FET ( Field Effect Transistor)

Transcription

1 NMO MO NMO MO Enhancement eletion Enhancement eletion N kanál Transistors tyes Field effect transistors Electronics and Microelectronics AE4B34EM MOFET MEFET JFET 7. ecture Uniolar transistor arameters Alications N N FET ( Field Effect Transistor) Few imortant advantages of FET over BJT Transistors 1. Uniolar device oeration deends on only one tye of charge carriers (h or e). oltage controlled evice (gate voltage controls drain current) 3. ery high inut imedance ( ) 4. ource and drain are interchangeable in most owfrequency alications 5. ow oltage ow Current Oeration is ossible (owower consumtion) 6. ess Noisy as Comared to BJT 7. No minority carrier storage (Turn off is faster) 8. elf limiting device 9. ery small in size, occuies very small sace in ICs 10. ow voltage low current oeration is ossible in MOFET JFET Junction Field Effect Transistor The JFET transistor has the control electrode of oosite semiconductor conductivity than the The N junction is reverse olarized. oltage on the gate modulates the ON (sace charge), thereby control the thickness of the and hence conductivity JFET Oeration Real JFET construction JFET Crosssection ON N rain ate N N ubstrate ubstrate ource N

2 When is small Oeration rincile When is zero Oeration rincial ON ON Outut or rain ( I ) Characteristics of njfet Inut characteristics NJFET for = 10 Circuit for drain characteristics of the n JFET and its rain characteristics. Nonsaturation (Ohmic) Region: I The drain current is given by I aturation (or inchoff) Region: I I and I I 1 Where, I is the short circuit drain current, is the inch off voltage imiting arameters of JFET Basic circuits with JFET Breakdown voltage Maimal ower dissiation ma Maimal ate current I Current sources Current source of I Current source I <I

3 Basic circuits with JFET etting of oerational oint for class A JFET as small signal amlifier Common source amlifier: JFET equivalent circuit Amlifier equivalent circuit MEFET Transistor N junction () is made by chottky diode Tyical comonents on aas Alications: High frequency amlifiers, switchers, etc. Ohmic contact chottky contact N emi insulating substrate Ohmic contact Enhancement eletion MOFET Transistor Transistor is driven by electric field Transistor structure Metal M oid Oide O emiconductor Metal is now mainly relaced olyi, oide io ery high entrance gate resistance, u to Transistor can be very small ~ 3 nm ery huge integration density ~ comonents on chi ery small static ower consumtion Enhanced mode transistor (conduction eists when = 0. NMO transistor structure MO transistor structure eletion mode Enhancement mode eletion mode Enhancement mode ource iffusion area olyi ate Oide io rain iffusion area ource iffusion area olyi ate Oid io rain iffusion area N N N ubstrate ubstrát N ubstrate N

4 MO caacitorno contact MO caacitorno gate voltage and no bias Bend diagrams: d o Q Q oide N A oide N A d o d ac e bi e o e s e s ac e m e s e m e s e s 0 n =n i bi = s m n o n Inversion layer formation conduction eletion region e( bi ) MO caacitor gate voltage and no bias Q oide d o d e s e s e o Q ac e s N N h h h h h h e m B ubstre N e eletion mode h Holes 0 bi = s m n o n MO caacitor gate voltage and no bias Q oide d o d Q ac MO caacitor in inversion mode fields and charges Q d o d oide Q B < s < B e( bi ) e s e s e o e s e m B < s < B E e Inversion layer Q bi = s m 0 Q n o n Q

5 1 t t 3 t e( bi ) MO caacitordee inversion Q Q oide i Q s > B d o d ac e s e s e s MO caacitor in dee inversion mode fields and charges Q d o d oide Q i Q e o E e m e 0 Mobile inversion charge Q Q n o n Q Q i Inversion layer formation conduction When is alied eletion region e e e e e e N N Conduction e e e e e e N N e electrons Triode region: ( > t, < t) oly i Oid rain Current: W I nco t I Triode region N N ef 3 1 e e e e e e N N 0 e

6 1 t t 3 t Active region: (>t, t) Active region (>t, t) rain Current: I I W nco Active region 3 t rain Current: I W nco rain current with length modulation: t 1 e e e e e N N I W nco t 1 0 Transconductance n B n W gm nco t W I nco I t Active region (>t, t) Active region (>t, t) length modulation arameter: Outut resistivity: 1 ro I Transition frequency: gm ft ( C C C ) mallsignal model in active region Body (bulk) transconductance: g s g B m f g g s m (tyical )