3.6. Metal-Semiconductor Field Effect Transistor (MESFETs)

Transcription

1 .6. Metal-Semcouctor Fel Effect rator (MESFE he Metal-Semcouctor-Fel-Effect-rator (MESFE cot of a couctg chael potoe betwee a ource a ra cotact rego a how the Fgure.6.1. he carrer flow from ource to ra cotrolle by a Schottky metal gate. he cotrol of the chael obtae by varyg the epleto layer wth uereath the metal cotact whch moulate the thcke of the couctg chael a thereby the curret betwee ource a ra. Source ate ra + + -chael p-type or em-ulatg ubtrate epleto rego Fgure.6.1 Structure of a MESFE wth gate legth,, a chael thcke,. he key avatage of the MESFE the hgher moblty of the carrer the chael a compare to the MOSFE. Sce the carrer locate the vero layer of a MOSFE have a wavefucto, whch exte to the oxe, ther moblty - alo referre to a urface moblty - le tha half of the moblty of bulk materal. A the epleto rego eparate the carrer from the urface ther moblty cloe to that of bulk materal. he hgher moblty lea to a hgher curret, tracouctace a trat frequecy of the evce. he avatage of the MESFE tructure the preece of the Schottky metal gate. t lmt the forwar ba voltage o the gate to the tur-o voltage of the Schottky oe. h tur-o voltage typcally.7 for aa Schottky oe. he threhol voltage therefore mut be lower tha th tur-o voltage. A a reult t more ffcult to fabrcate crcut cotag a large umber of ehacemet-moe MESFE. he hgher trat frequecy of the MESFE make t partcularly of teret for mcrowave crcut. hle the avatage of the MESFE prove a uperor mcrowave amplfer or crcut, the lmtato by the oe tur-o ealy tolerate. ypcally epleto-moe evce are ue ce they prove a larger curret a larger tracouctace a the crcut cota oly a few trator, o that threhol cotrol ot a lmtg factor. he bure chael alo yel a better oe performace a trappg a releae of carrer to a from urface tate a efect elmate. he ue of aa rather tha lco MESFE prove two more gfcat avatage: frt, the electro moblty at room temperature more tha 5 tme larger, whle the peak electro velocty about twce that of lco. Seco, t poble to fabrcate em-ulatg (S aa ubtrate, whch elmate the problem of aborbg mcrowave power the ubtrate ue to free carrer aborpto.

2 he threhol voltage,, of a MESFE the voltage requre to fully eplete the ope chael layer. h threhol voltage equal: φ qn ε (.6.1 where φ the bult- potetal a the thcke of the ope rego. h threhol voltage ca alo be wrtte a a fucto of the pch-off voltage : φ (.6. here the pch-off voltage equal: qn ε (.6. he ervato of the curret a MESFE tart by coerg a mall ecto of the evce betwee y a y + y. he curret ety at that pot ca be wrtte a a fucto of the graet of the chael voltage: J qv qn E qn ( y y (.6.4 he ra curret relate to the curret ety a the part of the MESFE chael that ot eplete. J( x ( y (.6.5 here the epleto layer wth at poto y relate to the chael voltage, (y, by: x ( y ε ( φ + ( y (.6.6 qn he equato for the curret ca ow be tegrate from ource to ra, yelg: y qn (1 φ + (.6.7 Sce the teay-tate curret the evce epeet of poto, the left ha term equal tme o that: qn ( ( φ + (.6.8

3 tegrato reult : + N q ( ( φ φ (.6.9 h reult val a log a the wth of the u-eplete chael ( x (y potve, amely for: (.6.1 h coto alo efe the quaratc rego of a MESFE. For larger ra voltage, the curret aturate a equal that at at, (.6.11 he correpog curret the aturato curret,,at : at N q, (φ (.6.1 A example of the reultg - charactertc how Fgure.6.. he correpog evce parameter are lte able.6.1. Fgure.6. ra curret veru ra-source voltage at a gate-ource voltage of.,.4,.6.8 a 1. olt for a lco MESFE wth bult- potetal of 1. hael parameter a evce meo are lte able ra-source oltage ( ra curret (ma

4 arameter Symbol alue hael wth 1 mm hael legth 1 m hael moblty 1 cm /- hael opg N 1 17 cm - hael thcke 115 m Bult- potetal φ 1 able.6.1 MESFE parameter ue to calculate Fgure.6.. he trafer charactertc of a MESFE how Fgure.6. a compare to a quaratc expreo of the form:, at ε w ( (.6.1 where w the average epleto layer wth the chael layer. he quaratc expreo yel the ame curret at φ for w 8. he cloe ft at tme ue to jutfy ug the mpler quaratc equato. (ra curret 1/ (A 1/ ate oltage (

5 Fgure.6. rafer charactertc of a MESFE. Show the quare root of the ra curret of the MESFE (ol le a a quaratc ft wth w 8 (otte le.