Integrated circuits using epitaxial diffusion and thin-film techniques

Greater freedom in circuit design, better isolation, and enhanced temperature stability can be achieved by a combination of epitaxial, diffused, and thin-film techniques for integrated circuitry. The example of an integrated differential amplifier whose performance represents the state-of-the-art even when compared with one made from discrete components is used to illustrate the advantages of this combination. The circuit uses several npn and pnp transistors. A new isolation technique achieves high breakdown voltages and low isolation leakage currents in the pA range. Several samples of the integrated circuit used as example have been successfully made. The complete circuit occupies an area of 60 × 120 mils and consists of 5 npn and 3 pnp transistors and 9 resistors, two of which are diffused. The described fabrication technique and the close spacing of individual circuit elements allows excellent matching of both types of transistors. The input impedance of the circuit is greater than 0.5 MΩ, temperature drift of the output is less than 100 µV/°C referred to the input, and linearity is about ±1%. The combination of diffused and evaporated resistors reduces the temperature coefficient of the circuit to very low values. The design allows the adjustment of the thin-film resistors by an abrasive method and by tapping, and of the diffused resistors by a variation of the diffusion process; the resistance adjustment makes it possible to reduce the output offset voltage to approximately zero. The design uses a new simplified constant-current source which consists of a two-emitter transistor and two resistors. This eliminates the need for temperature-compensating diodes. Structural details of the integrated circuit and the advantages of this method are presented.