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Status on long-wavelength InP waveguide heterojunction phototransistors
Samuel Dupont, Vincent Magnin, Emmanuel Fendler, Filippe Jorge, Sophie Maricot, JeanPierre Vilcot, Joseph Harari, Didier Decoster.
Institut d'Electronique de Microélectronique et de Nanotechnologie, UMR CNRS 8520
Université des Sciences et Technologies de Lille
Avenue Poincaré, BP 69, 59652 Villeneuve d'Ascq, France
[email protected]
Abstract:
Heterojunction
Phototransistors
(HPT) are promising components for
microwave
photodetection.
Among
possible designs of HPTs, we have chosen
to develop a 3-terminations edge-coupled
waveguide device. We will present the
HPT design, fabrication, characterization
and state of art.
1. Introduction:
Transistors can convert optical
signals into electrical signals, as the
materials they are made of are able to
absorb light. Devices that exhibit a special
design to collect light are called
phototransistors. Several approaches have
been studied for efficient light coupling:
vertical illumination through a transparent
electrode or an electrode window, back
illumination, butt coupling, or waveguide
feeding. Si, GaAs and InP based devices
have also been developed, the last material
line being compatible with 1.3 µm and
1.55 µm optical telecommunications based
operations. Theses applications request
both efficient response and high speed
operation. For example, microwave
photonics and high bit-rate optical
communication systems (  40 Gbit/s) will
require efficient transponders, able to
operate up to the millimetre-wave
frequency range. In this context, high
frequency
Heterojunction
bipolar
PhotoTransistor (HPT) is seen as a
possible transducer for advanced photonic
circuits (clock recovery setups or local
oscillator signal generation in distributed
antenna systems). This device is
compatible with HBT technology and can
indeed find direct applications in
fundamental circuits such as injection
locked micro/millimetre wave oscillators
[1-2].
Among the possible designs of
phototransistors, we have developed a side
illuminated 3-terminations InP-InGaAsP
HPT. We will present the influence of the
different parameters on the device
performances; the parameters evolution
can have contradictory effects on the
different figures of merit of the HPTs
(some benefice on the cut off frequency
can be accompanied with a reduction of the
efficiency as a well known example). The
optimisation is thus subject to various
trades-offs. We will present the
optimisation of the developed device, its
fabrication and characterization. After
what, we will draw the state of art of
phototransistors.
2. Device definition:
Studies of phototransistors have
been reported early as they provide gain,
compared to photodiodes, without adding
much excess noise, compared to avalanche
photodiodes [3]. The easier phototransistor
to develop is the 2-terminal device as it
does not include a base terminal, but the
gain of this device is usually small. The
use of a 3-terminal device ensures to obtain
the best gain-bandwidth product. For a
floating base, holes are accumulated in the
base resulting in a base/emitter barrier
diminution, for a 3-termination device the
base potential can be kept constant. For a
NPN simple heterojunction type the
structure consists of a N+ emitter layer, a
P++ base layer and one or several N layers
to constitute the collector. Emitter and subcollector are composed of InP whereas
base and collector are composed of
InGaAs with a lower gap energy to ensure
light absorption. HPT are subject to the
main HBT problems, that is to say: - the
necessity to use thin base/collector layers
to lower the carriers transit time; - to
reduce the device area in order to minimize
the base/collector capacity; - to get a strong
base doping level to decrease the access
resistance; - Kirk effect which corresponds
to the reduction of the collector field with
increasing the free carriers densities; avalanche effect that can occur at the end
of the collector and can lead to the device
destruction; - base carriers recombination
that reduces the gain; … Those are well
known phenomenons extensively explored
with HBT and that are to be taken into
account to develop a HPT. It appears that
high speed HPTs need thin base/collector
layers and a small area like high speed
HBTs.
These features are important;
however we would like to focus on the
HPT specificity. A cautious design of the
device should lead to the best
optical/electrical conversion efficiency;
two main light coupling solutions are
possible: vertical or lateral. Vertical
illumination leads to the classical
efficiency/rapidity trade-off, thin absorbing
layers are required to get a high cut-off
frequency but the conversion efficiency is
reduced. Efficient and fast devices have
been recently achieved with a back
illumination (350 nm absorbing layer, 0.26
A/W, and 80 GHz optical gain cut-off
frequency [1]). Lateral illumination of the
device has been proposed to get better
conversion efficiency [4]. Taking a given
structure we can evaluate the benefit
arising from lateral illumination. We have
evaluated the following structure inspired
of [5]: 0.3 µm of N InP (emitter); 0.1 µm
of P+ InGaAs (base); 0.28 µm of InGaAs,
0.2 µm of N InP and 0.2 µm of N+
InGaAs, (collector); InP substrate. For a
vertical
illumination,
an
internal
responsivity of 0.37 A/W at  = 1.55 µm
is expected. BPM simulations (fig. 1a) of
this device show that 0.64 A/W internal
responsivity could be obtained with a
device of 6x4 µm² using a lensed fibre.
air
InGaAs
substrate
(1.a) Light propagation and absorption in
an edge coupled phototransistor.
air
InGaAs
substrate
(1.b) Effect of a quaternary layer to
improve optical coupling.
Fig. 1 BPM simulations of light
propagation in an edge coupled
phototransistor; (a) without and (b) with
quaternary layer
The opto-electrical conversion
enhancement could give about 5 dB power
improvement at the HPT output. This
illustrates the side illumination benefits
over the vertical configuration on the
devices performances, in spite of a more
difficult fabrication process. Tolerance to
the fibre position has also been studied
with a lensed fiber, a misalignment of +/0.5 µm reduces the responsivity of about
10 % (fig. 2).
Responsivity (A/W)
1
0.8
TE
TM
0.6
to develop an equivalent circuit model of
the HPTs we have fabricated (Fig. 3). The
model is a modified HBT model including
a current source taking into account the
photo-generated carriers and laser RIN;
and also the elements of the base circuit
[7].
0.4
E
0.2
0
-4
-2
0
2
4
C
B
Injection offset (µm)
Fig. 2 Internal responsivity of side
illuminated HPT versus injection offset, TE
and TM modes
Taking the device structure
mentioned above, better optical coupling
could be obtained with a 0.5 µm InGaAsP
layer inserted under the collector to
improve the waveguiding properties of the
device (0.74 A/W for TM), in fig. 1b light
coupled to the substrate is reduced. Even
more efficient light collection can be
achieved with a modified structure: - first
the absorbing layer should be thicker (0.5
µm) and the quaternary confinement layer
as well (0.7 µm); - secondly the device
should be longer (8 µm); this device has
been fabricated and characterised (§ 3).
The optical gain cutoff frequency should
be lower than 80 GHz but, thanks to the
improved
waveguiding
properties,
theoretical evaluations of the device
predict that 90 % of the light will be
absorbed (at  = 1.55 µm), resulting in an
internal responsivity of 1.1 A/W; output
power should take advantage of such a
good conversion coefficient. Efficient light
collection can be of interest as well for
high CNR demanding applications, as
CNR increases with more incident optical
power [3, 6].
3. Device characterisation:
On wafer measurements of 3x15
µm² devices give ft and fmax of 60 GHz and
45 GHz respectively at 13.5 mA. S
parameters are extracted and are employed
Cleaving axis
C
B
E
Fig. 3 Micrography of the 3T-HPT before
cleaving
Fig. 4 shows the opto-microwave
characterisation of a 3x8 µm² HPT up to
40 GHz. Plots extrapolation shows that it
has an optical gain cutoff frequency of
about 45 GHz.
Fig. 4 Opto-microwave characterisation of
the 3T-HPT
4. State of art:
Here is presented a non exhaustive
collection
(but
representative)
of
phototransistors performances. Fig. 5 is a
plot of the HPTs optical gain cutoff
frequency versus emitter/base junction
area. Waveguide HPTs are plotted with
triangles; vertically illuminated ones by
disks; empty figures point out 2T-devices.
A 1/S curve is also plotted (dashed line).
This curve is consistent with emitter/base
capacitance frequency limitations; this plot
seems to fit the upper limit of the devices
performances, all designs considered, with
a good agreement. The ultimate HPT size
reduction is limited by the optical spot
size.
[3] P. Chakrabarti, N.K. Agrawal, P. Kaira,
S. Agrawal, G. Gupta, “Noise modelling of
an InP/InGaAs Heterojunction bipolar
phototransistor”, Opt. Eng., vol. 42, no. 4,
(2003), pp. 939-947.
100
90
NTT 2001
80
Optical ft , GHz
70
IEMN 99
60
CNET 99
50
40
IEMN 96
CNET 97
BT 93
30
NTT 94
20
NTT 95
10
0
0
20
[2] J. Lasri, A. Bilenca, G. Eisenstein, D.
Ritter, “Optoelectronic mixing, modulation
and injection locking in millimetre-wave
self oscillating InP/InGaAs heterojunction
bipolar phototransistor- Single and dual
transistor configuration”, IEEE Trans.
Microwave Theory Tech., vol. 49, no. 10,
(2001), pp. 1934-1938.
40
Cincinnati 98
IEMN 96
CNET 96
Naval Research
ATR 93
Laboratory 91
Michigan
AT&T 91
University 93
60
80
100
120
140
160
Emitter-base junction, µm 2
Fig. 5 State of art of long wavelength
phototransistors; Optical gain cutoff
frequency versus emitter area; Dashed
line: (Surface)-1 -type evolution
Conclusion:
We have presented a long wavelength
InP/InGaAsP waveguide phototransistor.
Such device can present an excellent light
collection efficiency (more than 1 A/W
internal responsivity) and still present very
good microwave performances (> 40
GHz). An optical simulation of a high
optical gain cutoff frequency device
developed for vertical illumination shows
the possibility to improve light collection
by side illumination even for such delicate
device.
References:
[1] H. Kamitsuna, T. Shibata, K.
Kurishima, M. Hida, “Direct optical
injection locking of a 52 GHz InP-InGaAs
HPT oscillator IC for over-100-Gb/s halfor full-rate optoelectronic clock recovery”,
IEEE Photon. Tech. Lett., vol. 15, no. 1,
(2003), pp. 108-110.
[4] D. Wake, D.J. Newson, M.J. Harlow,
I.D. Henning, “Optically biased, edgecoupled
InP/InGaAs
heterojunction
phototransistors”, Electron. Lett., vol. 29,
no. 5, (1993), pp. 2217-2219.
[5] H. Kamitsuna, Y. Matsuoka, N.
Shigekawa, “Ultrhigh-speed InP/InGaAsP
DHPTs for OEMMICs”, IEEE Trans.
Microwave Theory Tech., vol. 49, no. 10,
(2001), pp. 1921-1925.
[6] A. Bilencia, J. Lasri, B. Sheinman, G.
Eisenstein, D. Ritter, “Millimeter-wave
generation and digital modulation in an
InGaAs-InP
Heterojunction
phototransistor: model and experimental
characterization of dynamics and noise”, J.
Lightwave Tecnol., vol. 19, no.9, (2001),
pp. 1340-1351.
[7] S. Dupont, M. Fendler, F. Jorge, S.
Maricot, J-P. Vilcot, D. Decoster, “Signal
to noise ratio enhancement using
heterojunction bipolar phototransistor by
base current compensation”, Microwave
Photonics, MWP’2000, Oxford.