Blog 7: How did Transistor Revolutionize the World?


Transistor are a type of semiconductor device that can be used for both conduction and insulation of current or voltage. Transistors basically act as switches/gates and amplifiers. The term transformer refers to the transmission characteristics and Ister average resistance characteristics provided for the connection. In other words, it is a switching device that regulates and amplifies electrical signals such as voltage and current. Transistors can also be described as small devices used to control or regulate the flow of electronic signals.

As a switch, it can be in one of two different states, on or off, to control the flow of electronic signals through electrical circuits or electronic devices. Transistors are one of the key components of most electronic devices that exist today. The transistor was developed in 1947 by three American physicists, John Bardeen, Walter Bratten, and William Shockley, and is considered one of the most important inventions in scientific history.

Parts of transistor:

A typical transistor consists of three layers of semiconductor material, more specifically terminals that connect to an external circuit to help carry current. The voltage or current applied to any pair of transistor terminals controls the current flowing through the other pair of terminals. The transistor consists of two PN diodes connected back to back. Transistors usually consist of three layers or terminals of semiconductor material, each of which can carry current. That is the emitter, the base, and the collector.

Base: This is used to turn on the transistor.

Collector: The positive lead wire of the transistor.

Emitter: Negative lead of the transistor.

The base is in the middle and consists of a thin layer. The right side of the diode is called the emitter diode and the left side is called the collector base diode. These names are given according to the general connection of transistors. The emitter-based junction of the transistor is forward-biased and the collector-based junction is reverse-biased, providing high resistance.

Diode connections are described in detail below.

Emitters- The section that provides most of the charge carriers is called the emitter. Since the emitter is always forward biased with respect to the base, most charge carriers are sent to the base. Emitter-base junctions are heavily doped and reasonably large, so large numbers of carriers are injected into the base.

Collector- The section that collects most of the charge carriers supplied by the emitter is called the collector. The collector-base junction is always reverse biased. Its main function is to remove the majority of the charge from the junction with the base. The collector path of the transistor is moderately doped, but it is large enough to absorb most of the charge carriers supplied by the emitter.

Base- The central part of the transistor is called the base. The base forms two circuits, an input circuit with an emitter and an output circuit with a collector. The emitter-based circuit is forward-biased and has low resistance to the circuit. The collector-base junction is reverse biased, providing higher resistance to the circuit. The base of the transistor is low-concentrated and very thin, giving the base the most charge carriers.

How have transistors revolutionized the tech world?

The transistor, invented at Bell Labs in 1947, quickly replaced the bulky vacuum tube as an electronic signal controller. The invention of the transistor is regarded as one of the most important developments in the history of the PC, driving the trend toward the miniaturization of electronic devices. Because these solid-state devices are much smaller, lighter, and consume much less power than tubes, electronic systems that use transistors are also much smaller, lighter, faster, and more efficient.

Transistors are also more powerful, require significantly less power, and unlike tubes, do not require an external heater. As the size of the transistors decreased exponentially, their cost decreased and more ways to use them were opened up. The integration of transistors with resistors and other diodes or electronic components has made ICs smaller. This miniaturization phenomenon is related to Moore’s Law, which doubles the number of transistors in a small IC every two years.

Types of Transistors:

There are two main types of transistors, depending on how they are used in the circuit.

Bipolar junction transistor (BJT):

BJTs are the most common type of transistor and are found in almost all electronic devices. The three terminals of the BJT are the base, emitter, and collector. The very small current flowing between the base and emitter can control the larger current flow between the collector and emitter terminals. In addition, there are two types of BJT. These include:

P-N-P Transistor: This is a type of BJT in which an n-type material is inserted or placed between two p-type materials. In such a configuration, the device controls the current flow. The PNP transistor consists of two crystal diodes connected in series. The right and left sides of the diode are known as collector-based and emitter-based diodes, respectively. P-N-P transistors, and holes are most charge carriers. For PNP transistors, the direction of current is from the emitter to the collector.

NPN transistor: In this transistor, there is a p-type material between two n-type materials. NPN transistors are basically used to boost weak signals to strong ones. In an NPN transistor, electrons move from the emitter to the collector region, causing current to flow through the transistor. This transistor is widely used in circuits. In N-P-N transistors, electrons are the main charge carriers. For NPN transistors, the direction of current is from collector to emitter.

BJT is a current steering transistor that has low input impedance and can pass a large current through the transistor. These transistors operate in three modes/ranges

Cut-off area

Active area

Saturated region

In the cut-off band, the transistor remains in the “off” state. Operate in the active region to use the transistor as an amplifier. And in the saturation region, the transistor acts as a switch.

Types of Configuration of transistor:

There are three types of configuration a common base (CB), common collector (CC), and common emitter (CE).

In Common Base (CB) configuration the base terminal of the transistor is common between input and output terminals.

In Common Collector (CC) configuration the collector terminals are common between the input and output terminals.

In Common Emitter (CE) configuration the emitter terminal is common between the input and the output terminals.

Field Effect Transistor (FET):

Field-effect transistors also have three main terminals called gates, drains, and sources. Unlike BJTs, these transistors are voltage control devices with very high input impedance. Impedances range from a few megaohms to much larger values, and a minimum amount of current can flow through them. These transistors use an electric field to control the flow of current from the source to the drain. FETs are unipolar transistors that conduct using N-channel FETs or P-channel FETs.

The main uses of FETs are low noise amplifiers, buffer amplifiers, and analog switches. The figure above shows an N-type FET. When the arrow points outward, it represents a P-type FET, and the voltage applied to the gate terminal controls the flow of current from the source to the drain. Due to their high impedance values, these transistors are ideal because they carry very little current from the circuit and do not interfere with circuit performance.

FETs are further divided into two main categories: JFETs and IG-FETs. MOSFETs, also known as metal oxide semiconductor FETs, are the most common type of IGFET. FETs are ideal for applications such as current limiters to prevent overcurrent from reaching the load device. Apart from that, FETs are also used as multiplexers, choppers, and phase shift oscillators. Purpose BJT is suitable for amplification, while FET is suitable for low noise amplification. In addition, FETs take up much less space than BJTs, so most electrical components use FETs in their ICs. Another important difference between BJTs and FETs is that BJTs are bipolar devices and FETs are unipolar devices. This means that FETs depend on either holes or electrons for operation, while BJTs otherwise depend on both.

Other Types:

Apart from these, there are many other types of transistors which include MOSFET, JFET, insulated-gate bipolar transistor, thin-film transistor, high electron mobility transistor, inverted-T field-effect transistor (ITFET), fast-reverse epitaxial diode field-effect transistor (FREDFET),  Schottky transistor, tunnel field-effect transistor,  organic field-effect transistor (OFET), diffusion transistor, etc

How do Transistors work?

Let’s see how the transistor works. It is known that BJT consists of three terminals (emitter, base, collector). This is a current operating device with two P-N junctions in a BJT.

One P-N junction is between the emitter and base regions and the second junction is between the collector and base regions. A very small current flowing from the emitter to the base can drive a fairly large current flowing through the device from the emitter to the collector.

In normal BJT operation, the base-emitter junction is biased forward and the base-collector junction is biased backward. When current flows through the base-emitter junction, current flows through the collector circuit.

 To illustrate how a transistor works, let’s look at an example of an NPN transistor. The same principle is used for PNP transistors, except that the current carrier is a hole and the voltage is reversed.

Operation of NPN Transistor:

Since the emitter of an NPN device is made of n-type material, most carriers are electrons. When the base-emitter junction is forward-biased, electrons move from the n-type region to the p-type region, and minority carrier holes move to the n-type region. When they meet, they connect and allow current to flow across the junction. When the junction is reverse-biased, holes and electrons move away from the junction, forming a depletion region between the two regions where no current flows. When a current flows between the base and the emitter, the electrons leave the emitter and flow into the base as described above.

Normally, electrons combine when they reach the depletion region. However, the doping in this area is very low and the base is also very thin. This means that most electrons can move within the region without recombination with holes. As a result, the electrons drift to the collector. This causes them to flow substantially across the reverse-biased junction and current to flow through the collector circuit.

Characteristics of Transistor:

Characteristics of the transistor are the plots that can represent the relation between the current and the voltage of a transistor in a particular configuration.

There are two types of characteristics.

Input characteristics: It will give us the details about the change in input current with the variation in input voltage by keeping output voltage constant.

Output characteristics: It is a plot of output current with output voltage by keeping input current constant.

Advantages of Transistor:

  • Lower cost and smaller in size.
  • Smaller mechanical sensitivity.
  • Low operating voltage.
  • Extremely long life.
  • No power consumption.
  • Fast switching.
  • Better efficiency circuits can be developed.
  • Used to develop a single integrated circuit.

Limitations of Transistors:

Transistors also have a few limitations. They are as follows:

  • Transistors lack higher electron mobility.
  • Transistors can be easily damaged when electrical and thermal events arise. For example, electrostatic discharge in handling.
  • Transistors are affected by cosmic rays and radiation.

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