1. ECE 875: Electronic Devices Prof. Virginia Ayres Electrical & Computer Engineering Michigan State University ayresv@msu.edu

2. VM Ayres, ECE875, S14 Lecture 39, 16 Apr 14 Chp 06: MOSFETs Aspects of realistic MOSFET operation (n-channel p-substrate) Substrate bias V BS Subthreshold swing

3. Substrate bias: HW08, Pr. 6.5: Answer: Quadratic => V T = -1.0 V OR +1.67 V Might expect +answer, but correct choice is: V T = -1.0 V Because also need: V G = +1.0 V > V T VM Ayres, ECE875, S14

4. From Streetman and Banerjee: n-channel: -++- signs: VM Ayres, ECE875, S14

5. From Streetman and Banerjee: p-channel: ---- signs: VM Ayres, ECE875, S14

6. “Fixed oxide charge Q f a major cause of need for V FB correction” : n-channel: -++ - Also: charges in oxide can increase over time/use: thermal cycling, radiation, etc. VM Ayres, ECE875, S14 signs:

7. Equilibrium pn junction Reverse biased pn junction VDVD Potential drop: V DS + Original: VM Ayres, ECE875, S14

8. Different reverse biased pn junction VDVD Potential drop: V DS + Apply negative substrate bias V BS (for n-channel) V BS Apply V BS : VM Ayres, ECE875, S14

9. Apply V BS : get  V T : (49) Original: No V BS With V BS VTVT VM Ayres, ECE875, S14

10. Different reverse biased pn junction VDVD Potential drop: V DS + Apply negative substrate bias V BS (for n-channel) V BS V BS is a way to shift V T (or shift it back to proper operation): VM Ayres, ECE875, S14

11. Example: What (why important) is the question?

12. What: find  V T-02 Why important: For two different substrate biases, what happens when you increase the substrate bias? Answer: Working tools: check given:

13. given

15. V BS = -3V V BS = -1V

16. What: find  V T-02 Why important: For two different substrate biases, what happens when you increase the substrate bias? Increase V BS x 3, get positive shift ~ x 2.4 Answer:

17. VM Ayres, ECE875, S14 Lecture 39, 16 Apr 14 Chp 06: MOSFETs Aspects of realistic MOSFET operation (n-channel p-substrate) Substrate bias V BS Subthreshold swing

18. How do you turn the MOSFET OFF: Reduce V G all the way to –V T for accumulation in the channel V G : -V T So consider an I D -V G graph instead of I D -V D

19. y = mx + b Why linear region I D :

20. How do you turn the MOSFET OFF: (n-channel) Reduce V G below threshold. No inversion layer. Want: I D = 0. But leakage current is still a problem: why: diffusion and EHP formation can’t be stopped. Subthreshold swing S is a metric for turning a MOSFET device OFF Test conditions: use a small V DS to get a current I D running. Then see how that current responds to changes in V G Motivation: It is hard to turn a device OFF. I D when the MOSFET is supposed to be OFF is called leakage current.

21. With V D = V D-test = constant, and V T = (different) constant I D = f(V G ) has form: y = mx + b Line with some distortion at top and bottom

22. IDID = 0 Straight line intercept

23. If the bottom distortion is examined carefully, it is small but it doesn’t = exactly 0. Small currents matter. Definition 01 for Subthreshold swing S: S =  V G /decade I D = 1/slope, not slope

29. Definition 01 for Subthreshold swing S: S =  V G /decade I D Definition 02 for Subthreshold swing S: