Physical Sciences For NET & SLET Exams Of UGC-CSIR. Part B and C. Volume-16. Contents

Transcription

1 Physical cieces For NET & LET Exams Of UC-CIR Part B ad C Volume-16 Cotets VI. Electroics 1.5 Field Effect evices Otoelectroic evices Photo detector Light-Emittig iode (LE) Oeratioal Amlifier Alicatios of OP-Ams Part B&C (erivatives) [Volume-16]

2 Physical cieces For NET & LET Exams Of UC-CIR VI.1.5 Field Effect evices INTROUCTION I the tye of trasistor, both holes ad electros lay art i the coductio rocess. For this reaso, it is sometimes called a biolar trasistor. The ordiary or biolar trasistor has two ricile disadvatages. First, it has low iut imedace because of forward biased emitter juctio. ecodly, it has cosiderable oise level. Although low iut imedace roblem may be imroved by careful desig ad use of more tha oe trasistor, yet it is difficult to achieve iut imedace more tha a few mega-ohms. The field effect trasistor (FET) has, by virtue of its costructio ad biasig, large iut imedace which may be more tha 100 mega-ohms. The FET is geerally much less oisy tha the ordiary or biolar trasistor. The raidly exadig FET market has led may semicoductor marketig maagers to believe that this device will soo become the most imortat electroic device, rimarily because of its itegrated-circuit alicatios. I this chater, we shall focus our attetio o the costructio, workig ad circuit alicatios of field effect trasistors. 1. Tyes of Field Effect Trasistors A biolar juctio trasistor (BJT) is a curret cotrolled device i.e., outut characteristics of the device are cotrolled by base curret ad ot by base voltage. However, i a field effect trasistor (FET), the outut characteristics are cotrolled by iut voltage (i.e., electric field) ad ot by iut curret. This is robably the biggest differece betwee BJT ad FET. There are two basic tyes of field effect trasistors: (i) Juctio field effect trasistor (JFET) (ii) Metal oxide semicoductor field effect trasistor (MOFET) To beig with, we shall study about JFET ad the imroved from of JFET, amely; MOFET. 2. Juctio Field Effect Trasistor (JFET) A juctio field effect trasistor is a three termial semicoductor device i which curret coductio is by oe tye of carrier i.e. electros or holes. The JEFT was develoed about the same time as the trasistor but it came ito geeral use oly i the late 1960s. I a JFET, the curret coductio is either by electros or holes ad is cotrolled by JFET has high iut imedace ad low oise level. Costructioal details. A JFET cosists of -tye or -tye silico bar cotaiig two juctio at the sides as show i Fig. (1). The bar forms the coductig chael for the charge carrier. If the bar is of -tye, it is called -chael JFET as show i Fig. 1(b). The two juctios formig codes are coected iterally ad a commo termial called gate is take out. Other termials are source ad drai take out from the bar as show. Thus a JEFT has essetially three termials viz., gate () source () ad drai (). 1 Part B&C (erivatives) [Volume-16]

3 Physical cieces For NET & LET Exams Of UC-CIR rai rai ate ate urface -Chael JFET urface -Chael JFET Figure 1 Note: It would seem from Fig. (1) that there are three doed material regios. However, this is ot the case. The gate material surrouds the chael i the same maer as a belt surroudig your waist. JFET olarities. Fig. 2(a) shows -chael JFET olarities whereas Fig. 2(b) shows the -chael JFET olarities. Note that i each case, the voltage betwee the gate ad source is such that the gate is reverse biased. This is the ormal way of JFET coectio. The drai ad source termials are iterchageable i.e. either ed ca be used as source ad the other ed as drai. V V V V The followig oits may be oted: Figure 2 (i) The iut circuit (i.e. gate to source) of a JFET is reverse biased. This meas that the device has high iut imedace. (ii) The drai is so biased w.r.t. source that drai curret I flows from the source to drai. (iii) I all JFETs, source curret I is equal to the drai curret i.e. I I. 2 Part B&C (erivatives) [Volume-16]

4 Physical cieces For NET & LET Exams Of UC-CIR 3. Pricile ad Workig of JFET Fig. (3) shows the circuit of -chael JFET with ormal olarities. Note that the gate is reverse biased. Pricile: The two juctios at the sides form two deletio layers. The curret coductio by charge carries (i.e. free electros i this case) is through the chael betwee the two deletio layers ad out of the drai. The width ad the hece resistace of this chael ca be cotrolled by chagig the iut voltage V. [Note: The resistace of the chael deeds uo its area of X-sectio. The greater the X-sectioal area of the chael, the lower will be its resistace ad the greater will be the curret flow through it.].the greater the reverse V, the wider will be deletio layers ad arrower will be the coductig chael. The arrower chael meas greater resistace ad hece source to drai curret decreases. Reverse will hae should V, decrease. Thus JFET oerates o the ricile that the width ad hece resistace of the coductig chael ca be carried by chagig the reverse voltage V. I other words, the magitude of drai curret ( I Workig. The workig of JFET is as uder: (i) Whe a voltage V ) ca be chaged by alterig V. is alied betwee drai ad source termials ad voltage o the gate is zero [ee Fig. 3(a)], the juctios at the sides of the establish deletio layers. The electros will flow from source to drai through a chael betwee the deletio layers. The size of these layers determies the width of the chael ad hece the curret coductio through the bar. (ii) Whe a reverse voltage V is alied betwee the gate ad source [ee Fig. 3(b), the width of the deletio layers is icreased. This reduces the width of coductig chael, thereby icreasig the resistace of -tye bar. Cosequetly, the curret from source to drai is decreased. O the other had, if the reverse voltage o the gate is decreased, the width of the deletio layers also decreases. This icrease the width of the coductig chael ad hece source to drai curret. V V V V Figure 3 It is clear from the above discussio ot curret from source to drai ca be cotrolled by the alicatio of otetial (i.e. electric field) o the gate. For this reaso, the device is called 3 Part B&C (erivatives) [Volume-16]

5 Physical cieces For NET & LET Exams Of UC-CIR field effect trasistor. It may be oted that a -chael JFET oerates i the same maer as the -chael JFET excet that chael curret carries will be the holes istead electros ad the olarities of V ad V are reversed. - V + V JFET biased for Coductio Notes. It the reverse voltage V o the gate cotiuously icreased, a state is reached whe the two deletio layers touch each other ad the chael is cut off. Uder such coditios, the chael becomes a o-coductor. 4. chematic ymbol of JFET Figure (4) shows the schematic symbol of JFET. The vertical lie i the symbol may be thought as chael ad source () ad drai () coected to this lie. If the chael is -tye, the arrow o the gate oits towards the chael as shows i Fig. 4(a). However, for -tye chael, the arrow o the gate oits from chael to gate [ee Fig. 4(b)]. -Chael JFET -Chael JFET Figure 4 5. Imortace of JFET A JFET acts like a voltage cotrolled device i.e. iut voltage ( V ) cotrols the iut curret. This is differet from ordiary trasistor (or biolar trasistor) where iut currat cotrols the outut curret. Thus JFET is semicoductor device actig like a vacuum tube [Note: The gate, source ad drai of a JFET corresod to grid, cathode ad aode of a vacuum tube]. Cotiued with...page 5 Owards Register Now, Ad et 25% iscout o This tudy Material Package 4 Part B&C (erivatives) [Volume-16]