Physics · Ch 14 — Semiconductor Electronics: Materials, Devices and Simple Circuits
Introduction
Introduction
14.1 Introduction
The story of modern electronics begins with a simple question: how do we control the flow of electrons? Every electronic circuit, from a pocket radio to a supercomputer, relies on devices that can regulate this flow with precision. Before 1948, the only practical way to do this was with vacuum tubes — bulky glass envelopes from which most of the air had been removed.
The Era of Vacuum Tubes
A vacuum tube works by heating a metal cathode until it glows red-hot. The heat gives electrons enough energy to escape the metal surface — a process called thermionic emission. These freed electrons then travel through the evacuated space toward a positively charged anode (also called the plate). By varying the voltage between the electrodes, you can control how many electrons make the journey.
The simplest vacuum tube is the vacuum diode, which has exactly two electrodes: a cathode and an anode. The triode adds a third electrode called a grid, placed between cathode and plate. A small voltage on the grid can control a much larger current from cathode to plate — this is the basis of amplification. More complex tubes followed: the tetrode (four electrodes) and pentode (five electrodes).
In a vacuum tube, electrons can only flow from the cathode to the anode — never the reverse. This one-way behaviour is why these devices are often called valves, like the valves that allow water to flow in only one direction through a pipe.
Why must the space between electrodes be evacuated? If air molecules were present, the fast-moving electrons would collide with them, losing energy and scattering in random directions. The vacuum ensures a clean, controlled path.
The Drawbacks of Vacuum Tubes
Despite their revolutionary impact, vacuum tubes had serious limitations:
- Bulky — each tube is a glass envelope the size of a small light bulb
- High power consumption — the cathode must be kept hot continuously, wasting energy as heat
- High operating voltages — typically around 100 V or more
- Limited lifetime — the hot cathode gradually wears out
- Low reliability — tubes are fragile and prone to failure
These problems drove engineers to search for a better alternative.
The Solid-State Revolution
The seeds of that alternative were planted in the 1930s, when researchers realised that certain solid materials — semiconductors — offered something remarkable. Unlike the vacuum in a tube, where electrons must be boiled off a hot cathode and travel through empty space, semiconductors allow charge carriers to move within the solid itself. No heating, no vacuum, no bulky envelopes.
What makes semiconductors special is that their electrical conductivity can be changed dramatically by simple external influences:
- Light — shining light on a semiconductor can increase its conductivity
- Heat — raising the temperature creates more mobile charge carriers
- Small applied voltages — a tiny voltage can control the flow of a much larger current
This last property — the ability of a small signal to control a large current — is exactly what made vacuum tubes useful for amplification. But semiconductors do it without the heat, the bulk, or the high voltages.
In vacuum tubes, electrons are supplied by a heated cathode and flow through an evacuated space. In semiconductor devices, charge carriers exist within the solid material itself and move through it. This fundamental difference — internal versus external charge supply — is what makes semiconductor devices smaller, cooler, more efficient, and more reliable.
The First Semiconductor Device
Long before the physics was fully understood, a crude semiconductor device was already in use. A naturally occurring crystal of galena (lead sulphide, ) with a metal point contact attached to it was used as a detector of radio waves.
Modern Semiconductor Devices
The devices we will study in this chapter are the direct descendants of that galena crystal, but built on a deep understanding of quantum mechanics and solid-state physics. The two most important are:
- The junction diode — a two-electrode device that allows current to flow easily in one direction but blocks it in the opposite direction
- The bipolar junction transistor — a three-electrode device that can amplify weak signals
We will also look at simple circuits that use these devices, showing how the theoretical principles translate into practical applications.
| Feature | Vacuum Tubes | Semiconductor Devices |
|---------|-------------|----------------------|
| Charge source | Heated cathode | Within the solid |
| Medium | Evacuated space | Solid crystal |
| Size | Bulky | Very small |
| Power consumption | High | Low |
| Operating voltage | ~100 V | A few volts |
| Lifetime | Limited | Long |
| Reliability | Low | High |
The replacement of vacuum tubes by semiconductor devices is not just a historical curiosity — it is happening even now. The cathode ray tubes (CRTs) that once dominated television and computer monitors are being replaced by liquid crystal displays (LCDs) driven by solid-state electronics. The same principle — controlling charge flow within a solid rather than in a vacuum — underlies everything from smartphone processors to solar cells.