Flicker Noise and CMOS scaling

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Does the Scaling of CMOS help to ease flicker noise?

The device processed in 0.13um has better noise performance than that in
0.35um?
 

Based on Flicker Noise: Vg = (k/(WL(Cox)f)

With the scaling, it would appear that the noise would seem to get worse.
 

By your equation, the Cox is often larger in advanced technology. So, if
the device size WXL and K factor is same, the flicker noise is smaller
in advanced technology.
 

Smaller aspect ratio with minimal gate length, L, ensures lower flicker noise.

Rgds
 

hrkhari said:
Smaller aspect ratio with minimal gate length, L, ensures lower flicker noise.

Rgds
Click to expand...

Hi hrkhari,

I can't realize why aspect ratio relates to flicker noise?
Would you explain more?
Thanks,
 

Vg = (k/(WL(Cox)f)


u see that there is an existance of WL? when uscale down a device, the size is smaller than before thus amplifying the Vg...

but this is not usually happen with scaling becasue the parameters used in new smaller technology is different from previous..the voltage supplied is far less than before and k is different...

Cox=εox/tox...when scaling is done, thikness is reduce and if same material is used than the oxide capacitance is increasing....

smaller gap of the gate channel and the top of the substarte encouraging flicker noise
 


I can't relaize the last sentence "smaller gap of the gate channel and the top of the substarte encouraging flicker noise"
Smaller gap means the thickness of gate oxide?
So, overall the flicker noise is worsen in advanced process?
 


For the same size, flicker noise of the device in 0.18um is smaller than that in 0.35um due to increased Cox. But if to use minimal length device(for constant W/L in different process), it is another story, maybe smaller maybe worse.
welcom correct if anything wrong.
 

In submicron technology the 1/f does not fall as in bipolar devices, the Knee extends in tens of MHz , 1/f falls very slowly in we go in deep submicron .. so yo will relatively higher 1/f noise when you smaller and smaller geometry. see Agilent 1/f measurement notes.
 

The generation of flicker noise depends in CMOS depends both on material quality and on the fields in the conducting region.
Historically, thinner oxide meant better quality material, and flicker noise (for equal area devices) tended to reduce as the geometry got smaller.
However, field orthogonal to the surface (that attracts carriers towards the conducting region) will also tend to capture traps in the surface region. High fields and high currents along the conducting direction will also tend to generate more traps. The combination of these effects started to dominate over material quality at the 0.5-um geometries. In addition, this was the geometry around which it became necessary to use P-doped gate material for P-FETs (in order to reduce short-channel effects), so the historical flicker advantage for P-FETs vanishes at about this point. Another factor is gate oxide: initially, a change from oxide to oxi-nitride tended to improve surface quality, but ultra-small-geometry transition to hi-K dielectrics caused further issues. Even more recently, deliberate straining of material has different implications for N-channel than for P-channel devices. BTW, somewhere around 0.5-um is also the geometry at which short channels can start to generate disproportionately more noise than longer ones - an effect that even the BSIM model is not capable of fitting accurately.
A final complication for RF designers relates to the models only being valid when the bias conditions are steady-state. The transistors in oscillators and power amplifiers are typically quite deeply modulated; the overall effect of this is that actual circuit noise of active devices (especially of MOSFETs) tends to be lower than simulations would suggest. Two of the factors described above contribute to this: the first that generation rates are higher at higher bias, and the second that traps that were captured at the surface under steady-state high-bias conditions are actually repelled from the surface at subthreshold. The consequence is that (compared with steady-state) flicker noise is increased during the low-bias part of the cycle, but is reduced during the high-bias periods; the net result is a (sometimes substantial) reduction in the level of flicker noise - because nearly all the noise is generated during the high-bias parts of the cycle.
 
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    covcst

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leo_o2 said:
Any layout skills can help to reduce 1/f noise?
Click to expand...
In general the circuit design (including choice of device types) determines nearly everything.

For MOS devices this can include shaping the gate so that the parts of the device that contribute most to the flicker take larger proportions of the area; this can be achieved either by using part-annular structures or by building from multiple elements. However, the best flicker-reduction that you can achieve in this way is equivalent to increasing the area by about 30%, so the additional complexity is rarely worthwhile.

For vertical bipolar transistors it is usually (not always) preferable to choose large octagonal emitters rather than multiple small emitters or long-and-narrow devices. This should be clear from the models provided by the foundry.

In some processes the periphery of the collector-base diode of the transistors is subject to additional flicker noise, though this is only important if the collector-base junction is forward biased; this noise can sometimes be reduced by special layouts - but this is in the realm of device design and you will also require special models: this is a task for specialists only. (BTW, the still widely used historic SPICE model handles collector-base flicker as part of the emitter-base flicker; if you are likely to use the devices in this regime you will need to check the status of the models)

If you can orient the devices to minimise sensitivity to strain you will reduce environmental contributions (possibly also some internal effects) that can introduce both additional popcorn and flicker. Generally, NFETs and Ndoped resistors are not significantly improvable (relative to standard orientation), but PFETs, lateral PNPs and P-resistors built into the substrate are better when the current-flow is at 45 degrees to the usual Manhattan axes - if design rules permit. However, with modern strain-enhanced MOSFETs the models won't match as you are likely to sacrifice device mobility.

More practically, good matching arrangements (common centroid and higher-order) can help with differential structures.
 
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    erikl

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Presumably we are looking at gate-referenced Voltage noise here? If so, this could apply to some processes and bias conditions; however, there are too many variables.
For example, the PMOS devices in some 0.35-um processes had N-doped gates, and consequently a buried channel that has much lower noise than enhancement MOSFETs available in 0.18-um processes (though I don't know if any of these processes are still available).
If we look at a single process that supports both low-voltage (0.18-um) and mid-Voltage (0.35-um) gates, the situation is still clouded. An exemplary manufacturer has 0.35-um PMOS devices with higher gate-referenced noise than the 0.18-um devices at all bias conditions, but the standard 0.35-um NMOS device simulates to be substantially the quieter at low bias and substantially noisier at higher levels (I'm suspicious that the "quiet" region is actually extrapolated, as I've no idea as to a specific mechanism that could be at work here). There are also NMOS devices available where the P-well is absent - and these provide substantially lower flicker noise than any of the standard (enhancement) structures.
Perhaps it is worth noting that the MOS process I have seen that gives best flicker noise (both for N-channel and for P-channel) is a 0.6-um process that appears to have been optimised specifically for this purpose.
In conclusion - it all depends! So there is no substitute for simulating and roughly optimising the performance in the most promising processes that are available to you. On top of this it is essential to check the region of validity with the foundry. Sometimes the foundry will provide graphs comparing simulations with measurements; if these don't cover your intended region of operation, you should assume that the devices were not measured in that region - unless reliably informed otherwise.
 
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