Difference Between BJT and FET

Main Difference – BJT vs. FET

BJT (Bipolar Junction Transistors) and FET (Field Effect Transistors) are two different types of transistors. Transistors are semiconductor devices that can be used as amplifiers or switches in electronic circuits. The main difference between BJT and FET is that BJT is a type of bipolar transistor where the current involves a flow of both majority and minority carriers. In contrast, FET is a type of unipolar transistor where only the majority carriers flow.

What is BJT

A BJT consists of two p-n junctions. Depending on their structure, BJTs are classified into npn and pnp types. In npn BJTs, a small, lightly-doped piece of p-type semiconductor is sandwiched between two heavily-doped n-type semiconductors. Conversely, a pnp BJT is formed by sandwiching an n-type semiconductor between p-type semiconductors. Let us have a look at how an npn BJT works.

The structure of a BJT is shown below. One of the n-type semiconductors is called the emitter (marked with an E), while the other n-type semiconductors is called the collector (marked with a C). The p-type region is called the base (marked with a B).

Difference Between BJT and FET - an_npn_BJT

The structure of an npn BJT

A large voltage is connected in reverse bias across the base and the collector. This causes a large depletion region to form across the base-collector junction, with a strong electric field that prevents the holes from the base from flowing into the collector. Now, if the emitter and the base are connected in forward bias, electrons can flow easily from emitter to the base. Once there, some of the electrons recombine with holes in the base, but since the strong electric field across the base-collector junction attracts electrons, most electrons end up flooding into the collector, creating a large current. Since the (large) current flow through the collector can be controlled by the (small) current through the emitter, the BJT can be used as an amplifier. In addition, if the potential difference across the base-emitter junction is not strong enough, electrons are not able to get into the collector and so a current will not flow through the collector. Due to this reason, a BJT can be used as a switch as well.

The pnp junctions work under a similar principle but, in this case, the base is made of an n- type material and the majority carriers are holes.

What is FET

There are two major types of FETs: Junction Field Effect Transistor (JFET) and Metal Oxide Semiconductor Field Effect Transistor (MOSFET). They have similar working principles, although there are some differences as well. MOSFETs are more commonly used today than JFETS. The way that a MOSFET works was explained on this article so here, we will focus on the operation of a JFET.

Just like BJTs come in npn and pnp types, JFETS also come in the n-channel and p-channel types. To explain how a JFET works, we will look at a p-channel JFET:

Difference Between BJT and FET - a_p-channel_JFET

A schematic of a p-channel JFET

In this case, “holes” flow from the source terminal (labelled with an S) to the drain terminal (labelled with a D). The gate is connected to a voltage source in reverse bias so that a depletion layer forms across the gate and the channel region where charges flow. When the reverse voltage on the gate is increased, the depletion layer grows. If the reverse voltage becomes large enough, then the depletion layer can grow so large that it can “pinch off” and stop the flow of current from the source to the drain. Therefore, by altering the voltage at the gate, the current from the source to the drain could be controlled.

Difference Btween BJT and FET

Bipolar vs Unipolar

BJTs are bipolar devices, in which there is a flow of both majority and minority carriers. 

FETs are unipolar devices, where only the majority carriers flow.

Control

BJTs are current-controlled devices.

FETs are voltage-controlled devices.

Use

FETs are used more often than BJTs in modern electronics.

Transistor Terminals

Terminals of a BJT are called the emitter, base, and collector

The terminals of an FET are called source, grain, and gate.

Impedance

FETs have a higher input impedance compared to BJTs. Therefore, FETs produce larger gains.

Image Courtesy:

“The basic operation of an NPN BJT in Active mode” by Inductiveload (Own drawing, done in Inkscape) [Public Domain], via

“This diagram of a junction gate field effect transistor (JFET)…” by Rparle at en.wikipedia (Transferred from to Commons by using ) [], via

About the Author: Nipun


Related pages


what does deus ex meanherbivore carnivore omnivore venn diagramtypes of smooch kissesnerd and geek differencealsatian characteristicsdifference between lentils and legumesdifference between balanced and unbalanced forcesmolecular structure of petroleum etherdefinition cuddlingthe difference between dietitian and nutritionistpossessive pronouns adjectivessn2 reaction conditionswhat is the definition of paralanguagestandard dialect definitionanorexia nervosa and bulimia nervosa differenceskancheepuram silk sarees in kanchipurampositive and normative economics exampleshyperosmolar solutiondifference between a nutritionist and dieticiantransverse binary fissionhow does cognitivism differ from behaviorismfigurative and literalbotanical name of coriander leafdifference between hunan and szechuandifference between hypoglycemia and diabetesdifference between light and compound microscopewhat is the difference between flu and food poisoningdifference between mole and mousecomparison of prokaryotes and eukaryotesprovisional balance sheet for bank loandefine homologous serieswhat is the difference between aldose and ketosecondescendingly meaningsecondary succession biologyeminent or imminentchemical formula for ethanoic acidwhat is an example of enjambmentmorals values and beliefsromanticism and gothicismrelish vs chutneynonessential appositivedefine endocytosis and exocytosisdifference between refraction and reflectiondeoxyribose molecular formulainverting amplifier definitionconfessionalism poetrywhat is the difference between chow mein and chop sueywhat is the difference between a serf and a slavedifference between shrubs and herbsdefinition consumer surplusdefinition of regular hexagonyield vs tensile strengthdistinguish between covalent and ionic bondsdifference between ldl and vldlcondenser and capacitorcpi equationhow to analyse the poemdifference between truvia and steviawhat is the difference between exocytosis and endocytosisdefine auntydefine amoebiccoenzymes and cofactors differenceare pigeon peas beansis it fiance or fianceecivil servants definitionpleurisy or pneumonianeurotic mental illnesssn1 vs sn2 reaction mechanismdefinition madamvitamin b12 vs b complexdifference between classical conditioning and operant conditioningdifference between monocot and dicot rootdefine boiling point of liquidreciprocal pronouns exampleskinds of adjectives and examplesan example of a compound complex sentenceis baking soda sodium carbonateevaporation chemistry definitionatomic weight and mass numberappendix versus attachmentdifference between adenoma and carcinomadifference between custard and ice creamformula for finding average velocity