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The two frequencies at which the linearity error is equal to b
can be derived from eqns. 2 , 4 and 8 to be
= k 2 sin
eo .(a*)'/*
(9)
fo
with
the total group delay disperThe frequency bandwidth 4,
sion T = A[tJ in this band and the compression ratio T .U
over which the linearity error is smaller than 6 are given by
T = 230ps of a phosphate Nd : glass laser for which 1, =
1060nm and
AA = 2Onm. fo = 2.8 x 1014Hz and
Af= 6 x 10"Hz can be compressed to the b i t of
AT = 170fs. It must be noted that,in this device, several passages are possible for compressing even longer pulse8 (for
example pulses with T . Af= loo00) before the pulse wmpression performance will be limited by the dispersion nonlinearitits.
In conclusion, we have proposed and analysed a new diffraction grating pair which exhibits a very l i n q group time
delay variation against frequency. The new device could be
useful for laser pulse compression systems over a wide bandwidth or very large compression ratios.
0IEE 1993
14th June 1993
D&ue 10.
P. Tournois (Thomn-CSF, Cedex 67, 9204.5 Paris Lo
France)
RefeEWeS
As an example, Fig. 2 shows the time delay trim (via the
dimensionless parameter ctdd + N cos 0') against normalised
L
:
1 -CY,
E 8.: 'Optical pulse codtpresslonwth dieiactlon gratings',
IEEE J . Quanrun! Electron., 1969, QES, pp. 454-458
2 WJNE. P.. STRICKLAND, D., BAW, P., PFswr, M., and MOUROU, 0.:
yjeneration of ultrahigh peak power pulses by chirped ampli6catlon', IEEE J . ~ u r u uElectron,
~,
1 9 8 8 , pp.
~ 399403
3 IUURNOIS, P., and LAGIW, M.: Thcory of a new parallel diffracton
grating delay Line for fincar FM pulse compmon', IEEE Trans.,
1971, .$U-18,
pp. 118-122
8 3
T!
'2
01
-
8.70 '5
........
0
.... .........................
~
.,,0L5
25
45
~ .
~
..........
.. ~
.......
65
85
105
\
125jJ45
.
""
DC AND TEMPERATURE DEPENDENT
CHARACTERISTICS OF InP DOUBLE
HETEROSTRUCTURE BIPOLAR
TRANSISTORS WITH QUATERNARY
COLLECTOR
S. P. McAlister, Z.-E.Abid, W. R. McKinnon
Fig. 2 variation of group time delay against frequency for
N sin 6' = 1.06
N = 1.5, B = 459, eo = 70.5"
frequency parameter &/pc for an angle of incidence B = 45" in
glass prisms with index of refraction N = 1.5 (N sin B = 1.06).
This time delay is linear around the angle Bo = 70.5" and the
normalised frequency afo/pc = 8.5. With such a diffraction
device, when the distance between the grating planes is
d = 6 mm,the linearly frequency modulated pulse of duration
T = 2ps of a titanium sapphire laser for which 1, = 800nm
and A1 = 2 0 0 1 fo~ ~= 3.75 x 10" HZ and 4=lOI4 Hz, can
be compressed to T = 1Ofs.
As another example, we show in Fig. 3 the time delay tllU)
(again via ctJd + N cos 8') against afpc for an angle of incidence 8' = 85" in prisms with index of refraction N = 2 (N sin
8' = 1.99). This time delay is linear around the angle 8, =
31.1" and the normalised frequency ofo/pc = 0.67. With this
device, when the device, when the distance d is equal to 1m,
the linearly frequency modulated laser pulse of duration
5h
20
8.m
1'
8.50
-2 5 1
-3 0
\
rn
Fig.3 Variation of group time delay against frequency for
N sin 6' = 1.99
N = 2 , =~w , e 0 = 30.1"
ELECTRONICS LETTERS 5rh August 1993 Vol. 29 No. 16
and
M. Davies
Indexing t m : Bipolar devices, Transistors, Semiconductor
devices and materials
The Lcttcr reports the 6rst study of the temperature dcpcndencc of the DC characteristics of InP-bad double heterostructure bipolar transistors which have a quaternary
collector. The quaternary layer was s p a a d away from the
basbcollector junction to improve the HBT characteristics.
The devices exhibited useful gain over seven orders of magnitude of Current and had breakdown voltages of -8V. The
DC gain decxcascd with d d s temperature whercas the
ideality factors inacad.
Early InP-based HBTs displayed good high frequency characteristics cfT > 1SOGHz) [l] but the devices had low breakdown voltages and operating currents due to the use of
latticamatched InGaAs in the collector. Reducing the doping
in the collector produces better results, as does a widebandgap collector [2, 31. The composite collector design [3]
where a spacer separates the widebandgap collector from the
base,has shown excellent results [4-61. The spacer width is a
delicate compromise: a thicker spacer improves the gain and
speed, but if it is too wide the HBT will behave as a single
heterojunction device and the improvement in the breakdown
performance will be lost.
We report HBTs made with InGaAsP quaternary collector, aiming to increase the breakdown voltage and the range
of useful gain, and to see whether wide-bandgap quaternary
material may offer improved performance. Stepgraded collee
tor structures using quaternary layers have been reported
recently [5, 7J The quaternary composition chosen has a
1.13 eV bandgap (1.1 pm photoluminescence wavelength),
compared with the 1.35eV bandgap for the InP emitter and
0.75eV for InGaAs. We studied the DC characteristics and
temperature dependent behaviour of our devices to determine
1415
.
,
the effects of the quaternary collector heterojunction. The
designed layer structure, grown on SI InP, was: 400nm
InGaAs subcollector (n-doped 1.8 x lo”), 250nm InGaAsP
quaternary collector (ndoped 2.5 x lo”), lOOnm InGaAs
spacer (n-doped 2 x 10l6), 6Onm InGaAs base @-doped
8 x lo”), 8nm InGaAs setback layer (undoped), 150nm
InP emitter (ndoped 4.3 x lo”), 150nm InGaAs emitter
contact layer (ndoped LO”). The n-dopant was S and the
pdopant Zn. All dopant levels are in ~ m - The
~ . InGaAs
composition was In,.,,Ga,.,,As.
The quaternary was
Ino.8,Gao.13Aso,2gPo.,l.
The wafer was grown using MOCVD
by Epitaxial Products International. SIMS analysis showed
out-diffusion of the pdopant into the spacer layer so that the
base width effectively increased by 20nm. If the subcollector is
highly doped the zinc diffusion can be high at the temperatures normally used during MOCVD growth [E].
Devices were fabricated using a triple-mesa process, followed by a single Ti-Pt-Au liftoff metallisation. The emitters
ranged in size from 400 to 8100pm’ and were probed directly.
Selective wet etching defined the emitter and isolation mesas,
hut it was necasary to use reactive ion etching to remove the
quaternary layer.
Typical results for large-area devices are shown in Fig. 1.
The common-emitter offset voltage was <0,2V. The
I
I
/
I
device current was kept low to minimise any current-stressing
effects which could give rise to changes that were not due to
varying the temperature. The temperature dependence is
collector current, A
pss,21
Fig. 2 Current gain asfwtetion of collector current for device of Fig. I
I
400
20 -
3
I
1
E 40-r-----40mA
I
I
I
I
I
I
I
I
I
I
I
f ”:
5r”
-
U
U
14
-
12-
temperature, K
Fa.3 Temperature dependence of the gain of a 70 x Wpm‘ device at
VcEof 1 V and I , of 30pA and variation of the collector ideality foctor
for the same device
(i) Gain against temperature
(U) Ideality factor against tempmature
collector voltage, V
1259/11
Fs.1 Common emitter characteristicsfor
base curren&sindicated ond
common base characteristicsfor a series of emitter currents, at 290 K
Device area 70 x 90pm’
top: common emitter characteristics
bottom: wmmon base characteristics
common-base turn-on is abrupt, not showing the slower
turn-on feature commonly seen when InP is used in the collector [6,9]. The common-base breakdown voltage is 8 V; this
breakdown is due to impact ionisation, predominantly in the
collector region, but is improved in the composite-coilector
structure. A Gummel plot gave ideality factors of 1.32 (base)
and 1.18 (collector) at 290K. Collector current leakage was
less than 10- l 1 A. The device shows useful gain over a wide
range of collector current (Fig. 2), better than is commonly
seen, for example in GaAs HBTs because of the reduced
surface recombination. The ideality factor for the collector
current is larger than expected. This might be related to the
collector heterojunction which can produce current blocking
[lo]. This blocking can be. suppressed by judicious deltadoping [4, 6, 9, lo] near the heterojunction. T h e current
blocking could increase the recombination probability in the
collector spacer layer and lead to ideality factors greater than
unity.
The commonemitter gain in the saturation regime (VcE=
1V) decreases with decreasing temperature (Fig. 3(i)). The
-
1416
similar to that in Si bipolar devices [ll] but different from
AIGaAs/GaAs HBTs [12]. Fig. 3(ii) shows the temperature
dependence of the collector-current ideality factor (obtained
from Gummel plots). The base current ideality factor, not
shown, follows the collector ideality curve closely with values
approximately 0 1 higher. The gain reduction could arise from
effects such as changes in the bandgaps, band offsets, donor
and acceptor ionisation, mobilities, diffusion coefficients,
recombination effects and so on. For our devices we believe
that the current-blocking at the heterojunction in the collector
is a major cause of the effects. This would become more
important at lower temperatures and thus could give both the
decrease in the gain and the increase in the collector ideality
factor, the latter due to increased recombination near the heterojunction. We note that the common-base breakdown
voltage decreased 8% for a 50°C rise in temperature above
room temperature, which is less than that seen in the
common-emitter breakdown behaviour of AIInAs/InGaAs
HBTs [13]. This decrease, along with an increasing gain, will
make the devices thermally unstable at high voltages. We also
saw that the common-emitter offset voltage did not vary down
to 90K. Because this offset is determined by the different
turn-on voltages for the emitter-base and base-collector junctions and thus related to the hand discontinuities,we conclude
that these band discontinuities are essentially independent of
temperature between 100 and 300 K.
-
ELECTRONICS LETERS 5th August 1993 Vol. 29 No. 16
Our HBTs with an InGaAsP collector have good I-V characteristics, breakdown voltages and useful current gain over
more than seven orders of magnitude of current. With
decreasing temperature the DC gain decreased whereas the
ideality factors showed an increase. We suggest that this is due
to the presence of the heterojunction in the composite collector.
K., NAKAJIMA,H., KOBAYASHI, T., MATSUOKA, Y., and
ISHIBASHI, T.: ‘InPjInGaAs double-heterojunction bipolar tran-
5 KURISH~MA,
sistor with stepgraded InGaAsP collector’, Electron. Lett., 1993,
29, pp. 258-260
6
KURUWMA, K., NAKAIIMA, H., KOBAYASHI, T., MATSU~W,Y., and
ISHIBASHI, T.: ‘High-speed InPjInGaAs double heterostructure
bipolar transistors with suppressed collector current blocling’,
Appl. Phys. Lett., 1993,62,pp. 2372-2374
7 OHKUBO, M., IKEIANI, A., UICHI, T., and KIKIJTA,
0IEE 1993
3rd June 1993
S. P. McAlister, 2.-E.Abid, W. R. McKinnon and M. Davies
(Imtitute of Microstructurd Sciences, National Research Council of
T.: ‘InGaAsjInP
double-heterojunction bipolar transistors with step graded
InGaAsP between InGaAs base and InP collector grown by
metalorganic chemical vapor deposition’, Appl. Phys. Lett., 1991,
59, pp. 2697-2699
8 KURISHIMA, K., KOEAYASU, T., and caw,& U,: ‘Abnormal redistribution of Zn in InPjInGaAs heterojunction bipolar transistor
structures’,Appl. Phys. Lett., 1992,a0, pp. 2496-2498
Canada, Ottawa K I A OR6,C d a )
9
Acknowledgments: We would like to thank E. Guzzo, R.
Barber and P. Chow-Chong for their technical help.
RITIER, D., HMIM, R. A., FTYGENSON, A., =KIN,
H., PA“,
M. B.,
and CHANDRASGKHAR, s.: ‘Bistable hot electron transport in InP/
GaInAs composite collector heterojunction bipolar transistors’,
References
a, and NOTIENBURG, R.: ‘Heterostructure bipolar transistors’, in urz, A. (Ed.):‘Indium phosphide and related materials:
Proaessing, technology and devices’ (Artech House, Boston, 1992),
pp. 379-406
2 SU, L. M., GROT% N., KAuMA“$ R, and S C H R m H.: ‘NpnN
double-heterojunctionbipolar transistor on InGaAsP’, Appl. Phys.
Lett., 1985,47,pp. 28-30
1
3
J A W
FWfCiENWN, A., RITIER, D.,HAMM, R. A., SMITH, P. R., MONTGOMERY, R.
K.,,“Y
R. D., TEMKIN, n,and PA“,
M. B.: ‘InGaAsPjInP
composite collector hetemstructure bipolar transistors’, Electron.
Left., 1992,- pp. 607-609
4
mGENWN, A., HAbW R A., SMITH, P. R, PINTO, M. R., MONTWMERY,
R. K., YAMSH, R. D., and TPMKIN, H.:‘A 144GHz InP/InGaAs com-
posite collector hetero-structure bipolar transistor’. IEDM Tech.
Dig., 1992, Article 4.2.1
INTEGRATED OPTICAL CIRCULAR GRATING
TAP POWER DIVIDER
S. I. Najafi, M.Fallahi, P. Lefebvre, C. W u and
I. Templeton
Appl. Phys. Lett., 1992,61,pp. 70-72
10 SUGRIRA, 0.. DWTAI, A. G., JOYNER, C. H., CHANDRASQCHAR, S., and
CAMPBELL, 1. c.: ‘High-current-gain InGaAsjInP double hetero-
junction bipolar transistors grown by metal organic vapor phase
epitaxy’, IEEE Electron Device Lett., 1988.9, pp. 253-255
MARnmu,R. u.: The temperature dependence of the dc base and
collector currents in silicon bipolar transistors’, IEEE Trans.,
1976,ED-23,pp. 1218-1224
12 CHAND, N., mcmit, R., HENDERSON, T., KLEM, J., KOPP, w., and
MoRKoc, H.: ‘Temperaturedependence of current gain in AIGaAs/
GaAs heterojunction bipolar transistors’, Appl. Phys. Lett., 1984,
45, pp. 1W6-1088
13 MALIK, R. I., C”D,
N, NAGJ., RYAN, R. w., ALAVI, K., and CHO,
A. Y.: ‘Temperature dependence of commondtter I-V and collector breakdown voltage characteristics in AlGaAs/GalnAs
HBTs grown by MBE: IEEE Electron Device Lett., 199213, pp.
11
557-559
substrate, resulting in a singlemode waveguide at 514nm
wavelength. The sample was coated with a 0.15- thick S O ,
layer to prevent contamination of reactive ion etch (RIE)
equipment with the sodium present in Corning 0211 glass.
Indexing term: Integrated optics, Wavelength division multiplexing
A new compact integrated optical tap and multiport power
divider is provided. A four-port device is made in a glass
substrate and is tested at 514nm wavelength.
Introduction: Waveguides with circular grating have potential
applications in making integrated optical circuits. Circular
grating surface emitting lasers have been recently demonstrated [I-31. In this Letter, we report the realisation of the
fist integrated optical circular grating tap power divider. The
motivation behind this work was to achieve a small size stackable device capable of extracting a s m a l l fraction of the signal
light and distributing this light in several waveguides. Such a
device has applications in, for example, signal processing and
optical computing.
I/
1528/1/
Principle and structure: Fig. 1 shows a schematic view of the
circular grating tap power divider. The device consists of a
central circular waveguide connected to four tapered waveguides, and a circular grating concentric with the central
waveguide. The input light is directed perpendicularly onto
the surface of the grating. The grating couples the light into
the central waveguide. Light propagates in the central waveguide and distributes into the tapered waveguides. The output
light intensity from the ports depends primarily on the coupling etliciency of the grating, the central waveguide radius, and
the input width of the tapered waveguide. To tap only a small
fraction of the signal tight, the grating should be shallow
and/or the grating period should be slightly different from the
ideal coupling condition at the signal light wavelength. In this
work, 514 nm was selected as the operation wavelength.
Fabrication: A slab waveguide was made by potassium ion
exchange [4] at 400°C for 15min in a Corning 0211 glass
ELECTRONICS LETERS
5th August 1993 Vol. 29 No. 16
Fig. 1 Schematic diagram of circular grating tap power divider
The sample was then coated with a 300A aluminium layer
and a PMMA photoresist. The circular grating with 0.36pm
period and 300pm diameter was written in PMMA by a
focused ion beam lithography. The period of the grating was
chosen to be 7% larger than the ideal coupling period. T h e
grating was etched by RIE in the aluminium layer (using
BCI, : He gas mixture) and then in glass (using CF, : 0, gas
mixture). The grating depth was 0.1 pm. A thin layer of aluminium was defined by a liftoff technique to pattern the fourport structure on the sample. RIE using CF, : 0, gas mixture
was again used (with aluminium as a mask) to etch 1 pm the
slab waveguide and create the central and tapered channel
waveguides. Finally, aluminium was removed. Fig. 2 illustrates an optical microscope photograph of the fabricated
device. The diameter of the central waveguide was 1mm. T h e
1417
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