1. Spring 2007EE130 Lecture 38, Slide 1 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

2. Spring 2007EE130 Lecture 38, Slide 2 Problem with the “Square Law Theory” Ignores variation in depletion width with distance y

3. Spring 2007EE130 Lecture 38, Slide 3 Modified (Bulk-Charge) Model saturation region: linear region:

4. Spring 2007EE130 Lecture 38, Slide 4 The expression that was previously derived for V T is the gate voltage referenced to the body voltage that is required reach the threshold condition: MOSFET Threshold Voltage, V T Usually, the terminal voltages for a MOSFET are all referenced to the source voltage. In this case, and the equations for I DS are

5. Spring 2007EE130 Lecture 38, Slide 5 Note that V T is a function of V SB : The Body Effect where  is the body effect parameter When the source-body pn junction is reverse-biased, |V T | is increased. Usually, we want to minimize  so that I Dsat will be the same for all transistors in a circuit

6. Spring 2007EE130 Lecture 38, Slide 6 MOSFET V T Measurement V T can be determined by plotting I DS vs. V GS, using a low value of V DS I DS V GS

7. Spring 2007EE130 Lecture 38, Slide 7 Channel Length Modulation Parameter, Recall that as V DS is increased above V Dsat, the width  L of the depletion region between the pinch-off point and the drain increases, i.e. the inversion layer length decreases.

8. Spring 2007EE130 Lecture 38, Slide 8 P-Channel MOSFET The PMOSFET turns on when V GS < V Tp –Holes flow from SOURCE to DRAIN  DRAIN is biased at a lower potential than the SOURCE In CMOS technology, the threshold voltages are usually symmetric: V Tp = -V Tn P+P+ P+P+ N GATE VSVS VDVD VGVG I DS VBVB V DS < 0 I DS < 0 |I DS | increases with |V GS - V Tp | |V DS | (linear region)

9. Spring 2007EE130 Lecture 38, Slide 9 PMOSFET I-V Linear region: Saturation region: m = 1 + (3T oxe /W T ) is the bulk-charge factor

10. Spring 2007EE130 Lecture 38, Slide 10 Small Signal Model Conductance parameters:

11. Spring 2007EE130 Lecture 38, Slide 11 Inclusion of Additional Parasitics

12. Spring 2007EE130 Lecture 38, Slide 12 Cutoff Frequency f max 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 |i out / i in | = 1  Increased MOSFET operating frequencies are achieved by decreasing the channel length