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Chapter 7_Part 3_Conclusion module concludes the basic electrical properties of MOSFET by discussing the latch-up problem and its remedies.

SSPD_Chapter 7_Part 3_Basic Electrical Properties of MOSFET_concluded.

7.3.16. Latch-Up problem in CMOS and its remedy.

[This part is adapted from the lecture series of Steve McGarry,Department of Electronics, University of Carleton, Ottawa. e-mail:smcgarry@doe.carleton.ca]

CMOS logic is fabricated either by P-Well or N-Well Process or by Twin-Well Process. In any of these processes we have a large number of junctions as . In Figure we give the cross-sectional view of CMOS fabricated by N-Well process. These multiple junctions give rise to three-junction four-layer device which behaves like a SCR. This we call parasitic SCR.

This arises due to cross coupling of P+NP and N+PN transistors. How these parasitic transistors action arise is shown in Figure

If this parasitic SCR is turned ON by the glitches on the power bus or by irradiation, then a low resistance conducting path is created between Positive Voltage Bus (V DD ) and GND Bus (V SS ). This causes irreversible damage to the IC chip. This precisely is SCR Latch-Up problem in CMOS. By careful control during fabrication, the parasitic SCR can be prevented from latching up under all conditions.

In MOS structures the source is always shorted to the body. As seen in the above Figures, Source of NMOS i.e. N+S is connected to P+body by a metal interconnect and P+body makes an ohmic contact to P-substrate. Similarly Source of PMOS i.e. P+S is connected to N+body by a metal interconnect and N+body makes an ohmic contact to N-Well. This ensures that Source-Body bias is zero which leads to zero BODY EFFECT hence threshold voltage of the MOS is held constant.

The equivalent circuit model is given in Figure

As seen in the Figure, the positive voltage bus acts as the Anode of the parasitic SCR and the GND bus i.e. V SS bus acts as the Cathode of the parasitic SCR. For discussion purpose we assume that R substrate and R well are infinite . Under normal condtions voltage between the two buses is 5V or less hence SCR doesnot fire and there is no latching. Why does SCR fire above break-over voltage ?

By inspection of the Circuit Model of SCR in Figure we get the following equations:

We further see that:

Where I CBO = Collector-Base Junction reverse leakage current with Emitter open.

Adding the two above equations, we get:

By rearranging the terms we get:

We know that I E1 = I E2 = I E by Kirchoff’s Current Law;

Also I C1 + I C2 = I E2 = I E ;

Therefore rearranging the terms we get:

Forward Current Transfer Ratio of BJT is α F = γ×β*×M

Where γ = injection efficiency. It falls at low currents due to recombination in depletion layer and it also falls at high current due to conductivity modulation. It is at the maximum value at moderate currents.

Also β* = base transport factor. This improves as Base Width becomes narrower which it will at higher voltages of V CE (forward active mode).

Questions & Answers

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scanning tunneling microscope
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Bob Reply
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The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
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Damian Reply
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yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
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nano basically means 10^(-9). nanometer is a unit to measure length.
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s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
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Devang Reply
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fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
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Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
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carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
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