The Key Difference Between Transistors and Thyristors

While much of the world has at least heard the term “transistor” before, their function and importance aren’t exactly common knowledge. Knowledge of thyristors and their purpose is arguably even more vague and yet, both diminutive electrical components are integral to many of the electric-powered devices that are the backbone of our societies and economies. Though transistors and thyristors do not represent the 'flashiest’ of technologies, they are nonetheless crucial components of today’s electronic-driven world.

Whether you plan on making a career out of working with electronic devices or have an interest in how the world around you works, getting familiar with transistors and thyristors is a good place to start. In this article, we’ll define both components and list the main differences between them.

What is a Transistor and What Does it Do? 

A transistor is an electronic component that is both small and generic looking enough that it may not seem to be all that impressive to the uninitiated eye. Fortunately, the size and “plainness” of these components is by no means an indicator of their importance. In truth, a transistor is very much an integral electronic component as any other contained within an electronic device. The transistor can play two very important roles – as an amplifier or a switch. In the former role, a transistor can take a small amount of input current and turn it into a greater amount of output current, effectively acting as a current “booster”. Using a transistor as a booster has many industrial uses. Consider for a moment a large scale plant that relies on optical fiber communication between field devices and the control room. Transistors in a receiver can be used to boost the signal, creating better quality data. 

As a switch, transistors perform an equally important task. For example, a small amount of current entering one part of the transistor can allow a much larger current to pass through another part of it. In this way, a transistor is operating as a “switch”. 

The Different Types of Transistors

There are many, many types of transistors to become familiar with. Below is a brief description of a selection of them. 

• Bipolar Junction Transistor, or BJT: These types of transistors are controlled by current and come in two different types PNP (Positive-Negative-Positive), or NPN (Negative-Positive-Negative). Depending on the type, the current will either flow from the collector to the emitter (NPN) or from the emitter to the collector (PNP).
   
• Diffusion Transistor: Diffusion transistors are a form of BJT transistors. These are formed by precisely diffusing impurities into a semiconductive material as a way of altering the electrical properties of the transistor. 
   
• Avalanche Transistor: Another type of Bipolar Junction Transistors, avalanche transistors operate in avalanche breakdown region, meaning that the transistor has the ability to switch very high currents exceptionally rapidly, requiring less than a nanosecond to rise and fall.
   
• Schottky Transistor: This is a transistor that is combined with a Schottky diode, which prevents the saturation of the transistor by the diversion of extremely high input current.
   
• Darlington Transistor: Darlington transistors are made by combining two different transistors. In doing so, the Darlington transistor has a significantly greater ability to gain current.
   
• Field Effect Transistor: These transistors are controlled by voltage. They have high impedance, which tempers the amount of current that passes through them. Field effect transistors can not operate as an amplifier, unlike other BJTs.
   
• Multiple Emitter Transistor: In this type of transistor that is generally used at the inputs of integrated circuit TTL NAND logic gates, with input signals being applied to all emitters. 

What is a Thyristor and How Does it Work? 

Thyristors are made of four layers of semiconductive materials. This “wafer” of materials consists of alternating positive and negative-type layers, resulting in a P-N-P-N configuration. Like transistors, thyristors are used to perform switching operations, but unlike transistors, thyristors can be fully switched on or off (transistors for their part, operate between on and off states). 

The three states of thyristor operation include forward conducting, forward blocking, and reverse blocking. The state in which they operate will vary based on the application. 

• Forward conducting state:  The forward blocking state is generally considered to be a thyristor’s primary operational state. When in this mode, the thyristor will change and remain in a conducting mode until the current that is being applied falls below the holding current.
   
• Forward blocking state: In the forward blocking state, the thyristor will effectively block the flow of current regardless of the voltage being applied in the direction that would normally signal a diode to conduct the voltage.
   
• Reverse blocking state: In this state, if the current tries to navigate through the thyristor in the opposite direction it will be blocked by a diode which in turn will prohibit thyristor activation.
   

Additionally, thyristors can also perform as a circuit breaker in device power circuits. In doing so, they can prevent disruptions to power supplies when the thyristor gate is adjoined with a Zener diode. In the event voltage exceeds that of the Zener voltage, the thyristor will automatically turn off the supply of power to the ground and will activate a circuit breaker upstream of the power supply.

The Different Types of Thyristors 

Like transistors, there are several different types of thyristors. The most common of which are outlined below:

•  Silicon-Controlled Rectifier (or SCR): The switching application of this type of thyristor works like this. In the event the cathode is negatively charged relative to the thyristor’s anode, current will be prohibited from flowing until a pulse is applied to the thyristor’s gate. At that point, the SCR thyristor will conduct current between the anode and cathode until the voltage between the two is either reversed or falls below the holding current. 

• Reverse Conducting Thyristor (or RCT): The RTC will typically only allow current to travel in the forward direction and prevent the passage of reverse direction currents. Having said that, RCTs do allow electricity to be conducted in the reverse direction (with an improved commutation) because they include an integrated SCR with a reverse diode - this eliminates the undesired loop inductance and reduces reverse voltage transients.
   
• Light Activated Silicon-Controlled Rectifier (or LASCRs): When light particles strike the reverse-biased junction, the number of electron/hole pairs in the transistor goes up. If the light’s intensity exceeds the thyristor’s critical value, the thyristor will be put into the on state. 

Main Points of Difference between Transistors and Thyristors

Thyristors, when compared to transistors, have a specific design that allows them to operate using higher currents and voltages. Since thyristors can withstand such high-power loads, they are better suited than transistors in high powered applications (this of course means that since transistors function best in low voltage and current, they are better suited for low-power applications). For example, thyristors are better suited for things like high voltage power lines or power switches in a factory, whereas transistors are most commonly used in computer memory chips. 

As for composition, transistors have three semiconductive layers made up of N-Type and P-Type materials, compared to the four-layer composition found in thyristors. Thyristors require more time than transistors to alternate between their respective on/off states, are more expensive to manufacture, and are relatively heavier compared to transistors. 
 

Small Components, Large Impact

It’s clear that even the smallest components of electronic devices can have a large impact on how the world works. Two of those devices, transistors and thyristors, while small, are integral to protecting and powering the circuits in the devices and machinery that have built modern societies.

Knowing your way around a circuit and the components therein is integral to understanding larger and more complex technologies. If you’d like to learn more about transistors and thyristors, consider reviewing our the Electromechanical Technician Certificate Program.