Patent GB-810452-A (GB)
Title: Improvements in or relating to signal translating apparatus and circuits employing semiconductor bodies
IPC: H03K3/45
Filed: 1956-02-17
Abstract: 810,452. Semi-conductor devices. WESTERN ELECTRIC CO. Inc. Feb. 17, 1956 [Feb. 18, 1955 (4)], No. 5013/56. Class 37. [Also in Groups XXXIX and XL (c)] A semi-conductor device comprises a body of semi-conductor material provided with a pair of electrodes the resistance between which can be varied by charging a ferro-electric body disposed near the semi-conductor via an electrode mounted on it. In the device shown in Fig. 1 the ferro-electric body overlies the junction 12 between a low-resistivity P-type zone and a higher resistivity N-type zone so that when a sufficiently high positive voltage is applied to electrode 20 negative charge carriers are drawn to the region beneath the ferro-electric body to provide an extension of the N-type zone along the surface. The area of the PN junction and hence its reverse conductance are thereby increased. The above effect occurs for any position of the ferro-electric body 19 between that in which only a small part of it lies over the P region and that in which it lies only over the P region with its nearest edge within an electron diffusion length of the j unction. If the P zone is of higher resistivity than the N zone the low reverse impedance condition is obtained with electrode 23 positive if the ferro-electric body is mainly over the P-type zone but with the electrode negative when it overlies more of the N- type zone. If an intrinsic zone is introduced between the P and N zones the effect is enhanced since the normal reverse impedance is higher while in the polarized condition it is similar to that of the PN device. When the voltage is removed from electrode 20 the ferroelectric remains polarized on account of hysteresis and the low reverse impedance is retained until a voltage of opposite polarity sufficient to overcome the polarizing field is applied. It is stated that the reverse impedance of a semi-conductor to metal rectifying contact, e.g. a point contact, may also be altered in the above manner. The unipolar transistor shown in Fig. 8 comprises a thin layer 115 of N-type Ge on a block 116 of P-type Ge, the electrodes 117, 118, 119 being non-rectifying. Application of a high negative voltage to electrode 124 produces an electric field which, by drawing holes into the surface region of layer 115 alters its conductivity type to give a further reverse biased PN junction. The resistance of the conduction path between electrodes 117, 118 which are preferably placed so as to remain on the unconverted part of the layer is thus increased by the consequent reduction of its cross-section. This change in resistance may be increased by making the surface region of the layer, which is removed from the conduction path by the field, of the highest conductivity though this also has the adverse effect of reducing the depth of penetration of the field. The conductivity of the path may also be controlled by varying the reverse bias of junction 114 and hence the depth of a carrier depleted region in the layer 115. The parts of the surface layer beyond the influence of the field may be of increased crosssection and enhanced conductivity. In the Fig. 14 arrangement the N-type surface layer has only a single ohmic connection 314. In the absence of a polarizing voltage on electrode 319 the device comprises a reverse biased PN junction connected in series with a load 317. When a sufficient negative voltage is applied at 319 the field sweeps electrons out of the N-type region and converts it to P-type thus eliminating the junction and greatly increasing the load current. In order to allow maximum penetration of the field into the semi-conductor, the surface region of layer 312 should be of lower conductivity than the region adjacent junction 313. This may be achieved by first diffusing arsenic into the surface of a P-type crystal and then in a second process diffusing some of it out again. In a modification of this arrangement the surface layer is omitted so that in the unpolarized condition of the ferro-electric a lowresistivity path is provided between electrodes 314, 315 the effect of the intense field in this case being to induce a surface layer of N-type conductivity and hence introduce a reverse biased junction into the load circuit. In the embodiment shown in Fig. 11 an NPN junction body is used with the ferro-electric body overlying the P-type zone and the load RL connected between the N zones. Application of a positive voltage at 223 causes a channel of N- type material to be formed across the surface of the P-type zone thereby short-circuiting both junctions. The polarizing voltage may be applied between electrode 223 and a further non-rectifying electrode on the opposite side of the P-type zone rather than via electrodes 223, 215 to give synmetrical operation of the device. The P-type zone should be thicker than the minority carrier diffusion length e.g. 5 mils. in a Ni doped layer. In all the above devices Ge, Si, a Ge-Si alloy, as intermetallic compound of the AIII Bv type, Te, or Se may be used as the semi-conducting material. The most suitable ferroelectric material is guanidinium aluminium sulphate hexahydrate though other isomorphic crystals containing the guanidinium ion as described in Specification 810,451, barium titanate, Rochelle salt, ammonium dihydrogen phosphate, and ammonium lithium tartrate may also be used. Any gap existing between the surfaces of the ferro-electric body and the semi-conductor which should however be ground and mechanically and chemically etched to match may be filled with a high dielectric constant wax or liquid e.g. ethylene cyanide or nitrobenzene to increase the field strength for a given polarizing voltage. In any of the devices described the conductivity types of the various zones may be reversed with corresponding reversals of the biasing and polarizing voltages. A device of the type shown in Fig. 8 may be made by subjecting the surface of a P-type Si body to boron oxide or phosphorous oxide as described in Specification 782,662. A body of the type required in the Fig. 11 device may be made by withdrawal of a seed crystal from a. melt of varying composition by alloying Pb-As bodies to opposed faces of a 44 ohm cm. P-type crystal of Ge, or by diffusion and alloying processes described in Specifications 782,662 and 759,012 respectively. All the devices described possess the property of non-destructive readout.