Concept of Semiconductor Electronics

Devices in which a controlled flow of electrons can be obtained are the basic building blocks of electronic circuits. Before the discovery of the transistor in 1948, such devices were mainly vacuum tubes, also called valves.

These vacuum tube devices included the vacuum diode, triode, tetrode, and pentode. The vacuum diode has two electrodes, the anode and cathode, while the triode has three electrodes: cathode, plate, and grid; the tetrode and pentode have four and five electrodes, respectively.

In a vacuum tube, electrons are supplied by a heated cathode. The controlled flow of these electrons in vacuum is obtained by varying the voltage between the different electrodes.

A vacuum is necessary in the inter-electrode space; otherwise, moving electrons may lose energy by colliding with air molecules in their path. In these devices, electrons can flow only from the cathode to the anode, that is, only in one direction. Therefore, such devices are generally referred to as valves.

Types of Vacuum Tubes:

• Vacuum diode: a two-electrode device with an anode and a cathode.
• Triode: a three-electrode device with a cathode, plate, and grid.
• Tetrode: a four-electrode device.
• Pentode: a five-electrode device.

The provided content states that vacuum tube devices have the following drawbacks:

• They are bulky.
• They consume high power.
• They generally operate at high voltages of about 100 V.
• They have a limited life.
• They have low reliability.

The development of modern solid-state semiconductor electronics goes back to the 1930s. At that time, it was realised that some solid-state semiconductors and their junctions offered the possibility of controlling both the number and the direction of flow of charge carriers through them.

Simple excitations such as light, heat, or small applied voltage can change the number of mobile charges in a semiconductor.

In semiconductor devices, the supply and flow of charge carriers are within the solid itself. This is different from vacuum tubes, where mobile electrons are obtained from a heated cathode and made to flow in an evacuated space or vacuum.

The content clearly states that semiconductor devices do not require external heating or a large evacuated space. It also states that they are small, consume low power, operate at low voltages, and have long lifetimes and high reliability.

The material also mentions that, much before the full implications of semiconductor devices were formally understood, a naturally occurring crystal of galena, or lead sulphide (PbS), with a metal point contact attached to it, was used as a detector of radio waves.

Even Cathode Ray Tubes (CRT), which work on the principle of vacuum tubes and were used in television and computer monitors, are being replaced by Liquid Crystal Display (LCD) monitors with supporting solid-state electronics.

• Electronic circuits are built using devices that allow controlled flow of electrons.
• Before 1948, vacuum tubes or valves were commonly used.
• Electrons in vacuum tubes come from a heated cathode and move in the vacuum.
• Vacuum tube devices are bulky, high-power, high-voltage, with limited lifetime and low reliability.
• Semiconductor devices work within the solid itself.
• Semiconductor devices do not require external heating or a large evacuated space.
• Semiconductor devices are small, low-power, low-voltage, long-life, and highly reliable.