Transistor & Types of Transistors , what is NPN Transistor

Introduction to Transistor:

Earlier, the critical and important component of an electronic device was a vacuum tube; it is an electron tube used to control electric current. The vacuum tubes worked but they are bulky, require higher operating voltages, high power consumption, yield lower efficiency and cathode electron-emitting materials are used up in operation. So, that ended up as heat which shortened the life of the tube itself. To overcome these problems, John Bardeen, Walter Brattain and William Shockley were invented a transistor at Bell Labs in the year of 1947. This new device was a much more elegant solution to overcome many of the fundamental limitations of vacuum tubes.

Transistor is a semiconductor device that can both conduct and insulate. A transistor can act as a switch and an amplifier. It converts audio waves into electronic waves and resistor, controlling electronic current. Transistors have very long life, smaller in size, can operate on lower voltage supplies for greater safety and required no filament current. The first transistor was fabricated with germanium. A transistor performs the same function as a vacuum tube triode, but using semiconductor junctions instead of heated electrodes in a vacuum chamber. It is the fundamental building block of modern electronic devices and found everywhere in modern electronic systems.

Transistor Basics:

A transistor is a three terminal device. Namely,

Base: This is responsible for activating the transistor.

Collector: This is the positive lead.

Emitter: This is the negative lead.

The basic idea behind a transistor is that it lets you control the flow of current through one channel by varying the intensity of a much smaller current that’s flowing through a second channel.

Types of Transistors:

There are two types of transistors in present; they are bipolar junction transistor (BJT), field effect transistors (FET). A small current is flowing between the base and the emitter; base terminal can control a larger current flow between the collector and the emitter terminals. For a field-effect transistor, it also has the three terminals, they are gate, source, and drain, and a voltage at the gate can control a current between source and drain. The simple diagrams of BJT and FET are shown in figure below:

  Bipolar Junction Transistor (BJT)

      Field Effect Transistors (FET)

Comparison of Thyristor and Transistor

As already mentioned, transistors and thyristors are both semiconductor devices. They are now widely employed in switching operations because of their numerous advantages such as noiseless operation owing to absence of moving parts, very high switching speed (say 109 operations per second), high efficiency, low maintenance, small size, little weight and trouble free service for long period, large control current range (say from 30 A to 100 A) with small gate current of few mA over mechanical switches or electro-mechanical relays. However, transistors and thyristors both have their own areas of applications. Thyristors have some advantages over transistors, as enumerated below:

1. Thyristor is a four-layer device while the transistor is a three-layer device.

2. Due to difference in fabrication and operation it is possible to have thyristors with higher voltage and current ratings.

3. Rating of a transistor is always in watts while that of a thyristor is in kWs i.e. thyristors having better power handling capacity.

4. A thyristor needs only a pulse to make it conducting and thereafter it remains conducting. On the other hand a transistor needs a continuous current for keeping it in a conducting state.

5. Internal power losses in a thyristor are much smaller than those in a transistor. Transistors, however, have lower voltage drop, and need no turn-off circuit. Power transistors are linear devices better suited for some linear power applications such as audio and high frequency power amplifiers, regulator systems etc.

NPN Transistor

Definition: The transistor in which one p-type material is placed between two n-type materials is known as NPN transistor. The NPN transistor amplifies the weak signal enter into the base and produces strong amplify signals at the collector end. In NPN transistor, the direction of movement of an electron is from the emitter to collector region due to which the current constitutes in the transistor. Such type of transistor is mostly used in the circuit because their majority charge carriers are electrons which have high mobility as compared to holes.

Construction of NPN Transistor

The NPN transistor has two diodes connected back to back. The diode on the left side is called an emitter-base diode, and the diodes on the left side are called collector-base diode. These names are given as per the name of the terminals.

The NPN transistor has three terminals, namely emitter, collector and base. The middle section of the NPN transistor is lightly doped, and it is the most important factor of the working of the transistor. The emitter is moderately doped, and the collector is heavily doped.

Circuit Diagram of NPN Transistor

The circuit diagram of the NPN transistor is shown in the figure below. The collector and the base circuit is connected in reverse biased while the emitter and base circuit is connected in forward biased. The collector is always connected to the positive supply, and the base is in negative supply for controlling the ON/OFF states of the transistor.