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current can be reduced significantly by increasing the barrier
thickness. However, for a thicker bamer, the thermionic emission
current becomes important and the base current is less sensitive to
the barrier thickness, as can be seen in Fig. 2. This base current
behaviour makes the current gain increase rapidly with barrier
thickness for thin barriers and this rate of increase reduces for a
thicker barrier, as shown in Fig. 1.
Current-injection ECL/CML for AIGaAs/
GaAs HBTs
T.Y.K. Wong
Inde.Ting terms: Hett'rojuncrion bipolar transistors, Emitter-coupled
logic
A novel 'current-injection' circuit topology is presentec that
requires lower supply voltage, and demonstrates smaller
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. lb
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120
2 10
165
2 55
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liz413/
Fig. 3 Collector and base transfer characteristics of TEBT with 200 and
500A barrier thickness at V,, = 2.75 V
Introduction; An ECLICML structure is commonly used in high
speed digital circuits [I, 21. Fig. 1 shows an input stage of a classical 2-level logic. A level 1 (LVI) differential pair 4 1 - 4 2 or 4 3 - 4 4
can be enabled, by switching on cascaded level 2 (LV2) transistor
4 5 or 46. The propagation delay from LV2 inputs (INZININ2) to
LV1 outputs (OUTAINOUTA and OUTB/NOUTB) is the sum of
the delays through transistors LVI and LV2. The output current is
11 less two base currents.
6
NOUTA
200A
_ _ - - 500'4
~
propagation delay and outperforms classical 2-level emittercoupled logicicurrent-mode logic (ECUCML) in AlGaAdGaAs
heterojunction bipolar transistor (HBT) technology. Test circuits
were wafer-probed at IOGbitis with full functionality, and
operating clock frequency up to 20GHz is recorded.
OUTA
NOUTB
OUTB
Q GNINlA
& , IN16
Q @
Fig. 3 shows the base and collector transfer characteristics of
the TEBT with 200 and 5008, bamer thickness. Both the base and
collector current of the TEBT with 5008, barrier are less than
those of the TEBT with 200A barrier. However, the discrepancy
of base current is smaller because thermionic emission dominates
the base current. The barrier thickness of 500A severely retards
the tunnelling electrons and the collector current apparently
reduces. As a consequence, the current gain falls off for barrier
thickness increasing from 200 to 500A.
In summary, we have studied the current gain Characteristics of
TEBTs with bamer thickness varying from 25 to 5008,. The current gain increases rapidly with barrier thickness in the range 25508, and this increment slows down in the range 50-200A. On the
other hand, the current gain decreases as the barrier thickness
increases from 200 to 500A.
Fig. 1 Input stugr of classical two-level ECL/CML
Fig. 2 shows an altemative method to implement 2-level ECL/
CML. It comprises LVI differential pairs 4 1 - 4 2 and 4 3 - 4 4 , LV2
current-injection transistors 45-46, and constant-current sources
I1 and 12. In order for 45 or 4 6 to inject current into I1 or I2 to
NOUTA
NOUTB
OUTA
OUTB
Acknowledgment: This work is partially supported by the National
Science Council of the Republic of China under contract number
No.NSC82-0404-E009-375.
VSS
0 IEE 1993
12 August IY93
Fig. 2 Input stage of current-injection tu,o-kvel ECUCML
Electronics Letters Online No: 19931218
H. R. Chen, C. P. Lee, C. Y. Chang, K. L. Tsai and J. S. Tsang
(Department of Electronics Engineering and Institute of Electronics,
National Chiao-Tung University, Hsinchu, Taiwan, Republic of China)
References
CAPASSO, F ,
MOHAMMED, K ,
CHO, A Y ,
HUTCHINSON, L.:
'Effective mass filtering:
HULL. R ,
and
Giant quantum
amplification of the photocurrent in a semiconductor superlattice',
Appl. Phys. Lett., 1985, 47, pp. 42M22
xu. J., and SHUR, M : 'A tunnelling emitter bipolar transistor', IEEE
EIPctron. Device Lett., 1986, 7 , pp. 4 1 H 1 8
LEVI, A F.J., NOlTENBURG, R N , CHEN. Y K , and CUNNINGHAM. J E :
'AIAdGaAs tunnel emitter bipolar transistor', Appl. Phys Lett ,
1989,54, pp. 225&2252
MALIK, R . J , LUNARDI. L.M., RYAN, R w., SHUNK, s c , and FEUER, M D :
'Submicrometre scaling of AlGaAsiGaAs self-aligned thin emitter
heterojunction bipolar transistors (SATE-HBT) with current gain
independent of emitter area', Electron. Lett., 1989, 25, pp. 1175-
1177
NAJJAR, F E , RADULESCU, D C , CHEN, Y K., WICKS, G W , TASKER, P 1 ,
and EASTMAN, L F : 'DC characterization of the AlGaAsiCaAs
tunnelling emitter bipolar transistor', Appl. Phys. Lett., 1987, 50,
pp. 1915-1917
1884
disable 4 1 - 4 2 or 4 3 - 4 4 , the base voltage of 4 5 or 4 6 needs to be
higher than the logic high applied to the bases of 4 1 - 4 2 or 4 3 - 4 4
by a few times the thermal voltage V,. To enable 4 1 - 4 2 or 4 3 - 4 4 ,
the base voltage of 4 5 or 4 6 needs to be lower than the logic high
applied to the bases of 4 1 - 4 2 or 4 3 - 4 4 by a few times V,.
Because LVI and LV2 transistors are not in cascade, Fig. 2
requires -I VBE,"less voltage headroom than Fig. 1. Owing to differential switching between LV1 and LV2 transistors, the propagation delay from LV2 inputs to LV1 outputs is only one transistor
delay, as against two transistor delays in Fig. I. The output current is I1 less one base current, which is less sensitive to the forward current gain br.
E.xperiment: Two test circuits incorporating current injection have
been fabricated using the BNR HBT process [3]. Test circuit 1 is a
decision circuit compnsing three buffers, one master-slave flipflop (FF) with current injection and one output cell. A die
photograph is shown in Fig. 3. Test circuit 2 is an fi2 frequency
divider comprising one F F with current injection, one buffer and
one output cell. The external clock is unbuffered, allowing clock
amplitude and offset to be adjusted externally. Both test circuits
use a standard pad-out, with predefined positions for power,
ground and low- and high-speed signals. High-speed signals are
ELECTRONICS LE77ERS
14th October 1993
Vol. 29
No. 21
Conclusions: A current injection topology has been introduced. An
ECLiCML FF with current injection has demonstrated superior
speed performance compared to its classical counterpart. Sharing
the same power bus between 1- and 2-level logic cells can be realised as they have the same power supply headroom, simplifying
power bus routing. Wafer probe yield has shown that the circuit is
consistently reproducible and suitable for production.
Acknowledgments: This work was supported by the HBT program
at BNR, Ottawa. The author would like to thank J . Sitch at BNR
for valuable discussions.
0 IEE 1993
23 August 1993
Electronics Letters Online No: I9931 I81
T. Y. K. Wong (Bell-Northern Research Lid., P.O. Box 3511 Station
C, Ottawa, Ontario, Canada K1 Y 4 H n
References
1244131
ASBECK. P M , MILLER. D.L, ANDERSON, R 1 , DEMING, R N., CHEN, R T I
LIECHTI, c A . and EISEN. F H : ‘Application of heterojunction bipolar
Fig. 3 Die photograph of decision-circuit test chip
transistor to high speed, small-scale digital integrated circuits’.
Tech. Dig. IEEE GaAs IC Symp. 1984, pp. 133-136
ICHiNO, H :
‘20-Ghis digital SSl‘s using AlGaAsiGaAs
heterojunction bipolar transistors for future optical transmission
systems’, IEEE J. Solid-State Circuits, 1993, 28, (2) pp. 115-122
LESTER. T , SURRIDGE, R K , EICHER, S , HU. J , ESTE, G., NENTWICH, H.,
MAcLAuRIN. B., KELLY, D , and JONES. I : ‘A manufacturable process
for HBT circuits’. Proc. IOP 20th Int. Symp. on Gallium Arsenide
and Related Compounds, 1993, (Germany), to be published
5OQ terminated and have a ground pad on each side, allowing
10Ghitk wafer-probing. The standard transistor used in the
circuits has a 2 x 2~ emitter with and fm, of 60GHz at 4 x
1WAlcm’ collector current density.
Results and discussions; Table 1 lists the speed performance of
classical and current-injection ECLiCML FFs from postlayout
simulation. Owing to the difference in switching delay between the
two FFs, the propagation delay I&, set-up time I , and hold time I ,
differ by -lops. 7‘’+ t, for both FFs are the same. For circuits
where tF limits speed performance, such as frequency dividers, the
current-injection FF offers a significant speed advantage.
Table 1: Simulated speed performance of classical and currentinjection ECLiCML flipflops
ECLiCML flipflop
‘Pd
PS
PS
PS
Classical
Current injection
41
30
17
27
IO
0
1,
Electron saturation velocity in Ga,,lnP,,,P
measured in a GalnP/GaAs/GalnP doubleheterojunction bipolar transistor
W. Liu, T. Henderson, E. Beam 111 and S.K. Fan
[h
Indexmy terms. Heterojunctron bipolar trawisiors, Carrier mobiliiy
The first (not necessarily typical) performance measurements on
Ga,51%,P/GaAs/Ga,, JQ ,P
double-heterojunction bipolar
transistors are reported. The measured cutoff frequency and
maximum oscillation frequency are 25 and 35GHz, respectively.
From various masurements of f under various bias conditions,
the value of the electron saturation velocity in GalnP is
detemned to be 4.4 x 106cm/s.
The power dissipation of the classical ECLiCML FF (6.5V supply) and the current injection FF (5V supply) were simulated to be
l00mW and 98mW, respectively.
2Ops /division
Fig. 4 Eye diagram of decision circuit
Fig. 4 shows the eye diagram of the decision circuit at a wafer
probe through the output cell. At le9bit error rate, ts + I , for the
wire-bonded parts were measured to be 29ps at 2*O-I pseudorandom binary sequence, 25ps at 215-1 and 19ps at 2’-l. Although
designed for IOGHz operation, the wafer probe yield at I5GHz
was 35%.
The maximum operating frequency of the fi2 circuit was measured to be 20GH2, limited by the maximum frequency generated
by the test setup. A neighbouring classical ECLiCML frequency
divider demonstrated a maximum operating clock frequency of
I8GHz.
ELECTRONICS LE77ERS
__
-~ -
-
14th October 1993
Ga,,In,,P lattice-matched to GaAs is being actively studied as an
alternative material to AIGaAs. The GalnP/GaAs material system
is also promising for double-heterojunction bipolar transistors
(DHBTS). Studies comparing the common-emitter VV characteristics of GaInPEaAs SHBTs and DHBTs show an increase in
breakdown voltage when GaInP replaces GaAs as the collector
material [I]. However, the published DHBT data thus far are
merely DC results. There was no mention of the value of the electron saturation velocity ( v J in GaInP. If the v, value is much
smaller than that in GaAs, then the replacement of collector material would degrade the transistor’s high-frequency performance. In
this work we determine experimentally the value of U, in GaInP
by measuring the cutoff frequencies in a GaInPGaAsiGaInP
DHBT under various bias conditions. This work also represents
the first report of the high-frequency performance of GaInF’
DHBTs, demonstrating, a measured fr of 25GHz and an f, of
35GHz. (It should, however, be emphasised that very few samples
were measured)
The DHBT wafer under investigation was grown by metal
organic molecular beam epitaxy (MOMBE) [2], and the grown
GaInP layers are disordered as verified by transmission electron
diffraction. The epitaxial structure consists of a 2000A n+ GaAs
cap layer doped at I x 1019cm-1,
a 200A n+ GaInP layer doped at 1
x 1018cm-3,
a 7008, n- GaInP active emitter layer doped at 5 x IO1’
cm-’, a 8008, p’ GaAs base layer doped at 3 x 1019m-3,a 2008,
Vol. 29
No. 21
1885
~~
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