Patent GB-1408627-A (GB)
Title: Semiconductor devices
IPC: H01L21/761
Filed: 1972-11-17
Abstract: 1408627 Semi-conductor devices PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 17 Nov 1972 [22 Nov 1971] 53204/72 Heading H1K A monocrystal substrate 10 of P-type is surface pre-diffused with a first impurity substance to produce a surface region 12a of opposed conductivity patterned to form part of the collector of a first LF transistor T 1 , and a subsequent prediffusion of N-type opposed conductivity with a substance of greater diffusion is effected in the form of annular band 13a overlying the periphery of the surface region 12a to form the collector boundary. A P-type epitaxial layer 14 is then deposited on the surface of the substrate 10 and the in-diffused impurities at 12a, 13a then out-diffuse into layer 14 and in-diffuse into substrate 10 to produce collector region 12 and collector boundaries 13 (Figs. 1a, 1c). A further N-type diffusion into the outer surface of layer 14 at a location spaced from that of the first transistor defines an island in which a second transistor T 2  may be formed, and a second P- type epitaxial layer 16 thinner than the layer 14 but of the same conductivity is deposited thereon. At its surface further N-type regions respectively situated above the central part of the collector region of T 1  and as an annulus overlying the collector boundaries thereof, are deposited and further in-diffused to produce regions 13, 17, 18 (Fig. 1f) of which region 17 provides the emitter, 13, 18, the collector boundaries, while region 19 provides the base of the first transistor enclosed between walls 13, 18 and part 12 of the underlying collector zone. Region 15 extending through layer 16 into layer 14 receives a P-type diffused region 21 into which N-type impurity is diffused to form region 22, so that region 15 defines the collector, region 21 the base and region 22 the emitter of a second transistor T 2 . Simultaneously, N-type high concentration diffusion forms collector and emitter contact zones 23, 24 of the first transistor (Fig. lh). The collectors of the two distinct transistors are positioned below the respective base regions at substantially different levels so that the base thickness of T 1  may exceed that of T 2 , to ensure substantially differing transition frequencies, and the impurity concentration of the collector of T 2  decreases towards the surface, while the device may have a configuration (Fig. 2) in which the emitter 24 of T 1  is surrounded by base 19 and collector region 23 while emitter 22 of T 2  is surrounded by base 21 and collector region 15. In a modification (Figs. 3, 4, not shown) NPN transistors T 1  and T 2  are formed as before with the addition of a third PNP power trausistor T 3  complementary to T 1 ; insulated therefrom by a cup-shaped zone formed by diffusion of two distinct N-type impurities of different diffusion constants into the substrate and its overlying epitaxial layers to form an enclosure having encircling walls and a bottom. In fabrication, the substrate is of P-type monocrystal Si and As is indiffused to form the collector of T 1  and the base of the isolation zone of T 3 ; while the collector walls of T 1 , the emitter of T 1  and the walls of the isolation zone of T 3  are formed by indiffusion of P. The island of T 2  is formed by As indiffusion, as is the zone in which the base and emitter regions of T 3  are formed. The base of T 2  and emitter of T 3  are formed by indiffusion of B. The respective conductivity types may be inverted.