Bipolar Transistor Including Optical Waveguide

This invention relates to optical devices and more particularly to transistors which include optical waveguides.

Devices are known in which electronic and optical functions are combined. However, often these require high switching voltages for their operation because of low electro-optic constants or because other electro-optical interactions attenuate the beams.

The present invention arose in an attempt to develop a device combining electronic and optical effects to give improved performance compared to previously known devices.

According to the invention there is provided an optical device comprising a bipolar transistor which includes an optical waveguide. A device in accordance with the invention may be made particularly compact and is able to operate at relatively low drive voltages. It is believed that such devices may be used with advantage in both linear and non-linear applications. It is preferred that the base layer of the transistor includes the optical waveguide. Where the device is arranged to operate in a linear manner, for example as a switch or a modulator say, only a fraction of the switching or modulation power is required to be fed into the base region of the transistor and the power requirements for electro-optical control of the phase of light through the waveguide are low. The electronic gain in the optical waveguide region ensures that a much larger current or voltage is dropped across the electro-optical interaction region than is fed in at the base electrode, the extra power being drawn through the dc supply rails. This offers significant advantages over conventional p-i-n structures in which the full drive power must be supplied to each element, a requirement which is difficult to meet above a few GHz.

It is preferred that the device includes a ridge which is extensive from a substrate and which is arranged to act as one of the emitter or collector electrode, the ridge defining the lateral extent of the optical waveguide. Vertical confinement within the optical waveguide is achieved by the difference in the refractive indices of layers in the structure. This configuration is particularly advantageous as the ridge offers not only optical lateral confinement but also, where the ridge is the emitter, allows easy access for the base contact.

The transistor may be an n-p-n structure or a p-n-p device. Advantageously the transistor is a heterojunction transistor in which adjoining regions are of similar crystal structure. Such a structure may be relatively easily fabricated as it does not require re-growth steps. In a particularly advantageous embodiment of the invention, the base layer is a ternary or quaternary of InP and is sandwiched, in the region of the waveguide, between InP layers.

Where the device is arranged to operate in a linear fashion, for example, as a switch, typically the base layer has a bandgap energy which differs significantly from the photon energy, for example, the bandgap and operating wavelengths may be 200 nm apart.

However, it may be preferred for some applications of a device in accordance with the invention to arrange that the band gap wavelength of the light guiding layer is substantially similar to the wavelengths of the interacting radiation. This enhances the optical/electrical coupling and enables a highly non-linear device to be produced. It may be advantageous for the optical waveguide to be arranged to act as a laser cavity.

According to a feature of the invention, an optical arrangement includes two devices in accordance with the invention arranged such that their emitters are electrically coupled to form a long-tailed pair. Such an arrangement minimises any difficulties which may occur because of large amounts of charge being transferred through the structure. The input signal is normally supplied between the base and the current drain is fixed. Thus, in operation, one transistor must switch off as the other turns on, giving a sharp switching characteristic. Also, as neither transistor saturates, fast switching may be achieved, thus avoiding charge build-up in the transistor bases and a guasi digital response is obtainable when the arrangement is used in linear devices such as switches. In one embodiment of the invention, the two devices are optically coupled, for example, they may be formed on a common substrate. Advantageously, the optical waveguides of the devices together form a complementary pair in which light transmitted along the waveguides may be arranged to interact. For example, in Mach-Zehnder interferometers light in one guide is retarded relative to that in the other, so that a phase difference accumulates along the structure and gives rise to constructive or destructive interference when the two light signals are combined. Conveniently, the refractive index in one guide is increased and that in the other decreased. Another type of arrangement in which complementary pairs of waveguides are included is a reversal switch.

In one preferred embodiment, the arrangement is a bistable gate. The electronic structure of the long-tailed pair provides a linear small signal response and good limiting/clipping properties. Clipping is achieved because only a certain amount of current may be drawn through either collector. This enables the arrangement to be operated as a binary switch, as overdriving the bases squares off the voltage/current response of the circuit. The electronics may therefore be used to compensate for the "non-binary" switch characteristic of the optical circuit.

The device in accordance with the invention may be a phototransistor, that is, one which is optically triggered. This gives high speed control of switching circuits without parasitic capacitances and may be used to introduce negative feedback to compensate for fabrication variations.

The invention is now further described by way of example only with reference to the accompanying drawing in which:

FIG. 1 schematically shows an optical device in accordance with the invention; and

FIG. 2 is a schematic circuit diagram of a long-tail pair arrangement in accordance with the invention.

With reference to FIG. 1, an n-p-n transistor in accordance with the invention comprises an n.sup.+ doped substrate 1 on which an n.sup.- InP layer 2 is epitaxially grown. A layer of p-doped InGaAsP 3 is grown on the epitaxial layer 2. Another epitaxial layer of n.sup.- doped InP 4 is grown as a ridge on the p-doped layer 3. An n.sup.+ doped capping layer 5 is laid down on top of the epitaxial layer 4. The emitter contact of the transistor is made via the capping layer 5 and the collector contact through the substrate 1. The base contact is made to the p-doped layer 3. Because the p-doped layer 3 has a higher refractive index than the InP layers 2 and 4 in the region of the ridge, any light transmitted along the InGaAsP layer 3 is substantially confined by the vertical refractive index difference in the region of the ridge. The ridge also defines the lateral confinement of the light. Current injected via the base contact controls the electric field in the region of the ridge 4 and hence the refractive index of the layer 3 by means of the Pockels effect, Kerr effect and by current injection/depletion. This enables the passage of light transmitted along the waveguide to be controlled. By controlling the current applied to the base 3, the mode velocity in the waveguide may be controlled. It is possible to achieve relatively large changes in the phase of light transmitted through the device. The device may be arranged to be optically triggered.

With reference to FIG. 2, two devices 6 and 7 similar to that illustrated in FIG. 1 are arranged as a long-tailed pair in which their emitters 8 and 9 are connected. The devices 6 and 7 may be formed on the same substrate and their optical waveguides coupled. The arrangement enables high switching speeds and a digital-type response to be achieved. It also may give enhanced non-linearity for optical/optical switching where desired.

In an alternative embodiment of the invention, the emitter contact is made through the substrate and the capping layer of the devices acts as the collector contact. This is particularly advantageous in the long-tailed pair arrangement.