Really nice graph.
If the gate extends the entire cell, it will be like your first graph. If the gate do no extend entire cell, then the second picture is more correct.
I have a post on this somewhere on the forum, let me dig it out.
(found it http://quantumwise.com/forum/index.php?topic=1597.msg8232#msg8232
If you dont have a gate at the bottom, you have to set the two options - you can set Dirichlet Boundary Condition which is this case will have the same effect as
setting the bottom gate. You will then be able to calculate the field simply by taking the gate voltage and divide it with the length from the edge of your gate to other side.
You can also go with Neumann Boundary Condition, then you will have to calculate the electrostatic potential on the bottom and then you can substract this from the gate voltage and divide with the length to get the field.