Basics of Transistors

Branch : Electronics and communications engg. Subject : Electronic Component and Devices-I Topic Sub Topic : Basics of Transistors : Construction details, Symbols of Transistors 1 OBJECTIVES : On completion of this period, you would be able to understand : • What is a transistor? • How it is formed? • Symbols of transistors. 2 RECAP • What are the extrinsic semi conductors? • What is meant by a Hole? • What is major difference between P type to N type materials? 3 INTRODUCTION • The Transistor was invented in 1947 by Dr.John Bardeen, Dr.walter Brattain and Shockley at Bell Laboratory in U.S.A. • It is the invention of the transistor that has brought revolutionary changes in the field of Electronics. 4 Dr. Bardeen (left) • Dr. Shockley (seated) • Dr. Brattain. • 5 • Transistor is an acronym for Transfer Resistor. • Transistor is an active device. • Transistor is a semiconductor device made of Germanium (Ge) or Silicon (Si). 6 Basically, there are two types of transistors 1.Bipolar Junction Transistors (BJTs), 2.Field Effect Transistors(FETs). 7 FORMATION OF TRANSISTORS : • From the previous chapter you got an idea about PN Diode • Basically Transistor Consists of two back-to-back PN junctions (diodes) manufactured as a single piece of semiconductor crystal. 8 • These two junctions give rise to three regions namely Emitter (E), Base (B) and Collector (C). 9 • A junction transistor is simply a sandwich of a thin layer of one type (either N type or P type) of semiconductor material between two layers of other type of semiconductor materials. • Transistors are of either PNP or NPN. 10 Symbol of Transistor 11 12 Fig 1.Formation of pnp Transistor 13 • Fig (1) Shows a thin layer of N type material Sandwiched between two layers of P type materials it is PNP transistor. 14 Fig 2. Formation of npn Transistor 15 • Fig (2) shows an NPN transistor consisting of a thin layer of P type material sandwiched between two layers of N type materials. 16 17 • The two junctions give rise to three regions. • The middle region is called Base and the outer two regions are called as Emitter and Collector. • Three terminals were taken from three regions, namely E, B & C. 18 • Although Emitter & Collector are made of same material their functions cannot be inter changed because they have different physical & electrical properties. • Emitter : It is that region of the transistor which emits majority charge carriers into Base region. 19 • The doping concentration of Emitter is more than both Base and Collector. • Physical size of Emitter is less than Collector but more than the Base. 20 Base : • It is the middle region of the transistor. • It is very thin and lightly doped as compared to either the Emitter or Collector. 21 Collector : It is that region of the transistor which Collects the majority charge carriers emitted by the Emitter Passing through the Base region. 22 • Collector region is made physically larger than Emitter and Base to dessipate the heat generated by high velocity charge carriers. • It's doping concentration is greater than Base region and less than that of Emitter region. 23 • The PN junction between Emitter and Base is called Emitter junction and the junction between Base and Collector is called Collector junction. • The horizontal line represents Base (B) while the two inclined lines represents the Emitter (E) and Collector (C). 24 • Emitter is distinguished from the collector by an arrow head. Arrow head on the Emitter represents the direction of conventional current. • In PNP transistor arrow points towards the Base meaning that Emitter is at positive potential with respect to Base. 25 • In NPN transistor arrow points from Base to Emitter meaning that the Base (and Collector as well) is at positive potential with respect to Emitter. 26 Summary • Transistor is a semiconductor active device. • • • • Transistors are made of Germanium (Ge) or silicon (Si). Transistor is a 3 layer,2 junction, 3 terminal device. Transistors are of either PNP or NPN. PNP is complement of NPN. 27 Quiz Transistor is an ___________ device. active 28 heavily Emiter is ___________ doped and moderate area. has________ 29 thin Base is ____________ and _________ lightly doped. 30 larger in area and Collector is ________ moderately doped. ________ 31 Arrow head on Emiter points the direction of Conventional current ______________ 32 Frequently Asked Questions 1. What is a transistor ? 2. Explain how the transistor is formed ? 3. Draw the symbols of transistor ? 33