Electromechanical transducer

1,088,793. Semi-conductor devices. STANDARD TELEPHONES & CABLES Ltd. Dec. 17, 1965 [Dec. 23, 1964], No. 52225/64. Heading H1K. [Also in Division H4] A microphone or other transducer comprises a semi-conductor element biased for flow of electric current therein and a movable member spaced from the element, the member being biased with respect to the element to cause an electrostatic field to be set up in the element. Means responsive to an applied mechanical force is provided to vary the spacing of the member from the element. The strength of the field in the element varies in accordance with the spacing of the member from the element and so varies the value of the electric current flowing in the element. In one microphone (Fig. 3) a field effect transistor has a movable gate electrode comprising an aluminium layer 21 evaporated on a dielectric diaphragm 22 fastened at its edge to the microphone enclosure. Two P-type regions 16, of side-by-side strip configuration (Fig. 6, not shown), are diffused in an N-type block 14 and block 14 is covered by slicon oxide insulation 17, as is half of each region 16. A metal coating 18 connects one P-type region 16 to a positive potential 19 (source) to which is also connected block 14. The other P-type region is connected to a negative potential 20 (drain). In a modification, Figs. 4, 5 (not shown) the P-type regions are formed as a central region (16a) and a surrounding annular region (16b). The gate electrode is an annular layer (21). In each embodiment an aluminium strip 23 extends from the layer 21 to the edge of the diaphragm 22 for connection to a bias potential 24. In another embodiment, Fig. 1 (not shown) the reverse biased P-N junction of a planar epitaxial transistor has a biased electrode (4) near the base-collector junction. The biased electrode (4) is coupled to a diaphragm. An electrical output signal is taken from a resistor (6) in series between base (1) and collector (2). A similar effect is obtained by using the junction between emitter (7) and base (1). Alternatively a P-N junction diode (8), Fig. 2 (not shown, is reverse biased by a battery (9). A movable electrode (10) fastened to a diaphragm (11) is biased by a battery (12) and an ou put signal responsive to the modulation of the spacing of the electrode (10) is taken from a resistance (13) in series between the positive and negative electrodes of the diode.