What is a Point-Contact Transistor

The world’s first transistor was the point-contact transistor, but it was soon replaced by the bipolar junction transistor (BJT), and today it has become a relic in museums. Its existence was so short that many textbooks ignored its existence. In this article from TechSparks, we delve into some interesting aspects of the point-contact transistor, including its birth, principles, and the reasons for its replacement.

point-contact transistor

Birth of the Point-Contact Transistor

In the early development of electronic technology, the main components were electron tubes, utilizing the characteristics of a low molecular density and minimal resistance due to collisions in a vacuum environment, allowing relatively free electron movement for amplification, rectification, and switching functions. Although these functions could be achieved, electron tubes were found to have high power consumption and a short lifespan. Scientists began searching for more advanced alternatives.

electron tubes

Scientists observed that the electron surface layer on germanium semiconductors had a certain blocking effect on the electric field. Two tiny gold foils were fixed on the semiconductor surface, achieving a conduction effect similar to electron tubes. Although people could not explain the reason at that time, the point-contact transistor was born.

This research originated from Bell Labs, but when applying for a patent, Bardeen and Brattain did not include Shockley, leading to Shockley’s dissatisfaction. Driven by resentment, Shockley designed the bipolar junction transistor, a completely new structure, in less than half a year. In comparison, the point-contact transistor was more complex and had inferior performance, resulting in its elimination. Nevertheless, the point-contact transistor is still considered the world’s first transistor.

Bell Labs

Principle of the Point-Contact Transistor

A point-contact transistor typically consists of two semiconductor materials with a point contact and a substrate. “Point contact” means that a metal needle needs to make contact with the semiconductor material. It utilizes changes in the potential difference at the point of contact to control the flow of current. When there is no input signal, the current flows through the space charge region between the point contact and the substrate.

The amplification of the point-contact transistor is achieved by adjusting the width of the space charge region. The width of this region can be controlled by adjusting the pressure on the point contact. When a small input signal is applied to the point contact, the space charge region narrows, reducing resistance, making it easier for the current to flow, achieving amplification of the input signal.

The amplified signal can be obtained at the output port connected to the load circuit of the transistor. The load circuit helps convert the amplified signal into the form required for practical applications, such as audio signals or current drivers.

Although the point-contact transistor appears bulky and rudimentary compared to modern transistors, revealing a sense of cheapness, it is more power-efficient and avoids the heating electrode dissipation issues associated with electron tubes.

Point-Contact Transistor vs Bipolar Junction Transistor

The point-contact transistor relies on the contact of a metal point with the semiconductor surface to eliminate the potential barrier through the application of an external voltage, increasing the output current. The amplification principle of the bipolar junction transistor is almost the same as that of the point-contact transistor. The difference lies in the fact that charges flow not only through the surface thin layer but also through the bulk semiconductor. It uses two semiconductor interfaces to form a sandwich-like structure: emitter, base, and collector.

Point-Contact Transistor vs Bipolar Junction Transistor

History has proven that the point-contact transistor is far inferior to the bipolar junction transistor, leading to its elimination. One of the main reasons is its low reliability. Over prolonged use, the point-contact transistor is susceptible to oxidation and chemical corrosion, reducing its stability and lifespan.

Secondly, with the rapid development of electronic device technology, the pursuit of efficiency and speed has become the mainstream trend. The point-contact transistor shows shortcomings in this regard. Its structure exhibits capacitance effects, which may result in additional capacitance at high frequencies, affecting the transistor’s frequency response. Additionally, since the current mainly flows through the space charge region at the point of contact, it limits the charge transfer speed.

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