Bipolar transistor including optical waveguide

An optical device comprises a heterojunction bipolar transistor which includes a light guiding layer to which the base contact is made. A ridge is included adjacent to the light guiding layer and acts as the emitter or collector of the transistor, the ridge also defining the lateral extent of the light guiding region. Current injected via the base contact controls the electric field in the region of the ridge and hence the refractive index of the layer controlling the passage of light transmitted along it.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
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.