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Workshop  Phototransistors September 9, 2003
Budapest Hungary
InP-based Phototransistors and comparison of
performances to those of PIN and UTC
photodiodes Carmen Gonzalez Alcatel RI -
Laboratoire OPTO Carmen. Gonzalez_at_alcatel.fr
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Outline
InP/InGaAs-based bipolar phototransistor Photo-HB
T performances as a direct photodetector Opt
oelectronic integrated circuits OEIC - O/E
narrow band amplifier - O/E upconverting
mixers Performances of - Top illuminated
PIN photodiode - Back illuminated UTC
photodiode Summary
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Photo-HBT developed at OPTO
Choice of material to maximize carrier velocities
Material System InP (despite of cost) vs
GaAs InP, InGaAs Higher electron velocities
and lower surface
recombination velocities than GaAs InGaAs
Narrow band gap Eg 0.75 eV, compatible with the
detection of 1.30 and 1.55 µm
wavelength light
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Photo-HBT developed at OPTO
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Photo-HBT as a direct photodetector
Emitter area 9 µm2 Base area
44 µm2 Optical window area 16 µm2
RDC 0.2 A/W
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Saturation characteristics
_at_ -1dB -21 dBm
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Analog noise characteristics
Input noise current spectral density ltIingt
(pA/Hz0.5)



photo-HBT
CHAIN

Spectrum



b-tee
Analyser
3
2
LNA
C

lt
I
gt

in
G 50 dB
P
(dBm/Hz)



NF lt 5 dB
1
sys_noise
B
RBW 2
MHz

E
I
b
_
tot



I
I

I
b
_
tot
b_
elec
ph
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Analog noise characteristics
Input noise current spectral density
_at_ 40 GHz Ic ltIingt (mA)
(pA/Hz0.5) 2 50 10
66
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Digital noise characteristics
BER at 10-9 C/N 24.3 dB
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OEIC using photo-HBTs
An O/E narrow-band amplifier at 28 GHz 2 cascode
cells with 1 photo-HBT3HBTs
Transimpedance Gain 50 dB? _at_ 28 GHz
BER at 10-9 24.3 dB
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Optoelectronic mixers using photo-HBTs
Upconversion mixer from 2 GHz to 28 GHz and to 42
GHz

G
Mixer
conv
28 GHz 17.8 dB
42 GHz
9.2 dB
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Top illuminated PIN photodiode
The main attraction of this device is
its compatibility for integration with SHBTs. For
example, this photoreceiver realized by D. Huber
et al., IEEE JLT 2000
PInGaAs
InGaAs
NInGaAs
Trade-off between efficiency and speed
(bandwidth) The better performances with
a Photoabsorption layer of 400nm ? Bandwidth
30 GHz ? R 0.30 A/W
PIN photodiode
SHBT-based preamplifier
SHBT Base-collector homojunction PIN
homojunction ? Bandwidth 53 GHz ?
Transimpedance gain 44.3 dB
Photoabsorption layer
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PIN photodiode
For higher-speed operations, waveguide or
travelling-wave photodiodes are proposed. TW-PD
with bandwidth of 100 GHz has been
reported. However, photoreceivers based on
edge-coupled PIN PDs exhibit similar characteristi
cs to those obtained with top-illuminated PIN-PD
Top-illuminated phototransistors, which offer
internal gain, could greatly reduce the need for
preamplification, with circuits less complicated
than those associated to PIN PD, in particular
at millimeter wave frequencies.
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Back illuminated UTC photodiode
In high-speed optical systems ? optical
pre-amplifier can be installed directly in front
of
the photoreceiver Need of photodiodes with
? broad bandwidth, high responsivity and high
output power
Uni-traveling-carrier photodiode principle
Separate absorption and space-charge region ?
high carrier density Electrons only contribute to
drift current ? transit time improved
Proposed by T. Ishibashi et al., NTT, 1997
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Back illuminated UTC photodiode
p Base contact
p Barrier
p Base absorption layer
Shimizu et al. IEEE PTL, vol 10, pp. 412, 1998
Composite collector
Since UTCs require a thin absorption layer ?
Responsivity is relatively low lt 0.2 A/W ? Need
of edge-illumination for improving R
Sub-collector
InP-Substrate
The responsivity is generally the same as that
of a PIN-PD for the same absorption layer
thickness
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UTC refracting-faced photodiode
To achieve higher responsivity in edge
illuminated configuration
Output power dependance on input light
UTC photo-HBT
20 GHz 19 GHz Popt 2.5
dBm 2.5 dBm PRF -30 dBm
-21 dBm
Fukushima et al. EL, vol 37, pp 780-781, 2001
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Summary - Future prospect
High speed photo-SHBT based on InP technology
Frequency performances up to the mm-wave band
Compatible with SHBT technology for
monolithically integrated photoreceivers
(amplifiers, mixers, oscillators) It is well
suited for performing more complex O/E functions,
as mixing or selft-oscillation, at high frequency
and high bit rates