Posted on 27 December 2019

Semiconductor Precursors


Since the advent of electrical engineering, common tasks faced by the electrical engineer have included the conversion of mechanical energy to electrical energy, conversion of electrical energy to other forms of energy such as mechanical energy, heat, light, or other electromagnetic radiation. One challenge that has been of particular importance throughout the history of electrical engineering is the conversion of alternating current into direct current and vice versa as well as signal amplification.

Various methods were used to solve these problems before the discovery of semiconductors. Below are several examples of technologies used to solve electrical problems prior to the semiconductor revolution.

Mechanical Rectifiers

Mechanical rectifiers are mechanical devices used to convert alternating current to direct current.

Needle rectifier

In a needle rectifier, a synchronous motor drives the rectifying pins in rhythm with the line frequency. Every time the line voltage is near its maximum value, the pin is positioned in such a way that the line is connected to the user end.

Synchronous motor needle rectifier

Figure 1. Needle rectifier

The needle rectifier was used in test bays at least until the 1960s.

Rotary Converter

Another example of a mechanical rectifier is the rotary converter, which can actually function as an inverter as well. A rotary converter is an electrical machine that operates similar to a combination of a motor and a generator. Rotary converters, however, have an advantage over an actual motor-generator combination since some of the electrical energy is transferred from the source to output without being converted into mechanical energy and then back to electrical energy.

An AC source is connected to a set of slip rings that are connected to the rotor windings of the DC generator. The DC generator rotates in a stationary magnetic field. This produces an alternating current in the rotor windings which is rectified by a commutator to produce a DC output.

Rotary converter

Figure 2. Rotary converter

Rotary converters are still used today for example in traction substations for conversion of 50Hz to 16 2/3 Hz alternating current.

Vacuum Tubes

An electron tube is an electronic component made of a vessel containing different electrodes with at least one anode and one cathode. The main characteristic of the vacuum tube is that the current conduction inside the tube between the anode and the cathode does not take place in an electrical conductor but instead by the flow free electrons.

Vacuum tubes

Figure 3. Vacuum tubes


Diodes are the simplest type of vacuum tube. The electrons are emitted from a heated cathode and fly through a vacuum to a positively charged anode. Current flows in only one direction since the anode, which is not heated, does not emit any electrons. Diodes, which were invented by John A. Fleming in 1904, are used to rectify current.


In addition to an anode and a cathode, a triode contains one more electrode, a control grid. The number of electrons flowing between the cathode and anode depends on the voltage applied to the control grid. Current flow through the triode can thus be controlled by the voltage on the control. Triodes are therefore mainly used for amplification of small voltages.

Vacuum tubes - diode and triode

Figure 4. Vacuum tubes: a) diode b) triode

The triode was invented in 1906 by the Austrian Robert von Lieben and the American Lee de Forest.

Hundreds of types of vacuum tubes were constructed, some with extremely complex structures. Vacuum tubes were commonly used in high power transmitters. Also, precursors of integrated circuits were realized by use of vacuum tubes in combination with resistors and capacitors.

Today, vacuum tubes have been replaced by power semiconductor elements to a large extent. Vacuum tubes are still used however in certain applications such as picture tubes in televisions or as cathode ray tubes in oscilloscopes.

Mercury Rectifiers

Mercury, or quick silver, rectifiers were the first high power rectifiers ever made.  The mercury rectifier was made up of a metal or glass vessel containing mercury and a carbon electrode. Current was transferred using the mercury vapour. Mercury rectifiers were commonly used between 1902 and 1975. The are still occasionally used today for high voltage current transmission.

mercury rectifier

Figure 5. Mercury Rectifier

A thyratron, which is a variation of the mercury rectifier, can be used for phase angle control in much the same way as a thyristor.


Figure 6. Thyratron

The mercury HVDC converter "Nelson River" in Canada was the largest of its kind. These mercury rectifiers were used between 1971 and 1975 and are still in partial use today. Part of the plant's components have been refitted with thyristors. The plant can handle a reverse voltage of 150kV and a maximum current of 1800A.


For more information, please read:

What is a Semiconductor?

Rectifier Diode Basics

P-N Junction Diode

Thyristor Basics


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