Basic ElectronicsQ1:Introduction to BJTS, FETS, JFETS and MOSFETS?Q2: Difference between BJTS,FETS , JFETS and MOSFETS?

Created by hamza shehzad(hemi)Shehzadhamza436@gmail.com

Q1Introduction to BJTS:If we know join together two individual signal diodes back-to-back, this will give us two PN-junctions connected together in series that share a common P or N terminal.The fusion of these two diodes produce a three layer , two junction , three terminal device

forming the basis of a Bipolar transistor Junction(BJT).Transistors are three terminal active devices made from differebt semiconductor material that can act as either an insulator or a conductor by the application of small signal voltage.The bipolar transistors have ability to operate within three different regions.

Active Region: The transistors operate as an amplifier and Ic = β.Ib

Saturation: The transistor is “Fully On” operating as switch and Ic =I

Cut-off :The transistor is “ Fully Off” operating as switch and Ic=0

The transistor are also called Bi-polar because the main flow of electrons through them takes in two types of semiconductor material PN as the main current goes from Emitter to collector.In other words, two types of charge carrier electrons and holes comprise the main current through the transistor.Bi-polar transistor consists of either P-N-P or N-P-N semiconductor “sandwich” structure.The three leads of bipolar transistor Emitter, Base and Collector.The basic function of BJT is to amplify current.This allow BJTS to be used as amplifies or

switches ,giving them wide applicability in electronic equipment,including computers,televisions,mobile phones,audio amplifies, Industrial control and radio transmitter.

Introduction to FETS: So far we have discussed the circuits applications of ordinary transistors, in which both electrons and holes take part.That’s why these are sometimes called the bipolar transistors.

Drawback’s: Low impedance because of Forward biased

Emitter Junction. Considerable noise level. Both of these drawbacks have been overcome,to

a great extent in the field effect transistor(FETS) which is an electric field or voltage controlled device.FETS because of possessing all the advantages that tubes and ordinary transistors (BJTS) have, are replacing both the vaccum tubes and BJTS in applications.

There are two categories of FET’S namely: 1.Junction Field effect Transistors(JFET’S) 2.Insulated gate field effect(IGFET’S) Commonly known as the metal-oxide semiconductor field effect transistors(MOSFET’S OR MOST’S)

Introduction to JFET’S: A transistor is a linear semiconductor device that control current with the application of a lower power electrical signal.Transistors may be roughly divided into categories Bipolar and Field effect.All Field effect transistors are unipolar rather than bipolar devices.That is , the main current through them is comprised either of electrons through an N-type semiconductor or holes through a P-type semiconductor.

In a junction field effect transistor or JFET, the controlled current passess from source to drain,or from drain to source as the case may be.The controlling voltage is applied between the gate and source.Note how the current does not have to cross

through a PN junction on its way between source and drain the path is called an channel is an uninterrupted block of semiconductor material.Generally N channel JFETS are most commonly used than P-channel.

When no voltage applied between gate and source , the channel is a wide open path for electrons to flow.However if a voltage between gate and source of such polarity that is reverse biases the PN junctuion.The fow between source and drain connections become s limited.or regulated just it was for bipolar transistors with a set amount of base current.Maximum gate source voltage”pinches off” all current through source and drain.

Introduction to MOSFET’S: The n-type Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) consists of a source and a drain, two highly conducting n-type semiconductor regions which are isolated from the p-type substrate by reversed-biased p-n diodes. A metal (or poly-crystalline) gate covers the region between source and drain, but is separated from the semiconductor by the gate oxide.

 It is the applied voltages which determine which n-type region provides the electrons and becomes the source, while the other n-type region collects the electrons and becomes the drain. The voltages applied to the drain and gate electrode as well as to the substrate by means of a back contact are refered to the source potential

 where the gate length, L, and gate width, W, are identified. Note that the gate length does not equal the physical dimension of the gate, but rather the distance between the source and drain regions underneath the gate. The overlap between the gate and the source and drain region is required to ensure that the inversion layer forms a continuous conducting path between the source and drain region. Typically this overlap is made as small as possible in order to minimize its parasitic capacitance.

Q2

Difference b/w BJT’S and FET’s:BJT is consists of two PN junctions (a junction made by connecting a p type semiconductor and n type semiconductor). These two junctions are formed using connecting three semiconductor pieces in the order of P-

N-P or N-P-N. There for two types of BJTs known as PNP and NPN are available.

Three electrodes are connected to these three semiconductor parts and middle lead is called ‘base’. Other two junctions are ‘emitter’ and ‘collector’.

In BJT, large collector emitter (Ic) current is controlled by the small base emitter current (IB) and this property is exploited to design amplifiers or switches. There for it can be considered as a current driven device. BJT is mostly used in amplifier circuits.

FET is made of three terminals known as ‘Gate’ ‘Source’ and ‘Drain’. Here drain current is controlled by the gate voltage. Therefore, FETs are voltage controlled devices.

Depending on the type of semiconductor used for source and drain (in FET both of them are made of the same semiconductor type), a FET can be an N channel or P channel device. Source to drain current flow is controlled by adjusting the channel width by applying an appropriate voltage to gate. There are also two ways of controlling the channel width known as depletion and enhancement. Therefore FETs are available in four different types such as N channel or P channel with either in depletion or enhancement mode.

Difference b/w BJT’S and MOSFET’S:

The transistors BJT and MOSFET are both useful for amplification and switching applications. Yet, they have significantly different characteristics.

BJT, as in Bipolar Junction Transistor, is a semiconductor device that replaced the vacuum tubes of the old days. The contraption is a current-controlled device where the collector or emitter output is a function of the current in the base. Basically, the mode of operation of a BJT transistor is driven by the current at the base. The three terminals of a BJT transistor are called the Emitter, Collector and Base.

A BJT is actually a piece of silicon with three regions. There are two junctions in them where each region is named differently ‘“ the P and N. There two type of BJTs, the NPN transistor and th PNP transistor. The types differ in their charge carriers, wherein, NPN has holes as its primary carrier, while PNP has electrons.

The operation principles of the two BJT transistors, PNP and NPN, are practically identical; the only difference is in biasing, and the polarity of the power supply for each type. Many prefer BJTs for low current applications, like for switching purposes for instance, simply because they’re cheaper.

Metal Oxide Semiconductor Field-Effect Transistor, or simply MOSFET, and sometimes MOS transistor, is a voltage-controlled device. Unlike the BJT, there is no base current present. However, there’s a field produced by a voltage on the gate. This allows a flow of current between the source and the drain. This current flow may be pinched-off, or opened, by the voltage on the gate.

In this transistor, a voltage on an oxide-insulated gate electrode can generate a channel for conduction between the other contacts ‘“ the source and drain. What’s great about MOSFETs is that they handle power more efficiently. MOSFETs, nowadays, are the most common transistor used in digital and analog circuits, replacing the then very popular BJTs.

Difference between BJT’S and JFET’S:

BJT, or Bipolar Junction Transistor, was the first kind to be commercially mass-produced. BJTs conduct using both minority and majority carriers, and its three terminals have corresponding names ‘“ the base, emitter, and collector. It basically consists of two P-N junctions – the base-collector and the base-emitter junctions. A material called the base region, which is a thin intervening semiconductor, separates these two junctions.

Bipolar Junction Transistors are extensively useful in amplifying devices, because collector and emitter currents are effectively controlled by the small current at the base. They are named as such, because the current which is controlled, goes through two types of semi-conductor materials ‘“ the P and N. Current, essentially, consists of both hole and electron flow, in separate parts of the bipolar transistor.

BJTs basically function as regulators of currents. A small current is regulating a larger current. However, for them to properly operate as current regulators, the base

currents and the collector currents must be moving in the right directions.

FET, or Field-effect Transistor, also controls the current between two points, but it uses a different method to the BJT. As the name suggests, FETs’ function is dependent on the effects of electric fields, and on the flow, or movement, of electrons in the course of a particular type of semi-conductor material. FETs are sometimes referred to as unipolar transistors, based on this fact.

FET uses either holes (P channel), or electrons (N channel), for conduction, and it has three terminals – source, drain, and gate – with the body connected to the source in most cases. In many applications, FET is basically a voltage controlled device, due to the fact that its output attributes are established by the field that is depend on voltage.