Optical: systems and elements – Optical modulator – Light wave temporal modulation
Patent
1992-02-26
1993-09-28
Lobo, Ian J.
Optical: systems and elements
Optical modulator
Light wave temporal modulation
G02F 1015
Patent
active
052490742
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a combined bipolar junction transistor and ah optical modulator.
This invention addresses the need for optical modulators driven by the low voltage swings available from silicon digital integrated circuits. This is achieved by combining the functions of a resonant-optical absorption modulator with a bipolar junction transistor. Applications include optical interconnection of integrated circuits.
DESCRIPTION OF RELATED ART
Existing electro-optical absorption modulators require high driving voltages to achieve useful changes in transmission or reflection. These high drive voltages are not available from silicon large scale integrated circuits--requiring the use of buffer amplifiers, adding to complexity and power consumption. Existing approaches (W.Q.Li et.al. Elec. Lett. 25, (1989,476-477) to the integration of a modulator and a transistor have not provided voltage gain, nor has advantage been taken of optical resonance to reduce the voltage swing required on the modulator.
SUMMARY OF THE INVENTION
According to the invention there is provided a combined bipolar junction transistor and an optical modulator comprising a plurality of semiconductor layers providing an optical mirror for said modulator, a collector for said transistor formed upon said plurality of semiconductor layers, said collector also forming an optical absorber of said adulator, a base of said transistor formed upon said collector, an emitter of said transistor formed upon said base and a metallic contact for said base, said metallic contact providing a function of an optical reflector for said modulator.
Preferably the collector layer is formed from a plurality of semiconductor layers. A part of the base metallic contact may be replaced by another reflective material. Further, a part of the base contact may be removed to allow transmission of light. The semiconductor layers are preferably made of elements in chemical groups III and V. The preferred alloys for the semiconductor layers are: GaAs, GaAlAs, GaInAs, GaInAsP, AlInAs and GaAlInAs. Preferably the device is made on a substrate of GaAs, InP or Silicon.
The advantages of this invention are the provision of a transistor giving voltage gain together with optical folding (that is, resonance) of the modulator which reduces the voltage swing required for a given change in optical transmission or reflection. The transistors can also be used as the basis for electronic circuits to act as drivers for the modulators or as receiver amplifiers when using the modulators as light detectors. The transistors can also make logic circuits for coding or decoding the data to be transmitted or received.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described further by way of example with reference to the accompanying drawings in which:
FIG. 1 illustrates the cross section of the device according to the invention; and
FIG. 2 illustrates an exemplary circuit of the device embodying the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In the cross-section shown in FIG. 1 the transistor optical modulator comprises a semiconductor layer (1) which forms the emitter of a bipolar junction transistor. The said emitter can be n-type or p-type. A contact (2) is deposited on the emitter layer. The said emitter is formed upon a base layer (3) which is of the opposite doping type to the emitter layer and said base layer can be of the same or different semiconductor alloy as the emitter. A metallic contact (4) is deposited onto the base layer. The said base layer is formed upon a semiconductor layer (5) which is of opposite type to the base and so forms the collector of a BJT. Said collector can be of the same or different alloys as the base or emitter. The collector layer also functions as the absorptive medium for an electro-optical modulator. Said collector layer is formed upon a plurality of semiconductor layers (6), of the same doping type as the collector, which are so arranged as to form an opti
REFERENCES:
patent: 4872744 (1989-10-01), Abeles et al.
patent: 4997246 (1991-03-01), May et al.
Electronic Letters, vol. 25, No. 7, Mar. 30, 1989, W. Q. Li et al, "Integrated multi-quantum-well controller-modulator with high gain for low-power photonic switching", pp. 476-477.
IEEE Photonics/Technology Letters, vol. 1, No. 3, Mar. 1989, New York, "Field-effect transistor self-electrooptic effect device: integrated photodiode, quantum well modulator and transistor", D. A. B. Miller et al., pp. 62-64.
Okada et al., "Optical Intensity Modulator for Integrated Optics by use of a Leterojunction bipolar transistor waveguide structure," Appl. Phys. Lett., vol. 55, #25, Dec. 18, 1989.
GEC--Marconi Limited
Lobo Ian J.
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