<< Chapter < Page Chapter >> Page >
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

what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
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.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Got questions? Join the online conversation and get instant answers!
QuizOver.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Solid state physics and devices-the harbinger of third wave of civilization. OpenStax CNX. Sep 15, 2014 Download for free at http://legacy.cnx.org/content/col11170/1.89
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Solid state physics and devices-the harbinger of third wave of civilization' conversation and receive update notifications?