Since the depletion type or depletion mode mosfet is "normally close", does it mean it does not require any gate voltage to "turn ON"?
i.e current can flow in the drain-source circuit easily without any gate turn on voltage. And hence no threshold voltage is applicable?

 

Yes

 

Since the depletion type or depletion mode mosfet is "normally close", does it mean it does not require any gate voltage to "turn ON"?

With zero Vgs a depletion mode MOSFET is not "normally closed." In most cases a depletion mode MOSFET will be partially conducting at zero Vgs and (for an N-channel device) will require a negative Vgs to turn it OFF. A positive Vgs will make it conduct more than at zero Vgs.

i.e current can flow in the drain-source circuit easily without any gate turn on voltage. And hence no threshold voltage is applicable?

Wrong; it's not that there's no gate threshold voltage, it's that Vgs(th) is negative.

Think of a depletion mode MOSFET as acting just like an enhancement mode MOSFET, except that the entire Id vs. Vgs curve has been shifted toward more negative Vgs, as in this graph from the BSS139 datasheet:

 

With zero Vgs a depletion mode MOSFET is not "normally closed." In most cases a depletion mode MOSFET will be partially conducting at zero Vgs and (for an N-channel device) will require a negative Vgs to turn it OFF. A positive Vgs will make it conduct more than at zero Vgs.


Wrong; it's not that there's no gate threshold voltage, it's that Vgs(th) is negative.

Think of a depletion mode MOSFET as acting just like an enhancement mode MOSFET, except that the entire Id vs. Vgs curve has been shifted toward more negative Vgs, as in this graph from the BSS139 datasheet:

View attachment 209405

Ok I somehow now get it, I'll fully understand it when I start experimenting with it.
I like your way of explanation. I donno if this is acceptable, but i will like you to view my previous thread on self oscillating flyback converters. I posted it just yesterday.

https://forum.allaboutcircuits.com/...switching-power-supplies.170463/#post-1520872

 

Here's a review. Apropos on or slightly on and threshold, checkout page 4: http://www.aldinc.com/pdf/IntroDepletionModeMOSFET.pdf