- Published 14 Jun 2023
- Last Modified 25 Mar 2024
- 8 min
Guide to Discrete Semiconductors
What are discrete semiconductors and how do they work?
It’s hard to imagine a world without discrete semiconductors. In this guide, we’re looking at this vital component in a whole host of circuit types. We’ll cover what they are, how they differ from regular semiconductors, and the various types.
Discrete Semiconductor Definition
What is a Discrete Semiconductor?
When we’re asking what a semiconductor is, we first need to look at three definitions used in electrical hardware: conductor, insulator and semiconductor.
- Conductor: a material that allows the flow of electrons, and therefore an electric current, when a voltage is applied. A conductor has low resistance. Some conductors are better than others at their job, which is why we see them used in wiring. Copper and aluminium are good conductors, for example. Silver and gold are among the best, but they are also precious metals, which limits their use economically. Conductors get less effective as they get hotter
- Insulator: a material that has high resistance, to the point where it can be used to stop an electric current from flowing. Plastic, rubber, glass and wood are good insulators. Plastic is most commonly used to insulate wiring because it’s also flexible and waterproof
- Semiconductor: a material that falls between good conductors and good insulators. That is, they will have low resistance in some circumstances and high resistance in others, or they’ll simply conduct electricity, but not very efficiently. Those qualities mean they are useful in circuits as they can be used to perform actions like amplifying or switching, or manipulating the electric current in some way. Examples of good semiconductors include silicon, germanium and tin, but when made into compounds or alloys, there are dozens of potential semiconductor materials. Semiconductors increase their conductivity as they get hotter
That brings us to discrete semiconductors. As the name suggests, they are a specific type of semiconductor. In this case, it means a semiconductor device that has just one fundamental function, rather than more complex functions, as would be the case with an integrated circuit semiconductor.
Examples of discrete semiconductors include thyristors, Zener diodes, bipolar transistors, bridge rectifiers and others – we’ll go into more detail on the various types below.
Discrete semiconductors include a variety of components, including transistors, valves, rectifiers, and more. Some are sold as modular accessories that can be easily integrated into existing circuits.
Some discrete semiconductors are constructed in a way that lets them perform a function similar to what two different devices hooked together in a certain configuration would perform. This allows space to be saved on a circuit board and access to key functionalities that would otherwise require multiple components by simply providing a circuit with a single component.
In discrete semiconductors, much of their functionality depends upon the arrangement of the semiconductors within the device. Thyristors, for example, have four layers of semiconducting material with n- and p-type materials sandwiched together in alternating layers.
Types of Discrete Semiconductors
Discrete semiconductors are very basic parts of electrical engineering and there are numerous types. Here are some of the most common types of discrete semiconductors available for circuits, with a brief description of their tasks.
Thyristors
A thyristor is a semiconductor with two stable states – it is “bistable”. Inside any thyristor are four layers of semiconducting material, consisting of alternating layers of n- and p-type semiconductors. The state can be changed by applying or removing a current. However, once changed, it remains in that state until it receives another signal. It can therefore be used as a switch or a latch. Some of their everyday uses are dimmer switches and power controllers.
Zener Diodes
Zener diodes allow the flow of electric current in the opposite direction to normal, in response to the current reaching a certain voltage (known as the Zener voltage). They are available in different Zener voltages or variable voltage designs. Zener diodes are commonly used in roles where they provide a limiting factor on voltage, and protection against too much voltage on a circuit or a voltage reference. These are very common components across electronic devices and are particularly useful in power regulation and circuit overload protection.
Bipolar Transistors
These devices are frequently incorporated into analogue circuits. A bipolar transistor in a circuit can be used as a switch by allowing a large voltage to pass through it when a small signal voltage is applied to it. They are termed bipolar because they have carriers that utilise both negative and positive charges. In semiconductors, this means that they have both electrons and holes involved in their operation. Bipolar transistors are used in integrated circuits heavily but are also sold as discrete units.
Bridge Rectifiers
A bridge rectifier is built out of diodes arranged in a specific configuration - the namesake bridge. This bridge converts an input of alternating current (AC) into direct current (DC), a basic function for most electronic devices. These devices work with a two-wire AC input, have two diode drops and offer full-wave rectification.
MOSFET Transistors
MOSFET stands for Metal Oxide Semiconductor Field-Effect Transistor. This type of transistor is similar to a bipolar transistor in that there’s an input voltage. However, with a MOSFET transistor, that input voltage can be tiny, compared to the larger voltage required by the bipolar type.
These are field-effect transistors with four terminals, though three terminals are most commonly hooked up. In many cases, two terminals are short-circuited within the device. This arrangement, in fact, is so common that electrical diagrams show three terminals. The devices include body, drain, gate, and source terminals in their designs. They are the most common transistors in use.
TRIACs
A TRIAC (Triode for Alternating Current) is a type of thyristor. TRIACs function as electronic switches, being bidirectional and allowing current to be conducted across them in either direction. They allow current to flow in both directions once triggered by an input voltage. When it has been switched, it will remain in that state until another trigger switches it. The devices have similar functionality to a pair of silicone-controlled rectifiers in an inverse parallel configuration and connected together at each of the rectifier's gates.
JFET Transistors
Junction-gate Field-Effect Transistors (JFETs) are field-effect transistors that use three terminals to control current flow in a circuit. The three terminals are source, drain, and gate. The first two are the main channels through which current flows. The gate is the means by which current is pinched, either to restrict or block it altogether. They are frequently used in switching applications, though they can also be used to provide resistance dependent on voltage.
Schottky Diodes
A Schottky diode is a device made by fixing a semiconductor to a metal and is named after the German physicist Walter Schottky who theorised and experimented with the concept back in the 1930s. When used as a switch, Schottky diodes benefit from very fast switching times and low turn-on voltage. They’re useful in all manner of RF applications and power rectifiers, and also for preventing power discharging from batteries used with solar panels when the sun is not shining on them.
DIACs
A DIAC is a Diode for Alternating Current. Like the TRIAC above, it is used as a switch and will retain its state until the signal voltage falls below a predetermined holding current, at which point it will revert to a high resistance state, cutting off the circuit. They find uses in dimmer switches and power overload protection systems.
Varactor Diodes
Varactor diodes are also known as Varicap diodes, variable capacitance diodes, and tuning diodes. These semiconductor devices are used as capacitors controlled by the voltage applied to them. They are particularly useful in transmitters, TV sets, phones, voltage-controlled oscillators, and other devices that utilise FM transmitters or receive FM transmissions. These devices change their capacitance as the voltage applied to them increases or decreases. They will only allow AC to flow through them, not DC, with the specific characteristics determined by the doping profile.
How Do Discrete Semiconductors Differ from Other Semiconductors?
Many types of semiconductors are sold as part of circuits, often on an IC. These circuits are usually capable of carrying out any of a number of different functions in a device. This differentiates them substantially from discrete semiconductors.
A discrete semiconductor has a basic function that cannot be divided into other functions. An IC, for example, may have a transistor, a diode, and other components on it. These components can all perform different functions on their own or, working in conjunction as a circuit, can complete several functions together. A discrete semiconductor, however, can only perform one function. For instance, a transistor is always a transistor and can only perform transistor functions.
In today's world, most semiconductors are purchased as part of integrated circuits. For some applications, however, a discrete semiconductor provides the most appropriate possible solution to an engineering need. Therefore, discrete semiconductors have a crucial role in the electronic components marketplace.
As these are discrete devices, some of them need to be purchased along with other devices to achieve the desired functionality for which they are intended in any given circuit. Semiconductor devices perform functions ranging from regulating power to working as switches, and more. For some engineering needs, the ability to purchase discrete semiconductors makes prototyping and production affordable.
