difficult
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Phototransistor configuration

• Common Base
• Hole current flows to ground
• ? no current gain

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Phototransistor configuration

• Common Collector
• High current gain
• Low output resistance limits performance.

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Phototransistor configuration

• Common Emitter
• High current gain.
• Bandwidth limited by Miller effect

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Miller Effect
Integrated PIN HBT
Phototransistor
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Cascode Configuration
Performance comparable to that of a PIN HBT
?
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Kirk effect
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Kirk effect (cont.)
Associated time constant for a 110? emitter and
a 10? diameter optical window
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Photodetectors bandwidth limitations

• Carrier transit time
• RC of detector capacitance and amplifier input
  resistance

M. Agethen et al. IPRM 2002
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Photodetectors quantum efficiency
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Ideal Amplifier
Only transit time limits performance
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Base-collector junction in a phototransistor

• GaInAs active layers
• Base layer highly resistive -

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A top illuminated PIN as a notch filter
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Additional RC filter bandwidth limitation in a
top illuminated PIN diode / phototransistor.
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RC network in a PIN detector
Physical structure
Electrical equivalent circuit
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Spot size radius 12.5?
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Spot size radius 12.5?
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Spot size radius 12.5?
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Spot size radius 12.5?
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Spot size radius 6?
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Spot size radius 6?
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Top illuminated photo-transistoroption 1
Optical window
Emitter
Contact To base
Base Metal
Base Mesa
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Top illuminated photo-transistoroption 2
Optical window
Contact To base
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Spot size radius 12.5?
2?
12.5?
Internal contact
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Model of top illuminated detector
Solution of current equations is difficult
- distributed photocurrent

• Photodiode capacitance / area

Yet for a known capacitance value, a single pole
fit gives good results
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(No Transcript)
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High efficiency phototransistors

• Cover optical window with conducting transparent
  ITO (Indium Tin Oxide) layer.
• Place internal and external contact to the diode
  
• Backside illumination.

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Overcoming the limitations

• Incorporating novel structures in
• photo-transistors
• Wave guide photodetectors
• Distributed phototransistors
• Resonant-cavity-enhanced photodetector.
• Uni-traveling-carrier photodiode.

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Conclusions

• In the cascode configuration, photo-HBT have
  comparable performance to PIN detector HBT
  processed from the same layers.
• PIN detector HBT processed from different
  layers will have superior performance.
• The highly resistive base layer produces an
  internal filter in the top illuminated PIN
  detector.
• The influence of the filter should be included in
  the model of the detector.