l4-mosfet-iv

8/9/2019 L4-mosfet-IV

1/19

Lecture-4-MOS-Model

MOSFET

Now add s ource and d r ain t o MOS c apa c itor. Th e n ew st ruc ture is

c all e d MOS t r ansist or.Two typ e s of MOS t r ansist or s : NMOS and PMOS

Th e distan ce b e twee n S and D is c all e d c hann e l le ngth L and lat er alex te nt of c hann e l is w idth W . b o th W and L a re ver y imp or tantpa r am e ter s . Th e c hann e l is for m e d th rou gh appli e d gat e vo ltag e b e twee n S and D .


2/19

Lecture-4-MOS-Model

N-C hann e l MOSFET Op er ati o n

Wh e n V gs > V to is appli e d o n gat e inv er si o n lay er is for m e d and s ource to d r ain a re co nn ec te d th rou gh a n -typ e c hann e l. Gat e Vo ltag e is appli e d w ith re sp ec t t o s ource th e n Vt o is sam e as inc as e of MOS Capa c itor.


3/19

Lecture-4-MOS-Model

N-C hann e l MOSFET Op er ati o n : I -V Cur ve (1)

NMOS in Lin e a r Re gi o n :

if small d r ain v o ltag e is appli e d, d r ain curre nt f low s th rou gh th e co nd uc ting c hann e l.As d r ain v o ltag e in cre as e s, d r ain curre nt als o in cre as e s lin e a r ly w ithvo ltag e. Th e c hann e l re gi o n a c ts as v o ltag e co nt ro lle d re sist or. This o p er ati o n m o d e is c all e d lin e a r m o d e.


4/19

Lecture-4-MOS-Model

N-C hann e l MOSFET Op er ati o n : I -V Cur ve (2)NMOS in sat ur ati o n R e gi o n :

As d r ain v o ltag e in cre as e s again, th e gat e vo ltag e is n o t s uff ic ie nt t o maintain c hann e l b e low th e gat e.Th e c ha r g e and c hann e l d e pth sta r t t o d ecre as e at d r ain e nd . AtVds =Vdsat th e c ha r g e and d e pth at d r ain e nd b eco m e zero. This p o int is c all e d pin c h off p o int . Be yo nd pin c h -off d e pl e tio n re gi o nfor ms adja ce nt t o d r ain and g row s t o s ource w ith in cre asing V ds .This o p er ati o n m o d e is c all e d sat ur ati o n m o d e. Ne a r pin c h -off s ec tio nhigh f ie ld re gi o n for ms b e twee n c hann e l-e nd and d r ain . Elec tro nsa rr iving at this e nd inj ec te d t o d r ain re gi o n and a cce ler at e d t o d r ain .


5/19

Lecture-4-MOS-Model

N-C hann e l MOSFET Op er ati o n : I -V Cur ve (3)

3-D analysis of this syst e m is v er y co mpl ex to e stablish th e I-V re lati o n of NMOS .

S e ver al app rox imati o ns b e mad e to simpli f yth e p ro bl e m .

Th e G r ad u al -C hann e l App rox imati o n (G CA )is u s e d w hi c h co nv er ts 3D p ro bl e m int o 1-Dfor analyti c d er ivati o n of I-V.


6/19

Lecture-4-MOS-Model

G r ad u al Chann e l App rox imati o n (G CA )


7/19

Lecture-4-MOS-Model

G r ad u al Chann e l App rox imati o n (G CA )(2)

This m e ans e nti re c hann e l re gi o n is inv er te d .Ass u m e Q I(y) is t o c ha r g e in inv er si o n lay er w hi c h is fu n c tio n of Vgsand V c s (y) and c an b e d ef in e d as

Inv er si o n lay er thi ck n e ss va r ie s fro m S t o D. n ow co nsid er in cre m e ntal re sistan ce dR of di ffere ntial c hann e l re gi o n and ass u m e co nstant s urf a ce m o bility n of e lec tro ns in c hann e l.


8/19

Lecture-4-MOS-Model

G r ad u al Chann e l App rox imati o n (G CA )(3)Now ass u m e curre nt d e nsity is u ni for m a cro ss th e s e gm e nt .Vo ltag e d ro p al o ng th e in cre m e ntal s e gm e nt in y -di rec tio n is

Arr ang e and int e g r at e al o ng th e c hann e l : 0


9/19

Lecture-4-MOS-Model

G r ad u al Chann e l App rox imati o n (G CA )(4)


10/19

Lecture-4-MOS-Model

Curre nt V o ltag e Cha r a c ter isti c

Giv e n : W =20 m, L=2 m, Vt0=1 .0V, k=0 .42 m A / V2


11/19

Lecture-4-MOS-Model


Sat ur ati o n re gi o n curre nt equ ati o n .


12/19

Lecture-4-MOS-Model


Sat ur ati o n re gi o n curre nt equ ati o n .


13/19

Lecture-4-MOS-Model

Chann e l Le ngth M o d u lati o n

For Vds > Vdsat


14/19

Lecture-4-MOS-Model

Chann e l Le ngth M o d u lati o nElec tro ns n ow hav e to tr av e l L le ngth . On e c an wr ite curre nt equ ati o nagain re pla c ing L by L . GCA still h o lds .

Simpli f y th e ex p re ssi o n


15/19

Lecture-4-MOS-Model

Chann e l Le ngth M o d u lati o n


16/19

Lecture-4-MOS-Model

S u bst r at e Bias E ffec ts


17/19

Lecture-4-MOS-Model

Curre nt -Vo ltag e S u mma r y

? A

T V GS V DS V T V GS V

DS V SBV T V GS V LW OX C nSat D I


DS V DS V DS V SBV T V GS V LW OX C nlin D I

T V GS V D I

""

!

"

!

!

,

)1(2))((2

.)(

,

)1(2))((22

)(

f or 0 NMOS

P Q

P Q


18/19

Lecture-4-MOS-Model

Curre nt -Vo ltag e S u mma r y

? A


DS V SBV T V GS V LW OX C p

Sat D I


DS V DS V DS V SBV T V GS V LW OX C p

lin D I

T V GS V D I

!

"

!

u!

,

)1(2))((2

.)(

,

)1(2))((22

)(

f or 0 PMOS

P Q

P Q


19/19

Lecture-4-MOS-Model

Nex t C lass T o pi c

MOSFET Inv er ter : Stati c Cha r a c ter isti c s

l4-mosfet-iv

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