Patent GB-1036837-A (GB)
Title: Improvements in or relating to electromechanical transducers
Inventor: DRAKE CYRIL FRANCIS
IPC: H03F3/185
Filed: 1965-04-13
Abstract: 1,036,837. Field effect transistors. STANDARD TELEPHONES & CABLES Ltd. April 13, 1965, No. 15677/65. Heading H1K. [Also in Division H4] An electromechanical transducer (e.g. a microphone) of the electrostatic type includes a vibratable front capacitor plate 17 (Fig. 6) spaced from a back capacitor plate formed by a surface 19 of a body 10 of semi-conductor material in which is formed a semi-conductor amplifier device for amplifying the electrical output of the transducer. The semi-conductor amplifier is preferably a junction field effect transistor. In one method of manufacture, a single crystal rod 10 (Fig. 2, not shown) of P- type silicon has epitaxially deposited around its periphery a layer 11 of N-type silicon followed by an overall layer 12 of P-type silicon. The rod is then cut into slices and each major surface of each slice has epitaxially deposited thereon a layer 13 (Fig. 4, not shown) of N-type silicon. A central circular region of each layer 13 is then removed together with a little of the underlying material 10, e.g. by a localized stream of fluid entrained abrasive particles (Fig. 5, not shown). There is thus obtained bulk material 10 of single crystal P-type silicon on opposite sides of which are rings 14 and 15 (Fig. 6) which act as a source and drain respectively and are interconnected by an annular channel 16. The vibratable plate 17 is a metal coated plastics diaphragm fastened by plastic solder 18 or conductive adhesive, the metal coating facing the ring 14. Alternatively, the metal coating is in front, the electrical connection being made separately. The back surface 20 may be covered by an insulating layer of SiO 2  and on this layer may be formed two solid circuit resistances, one resistance having a resistive impedance matching that of the transducer and used for connecting a source of negative potential to the gate and the other resistance acting as a load resistor for connecting the drain to a source of positive potential. In manufacture of an alternative form of field effect transistor, a slice 21 of single crystal N-type silicon has epitaxially deposited on one major surface a layer 22 (Fig. 9, not shown) of P-type silicon followed by a layer 23 of N-type silicon. Portions of layers 22 and 23 are air-abraded to leave a ring of the layers 22 and 23 (Fig. 10, not shown). Over this shaped major surface is epitaxially deposited a layer 24 (Fig. 11, not shown) of P-type silicon. The layer 24 is then removed by air-abrasion except on the sides of the ring 22, 23. The other major surface of the slice 21 is air abraded to leave a ring 28. There is thus obtained a slice 21 (Fig. 13) of single crystal N-type silicon having outer and inner rings 25, 26 interconnected by an annular channel 27, all of P-type silicon. The rings 25 and 26 act as a source and drain respectively. The slice 21 acts as the gate. The ring 28 has a front capacitor plate 29 insulatingly adhered to it. An insulating layer may be deposited on the surface region 32 to provide a site for laying down solid state resistances. In both constructions the cavity between the front and back capacitor plates may be connected to the external atmosphere by one or more apertures in the body material 10 or 21. Instead of a junction field effect transistor a MOS field effect transistor may be used, operated in either the enhacement or depletion mode. A complete solid state circuit may be laid down on the body material 10 or 21. In one form (Fig. 15, not shown), the transducer and field effect transistor feed, via a capacitor 33, into a transistor amplifier stage 34 and emitter follower low impedance output stage 35 including a capacitor 36, all of which are laid down on the body material. If the values of the capacitors 33 and 36 are too high for convenient integral construction they may be added separately. An alternative circuit (Fig. 17, not shown) laid down on the body material consists of a three stage directly coupled transistor amplifier 39, 40, 41 with negative feedback 42.