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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