Can we connect these two types of devices together if the threshold voltages are different? what problem would this connection cause?

 

One would switch at a lower gate-to-source voltage than the other, which should not be a problem in itself.

Why do you ask?

 

Yes - they can be connected in parallel, but there is no guarantee that they would share the current.
If they are being used in a linear mode, then the one with the lower threshold voltage will probably take all the current making the other superfluous.
In a switching mode you stand a better chance, but the current would be shared according to Rds(on) and temperture.
If the one with the lower Vgs(th) had higher Rds(on) the share of current will swap at some point. A feature you can take advantage of if you are biassing audio amplifiers.

 

This is an interview question, and I just want to know if can we connect these devices in parallel.

 

You would probably fail the interview if you gave a definite "yes" or a definite "no". The demonstration of your knowledge of the subtlety of the situation is what will make you successful.

 

If the supply of these two devices is different that can be a problem? for example, the HlVt supply is 1.8V and the ULVT device supply is 0.9V

 

The MOSFET is a voltage controlled current source - you can easily work out what will happen.

 

See

 

if we give in the input logic 0 that means there is a short connection between two different supply?

 

if we give in the input logic 0

To one gate or both?

 

both