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From Fig. 4 and Fig. 5, we can see that the equivalent
network of this structure can be considered to be a parallel
resonant circuit. The resonant frequency is f, = 1.27f,,, where
Ll is the cutoff frequency of the TE,, mode. Using the resonant characteristics of the parallel resonant circuit, we can
make filters of high selectivity and narrow pass band, which
are particularly useful in certain cases.
self electro-optic effect device (SEED) and its derivatives have
demonstrated great versatility for switching and logic applications? We have recently demonstrated a three terminal device
where an MQW region is incorporated in the base-collector
region of a n-p-i(MQW)-n heterojunction bipolar transistor
(HBT).5*6In this structure when light impinges on the device
at a particular wavelength, the photocurrent is amplified by
the transistor, and an enhanced negative resistance region can
be obtained. Along with the advantages of high gain, which
allows switching at low optical power levels consistent with
laser diode technology, the device has the advantage that it
can be efficiently switched optically or electronically.
We establish the MQW-HBT device as a simple flip-flop
programmable memory element. The basic concepts are
explained in Fig. 1. The device is conceived to form an
sinale cell
>
constant optical power
-10
L
0
40
20
60
80
E,
Fig. 1 Series reactance against
4444444
,
100
m
E,
Symmetric triple post configuration
1 = 1 . k ; d / a = 0.15
low voltage,
low transmit t once
progmmmoble memory
cells
Conclusion: The equivalent parameters of arbitrarily shaped
multipledielectric posts are found by the method of lines. The
analysis procedure shows advantages of simplicity and high
accuracy.
high voltage,
hlgh tmnsmittance
ma
a
Acknowledgment: This project is financially supported by the
NSFC of P.R.China.
X.-H. JIANG
S.-F. LI
Research Institute of EMF and Microwaves
Department of Radio Engineering
Southeast University
Nanjing, China
5th October I990
References
and AUDA, H. A.: ‘Multiple dielectric posts in a rectangular waveguide’, IEEE Trans., 1986, M’IT-34, pp. 883-891
2 ISE, K., and KOSHIBA, M.: ‘Equivalent circuits for dielectric posts in a
rectangular waveguide’, IEEE Trans., 1989, MTT-37, pp. 18231
HSU, CA. G.,
1825
3
COLLIN, R. E.: ‘Field
theory of guided waves’ (Maraw-Hill, New
York)
4
s. B., and PREGLA, R.: ‘Hybrid mode analysis of arbitrarily
shaped planar microwave structures by the method of lines’, IEEE
Trans., 1984, MTT-32, pp. 191-196
WORM,
PROGRAMMABLE MEMORY CELL USING
QUANTUM CONFINED STARK EFFECT IN
MULTI-QUANTUM WELL HETEROJUNCTION
BIPOLAR TRANSISTOR
Indexing terms: Cells, Memories
A three terminal bistable programmable memory cell which
can be read either optically or electrically is proposed and
demonstrated. The device is based on using Stark effect of
the excitonic transitions in a multiquantum well base region
of a heterojunction bipolar transistor. The single device can
be flipped (and held) from low transmittance (high voltage) to
high transmittance (low voltage) state and vice versa by a
varying base current signal.
The quantum confined Stark effect (QCSE) in multiquantum
wells (MQW) has been shown to lead to negative resistance In
the photocurrent-voltage characteristics in a p-i(MQW)-n
structure that can be exploited for swtching devices.’+ The
ELECTRONICS LETTERS 3rd January 1991
-1 1
~
- -
Vol 27 No 1
“t
v
“H
m
b
Fig. 1 Concepts of MQW-HBT deuice (IS a simpleflipflop p r o g r a m able memory elentent
a Proposed and demonstrated memory cell, capable of switching
and holding with appropriate base current signal
b lp curves responsible for device operation
element of a memory array which is illuminated with a constant and uniform photon flux with a wavelength that gives
the desirable I/V characteristics. The basic memory cell is
illustrated in Fig. la and consists of the n+-pqMQW)-n HBT
in series with a p-i(MQW)-n modulator and a resistive load.
The operating principles of the device are schematically shown
when the base current is 1;.
in Fig. lb. At a photon flux C,,
the photocurrent-voltage curve provides two stable points for
the load line. The high voltage point V, also corresponds to
high transmittance through the MQW region, and the low
voltage stable point at V, corresponds to a low transmittance.
If the base current is made near-zero, the load line has only
one stable point at A and when the base current is restored to
I:, the stable point at bias V, is set. If the base current is
made higher (I;) there is again only one stable operating point
at E . Now, when the holding base current 1; is restored the
low voltage point V, is chosen. The state of the device can
thus be efficiently altered and maintained by the base current.
The device described above is fully compatible with HBT
digital technology and only requires a constant uniform
optical illumination. Also it is very simple in that it requires
only one transistor, unlike a conventional flip-flop circuit. It
can also be read either electronically (through the voltap
levels) or by the transmittance through the MQW region.
The layer scheme of the heterostructure for the c o n t m l k /
modulator, grown by MBE, is shown in Table 1. The impor31
tant features of the heterostructure are the following: The
0.4pm thick AI,.,Ga,.,As layer serves as an etch stop layer
for selective substrate removal under the modulator. The collector region has the 0.6pm undoped GaAs/AI,.,Ga,.,As
MQW, which forms the essential element for the QCSE
modulator and detector in the HBT. The 8008, undoped
graded layer above the MQW is to ensure that carriers
emitted from the base gain sufficient energy to travel across
the first few barriers of the MQW. A 1508, thick undoped
GaAs layer is included after the Bedoped base region to
prevent possible dopant out-diffusion during epitaxy. The
measured absorption spectrum of the MQW at room temperature reveals clearly the HH and LH excitonic resonances.
measured switching and hold characteristics of the device
when a load of 2.0Mn was connected, as shown in Fig. l a .
The applied bias value was 27V. Fig. 2B clearly shows that
when the base current is changed from 0.95pA to 0 4 p A and
back to 0.95pA the high voltage (low transmittance) state is
produced and held. On the other hand, when the base current
is increased to 1.2pA and then brought back to 0,95pA, the
low voltage (high transmittance) state is produced and held.
The high built-in gain of the MQW-HBT device (-60) allows
a very good noise tolerance. These details will be discussed
elsewhere.
Table 1 MBE layer sequencefor integrated
controller/modulator
Layer
Thickness
Type
Doping
Cap
Emitter
Subemitter
Spacer
Base
Transit layer
MQW
Stop layer
Collector
Etch stop
Suhntrate
0.02
0.20
0.015
0.015
0.10
0.08
0.60
Superlattice
0.30
n+
2 x 10"
7 x 10"
7 x io''
0.40
U
Irm
AlAs
fraction
m - 3
n
n
0
0.3
(M.3
0
0
Graded
(M.3
U
p+
1 x 10'8
U
U
U
n
2 x 10''
0.3
. .
T
.. ..
0
n
SI
U = undoped
SI = semi-insulating
Device fabrication starts with the formation of emitter and
collector mesas by
etching in a solution
of
H,PO, : H,O, : H,O. Emitter and collector contacts are
formed by electron beam evaporation of Ge/Au/NiPi/Au and
subsequent lift-off in acetone. The base contact is formed by
deposition of Zn/Ni/Au. Both contacts are alloyed at 450°C
for 60 seconds, though separately, to form the respective
ohmic contacts.
In Fig. 2A we show typical measured current/voltage characteristics of the controller for an incident fixed optical power
of 1OpW at I = 85308,, which corresponds to an energy lower
than the HH exciton resonances. As can be seen from this
Figure, the I/V characteristics can be shifted as expected by
changing the base current. In Figs. 2B and 2C we display the
80
E
Fig. 2 8 Switching demonstration ofjhbricated device circuit
A sequence of pulse I,, V, and transmission Tare shown
Applied bias = 27 V
Load = 2OMn
In summary, we have proposed and demonstrated a novel
programmable optical/electronic memory compatible with
digital HBT technology. The device owes its simplicity to the
tailored photocurrent-voltage curves resulting from the
quantum confined Stark effect.
r-
l2
11
10 -
98-
2 7
A
l5[
>
U
-
:t
3c
= 160 n A
= 80nA
21
0 7
I
08
I
I
I
I
J
09
1
11
12
13
IE. PA
C
64449 2
lJ
Fig. 2C Holding demonstration o f f i r i c a t e d device circuit
The bistable flip-flop's characteristicsare highlighted
Load = 20mn
Applied bias = 27 V
Acknowledgment: This work was supported by the Air Force
Office of Scientific Research under contract AFOSR-88-0168.
0
5
10
20
15
"CE
25
30
'
A
Fig. 2A Measured IIV characteristics o f M Q W-HET device
The input optical power is lOpW at 1 = 8530A
32
W . 4 . LI
S. GOSWAMI
P. BHATTACHARYA
24th Octoher 1990
I. SlNGH
Department of Electrical Engineering and Computer Science
The University ofMichigan, Ann Arbor, M I 48109-2122, U S A
€1 XCTRONICS LE77ERS
3rd January 1991 Vol. 27 No. 1
References
MILLER, D. A. B., CHEMLA, D. S., DAMEN, T. C., MSSARD, A. C., WIEGMANN, w., WOOD, T. H., and BURRUS, c. A.: ‘Electric field depen-
dence of optical absorption near the bandgap of quantum well
structures’,Phys. Rev. B, 1985,32, pp. 10431060
MILLER, D. A. B., CHEMLA, D. S., DAMEN, T. C., WOOD, T. H., BURRUS,C.
A., GOSSARD, A. c., and WIFGMANN, w.: ‘The quantum well selfelectrooptic effectdevice: optoelectronic bistability and oscillation,
and self-linearized modulation’, IEEE J., 1985, QE21, (9), pp.
1462-1476
MILLm, 0.A. B., CHEMLA, D. S., DAMEN, T. C., WOOD, T. H., BURRUS, C.
A., GOSSARD, A. c . , and WLEGMANN,
w.: ‘Novel optical level shifter
maintained. It is common in FEM practice to be able to
identify corresponding mesh points in pairs of planes constituting a periodic boundary. Each pair of points is then assigned to a single nodal (vector) unknown, thus forcing periodicity
in the FEM trial function Ho. In the Galerkin weighted
residual option, when the weight functions are selected from
the shape functions defining the trial Ho, the required periodicity of WOautomatically ensues. Thus a finite element procedure relating to periodic structures such as the helix of a
travelling wave tube may be formulated using the residual
and self-linearized optical modulator using a quantum well selfelectrooptic effect device’, Opt. Lett., 1984,9, pp. 567-569
WHEATLEY, P., BRADLEY, P. I., WHITEHEAD, M., PARRY, G., MIDWINTER,
I. E., MISTRV,
P., PATE, M. A., and ROBERTS, I. s.: ‘Novel
nonresonant optical logic device’, Electron. Lett., 1985, 23, pp.
92-93
LI, w.-Q., HONG, s.-c.,OH,
1. E., SINGH, I., and BHATTACHARYA, P. K.:
‘Integrated multiquantum well controller-modulator with high
gain for low power photonic switching’, Electron. Lett., 1989, 25,
pp. 476477
HONG, s., and SINGH, 1.: ‘Theoretical investigation of an integrated
all-optical controller-modulator device using QCSE in a multiquantum well phototransistor’, IEEE J. Quantum Electron.. 1989,
25, p. 301
[(P” x W O )(e;’V“
.
- k’ WO. k H o ) l dR (2)
where
(3)
with a corresponding expression for V“
an FEM matrix equation
P(j&W
FINITE ELEMENT SOLUTION OF
TIME-HARMONIC MODAL FIELDS IN
PERIODIC STRUCTURES
Indexing term: Electromagneticfield theory
A weighted residual FEM formulation suitable for the elec-
tromagnetic modal analysis of periodic structures to give
numerical k fl relationships and field patterns is introduced. A two-dimensional realisation of the formulation
applying to open-sided ridged waveguides agrees well with
analysis. The general method described could be applicable
to practical travelling wave tube slow wave structures.
~
A weighted residual approach to solving boundary-driven
(deterministic) linear time-harmonic electromagnetic problems
by the finite element method (FEM) using a vector field variable, say H(x, y , z), requires setting to zero the residual
W). (e,-’V x H) - k Z W .@,H)] dR
+ k2Q%
x Ho. Eqn. 2 enables
=0
(4)
to be set up in standard fashion, where % represents a vector
of complex nodal values of the H-variable. The eigenproblem
represented by eqn. 4 is conveniently posed as if fi were given
and k sought, when it becomes of standard eigenequation
form. For lossless structures P is Hermitian, and the eigenvalues k corresponding to real j? are themselves real.
The use of a full vector variable is computationally expensive whereas some cases of interest may correspond to a transverse magnetic or transverse electric wave, for, say, TM cases
employing the two-component vector ( H x , H,, 0) as working
variable. As a preliminary ‘bench-mark‘ case the open-sided
periodic ridge waveguide of Fig. 1 has been studied and compared with the known analytical solution.’ Here the problem
symmetry admits of a pure TM wave represented by fields
either [H,(y, z). 0, 01 or CO, E& z). E,(y, z)]. Choosing to
work with the former and assuming E, = p, = 1 results in the
simplification
R
[(V x
x Ho)
=
(1)
f [ . W . VH
+j(W E
az - dw
az H )
R
+@’for a suitable number of vector weights Wand an appropriate
trial function H.’ The relative constitutive constants e, and p,
may be complex tensors, k is the free space wavenumber, R
represents the problem space whilst W and H are subject to
certain boundary and continuity constraints. It is further
shown in Reference 1 how the residual (eqn. 1) can be applied
to the uniform waveguide problem of establishing propagation constants j? and eigenvectors Ho(x, y) corresponding to
waveguide modes H = Ho(x, y ) exp ( - j s z ) at a given k. In the
waveguide analysis it is assumed that weight functions Wo(x,
y ) exp (+jj?z) can be constructed so that scalar products
between H a n d W, or their derivatives, are invariant along the
waveguide axis. Then surface integrals of such products cancel
between input and output ports, such planes having
oppositely direct normals. This cancellation allows the complete boundary surface enclosing R to be accounted for so
that eqn. 1 remains a valid residual.
A similar argument for obtaining the j? - k relationship and
eigenvectors Ho applies to waveguide structures with space
periodicity, provided the problem-space R is closed by a pair
of planes separated by the space period L. The vector Ho is
now a function of z as well as x and y , but by Floquet’s
theorem’ such z variation must be periodic over the length L.
If the weight functions W,(x, y, z ) are similarly periodic then
the cancellation referred to above occurs between planes
defining the periodic cell, so that the validity of eqn. 1 is again
ELECTRONICS LETTERS 3rd January 1991
~
Vol. 27
~~~
No. 1
1
k2)WH dR
(5)
The single x-component H here nevertheless represents a full
vector variable, so that assuming perfectly conducting corrugated guide walls, the appropriate boundary constraint at
such walls is homogeneous Neumann, allowing the scalar H
there to remain unconstrained.’ The finite element matrix
equation corresponding to the residual of eqn. 5 becomes
S%
- jj?Z%
+ (8’
- k 2 ) ) T x= 0
(6)
where S and T are the well-known arrays arising in the FEM
solution of the Helmholtz e q ~ a t i o n The
. ~ matrix Z, previously
\\\\\\\\\\ \ \ \ \ \\\\\\\\\ \ \ \ \
\ \
lLLzill
Fig. 1 Open-sided ridged periodic waveguide
33
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