School of Computer Science and Electrical Engineering 17/05/01 The bipolar Junction Transistor The term Bipolar is because two type of charges (electrons and holes)are involved in the flow of electricity The term Junction is because there are two pn unctions There are two configurations for this device n-type p-type n-type p-type n-type p-type Symbol NPN PNP Lecture 11 ENG1030 Electrical Physics and Electronics B./T Downs
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs School of Computer Science and Electrical Engineering 1 17/05/01 Lecture 11 The Bipolar Junction Transistor • The term Bipolar is because two type of charges (electrons and holes) are involved in the flow of electricity • The term Junction is because there are two pn junctions • There are two configurations for this device n-type p-type n-type NPN Symbol b e c p-type n-type p-type PNP Symbol b c e
School of Computer Science and Electrical Engineering 17/05/01 nPn and Pnp transistors NPN is more widely used majority carriers are electrons so it operates more quickly PNP is used for special applications we will concentrate on NPN The terminals of the transistor are labelled base Emitter and collector The emitter is always drawn with the arrow Collector Emitter Ba ase Emitter Collector Lecture 11 ENG1030 Electrical Physics and Electronics B./T Downs
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs School of Computer Science and Electrical Engineering 2 17/05/01 Lecture 11 NPN and PNP Transistors • NPN is more widely used – majority carriers are electrons so it operates more quickly • PNP is used for special applications – we will concentrate on NPN • The terminals of the transistor are labelled Base, Emitter, and Collector – The emitter is always drawn with the arrow. Base Emitter Collector Base Emitter Collector
School of Computer Science and Electrical Engineering 17/05/01 Asymmetry of Transistor Although in the nPn schematic, it looks like the collector and emitter can be reversed, in reality the device is very inefficient in reverse connection and has very little amplification gain) collector base emitter collector emitter base Is really n-type p-type n-type quite asymmetric NPN NPN Lecture 11 ENG1030 Electrical Physics and Electronics B./T Downs
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs School of Computer Science and Electrical Engineering 3 17/05/01 Lecture 11 Asymmetry of Transistor • Although in the NPN schematic, it looks like the collector and emitter can be reversed, in reality the device is very inefficient in reverse connection and has very little amplification (gain). n-type p-type n-type NPN is really collector base emitter collector base emitter NPN quite asymmetric
School of Computer Science and Electrical Engineering 17/05/01 Operation of nPn transistor In normal operation, the eb junction is forward biased and the BC junction is reverse biased The base region is very thin so the ratio L1: L2 is typically about 150:1 n Lecture 11 ENG1030 Electrical Physics and Electronics B./T Downs
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs School of Computer Science and Electrical Engineering 4 17/05/01 Lecture 11 Operation of NPN Transistor • In normal operation, the EB junction is forward biased and the BC junction is reverse biased • The base region is very thin so the ratio L1:L2 is typically about 150:1 DC DC n p n e b c Ie Ic Ib L1 L2
School of Computer Science and Electrical Engineering 17/05/01 Behaviour Forward biasing of the eb junction causes a heavy flow of majority carriers(electrons)from the n-type material into the base junction and also majority carriers (holes from the base region into the emitter region. We denote this current le The transistor is made so that nearly all the current le consists of a flow of carriers electrons from emitter to the base this is achieved by making the emitter much more heavily doped than the base n Lecture 11 ENG1030 Electrical Physics and Electronics B./T Downs
ENG1030 Electrical Physics and Electronics B.Lovell/T.Downs School of Computer Science and Electrical Engineering 5 17/05/01 Lecture 11 Behaviour • Forward biasing of the EB junction causes a heavy flow of majority carriers (electrons) from the n-type material into the base junction and also majority carriers (holes) from the base region into the emitter region. We denote this current Ie. • The transistor is made so that nearly all the current Ie consists of a flow of carriers (electrons) from emitter to the base. This is achieved by making the emitter much more heavily doped than the base. DC DC n p n e b c Ie Ic Ib