GB853421A - Improvements in or relating to transistors - Google Patents

Abstract

853,421. Unipolar transistors; electrolytic etching and plating. TESZNER, S. May 27, 1958 [May 27,1957; Nov. 6 ,1957], No. 16874/58. Classes 37 and 41 A unipolar field effect transistor comprises a rod of semi-conductor material provided at opposite ends with ohmic source and drain electrodes, and having a centrally disposed thinned portion the cross section of which increases continuously from the source end to the drain end provided with a surrounding rectifying gate electrode. The change in cross-section is preferably such that in operation both ends of the conductive channel through the gate electrode are closed simultaneously by the expanding space charge regions. The rod may be cylindrical with a conical thinned portion or in the form of a parallelipiped with a pyramidal thin portion. Such a transistor may be made from a cylindrical rod of 4 ohm.cm. N type germanium with a welded ohmic electrode at each end by rotating it about its axis in a jet of electrolytic etchant e.g. 0À02 N sulphuric acid using the circuit shown in Fig. 8. The etching current is unevenly distributed across the cross-section of the jet 67 on account of the adjustable potential gradient established along the bar by battery 69, thereby causing a graded etching which is deepest at the upper end of the etched section. After completion of the etching the rod is mounted in an arrangement similar to that shown in Fig. 8 but with the battery connections reversed, the etchant being replaced by a plating solution e.g. indium sulphate acidified with sulphuric acid. In this case the potential distribution along the rod controls the extent of the indium plating which forms a rectifying contact with the rod. If a parallelipiped form rod is used and reciprocated transversely instead of rotated during etching and plating a rod with a pyramidal plated central section is formed in similar manner. Such a device may alternatively be formed by electrolytic etching in an agitated bath of sulphuric acid or potassium hydroxide as shown in Fig. 12 using a ring shaped platinum cathode 19 connected in the Fig. 8 circuit. In this case prior to etching the germanium rod is provided with a cellulose varnish coating except in the central region. The plating process is carried out using a similar plating bath but with the electrical connections reversed. If desired an acidified solution of a salt of the metal to be plated, e.g. acidified indium sulphate may be used for both the etching and the plating process. The unipolar transistors described have greater voltage gain, transconductance and output power than otherwise similar transistors with thin portions of constant section and produce appreciably less distortion. Specification 808,734 is referred to.