Patent GB-901034-A (GB)
Title: Improvements in or relating to semi-conductor devices
IPC: H03K3/353
Filed: 1961-02-13
Abstract: 901,034. Semi-conductor devices; etching semi-conductors. TESZNER, S. Feb. 13, 1961 [Feb. 13, 1960], No. 5272/61. Classes 37 and 41. A field effect transistor is bobbin-shaped and has its source and drain electrodes at opposite ends a rectifying connection around the reduced portion and an inversion layer on the surface of the central hole. The Specification explains that the single rectifying junction connection is not able to make the space charge layer extend to the centre of the hole so as to cut off the current flow between the source and drain electrodes completely. The effect of an inversion layer on the inner surface is to accumulate charges of opposite sign to that of the material so that the space charge region extends outwardly from the hole. Fig. 5 shows a device of N-type germanium, silicon or an A 3 B 5  compound formed from a monocrystal, for example by ultrasonic cutting and having a recess or throat separating two end portions 9, 9<SP>1</SP>. The throat is hollowed out by the electrolytic process described in Specification 808,734 involving the rotation of the stub under a jet of electrolyte. An electrode 11 which may be of indium is plated on to the bottom of the throat immediately after the etching by a similar process and is spaced from the throat edges by spaces 7, 7<SP>1</SP> to reduce capacitance. Ohmic terminal electrodes are provided, one, 12, shaped like a ring and the other, 13, a welded metallic plate to allow access to the centre of the bobbin. Fig. 8 shows the apparatus used for reducing the thickness of the bobbin at the region a where the inversion layer is to be formed. The device is supported in a bath 36 of potash solution with a surrounding ring electrode 39 and the central disc electrode 40. Initially, during a cleaning step the ring 39 and disc 40 are negative and the bobbin 9 positive. The gate electrode is negative so as not to be etched. Next the region a is hollowed out by connecting disc 40 and bobbin 9 as before, the gate electrode being connected to the positive terminal through resistor 41. The gate electrode being positive with respect to the semi-conductor body minority carriers -are injected and go to region a where they reduce the potential barrier between the electrolyte and semi-conductive body to produce localised etching. An A.G. source 34 passes a current through the bobbin and voltmeter 35 to monitor the thickness reduction. The gate electrode edges are then cleaned by making electrode 12 positive and gate electrode 11 negative so that etching takes place on each side of the gate. A further cleaning step then takes place and the inversion layer is formed by immersion in strong boiling peroxide forming an oxide layer which fixes the electrons on the surface. The process continues until the layer has a thickness of 100 . The device may be partly embedded in a phenolic resin or a silicon base resin (48, Fig. 9) and is then welded on a base 46 which acts as a connection. The other connections are made through insulated pinches 42 through the base and a shield 47 is welded directly to the base circumference. In a modification (Fig. 6, not shown) a further ring around the neck of the device acts as an electrostatic lens as described in Specification 853,457. In a still further modification (Fig. 7, not shown), the bobbin is on a large semi-conductive base to increase the strength of the device and to increase the stray series resistance between the gate and source electrodes.