1. MOS: Device Operation and Large Signal Model

Transcription

1 1. MOS: ece Oerato ad arge Sgal Model Readg: Sedra & Smth Sec (S&S 5 th Ed: Sec ) ECE 10, Fall 011, F. Najmabad

2 Oeratoal Bass of a Feld-Effect Trasstor (1) Cosder the hyothetcal semcoductor below: (costructed smlar to a arallel late caactor) Electrcal cotact Metal Isulator P-tye semcoductor oet os Holes (majorty carres)

3 Oeratoal Bass of a Feld-Effect Trasstor () eleto Rego (o majorty carrer) Ierso layer ( chael ) eleto Rego (o majorty carrer) If we aly a oltage 1 betwee electrodes, a charge Q = C 1 wll aear o each caactor late. o The electrc feld s strogest at the terface wth the sulator ad charge lkes to accumulate there. Holes are ushed away from the sulator terface formg a deleto rego. The deleto rego creases deth as 1 s creased. If we crease 1 aboe a threshold alue ( t ), the electrc feld s strog eough to ull free electros to the sulator terface. As the holes are reelled ths rego, a chael s formed whch cotas electros the coducto bad ( erso layer ). Ierso layer s a rtual -tye materal.

4 Oeratoal Bass of a Feld-Effect Trasstor (3)

5 Oeratoal Bass of a Feld-Effect Trasstor (4) e eed to elmate currets flowg the -tye semcoductor for our dece to work,.e., curret flowg oly the chael whch acts lke a -tye materal Make the termals across the -tye semcoductor of -tye materal (set u dodes betwee termals ad -tye body ) Heay dog of the -tye termals rodes a source of free electros for the chael. Make sulator layer as th as ossble to crease the electrc feld. Body-source ad body-dra juctos should always be reerse bas for FET to work!

6 Chael wdth () s the smallest feature o the ch surface MOSFET cartoos for derg MOSFET characterstcs MOSFET mlemetato o a ch

7 NMOS - Characterstcs (1) To esure that body-source ad body-dra juctos are reersed bas, we assume that Body ad Source are coected to each other ad 0. o e wll re-exame ths assumto later For >, a chael s formed. The total charge the chael s Q = C C = C C t ε ε ε = t : Thckess of sulator : ermtty of sulator = 3.9ε = C : Caactace er ut area 0 ( = ) F/m (for SO )

8 NMOS - Characterstcs () e ow aly a small oltage betwee dra ad source. For >, a chael s formed. Electros (because -chael) wll moe from source to dra wth a elocty: drft = µ E = µ The Resultg Curret s dq dq dx Q Q = = = drft = µ dt dx dt Q = C (- ) = µ C ( t, )

9 NMOS - Characterstcs (3) he > ad s small: = = = µ C g - : wth Oerdre oltage g = µ C MOS acts as a resstace wth ts coductty cotrolled by (or ).

10 NMOS - Characterstcs (4) he s creased the chael becomes arrower ear the dra (local deth of the chael deeds o the dfferece betwee ad local oltage). Q Q = µ = C ( = µ C = µ C 0.5 [( ) 0.5 ] [ 0.5 ] )

11 NMOS - Characterstcs (5) he s creased further such that =, the chael deth becomes zero at the dra (Chael ched off ). = µ C µ [ ] [ ].5 = C = µ 0.5 C he s creased further, >, the locato of chael choff remas close to the dra ad remas armately costat.

12 NMOS - Characterstcs (5) For a ge (or )

13 NMOS - Characterstcs (Summary) k = µ C

14 Chael-dth Modulato The exresso we dered for saturato rego assumed that the ch-off ot remas at the dra ad thus remas costat. I realty, the ch-off ot moes slghtly away from the dra: Chael-wdth Modulato = 0.5µ C λ = 1/ A ( 1+ λ )

15 Body Effect Recall that ra-body ad Source-Body dodes should be reersed based. o e assumed that Source s coected to the body ( SB = 0) ad = B > 0 I a ch (same body for all NMOS), t s mossble to coect all sources to the body (all NMOS sources are coected together. Thus, the body (for NMOS) s coected to the largest egate oltage (egate termal of the ower suly). og so, chages the threshold oltage (called Body Effect ) t = + γ ( φ + φ ) 0 F SB F I ths course we wll gore body effect as well as other secodorder effects such as elocty saturato.

16 P-chael Ehacemet MOS (PMOS) A PMOS ca be costructed aalogous to a NMOS: (-tye body), healy doed -tye source ad dra. A rtual -tye chael s formed a P-MOS (holes are carrers the chael) by alyg a egate. - characterstc equatos of a PMOS s smlar to the NMOS wth the exceto: o oltages are egate (we swtch the termals to hae oste oltages: use SG stead of ). o Use moblty of holes, µ, stead of µ the exresso for

17 MOS Crcut symbols ad coetos NMOS PMOS

18 MOS - Characterstcs Equatos NMOS Cut - Off : Trode : Saturato : ad ad = 0 = 0.5µ C = 0.5µ C ( [ ( ) ] ) [ 1+ λ ] PMOS Cut - Off : Trode : Saturato : SG SG SG ad ad S S SG SG = 0 = 0.5µ C = 0.5µ C ( [ ( ) ] SG SG ) S [ 1+ λ ] S S *Note: λ = 1/ A for NMOS ad λ = 1/ A for PMOS.

19 MOS - Characterstcs Equatos NMOS ( = ) Cut - Off : Trode : Saturato : ad ad = 0 = 0.5µ C = 0.5µ C [ ] [ 1+ λ ] PMOS ( = SG )* Cut - Off : Trode : Saturato : ad ad S S = 0 = 0.5µ C = 0.5µ C [ ] S S [ 1+ λ ] S *Note: S&S defes = SG ad uses the PMOS formulas.

20 To Sole MOS Crcut: (wth arge Sgal Model) 1. Hyothess: assume oe of the modes of oerato for the MOSFET. Sole: Use the equatos for the selected mode to sole the crcut 3. Check: at the ed erform the check for the selected mode to erfy the hyothess 4. Redo: f the hyothess check fals, try aother hyothess ad start oer