Lecture 5: HT C Properties asic operatio of a (Heterojuctio) ipolar Trasistor Abrupt ad graded juctios ase curret compoets Quasi-Electric Field Readig Guide: 143-16: 17-177 1
P p ++.53 Ga.47 As.53 Ga.47 As asic Operatio of a ipolar Trasistor Emitter ase Collector 1) ase-collector juctio reversed biased (very small curret) ) Emitter-ase juctio forward biased. V E V C 3) Electros/holes diffuse betwee emitter ad base cotrolled by V E! 4) Electros flow out of collector C (V E ). 5) Some electros recombie i the base hole curret (V E ) 14-1-8 Lecture 6, High Speed evices 14
Commo Emitter Operatio C -V C =V CE -V E +.1 V CE(sat) V E =.816V + V E - V CE V E is the iput voltage the iput curret C the output curret - Kollektorström (A).8.6.4. V E =.81V V E =.798V V E =.781V V E =.V How do we calculate: C (V E ) (V E ) Curret gai b: C / 1 3 4 V ce (V) 14-1-8 Lecture 6, High Speed evices 14 3
Homo&Graded Short-base Juctios without recombiatio Emitter ase Collector d() d( )= E dp() o recombiatio t ~,E <<L,p C p qa d qa d pe E i G a i WG d qv exp kt be qv exp kt usually domiated by recombiatio! x be d d qv exp kt 1 C exp 1 b C p E qv kt pe E Large curret gai eve if >> E E E exp kt g From the i terms We will see that should be high: 1 cm -3 Typical HT has b ~ -1. 4
Example b for GaAs/P HT o recombiatio b C p pe E E E exp kt g P.53 Ga.47 As E g (ev) 1.35.76 µ,mi (cm /Vs) 15 µ p,maj (cm /Vs) 15 (cm -3 ) 1 17 - A (cm -3 ) 1 19 E (m) 15 (m) 3 5
ase curret compoets Emitter SCR ase 1 3 4 5 Collector 1. ack ijected holes egligible i a well desiged HT. Recombiatio i the space-charge-zoe 3. Recombiatio i the base bulk regio 4. Recombiatio at base cotact iterface 5. Recombiatio at base surface Every time a electro is lost replaced through the base curret. 6
Recombiatio terms for bulk base Radiative recombiatio Shockley-Read-Hall (SRH) Auger Radiative SRH Auger U U rad U SRH U Auger t ase: i <<<<p t : total recombiatio life time GaAs recombiatio time (t ) The base of a HT has high dopig level A > 1 19 cm -3. t ca become very short! t 1-1 ps for A = 1 19-1 cm -3 Lecture 5, High Speed evices 14 14-1-8 7
Homo&Graded Short-base Juctios with recombiatio Emitter ase Collector () ( )= p() qv exp kt 1 C exp 1 E qv kt qv exp kt E E o recombiatio t ~ <<L ζ C = 1 τ = 1/L x ζ C = x dc x = sih ζ C( x) sih ζ C ( ) sih x cosh x = ex e x = ex + e x C = qa c d dx x= E = qa c d dx x= 8
Homo&Graded Short-base Juctios: Curret gai Emitter ase Collector d() d( )= dp() dc x = sih ζ C( x) sih ζ C ( ) sih x cosh x = ex e x = ex + e x ase Trasport Factor: α T = C E 1 1 + τ cosh x d sih x dx 1 + x = cosh (x) + O x4 Commo emitter curret gai: τ b = Q C = qa E β = C 1 1 α T = τ x dx C τ τ b ase Trasport Factor High Gai: Thi ase High mobility Log life time 9
log ( C, ) deality Factor ipolar Trasistor Recombiatio iside space-charge regio, RLO qvbe exp kt C qa d i G a exp qv kt be, base exp qv kt be Recombiatio iside eutral base layer V E Total base curret: =,RLO +,base 1
log ( C, ) b 1 miute exercise Gummel Plot V E V E How does b vary with V be? 11
Abrupt Juctio f bi f bi E v Electro curret set by potetial spike ad diffusio across base (complicated) Hole curet set by diffusio Potetial barrier for electros ~f bi Potetial barrier for holes ~f bi +E v b C p pe E E E exp kt v 1
Correctio due to limited thermal velocity ase () ( )=?? Collector 5 x 17 4.5 4 v diff = / 3.5 J diff q v q, iffusio is due to thermal motio of electros: v diff < v thermal J diff = q( )v thermal : Must be some electros at for a diffusio curret to flow! J diff q / v th (1) ( )= diff v th 1 3 4 5 ase Thickess (m) 13 Vdiff (cm/s) kt * m 3.5 1.5 1.5 (1) Calculatio for GaAs base v th Moder HTs have <3 m So this correctio ca be importat! However more complicated math!
miute excercise (1) () d(x) = t =t Cosider two HTs with two differet miority carrier cocetratios i the base: 1) Are the collector currets the same? ) Which oe has the highest b? 14
ase electric field troduce a (quasi) electric field i the base regio diffusio ad drift For a costat electric field, e J c q x d dx qe kt x J ( x) c J c Same curret smaller (x) as compared with pure diffusio! J c kt qe x 1 exp x q qe kt Possible to achieve higher b Lower Q better high frequecy properties! 15