Lecture #23. Warning for HW Assignments and Exams: Make sure your writing is legible!! OUTLINE. Circuit models for the MOSFET

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1 Lecture #23 arning for H Assignments and Exams: Make sure your writing is legible!! OUTLINE MOFET I s. V characteristic Circuit models for the MOFET resistie switch model small-signal model Reference Reading Rabaey et al.: Chapter Howe & odini: Chapter 4.5 Lecture 23, lide 1 MOFET I s. V Characteristic Typically, V is fixed when I is plotted as a function of V Long-channel MOFET V = 2.5 V > V AT hort-channel MOFET V = 2.5 V > V AT Lecture 23, lide 2

2 MOFET V T Measurement V T can be determined by plotting I s. V, using a low alue of V : I (A) I = k n L V V T V 2 V 0 V T V (V) Lecture 23, lide 3 ubthreshold Conduction (Leakage Current) The transition from the ON state to the OFF state is gradual. This can be seen more clearly when I is plotted on a logarithmic scale: In the subthreshold (V < V T ) region, V > 0 I qv exp nkt This is essentially the channelsource pn junction current. (ome electrons diffuse from the source into the channel, if this pn junction is forward biased.) Lecture 23, lide 4

3 Qualitatie Explanation for ubthreshold Leakage The channel V c (at the i surface) is capacitiely coupled to the gate oltage V G : n poly-i n n letion region EVICE V G p-type i C V = ε CIRCUIT MOEL V G i C ox C 1 N A V c Lecture 23, lide 5 Using the capacitie oltage diider formula (Lecture 12, lide 7): V c = C ox Cox C The forward bias on the channel-source pn junction increases with V G scaled by the factor C ox / (C ox C ) Cox C C n = = 1 C C ox V G ox lope Factor (or ubthreshold wing) is defined to be the inerse slope of the log (I ) s. V characteristic in the subthreshold region: V > 0 n kt q ln(10) Units: Volts per decade 1/ is the slope Note that 60 mv/dec at room temperature: kt ln( 10) = 60 mv q Lecture 23, lide 6

4 V T esign Trade-Off (Important consideration for digital-circuit applications) Low V T is desirable for high ON current I AT (V - V T ) η 1 < η < 2 where V is the power-supply oltage but high V T is needed for low OFF current log I Low V T I OFF,low VT High V T 0 I OFF,high VT Lecture 23, lide 7 V The MOFET as a Resistie witch For digital circuit applications, the MOFET is either OFF (V < V T ) or ON (V = V ). Thus, we only need to consider two I s. V cures: 1. the cure for V < V T 2. the cure for V = V I V = V (closed switch) R eq V V < V T (open switch) Lecture 23, lide 8

5 Equialent Resistance R eq In a digital circuit, an n-channel MOFET in the ON state is typically used to discharge a capacitor connected to its drain terminal: gate oltage V G = V source oltage V = 0 V drain oltage V initially at V, discharging toward 0 V I ATn k n = 2 L ( V V ) 2 Tn C load Lecture 23, lide 9 The alue of R eq should be set to the alue which gies the correct propagation delay (time required for output to fall to ½V ): R eq 3 V 4 I ATn 5 1 λnv 6 Typical MOFET Parameter Values For a gien MOFET fabrication process technology, the following parameters are known: V T (~0.5 V) C ox and k (<0.001 A/V 2 ) V AT ( 1 V) λ ( 0.1 V -1 ) Example R eq alues for 0.25 µm technology ( = L): How can R eq be decreased? Lecture 23, lide 10

6 MOFET Model for Analog Circuits For analog circuit applications, the MOFET is biased in the saturation region, and the circuit is designed to process incremental signals. A C operating point is established by the bias oltages V BIA and V, such that V > V V T Incremental oltages s and ds that are much smaller in magnitude perturb the operating point The MOFET small-signal model is a circuit which models the change in the drain current (i d ) in response to these perturbations V BIA s I i d R G MOFET V ds Lecture 23, lide 11 V NMOFET mall-ignal Model G i d gs g m gs r o ds g i id = i gm o i gs i L λi k ( V V ) ds = g Lecture 23, lide 12 T m gs g o ds output conductance transconductance