Point-contact transistor

The point-contact transistor was the first practically realized bipolar transistor and is considered a pioneer of modern semiconductor technology. Subsequent studies showed, however, that it was indeed a thyristor. The foundations of modern semiconductor technology had not yet developed at the time of its development and all work was based largely on trial and error. The circuit symbol of the bipolar transistor to remind of the structure of the top transistor.

Development

In March 1942, the research team of Karl Lark - Horovitz was awarded the contract for the development of improved crystal detectors for use in the radars currently in the development at the University of Purdue by the Defense Advanced Research Projects Agency OSRD. The company Bell Laboratories was involved in military radar research. To prepare for the competition in the news sector after the war, founded Mervin Joe Kelly ( head of research at Bell) in July 1945, among other things, a research group Solid-State Physics, in which the group was led by William Shockley Semiconductor. The scientists in this group were: Walter Brattain, Gerald Pearson, Hilbert Moore, Robert Gibney. Later still came to him John Bardeen. This group tried to develop a theory for the effects in semiconductors, with the aim to develop a field-effect transistor based on the model of the patent by Julius Lilienfeld (1925 ). They built on the experience of the research team from Purdue group whose point contact diodes have been produced in large series.

In protracted experiments with electrodes in the immediate vicinity of the point contact diodes Brattain and Bardeen this found that the current was influenced by the diode while still held a reinforcement. On December 16, 1947, a demonstrable functional models, with a capacity gain of 100 -fold, in operation and presented on 23 December 1947 in the form of a microphone amplifier, the company's management. Until June 22, 1948 the discovery was kept secret and then inaugurated the technical staff and the military. Surprisingly, the invention has been released by the military on the secrecy so that they could be published on 25 June 1948 in the Physical Review. Already on 30 June 1948, reported in the daily press about the invention.

The large-scale production began in 1951 at the Bell Western Electric Company. In 1950, also at Bell, the junction transistor developed that ousted the point-contact transistor, due to its much higher reliability from the lower frequency range up to 5 MHz. Until about 1956, the point-contact transistor defended the frequency range up to about fT = 100 MHz, after which it was rapidly displaced by more modern drift transistor.

The table shows the technical data of a typical peak transistor from the 1950s. The characteristics of these transistors are given the technical development in the following decades, of course, not comparable with modern components. Because of the new opportunities, especially in terms of miniaturization, but was bought in the early years, the entire production from the military.

Structure and properties

On a metal base plate, which serves as the base terminal of the doped crystal is soldered junction- free. In the crystal, two 15 micron thick phosphor bronze wires are pressed. The spacing of these peaks amounts to about 30 microns. The system is sealed by the assembly Peephole in the metal tube and mounting the wire tip which is to form the collector terminal are welded by a short current pulse to the crystal. By the resulting layer structure, a four-layer diode with a control terminal ( thyristor ), whereby a current gain greater than one is obtained in the basic circuit is formed. This was not understood at that time and prepared for the amplifier development always problems because it could lead to unwanted switching of the transistor.

In addition, the Schottky contact of the emitter terminal was extremely sensitive to congestion and resulted in high-frequency precursors often due to electrostatic discharge failure. Also moisture problems were caused by the construction Kunststoffvergussmasse a major reason for failure. For example, had the hearing aid manufacturers Zenith Radio Corporation to recall all hearing in April 1953 and re- equip with miniature tubes instead of transistors.

Notes to the collector region

Since the point-contact transistor was quickly superseded by improved transistor types, his research has never been completed satisfactorily. The technique became known to the layer structure at the collector is as follows: Through strong surges of the single crystal is melted under the top and then recrystallized again. However, there remain gaps in the crystal lattice, which act as a p -type doping and form a barrier to the single crystal. Characterized the thickness of the base region is reduced. By heating simultaneously diffusing impurities from the wire tip and dope there a new, very thin n-type region. Characterized the current gain > 1 can be explained in the base circuit, which does not have a 'normal ' transistor. The process is difficult to control what the strong scattering explained and in some cases even to the absence of the effect of leads (see data above). The emitter does not require treatment and forms a Schottky contact.