Transistor | Definition, Types, Symbols, and History

The transistor is considered as the biggest invention of the 20^th century in electronics. The building block of modern embedded systems is a transistor that acts as a fundamental component to control the processing of data.

The transistor is a three-terminal semiconductor device that controls the flow of current. When voltage or current is applied to the input terminals of a transistor, it controls or amplifies the input signal to generate an output signal. It is made up of semiconductor materials such as germanium or silicon. A transistor is an essential component in integrated circuits (ICs), microprocessors, and microcontrollers, and in almost all modern electronics. This article will take you to the world of transistors, their types, symbols, and unfolding where the transistor journey has begun!

What is a Transistor?

A transistor is an electronic device that controls the flow of current. A typical transistor has two modes of operation, either acting as a switch or as an amplification device. In switch mode, it either allows or permits the flow of current. Whereas, in amplification mode, it amplifies the small input signal to generate a bigger output signal.

A transistor is made up of three layers of semiconductor material such as silicon and germanium. These layers are either PNP or NPN. A typical transistor has three terminals known as the Base, Emitter, and Collector. Such a type of transistor is known as Bipolar Junction Transistor (BJT) transistor. The typical BJT transistor symbol and figure are shown below.

Fig-1: Transistor Figure and symbol

Types of Transistors and Transistor Symbols

In a broader spectrum, transistors are classified into three major types, namely Bipolar Junction Transistors (BJT), Field Effect Transistors (FET), and Insulated Gate Bipolar Transistors (IGBT). These transistors are then further divided into subtypes. Each of the transistors is discussed in this section.

1.1  Bipolar Junction Transistor (BJT)

A BJT is a type of transistor in which current can either flow due to the free electrons or holes. BJT has three terminals: base, emitter, and collector. The small amount of current between the base and emitter terminal can control the large flow of current between the emitter and collector terminals. These transistors are further categorized as N-P-N and P-N-P Transistors.

Figure 2: BJT Transistors Symbols

In the P-N-P type, N-type semiconductor material is sandwiched between two P-type materials. This arrangement results in Base-Emitter Junction (Je) and Base-Collector Junction (Jc). In a typical application, the base-emitter junction is forward-biased and the base-collector junction is reverse-biased. In P-N-P type transistors, current flow is due to the holes being the majority charge carrier.

Whereas, in the N-P-N type, P-type semiconductor material is sandwiched between two N-type materials. In the N-P-N type transistor, current flow is due to the electrons as a majority charge carrier.  

When an input is applied between two terminals of the transistor, it amplifies the input at the output terminals. BJT has three terminals, one terminal acts as an input and one as an output. The other terminal acts as a ground. Therefore, BJT has three configurations:

1.     Common Emitter configuration (Emitter is ground, input at base, output at collector)

2.     Common Base Configuration (Base is ground, input at emitter, output at collector)

3.     Common Collector Configuration (Collector is ground, input at base, output at emitter)

1.2  Field Effect Transistor (FETs)

In FETs, there is no direct contact between the semiconductor layers like in BJT; rather, it uses an electric field to control the flow of current. It also has three terminals: gate, drain, and source. They are unipolar, unlike BJTs which are bipolars. FETs are extensively used in many applications due to high impedance (up to Mega Ohms), low power consumption, low heat dissipation, and high switching frequency range up to Mega Hertz. There are mainly two types of FETs i.e. MOSFET and JFET.

1.     MOSFET

Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is one of the widely used transistors. It is a voltage-controlled device. The oxide (Sio2) indicates that it has a thin insulated layer between the metal and semiconductor layers. Therefore, MOSFET uses an electric field to control the flow of current between metal and semiconductor layers. Unlike BJT, MOSFET can only use electronic (N Type) or holes (P Type) as a charge carrier in their operation.

Figure 3: N-Channel and P-Channel MOSFET Symbols

Due to the use of electric fields to control the flow of current, MOSFETs offer very high input impedance and low output impedance. Therefore, extensively used in power electronic circuits, Integrated Circuits (ICs), Operational amplifiers (Op-Amps), Oscillators, filters, and high switching frequency devices.

Figure 4: Typical MOSFET Through Hole Package

Like BJT, MOSFET Transistors also have three configurations of operation.

1.     Common Gate Configuration (Gate is Ground, Input at Source, Output at Drain)

2.     Common Drain Configuration (Drain is Ground, Input at Gate, Output at Source)

3.     Common Source Configuration (Source is Ground, Input at Gate, Output at Drain)

MOSFETs are further divided into Enhancement Type MOSFET, Depletion Type MOSFET, PMOS, and NMOS Transistors.

·       Depletion Type MOSFET: Depletion MOSFET, commonly known as D-MOSFET Transistor. When current flows between the source and drain, it is known as a channel. In D-MOSFET the channel is already being constructed in the process of manufacturing. So, it normally operates as ON without applying any gate voltage. Current flow from drain to source without applying any gate voltage. Therefore, a transistor in this condition is called an ON device. However, when the gate voltage is applied at the input of the transistor, its channel becomes resistive. Upon increasing the voltage, the channel current keeps on decreasing until the current from the drain to the source stops.

·       Enhancement Type MOSFET: Enhancement MOSFET, commonly known as E-MOSFET. The channel is not already created unlike in D-MOSFET. Normally, no current flows between the drain to source terminal. However, when the gate voltage is applied to transistor, the current keeps on increasing and makes the transistor channel less resistive.

·       PMOS & NMOS Transistors: Just like MOSFET, PMOS and NMOS also have three terminals gate, drain, and source. The main difference between PMOS and NMOS is that in NMOS transistors, source and drain layers are doped with N-type material. Whereas, in the PMOS transistor, Source and drain layers are doped with P-type material.

2.     JFET

JFET is a voltage-controlled device and stands for Junction Field Effect Transistor. JFET is one of the first transistors in FETs and the simplest one. The current flow in JFET is due to the majority charge carriers only, unlike in BJT current flow is due to both majority and minority charge carriers. Therefore, they are classified as N-JFET and P-JFET. It has three terminals gate, source, and drain.

In a typical operation, when gate voltage is zero, electrons easily travel from source to drain. However, when the gate voltage is applied across the source and gate, the P-N junction becomes reverse-biased and increases the width of the depletion layer. This will lead the JFET to pinch off the region (Completely OFF).

What Does a Transistor do?

A transistor is an electronic device that controls the flow of current. The transistor has two main functions, either used as a switch or an amplification device. The transistor has three modes of operation, i.e., cut-off, saturation, and active region. It works as a switch or amplifier based on the operation of the region. If a transistor is operated in the cut-off and saturation region, it acts as a switch. However, if it is operated in an active region, it acts as an amplifier.

·       Transistor as a switch: With the combination of saturation and cut-off region, a transistor is operated as a switching device. When it is in the cut-off region, the current does not flow at all and a transistor is in reverse biased condition. Therefore, it remains in the OFF state. When it is in a saturation region, current will flow, and a transistor will be in a forward bias condition. Therefore, it will go into the ON state. Transistor has extensive applications where it is operated as switching devices such as blinking of LEDs, DC motors, logic gates, high-frequency drives, precise power regulation, and relays.

·       Transistor as an Amplifier: When a transistor is precisely used in an active region, it is operated as an amplifier. The important factor contributing to the amplification is the gain (beta) of the transistor. It is normally mentioned in the datasheet of a transistor. The higher the gain, the higher the amplification of a transistor. Another factor also contributing to the performance of the amplifier is the ratio of input, output voltage, input and output resistance, current gain, and power gain. A transistor as an amplifier is extensively used in radio signals, wireless communication, operational amplifiers, audio devices, instrumental amplifiers, medical devices, and fiber optic communication.

How Do Transistors Work?

Transistors have revolutionized the modern world. In today’s world transistors are used everywhere from smartphones to rocket engines, modern processors, memory devices, and internet servers.

A typical transistor functions as a switch or amplifier. It is made with three layers of semiconductor materials i.e. N-Type and P-Type. So, how does a transistor work?

A typical transistor has three terminals: base, emitter, and collector. The purpose of the transistor is to control the flow of current. It controls the flow of current by using the principle of charge carriers. The majority of charge carriers are either electrons or holes. The three layers are placed together in such a way that there are two N Types and one P Type between them. This makes an N-P-N transistor and vice versa, which is true for a P-N-P transistor.

The fundamental operation of transistors is based on the base-emitter junction and base-collector junction. These junctions form when a base signal is applied at the base emitter terminal of a transistor. When a small amount of current is applied at the input, it allows a large current to flow from the base collector junction. This is known as transistor amplification. A transistor in amplification mode is achieved by making the base-emitter junction forward-biased and the base-collector junction reverse-biased.

When no base signal is applied at the input (base-emitter terminals), it makes both the base-emitter and base-collector junction reverse-biased. Therefore, no current will flow from the emitter to the collector and the transistor will be in OFF state. A transistor in this region of operation is referred to as a cut-off region.

When a base signal is applied at the input, it allows the current to flow from the emitter to the collector. Both the base-emitter and base-collector junction in this operation are forward-biased and the collector will be in an ON state. A transistor in this region of operation is referred to as a saturation region.

Transistor History

The origin of transistors is rooted back in thermionic vacuum tubes. Thermionic vacuum tubes were invented in 1907 and mainly used for radio technology and radar systems. These were the first type of transistors but consume excessive energy and are bulky in size. These vacuum tubes use an input signal to control the flow of current at the output by using the electrodes. 

5.1 When & Who Invented the Transistor

In October 1925, an Austrian scientist in Canada published a first-ever patent on a field effect transistor. However, his work was ignored at that time due to the lack of research articles published. However, during World War II, Bell Labs made efforts to produce a pure germanium crystal to use in radar and frequency mixer signals.

In 1947, John Bardeen and William Shockley at Bell Labs, New Jersey, United States, invented the first ever working transistor. Later on, in 1958, Bell Labs introduced the MOSFET transistor. The invention of MOSFET revolutionized modern electronics. It was the first planar transistor on which the drain and source are on the same surface. The discovery of MOSFET has then widely replaced the conventional transistors in almost all electronics, including processors, memory devices, and microcontrollers.

Conclusion

In conclusion, the Transistor is one of the major inventions of the 20^th century that has changed modern electronics. Modern embedded electronics such as processors, microcontrollers, and digital devices consist of transistors. Transistors are vital components in modern electronics, such as radars, fiber optical communication, medical devices, and instrumental amplifiers. Therefore, understanding transistor operation, its working principles, and types are crucial for engineers to design state-of-the-art applications.