Bipolar junction Transistor: Working and its Applications

Bipolar junction transistor (BJT):

  Bipolar junction transistors are those which contain both holes and free electrons for the transmission of electrons, as the diode is made up of two semiconductors join together to form a PN-junction, BJT is two diodes join together forming a three-layer, two junction semiconductor material. Physically it has three basic terminals through which it is attached in the circuit, these terminals are known as base, emitter, and collector.

Bipolar junction is a combination of Type semiconductor material sandwich between two N-type semiconductor material NPN bipolar transistors or N-type sandwich between two P-type transistors forming PNP transistor. N-type and P-type semiconductors are made by doping. Doping is a process through which semiconductors were treated through some other chemicals to enhance their properties like conductivity by injecting more free electrons and holes.

These transistors are used in the circuits in such a way that by providing a small current through them we can manage or control huge current passing through a circuit that’s why they can be used in switching or amplification of signals especially radio signals.

Based on the functionality BJT transistors are divided into two types PNP and NPN Transistors. Symbolic views of both the transistors are shown in the figure below. In both, the cases emitter-base junction is forward biased where the collector-base junction is reversed biased.

NPN Transistor working:

    AS the name represent the P-Type region is between two N-type regions to form an NPN transistor. The P-type region has an excess amount of holes as the majority charge carrier while the N-type region has an excess amount of electrons as the majority charge carrier. In an NPN transistor, the emitter is heavily doped whereas the base is moderately doped and the collector is lightly doped.

Working:

    NPN transistors are attached din the circuit in such a way that the emitter is connected with a negative terminal and is forward biased whereas the collector is attached with the positive terminal so the collector-base junction is reverse biased. The electrons from the emitter start traveling towards the base region where only 5 % of them were attached to the holes present there and cause emitter current to flow which is shown in the figure below.

The remaining 95 % of electrons which are left behind as the base layer is very small were traveled towards the collector where they cause collector current o flow. The motion of electrons in the circuit can be observed by the figure showing this illustration. Meanwhile, the Emitter current is equal to the base current and collector current in the circuit.

Working of PNP transistor:

    PNP transistor as the name represents P-type Semiconductor is between two N-type Semiconductors, P-type doping contains an excess amount of holes present in it. Emitter which is larger and heavily doped contains a large number of holes in it, the base is smaller in size having an excess amount of electrons as majority charge career and Emitter is again P-type and less doped structure.

The emitter-base junction is forward biased and the emitter is connected to the positive terminal where the collector is attached to the negative terminal and biasing of the collector-base junction is reverse biased. When the current has been applied the holes from the emitter start attracting towards the negative terminal which is the base side and produces emitter current, only 5 % of the holes produced emitter current.

The remaining holes are transferred to the collector region from where they are attracted toward the negative side of the source as shown in the figure in this way collector current is produced. So from here we observe that by using a small amount of electric charge a large amount of current is controlled. The biasing and the production of current can be observed...

MOSFET: Definition, Symbol, Working, Characteristics, Types & Applications

MOSFET:

    MOSFET is known as a metal oxide field effective transistor, they are an advanced form of FET as FET have some limitations in their operations, like impedance and frequency limitations, so MOSFETS are used in those circuits where high affectivity is required.

Construction:

Like other transistors, MOSFETs are also made up of semiconductors known as silicon. Semiconductor materials are partially conductive so their properties were increased by a process known as doping. Doping is a technique through which impurities were introduced in the Semiconductor devices to increase their functionality. 

Like phosphorus is added as an impurity in the silicon to increase its conductivity, as phosphorous has an excess amount of electrons present in it, which increases the number of free electrons in the silicon, which results in increasing its conductivity, this opening is known as P-type doping. Similarly when boron is added to the silicon as an impurity the number of holes increased. This doping is known as N-type doping. MOSFET is also a combination of these N-type and P-type Doing. 

MOSFET has four basic body parts known as Drain, Gate, Body, and source, and they were attached in the circuit with three terminals that are Gate, Body, and Source. MOSFET is made by injecting impurities in silicon and make Gate and Source as P and N-type semiconductor that is clearly shown in the figure.

 The construction of the gate terminal is such that a dielectric is placed above the silicon semiconductor with a metal-metal plate above the dielectric. Thus it makes a structure just like a capacitor. As capacitor contains a dielectric between two plats to store charge here Gate terminal acts similarly by using this combination. A clear glimpse of the structural view of MOSFET is shown in the figure.

Working:

    The main working Principle of the MOSFET is to control the flow of electricity or voltage between the drain and the source. The semiconductor is doped with either Type or N-type. When the voltage is applied between drain and gate, by connecting Positive terminal with the metallic plate, due to the presence of dielectric Positive charge is start storing in it.

Which further attaches the electron present in the semiconductor, and start pushing the holes downwards in the substrate. These electrons form a thick layer with the dielectric that helps in flowing electrons from source to drain. The thickness of the layer is controlled by the voltage applied.

Conversely when the negative potential is applied then the Holes were attracted towards the plate and electrons are pushed away towards the substrate, forming a depletion region in the substrate. Based on the operation and functionality, the MOSFET working is divided into two different modes known as.

• Enhancement mode
• Depletion Mode 

Each mode is further divided into two different types known as 

• P-channel
• N-channel

Characteristics:

    Drain characteristics:

        Drain characteristics of MOSFET for drain current and voltage between drain and source are shown in the graph. As we change the source gate violate we see an increase in drain current initially then become constant with the drain-source voltage increment.

P channel:

    As for the P channel, the substrate is N-type while drain and source are heavily doped with P-type, and when the negative voltage is applied the holes are attracted towards the gate and electrons start moving towards the Base or body terminal. The enhancement and depletion mode is depicted in the figures. For these conditions to fill it is mandatory to supply negative voltage to the gate terminal.

In depletion mode, the polarity of the battery is reversed.

N-channel:

The circuit depicts that in type channel the substrate is drain and source have N-type doping and when...

​ What is Diode & How it works?

Diode:

Diode is a semiconductor device that allows current to flow in only one direction, and shows infinite resistance in the other direction it is also known as a one-way switch. Diodes are installed in circuits mostly to a configuration that is known as forward biased and reverses biased.

The name Diode is derived from two words which are “di” and “ODE” which means two electrodes. The simplest form of the diode is the PN junction. Which conducts electricity when it is forward biased and acts as an open circuit when it is reverse biased. 

Symbol:

A diode is simply a PN junction that is combined by a specific method, p-junction contains an excess amount of holes in it which helps in the easy flow of electrons in forward biased. Where N-junction contains an excess amount of electrons in it. 

The forward error shows that the diode is forward-biased to ease the flow of electricity. Their polarity can find out by these terminals. When the anode is connected with the positive terminal it is forward biased and when connected to the negative terminal it is reverse biased.

Working Principle of Diode:

A diode is simply an N-type and P-type Semiconductor that is joined together. The N-type semiconductor has an excess amount of free electrons present in it, in other words, it has electrons as a majority charge carrier and holes as the minority charge carrier.

Where the other junction is P-type. P-type junction has holes as a majority charge carrier and free electrons are present in less amount in p-type that means holes are present in excess amount in P-type region.

Now when these two junctions are joined together, as they start transferring their majority charge carriers to each other, for example as p-Type junction has an excess amount of holes in it, when these junctions are combined these holes are starting transferring from P-type to N-type. These holes combine with the free electrons present in the N-type region and make positive ions there.

In the same way, the electrons from the N-type region start moving toward the P-type region and combines with the holes present there, and form uncovered negative ions in that region. This process is shown in the figure below.

This movement of holes and electrons results in the formation of a layer of positive charge in the N-type region conversely a layer of negative charge in the P-type region. The formation of these two layers left no space for charge carriers, which makes a deficiency of charge career in this region. This deficiency of charge carriers is known as the Depletion region between these two junctions this depletion region stops more low electrons and holes on either side. 

This means a potential barrier is created between these two junctions, the layer of positive charge in the N-type region repels the flow of holes from the P-type region and the layer of negative charge repels the movement of free electrons from the N-type region. Now if we want to pass out electrons from this PN- junction we have to overcome this potential barrier. Upon working diodes are mostly divides into two categories which are forward biased and reverse biased.

Types of Diode:

• Forward biased
• Reverse Biased

Forward biased:

When we attached the Positive terminal of the source with the P-type junction of the diode and the negative terminal with the N-type junction. We observe that initially, no current is passing through the diode, because the given voltage is not enough to break the potential barrier of the depletion region.

 To pass these majority charge carriers from their junction we have to increase voltage so that this potential barrier could be broken. As it is mentioned earlier this potential barrier could only be broken when the External voltage source is more than this potential barrier, the limit of this voltage is different for different materials, i-e is 0.7 volt for germanium.

When the voltage of the...