What is a Transistor and How does it work?

A transistor is a semiconductor gadget used for amplifying or switching electronic signals and electrical power.

It is built with a single piece of semiconductor material and consists of three terminals connected to the external circuit. If you provide any voltage through one end of a transistor, it changes the current flow through the rest of the terminals.

Transistors are an essential part of modern electronics. Without the invention of transistors, modern-day advanced electronics would not have reached where it is today.

Transistors use power to control power, and the power which is controlled (output power) is relatively much more than the power controlling (output power). So, transistors supply signal amplification.

Nowadays, industries manufacture transistors individually or as a whole together in an IC (Integrated Circuit).

History & discovery of transistor

The inventors of the transistor, Walter Brattain, John Bardeen, and William Shockley, received the Nobel Prize for physics because of their revolutionary invention back in 1956.

The three physicists had discovered the “transistor effect” by crisscrossing multiple layers of the element, Germanium (Symbol – ‘Ge‘) with silicon and then crossing them with the help of current. 

The three physicists were probably not aware of the weight of their discovery back then and were only helpful in FM radios. But, in the later years, the invention of the transistor paved the way for the rise of microelectronics.

The impact was so immense that from its discovery, till today we have manufactured and utilized nearly about sextillion (One with twenty-one zeros, 1000000000000000000000) transistors. 

Transistors have brought us everything from radios to smartphones. Within ten years of its invention, Gordon Moore predicted that the processing capability of transistors would double every eighteen months. Hence, the American computer scientist and entrepreneur stated “Moore’s first law” proving to be accurate in later years and went down in history.

However, the transistors evolved very swiftly and were used from radios to integrated circuits to logic gates. 

In the 1970s, Intel introduced newer technologies like lithography, the world’s earliest processor, and initial integrated memory. These were based on silicon transistors. But over the years, with the help of ion implantation, the manufacturing speed and quality of transistors have grown rapidly.

What will be the future of transistors?

Thanks to the research and progress of science, we already have the model for “tri-gate” transistors or known as 3D transistors. These will be much more efficient by having smaller dimensions, reduced energy absorption, and less power consumption. Hence, we can speculate that superconductors will be the future of transistors.

How does a transistor work?

Transistors are designed in such a way as to make them work as an amplifier or a switch. It is done by utilizing a tiny amount of electricity. With this electricity, the gate is controlled and maintains a larger electricity supply. The process is very similar to that of a valve controlling the speed and supply of water.

Transistors are made up of three parts – collector, base, and emitter. The base controls the supply of larger electricity while the collector carries it to the emitter. The emitter is the vent for that large electrical supply. The base adjusts and sends different current levels and thus maintains the amount of current that flows through the gate. Therefore, a tiny amount of current is only needed to maintain and control the massive amount of current.

This exact procedure is implemented while making binary code to use in digital processors. Though in this case, five volts of voltage threshold is used to unlock the collector gate. It is how we utilize transistor as a switch with binary function – “The switch is ON if the voltage supply is five volts and the switch is OFF when the voltage supply is less than five volts.”

Types of transistors

Vector isometric transistor and electronic parts icons infographic connection concept illustration

Transistors mainly fall into two main types depending on how they are constructed. Those two types are

  • Bipolar Junction Transistors (BJT) and,
  • Field Effect Transistors (FET)

There are a few more types which we will be seeing in a minute.

No matter which classification a transistor belongs to, every transistor has a specific arrangement of various types of semiconductors. The most widely used semiconductors for manufacturing transistors are Germanium, Silicon, and Gallium-Arsenide.

BJT and FET differ from each other on the basis that bipolar junction transistors need both majority and minority charge carriers to work. In contrast, Field-Effect Transistors only need the majority of charge carriers to operate.

Depending on these characteristics and properties, few transistors are used for switching purposes (MOSFETs), while the other transistors are used for amplification purposes (BJTs).

However, some specific types of transistors can be used for both amplification and switching purposes.

There are hundreds of types of transistors depending on their structure and making. So, it’s next to impossible to list all those. Here are the all popular types of transistors that are in use in the industry right now.

Bipolar Junction Transistor

Image
a) PNP schematic symbol, (b) layout (c) NPN schematic symbol, (d) layout. Source

Bipolar Junction Transistors (BJTs) are sometimes called Junction Transistors. The term “bipolar” included in the name refers to the fact that this type of transistor needs both electrons and holes for current conduction, and the term “junction” in the name refers to the fact that it contains PN Junction (both junctions). The Bipolar Junction Transistors have three terminals or gates, viz. Emitter (E), Base (B), and Collector (C). The BJT transistors are classified into two different types of transistors depending on their build. Those two types are NPN transistors and PNP transistors.

1. NPN transistors 

NPN transistor is named so as it contains two n-type semiconductor type materials and one p-type semiconductor material. The p-type semiconductor layer is thin and separates the two n-type semiconductors. Therefore, in NPN transistors, the electrons from the emitter to the collector carry a larger bulk of charge. 

2. PNP transistors 

PNP transistors contain one n-type semiconductor type material and two p-type semiconductor materials, thus the name PNP transistor. The n-type semiconductor layer is thin and separates the two p-type semiconductor materials. In NPN transistors, a larger bulk of charge is carried by the holes from the emitter to the collector, while the electrons carry a small charge. 

Field Effect Transistor

The Field Effect Transistor is another widely used transistor type. Just like the Bipolar Junction Transistors (BJTs), FETs also have three terminals. Those are called Gate (G), Drain (D), and Source (S). These transistors fall under Junction Field Effect transistors (JFETs) and Insulated Gate Field Effect Transistors (IG-FETs) or Metal Oxide Semiconductor Field Effect Transistors (MOSFETs).

Another fourth terminal is also used to connect it to the circuit, called the Base or Substrate. The Field Effect Transistors can adjust and modify the shape and size of a channel in between the Source (S) and the Drain (D), and it is done by applying a voltage at the gate (G) terminal. 

Junction-Field Effect Transistor

The Junction-Field Effect Transistors (JFETs) are probably the simplest and earliest Field-Effect Transistors. These are used as amplifiers, resistors, and switches and are voltage-controlled devices. JEFTs do not require basing current to operate. The Junction-Field Effect Transistors can also be categorized into two main subtypes viz. N–Channel and P–Channel.

N–Channel Junction-Field Effect Transistors

N-Channel JEFTs work due to the flow of electrons. A channel is made in the transistor when voltage is applied between gate (G) and Source (S). 

P–Channel Junction-Field Effect Transistors

P-Channel JEFTs work due to the flow of holes. A channel is made in the transistor when voltage is applied between Drain (D) and Source (S). 

Metal Oxide Semiconductor Field Effect Transistor

Image
Source

Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is the most famous transistor and is most widely used in the present-day industry.

The gate part of the transistor and the channel have a thin layer of metal oxide (usually, SiO2) between them. Hence, MOSFET is also called Insulated Gate Field Effect Transistors since the gate has insulation from the other region.

In addition, this transistor has an extra terminal called Body or Substrate, and this is the most important Semiconductor (Silicon) in which the Field Effect Transistor is forged. Metal Oxide Semiconductor Field Effect Transistors are so popular nowadays due to their low output impedance and high input impedance.

Mohamed M. Atalla and Dawon Kahng invented the MOSFET in 1959.

We can classify MOSFET as N–Channel transistors and P–Channel transistors. 

Applications and Uses of Transistor

  • Transistors are most widely used as switches and amplifiers.
  • Dual Gate MOSFETs are utilized in RF mixers/multipliers and RF amplifiers where two controlled gates are needed, and we put them in series connections.
  • Avalanche Transistors are used to convert high currents in nanosecond transition times or sometimes even less.
  • Transistors working as switches are used to make memory cards for mobile phones.
  • Heterojunction Bipolar Transistors (HBTs) are used in microwave communications since they give a high switching speed and can pick up signals of varying frequencies.
  • Isolated Gate Bipolar Transistors (IGBTs) are used as switches in appliances like trains, electric cars, refrigerators, air conditioners, and much more.

What’s next for Transistors?

With the advancement of sciences, many more transistors came into existence and are widely used in all fields of science and technology. We can also classify the transistors based on function. That includes – small-signal transistors, Small switching transistors, Power Transistors, High-frequency transistors, Phototransistors, and UniJunction Transistors (UJTs).

Maybe in the future, it would be quite possible to make transistors out of Titanium and Aluminum. It would be much more profitable as these are abundant, cheap, and metallic. Thus, It is our small explanation on transistors though the topic has more depth and variety.

We hope that it will help you understand what a transistor is and know its importance. 

Gaurav Tiwari

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