L4. making transformers - Transformers convey energy from one circuit to another, usually stepping AC voltage up or down at the same time. Two major components are the coils of wire, each wound with a specific number of turns. (The ratio of number of turns is the ratio of the voltage change between the circuits.) Another major component is the metal core that is magnetic and is normally made from an iron alloy. The usual method is to build the core from stacks of sheet iron ( laminations ) of the proper shape - a shape of the letter “E” is common with single-phase power - with the open side of the E closed in with straight strips that are laid alternately with the E’s. The magnetic permeability of the iron is important and the magnetic lines are carried around the coil in two directions. Iron oxide or another material provides the necessary insulation between the laminations. When the E-configuration is used, both coils are assembled over the center leg of the E. Fig. 13L4 illustrates this construction. When a “C” shape is used, separate coils are assembled on each side of the C. Coils are wound to the prescribed number of turns, first on semi-automatic machines that wrap the wire on bobbins or around mandrels of the proper size and shape. (See inductors .) The coil wire is insulated with a clear varnish and sometimes is square in cross-section. Protective insulating tape may be wrapped over the coils and between separate windings to provide protection and added insulation between core and windings or separate windings. The coils and laminations are then assembled together. Except for high mass-production transformers, lamination stacking and coil assembly to the laminations are manual operations, but some robotic handling has been employed. The end wires of the coils are stripped and connected to transformer terminals. The transformer is usually contained in a metal or plastic enclosure, and may be fully imbedded in a plastic material that adds insulation between windings of the coils and between the two coils. Larger power transformers are enclosed in a tank of oil that provides both insulation and cooling effect, especially if the oil is pumped to circulate. Transformers are most frequently found in power supplies for electrical and electronic equipment. They are also used extensively in audio and radio-frequency apparatus to transfer signals from one circuit to another, at the higher frequencies with an air core instead of magnetic laminations. In electronic circuits, many transformers have been replaced with transistor circuits.  

Fig. 13L4 A typical audio transformer before it is encapsulated.

L5. making discrete transistors and diodes - Transistors and diodes are solid-state electronic devices made from single-crystal semiconductor materials, usually silicon or other semiconductor materials containing gallium, aluminum, and arsenic. These devices are made with the same techniques used in the manufacture of integrated circuits as described above. Layers of silicon dioxide are grown. Layers of other materials are vapor deposited. Openings are made in the layers by photolithographic techniques and etching. Entire silicon layers and exposed areas are doped with impurities to establish the way electric current will flow. Open areas are doped by either diffusion or ion-implantation methods. Deposits of conductor, semiconductor, or dielectric layers, are limited by photolithographic techniques or by printing or stencilling methods. One difference between integrated circuit device manufacture and discrete device manufacture, however, is that the discrete transistors are found in applications where the electrical current and voltage levels may be higher and where larger circuit elements are needed to conduct the current. Additionally, the extremely small size of devices in integrated circuits is not normally required for discrete devices. Like integrated circuits, discrete transistors and diodes are made on wafers, are tested, cut from the wafer, are connected to terminals and assembled in a protective metal, ceramic, glass, or plastic package. The package is provided with external leads, pads, or other terminals for connection to the circuit where it is used. Fig. 13L5 illustrates the manufacturing sequence for production of discrete diodes, the operations for which are the same as for transistors except that diodes are inherently simpler, having two major components (base-collector or emitter-base) instead of three (base-emitter-collector).

Fig. 13L5 The operation sequence for making a set of diodes. The sketches on the left represent crosssections of the diodes as they are fabricated on a silicon substrate. The right hand sketches show the corresponding top views. Note that only a part of the substrate and its coatings are shown of the operation that produces a large quantity of diodes simultaneously. ( Reproduced with permission from Microlelectric Processing - An Introduction to the Manufacture of Integrated Circuits, by W. Scott Ruska, McGraw-Hill, 1987. )