Understanding MOSFET Operation and Beyond in Electronic Devices

Explore the intricacies of MOSFET operation, from the basics of channel charge to pinch-off points and beyond. Delve into the saturation region, overdrive voltage, and the unique characteristics of MOSFETs compared to bipolar transistors. Gain insights into channel length modulation effects and the differences between bipolar transistors and MOSFETs in terms of saturation and amplification.

• MOSFET
• Operation
• Saturation Region
• Electronic Devices
• Channel Length Modulation

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1. Lecture 15 October 13, 2023 1

2. MOSFET-Operation ???= ??? ??? - + ?? GND Gate (G) Source (S) Drain (D) Body (B) - + ?? Metal GND GND GND Oxide N+ N+ P+ V(x) x P- Substrate L 0 dx ? = ?? = ?(? ???) Channel charge: ??? = ? ??? ??? ?(?) ??? ??0 = ? ??? ??? ??? ??? = ? ??? ??? ??? ??? ??? = 0 When ???= ??? ??? Pinch off Point 2

3. Pinch off point ???= ??? ??? - + ?? GND Gate (G) Source (S) Drain (D) Body (B) - + ?? Metal GND GND GND Oxide N+ N+ P+ Pinch off V(x) x P- Substrate L 0 dx ??? = 0 Pinch off Point When ???= ??? ??? 3

4. Beyond Pinch off ??? ??? ??? - + ?? GND Gate (G) Source (S) Drain (D) Body (B) - + ?? Metal GND GND GND Oxide N+ N+ P+ Pinch off V(x) x P- Substrate 0 dx ?1 ? =?? Carrier velocity increases drastically ?? Electron shoot through depletion region and arrive at drain The device is in saturation region 4

5. Beyond Pinch off ??? ??? ??? ?=?1 ? ? =(??? ???) ???? ?? = ? ??? ??? V x ???.????(?) ?=0 ? ? =0 ??? ??? ???.? = ??? ??? ??? ???? ? ? ?2 2 0 2 ? ? ??? ??? 2 ???= ?? ??? ???=1 ? ? 2 2?? ?????? ??? ??? ??? Over drive voltage/ ????? ???> ????? for saturation region operation 5

6. MOSFET-Operation ??? non ?????????? ?????????? ???3 ???2 Similar looking plots as of ??vs ???of bipolar transistor ???1 ??? ???1 ??? ???3 ??? ???2 ??? 6

7. Bipolar Transistor and MOSFET Bipolar transistor MOSFET Device as switch or resistor Saturation ???< 0.3V Linear Triode ???< (??? ???) Saturation ??? (??? ???) Device useful amplification Active ???> 0.3V Definition of saturation in Bipolar and MOSFET is very different Can be confusing 7

8. Channel Length Modulation Effect - + ?? GND Gate (G) Source (S) Drain (D) Body (B) - + ?? Metal GND GND GND Oxide N+ N+ P+ Pinch off ? L P- Substrate 0 ? dx ? ??? Effect of channel length modulates with ??? 8

9. Channel Length Modulation Effect ? ?= ??? (In saturation) ? ?=? ? = (1 ???) ? Using Taylor series expansion ? ? = (1 + ???) ? ?? ??? 1 1 ? =1 ?(1 + ???) Original Saturation equation, ???=1 ? ? ?????? ??? 2 2?? 9

10. Channel Length Modulation Effect Substituting ?(1 + ???) 1 ? =1 Channel length modulation co-efficient ? ? ???????? ????????? ?? ? ?????? ? ??? ??????? ???=1 ? ? 2(1 + ???) 2?? ?????? ??? ??????? ??? ?????? ????????? ????????? ???3 ??? ???2 ???1 ???1 ??? ???3 ??? ??? ??= ? ? ???1 ??? 10