ECE 875: Electronic Devices Prof. Virginia Ayres Electrical & Computer Engineering Michigan State University

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

Substrate bias: HW08, Pr. 6.5: Answer: Quadratic => V T = -1.0 V OR 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

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

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

“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:

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

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

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

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

Example: What (why important) is the question?

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:

given

V BS = -3V V BS = -1V

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:

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

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

y = mx + b Why linear region I D :

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.

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

IDID = 0 Straight line intercept

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

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