ENG1002 Lecture Notes - Lecture 8: Organic Semiconductor, Electric Field, Direct And Indirect Band Gaps
30 May 2018
School
Department
Course
Professor
dep$on$movement$of$many$electrons$jumping$from$one$hole$to$another$(shld$
be$lower$that$for$electrons)
Number$MOST$IMPT$because$it$relies$on$activation$of$electrons$over$band$gap,$
which$is$the$process$most$impt$for$the$conductivity$of$pure$Si
HOWEVER
As$you$increase$temp,$probablity$of$jumping$over$increases$very$rapidly
-->$conductivity/resistivity$of$intrinsic$(pure)$semiconds$is$quite$sensitive$to$temp
If$a$device$made$of$pure$silicon$is$used$as$part$of$a$circuit,$it$would$work$
differently$at$10dC$and$30dC$-->$inconsistent/not$robust$devices$anymore
@$same$temp:
Ge$charge$carriers$ >$
Si$charge$carriers
!"#$%&'%()'!*%(+&,-(#+$')./01234)567/869):642106;<
Eg.$arsenic
Eg.$adding$a$few$elements$of$an$element$(1$extra/less$p$electron)$=$"doping"
Phosphorus atoms$will$occupy$sites$where$Si$atoms$were,$and$bond$with$4$Si$
atoms$like$pure$Si$did,$except$for$the$extra$free$electron$("floating$electron")
Has$special$energy$level$ falling$within$band$gap$but$very$close$to$
conduction$band$("donor$state"),$so$close$that$it$will$be$enough$for$
electrons$to$jump$frm$donor$level$ into$conduction$band$and$conduct$elec$
when$electric$ field$is$applied$@$rm$temp
n-type (negative-type)$-negative$charges$added$to$allow$us$to$make$
conducting$electricity$ easier
**no$electrons$jumping$frm$valence$band$to$conduction$band$+$no$holes$-->$
only$charge$carriers$are extra$electrons
Number$of$conducting$electrons$can$be$controlled$by$controlling$the$number$
of$P$atoms$added$(each$P$atom$gives$1$e)
Better$to$use$elements$with$less$shells$because$size$of$atoms$is$
smaller$-->$easier$ and$less$complicated
Eg.$aluminium,$gallium
Adding$Boron atoms$-only$3$electrons$in$second$shell$while$Si$has$4
Same$thing$as$n-type$except$reversed
Deficiency$of$1$electron$(AKA$hole)$occupies$special$energy$state$just$
within$band$gap$but$very$close$to$the$valence$band$("acceptor$state"),$
because$electrons$can$very$easily$jump$from$valence$band$into$the$
acceptor$level$ because$of$the$small$energy$diff,$but$NOT$into$the$
conduction$band
p-type (positive-type)$-a$material$that$can$conduct$electricity$ through$the$
migration$of$holes$(number,$charge$and$mobility)$and$conduct$elec$when$
electric$ field$is$applied$@$rm$temp
**can$control$conductivity$very$well
**temp$dependence$around$rm$temp$is$very$low$(PTO)
(like$alloys)
If$lots$of$atoms$added$into$Si,$lattice$will$be$disrupted,$affecting$mobility$of$
electrons$and$holes$as$function$of$concentrations$of$atoms$added
