Which is better for device matching - low VT or high VT?

[diarmuid]

Hello,

Maybe someone can solve my confusion regarding the above question.

My confusion stems from the following:

1.) From a design perspective I would say, larger Vt gives lower Vov and so, for a given Vgs and Vds, would push
the device further into saturation, yielding a higher output impedance and hence more ideal current source.

From that I would say higher VT is better for matching.

But...

2.) Today I heard lower VT is better for matching. Why is this? Is it because higher VT devices have doping
implants which are difficult to match?

So going from 1 and 2 above, say I want to design a simple current mirror. Do I go for high or low VT?

Any help greatly appreciated to alleviate my stumbling confusion!!!

All the best,

Diarmuid

 

[dick_freebird]

Lower VT means your channel is more easily influenced
by adjacent insulators' charge. Especially an issue on SOI,
but also a factor in edge related nonidealities.

 

[adi0803199]

From the above post I understand the possible confusion may have raised due to the usage of the variable V_t which in the design of current mirrors are referred to the Thermal Voltage given by the formula (K*T/q) .In the MOSFETs V_t is used to represent the threshold voltage which is given by another formula that is dependent on the concentration and energy levels.The latter symbol is used to design Voltage Controlled Current Source while the former is used to design Current Controlled Current Source.Hope this helps.