ecture #5 Design Project: Due in class (5 PM on hursday May 1 st 0 pt penalty for late submissions, accepted until 5 PM on 5/8 Your J design does not need to meet the performance specifications when and N are varied by +/- 10% Equation for E G assumes N E is in cm -3 and is in K Quiz#5 Results: (undergrad. s only N=60 Mean=0.5 Std.Dev.=3.6 30 0 10 0 EE130 QUZ SORE REND 1.6 17.9 18.9 17.9 0.5 Q1 Q Q3 Q4 Q5 EE130 ecture 5, Slide 1 OUNE NMOSFE - Effective mobility ransconductance PMOSFE - Subthreshold current EE130 ecture 5, Slide
deal MOSFE - haracteristics (Enhancement Mode NMOS ransistor inear region Saturation region EE130 ecture 5, Slide 3 Review: Qualitative Operation of the NMOSFE depletion layer he potential barrier to electron flow from the source into the channel is lowered by applying GS > Electrons flow from the source to the drain by drift, when >0. ( > 0. EE130 ecture 5, Slide 4 he channel potential varies from S at the source end to D at the drain end. (he ersion layer can be modeled as a resistor.
hen D is increased to be equal to G -, the ersion-layer charge density at the drain end of the channel equals zero, i.e. the channel becomes pinched off As D is increased above G -, the length of the pinch-off region increases. he voltage applied across the ersion layer is always Dsat = GS -, and so the current saturates: Dsat = = Dsat f is significant compared to, then will increase slightly with increasing > Dsat, due to channel-length modulation EE130 ecture 5, Slide 5 Q NMOSFE - haracteristics D > S urrent in the channel flows by drift hannel voltage (y varies continuously between the source and the drain qn Aε Si(ψ + ( y = F + ( y + ψ + hannel ersion charge Qdep( y ( y = oxe G F ( y ψ oxe ox EE130 ecture 5, Slide 6
1 st -Order Approximation Neglect variation of Q dep with y Q dep = qn ε (ψ + A Si S Q = oxe [ + ] G S where = threshold voltage at the source end: = F + S + ψ + qn ε (ψ + A Si ox S EE130 ecture 5, Slide 7 NMOSFE urrent (1 st -order approx. onsider an incremental length dy in the channel. he voltage drop across this region is dy dy dy d = dr = = = σ qµ n Q µ 0 dy = S D µ Q ( d D = µ Q d ( S = µ oxe GS in the linear region = Dsat = oxeµ ( GS in the saturation region EE130 ecture 5, Slide 8
= Q Effective Mobility v = Q oxe G where µ is the ective electron mobility = ( / µ ( µ E = Q µ he NMOSFE can be modelled as a resistor at low : R = = µ oxe( G EE130 ecture 5, Slide 9 µ vs. Effective Normal Field ( gs + t + 0./6 oxe (M/cm (NFE Scattering mechanisms: coulombic scattering phonon scattering (PFE surface roughness scattering ( gs + 1.5 t 0.5/6 ox e (M/cm EE130 ecture 5, Slide 10
is a function of S : = = 0 0 + γ he ody Effect qn Aε Si + ( ψ + S ψ oxe ( ψ + ψ S where γ is the body ect parameter hen the source-body pn junction is reverse-biased, increases. Usually, we want to minimize γ so that Dsat GS will be the same for all transistors in a circuit EE130 ecture 5, Slide 11 Problem with the Square aw heory Assumes that gate charge is purely balanced by ersion charge gnores variation in depletion width with distance y EE130 ecture 5, Slide 1
Modified Model m = oxeµ ( GS 3 since ε = 3 dm oxe where m = 1+ = 1+ Si ε Si O oxe dm EE130 ecture 5, Slide 13 Modified Model: Dsat & ransconductance saturation region: GS D Dsat = m Dsat = oxeµ ( m GS transconductance: g m = d /d GS g msat = oxeµ m ( GS EE130 ecture 5, Slide 14
MOSFE Measurement can be determined by plotting vs. GS, using a low value of EE130 ecture 5, Slide 15 P-hannel MOSFE he PMOSFE turns on when GS < p Holes flow from SOURE to DRAN DRAN is biased at a lower potential than the SOURE S G GAE P + P + N D n MOS technology, the threshold voltages are usually symmetric: p = - n EE130 ecture 5, Slide 16 < 0 < 0 increases with GS - p (linear region
inear region: 0 < PMOSFE - m = oxeµ p, ( GS + p GS Saturation region: > m = Dsat = oxeµ m < GS m p p p, ( GS p m = 1 + (3 oxe / dm is the bulk-charge factor EE130 ecture 5, Slide 17 Sub-hreshold eakage urrent e had previously assumed that there is no channel current when GS <. his is incorrect. onsider S close to ψ : here is some ersion charge at the surface, which gives rise to subthreshold current flowing between the source and drain: = µ oxe k ( m 1 e q q( G / mk (1 e q / k EE130 ecture 5, Slide 18
d(log10 S d GS k = ln(10(1 + q Sub-hreshold Slope S 1 dm oxe EE130 ecture 5, Slide 19 Design radeoff EE130 ecture 5, Slide 0