channel length modulation

[rogger123]

hi
can someone explain to me the concept of channel length modulation in mosfets. and how the Vds vs Ids curve is affected due to channelt lenght modulation
is hot electron effect and electron migration the same phenomenon?

 

[golfbumb]

Channel length modulation occurs when voltage between drain and source is greater than the voltage between drain and source required to put the device into saturation. This is somewhat simlar to the Early voltage that is a characteristic of bipolar junction transistors. The point at which the inversion charge is zero moves away from the drain terminal toward the source terminal. The effective channel length has decreased. If L effective decreases, there is more drain current than expected for a given Vds. At a given point on the curve you get more current for the same voltage than otherwise expected for.

Also, the output resistance changes inversely proportional to the channel length modulation factor. The higher the modulation, the lower the effective output resistance.

Electromigration occurs when metal interconnect lines in a device get hot enough that the grains in the metal can be moved around by the electrons flowing through the line. This is a common failure mechanism in devices that run at continuously too high temperatures. Voids form in the metal, increasing resistance and accelerating the problem until the line completely fails (becomes too ohmic to allow the device to function as designed).

Conversely, the hot electron effect occurs when there is high voltage between the source and drain and the electrons passing through the channel pick up speed and damage the oxide at the drain interface. The threshold voltage and electron mobility are changed over the life of the device.

 

[rogger123]

hi
when the length of the device decreases the current increases. but when the length of the channel(considering the channel carries the current from drain to source) decreases how does the current increase?

when the channel gets pinched off how does the current flow?
is the mosfet a minority carrier device or a majority carrier device?

 

[devrimaksin]

Here is the reason, for simplicity assume that
when the channel is formed, it is in brick form,
now for a given gate to source voltage you have
a given channel chanrge (Qin=f(Vgs)). Think as a resistor, for the same channel charge (or inversion layer charge Qin), if you reduce the length of the brick, because the Qin is constant the height of the brick will increase( because w is constant ) which in turn will lower your equivalent resistance. Yannis Tsividis's book about MOS transistor shows how to calculate the drain current in terms of Vgs and channle length

 

[devrimaksin]

The current flow from the pinched off region with the help of the electrical field (notice that that are is in depletion region).

Mosfet is a majority carrier device but this term is not really true (in sense of BJT device) because what is done is actually bending the energy band diagram with an applied voltage so that the donnor (or acceptor) propriety of the channel are is inverted
that is why it is called inversion layer (etc.) For an nmos orginally you just have to n-type diffusion island within a p-type body isolated from each other. Than you apply a voltage to the gate to invert the p-type region underneath (effectively make is n-type) and you have an n-type bridge betwwen the islands.

 

[rogger123]

hi
i do understand that with decrease in length and increase in width of the channel the resistance will drop and the current will increase..
my point is when we say the length of the channel decreases how does it remain in contact with the drain and source?
for example
let ue assume the length of the device is X. now the channel has a length equal to Y where X>Y.
if Y is less than X how does this channel connect the drain and the source to allow the flow of current.

during pinch of the channel does not carry the current but the underlying region (or the electrons in the entire p-substrate) carry the current?

 

[golfbumb]

rogger123, with respect to your last post:

In the extreme case:
Let us leave any gate voltage off for a moment. We have a source depletion region and drain depletion region. If Vds is increased beyond Vds (sat), what we have done is increased the size of the drain depletion region through the repulsive effects of electric potential. Go further and apply enough voltage and the depletion regions overlap and allow current to "punch through" from source to drain -- inversion layer or not. Drain current becomes independent of gate voltage.

Back up to a scenario short of punch through but Vds>Vds(sat). The drain terminal location is still fixed, but the effective drain depletion region has grown. The threshold voltage lowers because source and drain distances have decreased. There is a small linear increase in local resistance between drain contact and the shorter inversion channel that is created once the gate voltage is applied -- due to the distance across the [now bigger] well depletion region travelled -- but, since drain current is a square function of the difference between Vgs and Vt, the drain current increases faster than the increased local drain resistance decreases current flow. Repeated, drain current increases faster than the local resistance impedes it.

Local resistance between drain and channel may increase linearly, but total output resistance lowers by a square relationship.


.

 

[rogger123]

hi
i do understand u'll have taken great pain in explaining the concept to me. Either I am missing on a crucial concept or i haven't been able to explain my doubt correctly.
i have attached two diagrams of the mos in saturation and in tridoe. when the mos is in triode we see that there is a channel formed by electrons below the gate. this channel allows the current to flow from drain to source.
when we look at the channel when the device is in saturation we see that the same channel(formed by electrons) does not connect the source to the drain. my point is if this channel is pinched off(not connecting the source to drain) how does the current flow?

 

[sp]

for the channel length modulation explaination....i find tht this book explain the best....

Microelectronic Circuits 4th Edition - Sedra, Smith

i hav read it myself n find out tht is quite easily understandable.....it also hav nice diagram to explain wth it....

i take the channel length modulation part out n upload it here....

hope u like it...


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