Unit 2. Transistors and Voltage Amplification

Radio transmitters and receivers have existed since before the end of the nineteenth century. A practical form of wireless telegraph, attributed to G. Marconi, appeared in 1895, and successful transmission across the Atlantic Ocean was achieved in 1901. However, in the early part of the twentieth century, systems were limited by the lack of a means of voltage amplification. The appearance of a voltage amplification device, the vacuum tube, dramatically improved the concept, as microvolt signals could be boosted for receiving and transmitting.

In the middle of the twentieth century, the transistor appeared. The idea of transistors based on a sandwich of pn junctions (BJT) and a field-effect transistor based on pn junctions (JFET) and on a metal – oxide – semiconductor (MOS) structure (basically, a capacitor) were all understood at the time. However, pn-junction devices became a practical realization much sooner than the MOS structure, due to fabrication complications in producing the MOS device as well as perhaps a perceived lack of need. The JFET served as an interim field-effect transistor until the MOS technology evolved. It provided for a transistor with very high input resistance and was used extensively as the input transistors for BJT opamps.

A textbook on radio, Elements of Radio, published in 1948 (Marcus and Marcus, 1948), makes no mention of transistors. A 1958 text, (Millman, 1958), Vacuum-Tube and Semiconductor Electronics, gives equal weight to vacuum tubes and BJTs in electronic circuits but makes no mention of the field-effect transistor. Slightly later (Nanavati, 1963), in An Introduction to Semiconductor Electronics, as the title suggests, vacuum tubes are dropped completely and the only reference to a field-effect transistor is in one section of the last chapter and this refers to a junction field-effect transistor. In 1965, in his textbook Analysis and Design of Electronic Circuits, Chirlian devotes a small portion of the book to vacuum tubes, but most of the emphasis is on circuits based on the BJT (Chirlian, 1965). No mention is made of the field-effect-transistor. An example of a book in which BJTs and field-effect transistors of both types were finally given balanced treatment was published in 1979 (Millman, 1979). Textbooks tend to lag the industry a bit, and during the 1970s, MOSFET circuits were emerging rapidly, driven by the simultaneous development of integrated circuits. The four editions of a text on analog circuits by Gray and Meyer, (1977, 1984, 1993) and Gray, et al. (2001) serve well as a series through which we observe a transition from mostly BJT to, in the last two editions, more-or-less equal treatment of BJT and MOSFET devices. A recent textbook on the subject of analog integrated circuits (Johns and Martin, 1997) takes the approach that such circuits are now totally dominated by MOSFETS but includes some BJT applications. BiCMOS, a combination of MOSFET and BJT devices on the same integrated circuit, is growing in popularity as more ways of taking advantage of the superior properties of the two transistor types are developed.

Since the earliest transistors, there has been persistent competition between BJT and MOS transistors. It has been, to a large extent (along with many other considerations), a matter of power consumption versus speed; the BJT has been faster but is associated with high power consumption. The MOSFET has gradually taken over as the most important transistor, with increased emphasis on integrated circuits and improved speeds.