Mosfets have 3 regions of operation that we will concern ourselves:
The first is cut-off mode, when the mosfet does not conduct.
The second is the saturation mode, when the mosfet is switched on and is conducting with a very low drain to source resistance.
The third region is in between cut-off and saturation, and is known as the triode region. This is the region where the mosfet begins to conduct, but has a high channel resistance.
There are 3 key voltages that will determine what mode the mosfet is operating in. These are:
VGS, the voltage from the gate to the source, taken with the gate as positive and source as negative
VDS, the voltage from the drain to the source, taken with the drain as positive and the source as negative
Vt, the mosfet’s threshold voltage. This value is a device parameter, and is roughly given in the device’s data sheets. (Base on the Datasheet of the STP36NF06 MOSFET VGS is 10V to get saturation mode VDS is 25V and Vt is 2V)
For an N-Mos device:
Cutoff when VGS < Vt
Triode when VGS > Vt, and VDS < (VGS – Vt)
Saturation when VGS > Vt, and VDS > (VGS – Vt)
For you case here, you are using 9V supply so the MOSFET is in the triode region therefore have high channel resistance which the voltage drop in the Drain and Source is high.
Read this article for more detailed info on MOSFET http://roko.ca/robotics/h-bridge-fundamentals/4
Datasheet of the STP36NF06http://www.st.com/stonline/products/literature/ds/9619.pdf