“Is a resistor a semiconductor?” is a question many people have, and the answers you’re looking for are right here. Keep reading to find out what resistors are, how current flows through them, and what their connection to semiconductor devices is. We’ll also briefly touch on how the resistance value of resistors is calculated, the different types of resistors available for use in electronic circuits, as well as some of their applications. Let’s get to it!
Conductors, Insulators, And Semiconductors
Before we talk about what resistors are, let’s look at how solid materials are categorized in terms of how they allow or prevent an electric current flow through them.
Current Flow Through Conductors
Firstly we have conductors which naturally have free electrons that can act as charge carriers. They will begin to move when a voltage is applied, creating an electric current. Metals like copper, silver, and gold are examples of conductors.
Current Flow Through Insulators
Insulators don’t naturally have mobile electrons and do not conduct a current when a voltage is applied under normal circumstances. However, insulators do break down when a sufficiently high voltage is applied. This tears away some of the electrons from their atoms, making them mobile and able to conduct a current. Rubber, glass, and plastics are examples of insulators.
Current Flow Through Semiconductor Material
Semiconductors are solid materials that exhibit conductivity properties that lie between that of a conductor and an insulator. Devices made from such materials can restrict a current flow in one direction of the circuit while permitting it in the other. They also have other characteristics like sensitivity to light both visible and other parts of the electromagnetic spectrum. Silicon and Germanium have commonly been used as semiconductor materials.
Semiconductors can be doped, where another element is added to change the electrical properties by altering the crystalline structure of the original material. For example, Phosphorus can be added to Silicon which results in extra electrons (negative charge carriers), creating what is called an ‘n-type semiconductor’. Or Boron can be added instead to end up with a ‘p-type semiconductor’ which has mobile holes (positive charge carriers).
These two types of semiconductors can be used to make a p-n junction, one of the most elemental building blocks that make up a modern integrated circuit. This junction is what allows a current to flow in one direction, but restricts the flow in the opposite direction. Two junctions like this are used in a bipolar junction transistor.
Current Flow Through Other Types Of Materials
There’s another group of elements called metalloids which exhibit both metallic and non-metallic properties. Some of them like Silicon and Germanium are semiconductors, while others like Arsenic and Antimony are not. However, they too are essential elements needed for semiconductor manufacturing. To find out more about metalloids, read Why Metalloids Are Useful As Semiconductors In Modern Electronics.
What Is A Resistor?
Electrical resistance is how much a material opposes the flow of an electric current through it. Resistance is measured in ohm which is denoted by the Greek letter Ω. Conductors like copper have a very low resistance, while insulators have a very high resistance due to their electrical properties as discussed earlier. Semiconductors display variable resistance.
A simple resistor is a passive electrical component with two terminals and has a fixed resistance. It can be used to restrict current flow in a circuit, divide voltages, and more depending on how they are connected. This fixed resistance can change slightly with temperature or voltage. There can also be resistors with variable resistance.
Ohm’s law is what describes the behavior of an ideal resistor. According to this, the potential difference or voltage drop across the resistor is directly proportional to the current passing through it. Apart from the resistance they can add to a circuit, resistors are also rated for the power dissipated within them during current flow.
Axial lead resistors are some of the most commonly found discrete resistors used in electronic circuits. They can be placed in series or parallel throughout a circuit which changes how much resistance they ultimately contribute. Look at the circuit diagram below to see how a resistor achieves its basic function as an electronic component.
Is Resistor A Semiconductor?
While semiconductors and materials used to build resistors have resistance, they are not the same thing. Normally the function of a fixed resistor is to provide a fixed resistance. It is a passive component that consumes energy and cannot amplify or control the flow of current. A resistor opposes current flow equally in both directions, meaning it is not polarity sensitive. Its two terminals are identical.
A semiconductor component like a diode on the other hand is an active component. At a low potential difference, its resistance is very high. However, when a sufficient forward bias voltage is applied, the resistance drops dramatically in one direction, while resistance increases in the other. When forward-biased, the diode conducts, but when reverse-biased, it cannot conduct. This makes it a polarity-sensitive device, and how you apply voltage to its terminals makes a difference.
Furthermore, semiconductor devices like transistors can even amplify the current flowing through them. In conclusion, while resistance plays an important role in both types of components, their function within a circuit is not the same. Therefore a resistor is not the same as a semiconductor component.
Type Of Fixed Resistors
There are many types of fixed resistors used in the electronic industry. Let’s look at a few of them.
Carbon Composition Resistors
Carbon composition resistors (CCR) come cylindrical with embedded wire leads or metal end caps to connect them. The material that acts as the resistive element is a finely powdered carbon mixed with an insulating material like ceramic using resin. They were used mainly during the 1960s although they have been replaced by better resistors that provide better voltage dependence and tolerance.
Their main applications are surge protection and voltage pulse reduction because CCRs are non-inductive. Today, they are mainly used in power supplies, and as essential components used to refurbish vintage electronics. And finally, CCR can be printed onto circuit board substrates during PCB manufacture. For more on printed circuit boards, read The Consumer PCB And Why It’s Used For Consumer Electronics Applications.
Carbon Film Resistors
The manufacturing process of carbon film resistors is interesting. The carbon film is deposited on a substrate of insulating material. Then a helix shape is cut along it to create the long and narrow resistive path for the component. Since various shapes could be cut, a wide range of resistance values can be obtained. They also provide lower noise than carbon composition resistors.
Thick And Thin Film Resistors
Thick film resistors were very popular in the 1970s with most of the surface mount device (SMD) type resistors used today being made like this. Thin film resistors are a thousand times thinner than thick film resistors, and they differ in how the film is applied during their production.
Thin film resistors are made by vacuum deposition of the resistive element onto a substrate which is then etched to get the desired pattern much like in semiconductor or printed circuit board manufacturing. Screen and stencil printing methods are used for thick film resistors. Ceramic conductors also known as ‘cermet’ are used for the resistive material.
Metal Film Resistors
Many axial-leaded resistors use metal film as the resistive material. They are often coated with Nickel Chromium or cermet materials and may be manufactured in several different ways. It can be done similarly to thin film resistors but is sometimes done by cutting the helix-like we saw in carbon film resistors. Metal film resistors are known for their long-term stability, their tolerance, decent temperature coefficient, and lower noise. For high endurance demands that require resistors to work in extreme temperatures, and be reliable and stable, the metal oxide film can be used as well.
As the name suggests, these resistors are made by winding a Nichrome wire around a core made of ceramic, fiberglass, or plastic material. Then a layer of protective enamel is baked into this, and these resistors are capable of withstanding extremely high temperatures. A ceramic or aluminum outer with an insulating layer casing is added next. Wirewound resistors are typically used in high-power industrial applications such as circuit breakers, fuses, heat sinks, and even dynamic braking systems of electric railway cars.
Other Fixed Resistors
On top of the ones we discussed, there are others as well. There are the metal foil resistors developed by Felix Zandman and Sidney J. Stein which brought precision and stability to resistor design in the 1960s with a low-temperature coefficient. There are also ammeter shunts that are useful for measuring a larger current than what is traditionally possible with a current measuring device, grid resistors used in cranes and industrial equipment, and carbon pile resistors used for testing automotive batteries and radio transmitters.
Variable Resistor Types
Variable resistors allow users to change their resistance through different methods. This could be as simple as having separate tapping points, or even a sliding tapping point along the resistive material. A device like this with two terminals is called a ‘rheostat’. A resistor with three terminals with an adjustable tapping point connected to a radiating shaft or knob is called a ‘potentiometer’ since its primary function is to divide the voltage. The standard volume knob on any audio device is essentially a potentiometer.
There are thermistors whose resistance varies with temperature, making them useful in measuring it. Similarly, humistors have a resistance that varies with humidity, and photoresistors that vary with illumination.
Want To Learn More About Semiconductors?
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No. Resistors are passive components used in circuits to reduce the flow of current through them. They cannot amplify or control the direction of the current. Semiconductor components like diodes are made from semiconductor materials. They allow a current to flow in one direction when a forward bias is provided while opposing current in the opposite direction. They are also active components since transistors can amplify the current.
Fixed resistors come in a variety of types including carbon composition, carbon film, cermet thick film, cermet thin film, metal foil, metal film, wire-wound, and grid resistors to name a few. Rheostats, potentiometers, thermistors, humistors, and photoresistors are a few examples of variable resistors.