Patent GB-923104-A (GB)
Title: Improvements in or relating to semiconductive devices
IPC: H02H9/02
Filed: 1959-05-20
Abstract: 923,104. Semi-conductor devices. WES TERN ELECTRIC CO. Inc. May 20, 1959 [May 26, 1958], No. 17134/59. Class 37. A semi-conductor device comprises a PNPN body in which for a certain range of currents between terminals connected to the opposite end zones the current amplification factor a is greater than 1 and in which α falls to less than 1 above the upper limit of this range. Such a variation of α may be achieved by making the thickness between the junctions at the opposite faces of one of the intermediate zones variable. Part of the zone which is several diffusion lengths thick is separated from a part thinner than the diffusion length by a zone of high lateral resistance. Alternatively a similar variation in α is achieved by constructing one of the intermediate zones of a material in which the lifetime, and hence the transport efficiency, of minority carries decreases with increasing current density, and in which conductivity modulation occurs at high-current densities. This effect is achieved by making the zone of high-resisting N-type silicon containing indium trapping centres. The first method of varying α is used in the device shown in Fig. 1, which is analogous to the circuit shown in Fig. 9. The outer parts of the P-type zone correspond to the base zone of transistor 101 of Fig. 9 and their lateral resistance to resistor R, the inner parts correspond to the P-type zone of diode 103, and the zones 12A, 13 and 14 to transistor 102. The zones corresponding to transistor 101 of Fig. 9 have an α greater than 1 but a greater part of the emitter current is shunted by the diode as the current increases until the α of the diode-transistor parallel combination falls sufficiently to reduce the total α of the combination with the second transistor part 102 to less than 1 whereupon the impedance increases. The device therefore functions as a current limiter. The different zone geometries illustrated in Figs. 2 to 4 give rise to the same effect. If the α for very low currents is made less than 1, as by making the device of silicon, the characteristic exhibits high impedance initially and again when α falls due to the mechanism described above, and a low impedance for intermediate currents. Such a device may be used as a storage element. The device shown in Fig. 5 is analogous to the Fig. 9 circuit but with resistor R placed between the N-type zones of diode 108 and transistor 101. The thin part zone 51A forms resistor R and forms with zones 52, 53 the transistor 101. The remainder of zone 51 forms the diode with zone 52 and zones 52, 53, 54 form the second transistor section 102 The current at which the impedance rises to a high value is determined by the value of resistor R. In the above described devices this is determined by the geometry of the device, but in the Fig. 5 device the effective thickness and hence resistance of part zone 51A may be controlled by the reverse bias applied between electrodes 55 and 58. The Fig. 1 device is manufactured by the following steps. A monocrystalline 0.3 ohm cm. P-type silicon wafer is heated in phosphorus to yield a phosphorus-rich surface zone which is then removed from all but an annular zone on one face of the wafer. After a prolonged heating in air to increase the depth of diffusion from the annular zone the wafer is again heated in phosphorus and then in air to give the zone configuration of Fig. 10C. After removal of the N-type material from its edges, the wafer is masked and boron diffused into the exposed centre of its lower face to form P-type zone 14 (Fig. 1), after which electrodes 15, 16 are attached. In all the embodiments additional control electrodes may be provided on the intermediate zones.