Patent GB-1118826-A (GB)
Title: Optoelectric systems
IPC: H03H11/02
Filed: 1965-06-14
Abstract: 1,118,826. Semi - conductor. TEXAS INSTRUMENTS Inc. 14 June, 1965 [29 June, 1964], No. 25138/65. Heading H3T. [Also in Divisions G1 and H1] In an opto-electronic coupling system the detector and a subsequent semi-conductor device to which its output is delivered are integrated in a single substrate. An integrated circuit comprises a plurality of interconnected components in a body of semi-conductor material, the photo-sensitive detector having at least two regions of opposite conductivity types separated by a rectifying junction, and means are provided for generating optical radiation (including infra-red) directed on the photo-sensitive detector and having a wavelength such that at least a portion of the radiation is absorbed by the photo-sensitive detector to generate charge carriers which are collected at the rectifying junction. A wide-band amplifier, Fig. 17, includes a diode detector connected between base and ground of a transistor 60 which arrangement overcomes difficulties arising from the mal collection of charge carriers at the rectifying junction (42), Fig. 4 (not shown), and a resistor 111 to limit the base current to a value less than the photo-current generated by the diode in response to light from a signalling source 2 which may be gallium-arsenide. An additional transistor which may be N.P.N. or P.N.P. 64 connected in the Darlington configuration is provided to increase amplification and eliminate the Miller effect, the transistor not cutting off and on but having its degree of conduction changed by the detected light. Negative feed-back is provided between the collector of transistor 64 and the base of transistor 60, and between the collector of transistor 80 and the collector of transistor 64 through a transistor 72, the latter feed-back circuit including transistor 130. The feed-back to transistor 60 enables the value of resistor 111 to be considerably reduced in value and thus in size, and the feed-back to transistor 64 via transistors 130 and 72 holds the collector potential, and thus the base value of transistor 80, at a value preventing saturation of the transistor 80. A resistor 140 between the base of transistor 130 and the collector of transistor 72 prevents the charging of the Miller capacitance in transistor 130. The resistor 111 which is formed by a path in the silicon body base produces distortion at high frequencies due to its distributed capacity and the distortion is eliminated by the inclusion of capacitor 114 in the feed-back path to transistor 60, the eliminating circuit comprising resistors 110-113. A final emitter follower stage 86 to output terminals 91, 92 is provided. An analysis of the circuit by reference to a succession of simple embodiments in which the reason for the inclusion of the various components is explained is included. Practical embodiments of the circuits integrated into a silicon P base are described with reference to Figs. 4, 6, 10, 18, 19 and 20 (not shown) and in it the detector is shown as either a diode separated from the body by a rectifying junction or as a single diffused element in the body. Thermal stability is achieved by the use of identical transistors which mutually compensate for parameter variations, and the amplifier is said to be suitable for a range from D.C. to megacycles A.C. The light source is mounted above the detector diode from which it is separated by a layer of optically transparent material such as glass. Distributed capacitance introduced by the proximity of the light element may be eliminated by diffusing a P-type impurity into the N-type region 35, and into the top surface of the original substratematerial to form a P-type region 160, Fig. 19, overlaying the region 33. The original substrate is selectively overlayed by a layer of silicon oxide 166, e.g. the P-type detector region is not overlayed. Aluminium electrodes 168, 170 are formed respectively over the oxide layer and on the extreme exposed end of an N+ region to form a contact with cathode 33. The capacitor 114 is formed by three diffusions as follows: an N-type impurity is selectively diffused within an opening in the silicon oxide layer, a P-type region is diffused into this region and finally an N-type impurity is diffused into the P-type region. The resistor 111 may be formed by a double diffusion process. Fabrication of a transistor by a four-step diffusion process which isolates the P-type collector from the P-type substrate is also described. A differential amplifier including a number of the basic features of Fig. 17 and energized by two light sources is described with reference to Fig. 22 (not shown), and an alternative arrangement dispensing with one of the light sources and substituting a capacitor for the corresponding detector is referred to.