Lecture 38 OUTLINE The MOSFET Bulkcharge theory Body

Lecture #38 OUTLINE The MOSFET: • Bulk-charge theory • Body effect parameter • Channel length modulation parameter • PMOSFET I-V • Small-signal model Reading: Finish Chapter 17, 18. 3. 4 1 Spring 2007 EE 130 Lecture 38, Slide 1

Problem with the “Square Law Theory” • Ignores variation in depletion width with distance y 2 Spring 2007 EE 130 Lecture 38, Slide 2

Modified (Bulk-Charge) Model • linear region: • saturation region: 3 Spring 2007 EE 130 Lecture 38, Slide 3

MOSFET Threshold Voltage, VT The expression that was previously derived for VT is the gate voltage referenced to the body voltage that is required reach the threshold condition: Usually, the terminal voltages for a MOSFET are all referenced to the source voltage. In this case, and the equations for IDS are 4 Spring 2007 EE 130 Lecture 38, Slide 4

The Body Effect Note that VT is a function of VSB: where g is the body effect parameter When the source-body pn junction is reverse-biased, |VT| is increased. Usually, we want to minimize g so that IDsat will be the same for all transistors in a circuit 5 Spring 2007 EE 130 Lecture 38, Slide 5

MOSFET VT Measurement • VT can be determined by plotting IDS vs. VGS, using a low value of VDS IDS VGS 6 Spring 2007 EE 130 Lecture 38, Slide 6

Channel Length Modulation Parameter, l • Recall that as VDS is increased above VDsat, the width DL of the depletion region between the pinch-off point and the drain increases, i. e. the inversion layer length decreases. 7 Spring 2007 EE 130 Lecture 38, Slide 7

P-Channel MOSFET • The PMOSFET turns on when VGS < VTp – Holes flow from SOURCE to DRAIN is biased at a lower potential than the SOURCE VG VS • VDS < 0 VD GATE P+ IDS P+ N VB • IDS < 0 • |IDS| increases with • |VGS - VTp| • |VDS| (linear region) • In CMOS technology, the threshold voltages are usually symmetric: VTp = -VTn Spring 2007 EE 130 Lecture 38, Slide 8 8

PMOSFET I-V • Linear region: • Saturation region: m = 1 + (3 Toxe/WT) is the bulk-charge factor 9 Spring 2007 EE 130 Lecture 38, Slide 9

Small Signal Model • Conductance parameters: 10 Spring 2007 EE 130 Lecture 38, Slide 10

Inclusion of Additional Parasitics 11 Spring 2007 EE 130 Lecture 38, Slide 11

Cutoff Frequency • fmax is the frequency where the MOSFET is no longer amplifying the input signal – Obtained by considering the small-signal model with the output terminals short-circuited, and finding the frequency where |iout / iin| = 1 ® Increased MOSFET operating frequencies are achieved by decreasing the channel length 12 Spring 2007 EE 130 Lecture 38, Slide 12