First what is MOSFET: MOSFET stands for Metal Oxide Semiconductor FET. This device has same characteristics about FET i.e. same as FET but having a MOS structure. For MOSFET there are 2 types. One is enhancement type MOSFET and other is Depletion type MOSFET.
Depletion type MOSFET are normally ON device and Enhancement type MOSFET are normally OFF device and we can make enhancement MOSFET ON after we apply some voltage at the gate terminal greater than threshold voltage of the device. So the minimum voltage required to make the device On is called the Threshold voltage of the device. Now Depletion type of MOSFET is just the reverse of that ,this device is normally ON
device and by proper gate voltage this device can be made OFF.
About the structure of the MOSFET.
There are n-channel enhancement type MOSFET and p-channel enhancement type MOSFET.For n-Channel enhancement type MOSFET there is a p-substrate and within the p substrate there are 2 n+ regions are diffused .These 2 n+ regions will be source and drain. In between the source and drain there is a gate region. The structure of gate is Metal/Polysilicon then oxide and then p substrate.
Now the characteristics curve for the n-channel MOSFET are same as that of FET only in the curve there is a voltage at which gate to source voltage the device will remain in off state- I mean no current will flow when the device have gate to source voltage below the threshold voltage.
Now for the depletion type MOSFET there is a pre-existed channel in between the source and drain so forming a channel.There is a preexisted channel between the source and the drain that’s why when there is no gate to source voltage still we will get some current if some drain to source voltage applied.So this device is normally ON device.
Now there in depletion mode transistor say for n-channel depletion mode transistor FET i.e. MOSFET there is preexisted channel , this channel is made by ion implantation to implant some ions /electrons within the channel region that’s why the device is normally ON. Now when the proper gate to source voltage is applied then the device can be made OFF.