Lecture 6: III-V FET DC I - MESFETs. Metal-Semiconductor Junction Basic MESFET Operation

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Lecture 6: III-V FET DC I - MESFET Metal-Semiconuctor Junction Baic MESFET Operation Lecture 6, High Spee Device Fiel Eect Tranitor W L g V g Gate Source y Drain V DS N + N + x The gate
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Lecture 6: III-V FET DC I - MESFET Metal-Semiconuctor Junction Baic MESFET Operation Lecture 6, High Spee Device Fiel Eect Tranitor W L g V g Gate Source y Drain V DS N + N + x The gate electroe control the carrier concentration in the channel Source/Drain et the potential at the ource/rain ie Electron low rom ource to rain I DS an n(x,y) epen on geometry an tranport propertie. 2D problem (in x an y) Lecture 6, High Spee Device Fiel Eect Tranitor Metal Oxie Bulk MOSFET V g =1V n+ n+ p-type V D =1V SOI, Quantum Well MOSFET Metal Oxie n+ n - n+ Oxie or wie bangap Semiconuctor p-type, S.I. Inulating Metal MESFET V g =-1V Depletion region n+ n n+ p-type, S.I. Inulating Metal Wie ban emiconuctor HEMT n+ n+ n - p-type, S.I. Inulating Wie ban emiconuctor Lecture 6, High Spee Device Si v. III-V Fiel Eect Tranitor (FET) Si: µ n 13 V/cm 2. v at cm/ InGaA µ n 140 cm 2 /V! v at cm/ SiO 2 -Si excellent interace InGaA- GaO x InO x AO x poor interace III-V MOSFET are iicult to abricate Alternative: Semiconuctor to iolate the gate rom the channel Simple: MESFET Better: HEMT or III-V MOSFET SiO 2 /HSiO 2 Schottky Barrier n+ n+ Si p-type An Si MOSFET ue an oxie (SiO2, SiHO 2 ) to iolate the gate n+ GaA n n+ GaA Semi-Inulating A III-V Metal-Semiconuctor FET (MESFET) Lecture 6, High Spee Device Metal-Semiconuctor Junction Similar to a p + N junction! q m qc q n F b bi F b q q qc bi b m F b q n Schottky barrier height Buil in potential Too implitic! However, now we ignore that we terminate the crytal an create a lot o urace tate Lecture 6, High Spee Device Metal-Semiconuctor Junction II Experiment how only a very weak epenence o b a a unction o metal work unction b q b m Surace recontruction an urace eect create a large number o tate in the bangap, which pin the Fermienergy The energy poition o thee urace tate et the Schottky barrier height. For GaA, b V InP b 0.3V, InA b -0.1V, In 0.53 Ga 0.47 A b 0.1V bi F b q n b i a material parameter! Lecture 6, High Spee Device MESFET Structure implet FET tranitor V g (negative) Source Gate Drain N Depletion Region: n 0 b a y x Semi-inulating Schottky epletion uner gate moulate channel thickne b V DS caue current to low rom ource to rain, which can be moulate by V g Lecture 6, High Spee Device Metal-Semiconuctor Junction III r X ep a q y q y N X y y N q N X ep ep y 2 y 2 y ( X ) ( X ) ep ( 0) 0 ep Re. potential 0 y X ep 2 qn bi V a Depletion thickne, maximum X ep =a qn 2 a 2 Potential neee to ully eplete own to a Lecture 6, High Spee Device 2 minute exercie part 1 Depletion Ege V g =0 V g =-1V A B C D Black V g = 0V Which green plot correpon to V g =-1 V? Remember: negative bia increae the potential energy o an electron!! Lecture 6, High Spee Device 2 minute exercie part 2 V ub =1V V g V g =-1V V + ub - Depletion ege Black plot V g an V ub = 0V Which re plot correpon to V g =-1 V an V ub = 1V? Lecture 6, High Spee Device MESFET Operation V DS + - V g I I V c =0 V c (x) V c =V Reitive voltage rop along channel DV g DV g DV g V DS The potential uner the gate i et by the channel-gate potential V c (x) Lecture 6, High Spee Device Calculation o the current V DS + - V g (x,0)=0 I J n = qμ n n V J n = 0 2 φ x φ q y2 = V = ε N n Complicate 2D problem! x y GCA Graual Channel approximation: 2 /x 2 2 /y 2 2 φ y 2 = q N ε φ(x, b) J n = qμ n n x J n (x)/x = 0 X ep (x) varie lowly with x Electric iel in x-irection i mall Lecture 6, High Spee Device Drit Current i ch i x X x ch ep qwn 0 i a x n xbx I ch D x b x x V V (x) bi g c x Drit current Continuity x x a X ep a 1 x Graual channel approximation b(x) varie lowly with x b(x) i etermine by olving (y) Depletion E-iel L x L L I Dx qwn na 1 x qwn na x x 1 x x x Lecture 6, High Spee Device Saturation Voltage, Pinch-o V =0V V 0 V DS,at V =V DS,at V V DS,at At pinch-o, the epletion region reache the S.I region Our 1D-ecouple moel break own: ( 2 /x 2 0) we nee to olve 2D poion equation, D (x,y) (numerical olution neee) Reult: Channel o inite thickne orm at channel ege. Increae V rop inie thi region, or between channel-rain. Current remain inepenent o V ater V,at V DS, at bi V g Lecture 6, High Spee Device Current-Voltage Characteritic u( x) I D I - arb unit qwn x 0 L na L V V g 0 2 3 V g =0 3/ 2 V g =- V T 2 3 3/ Lecture 6 High Spee Device V V V DS, at T bi g bi bi qn 2 bi V V a g g 2 V V bi DS DS, at V g V DS V DS, at 1 V V DS DS, at Pinch-o voltage Threhol Voltage MESFET limitation I D, at qwn na L / 2 g m I D, at V g qwn na L 1 bi V g We want a high g m but: Poitive gate-voltage very high gate leakage! From electrotatic: a l/3 Increae N D but thi lower µ n ue to impurity cattering an increae gate leakage! Increae µ n by material choice: Nee to have a high Schottky barrier! (InGaA, InA can t be ue!) We can o better uing heterotructure or MOSFET! Lecture 6, High Spee Device
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