1415EEMv2resitexam1415Markscheme

《1415EEMv2resitexam1415Markscheme》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。

1、SBCYEAR2ENGINEERING2014–2015ResitMarkSchemeElectronicEngineeringMaterialsQuestion1(a)Forasemiconductordefinecarrierdriftandcarrierdiffusion.[4]Answer:Carrierdrift–electronsandholeswillmoveundertheinfluenceofanappliedelectricfield,sincethefieldexertsaforceonchargecarrie

2、rs.Carrierdiffusion–isduetothemovementofcarriersfromhighconcentrationregionstolowerconcentrationregions.(2)(2)(b)(i)SketchtheI-Vcharacteristicsforap-njunctionandexplainthedifferenceincurrentflowunderforwardandreversebias.[4]2marksforthegraphThecurrentflowsallthetimewhe

3、neveravoltagesourceisconnectedtothediode.Butthecurrentflowsrapidlyinforwardbias,howeveraverysmallconstantcurrentflowsinreversebiascase.Thisnonereversingbehaviourarisesfromthenatureofthechargetransportprocessinthe2typesofmaterials.(2)(2)(b)(ii)Statetwoexamplesoftheappli

4、cationsofap-njunction.[2]-Photodiodes-LEDs(marksgivenforanycorrectanswer)(1)(1)(b)(iii)Ifthesaturationcurrentofthep-njunctiondiodeis1*10-15A,whatisthereversecurrentbeforebreakdown?Isthemaincomponentofthiscurrentduetodriftordiffusion?Markthevalueforreversecurrentyouobta

5、inedonthegraphyoudrewinQ1(b)(ii).[3]Understrongreversebiasthecurrentisthesameasthereversesaturationcurrenttherefore1*10-15A.1markforcorrectly‘marked’diagramThemaincomponentofthiscurrentisduetodrift.(1)(1)(1)(c)Drawanenergyleveldiagramforap-njunctioninthermalequilibrium

6、.Onyourdiagramclearlylabelthedirectionsofelectrondrift,electrondiffusion,holedriftandholediffusion.[7]1markfor‘alignment’ofFermilevelinallregions2marksforcorrectp-typeregionincludinglabels2marksforcorrectn-typeregionincludinglabels2marksforcorrectdepletionregionincludi

7、nglabels(7)Question2(a)Definethefermi-diracdistributionfunctionandexplainhowitcanbeused.[3]Answer:(2)ItcanbeusedtodeterminetheprobabilityoffindinganelectronwithagivenenergyE.(1)(b)(i)Whatisthepurposeofaddingdopanttosilicon?[1]AnswerToincreasetheconductivity.(1)(b)(ii)I

8、sboronann-typeorap-typedopant?[1]P-type(1)(b)(iii)Intermsofelectronsandholesdescribewhathappenswhensiliconisdopedwith