Thickness of a transistor

Hi,
I am designing a charge pump circuit with thin oxide transistors with supply voltage 1.2 V, But, I am getting almost double output if I design the charge pump with thick-oxide transistors than the design with thin-oxide transistors.
My question is why this thing happen? and which transistor model is ideal for designing a charge pump circuit to work for a high voltage generator. And for an EEPROM how much high voltage is required for the write operation.

Charge pump efficiency has many detractors. The VT is a big
deal at low voltage, your 1.2V is barely turning on a high VT
transistor and 1.2V worth of gate swing can only establish
so much difference between "on" and "off" meaning you will
give up either charging forward efficiency, or leak-back
efficiency anywhere away from an optimum (which is still
less than ideal).

When I had the opportunity to do a charge pump on a flow
that had 0, 0.3 and 0.7V VT devices and a 3.3V supply, the
0.3V devices worked best. You probably do not have that
option. But you should spend some time looking at apples
and oranges, determine what in your PDK gives you the best
result and use what works.

I doubt that thick oxide per se is the advantage, but the
VT, or superior leakage at high Vds, or lower stray capacitance,
might be. Pick it apart and see where your losses are.