I researched the several websites since I know that Visual 6502 shows many NMOS depletion mode transistors. In other words, it is really PMOS transistor. It is converted into NMOS transistor by connecting gate and source of PMOS transistor.

Here is the picture below.

Take care,
Bryan

Since I haven't had much contact with depletion-mode MOSFETs, they've been a mystery to me. The Wikipedia article on MOSFETs says about them,
I figured it must be something like that. You can't just use a standard (enhancement-mode) MOSFET of the opposite polarity, because without gate voltage, it will not conduct. The amount of current a normal MOSFET will conduct depends mostly on the gate-to-source voltage, and is not very much affected by the drain voltage, especially relative to the gate. JFETs are of course depletion mode and are really nice, but unfortunately there aren't any power ones, because they way they have to be made makes them unable to get rid of much heat.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Don't mistake depletion-mode nMOS with ordinary pMOS - they are not the same at all. All nMOS transistors have increasing conduction as the gate voltage increases, whereas pMOS have increasing conduction as the gate voltage decreases. The point about depletion-mode nMOS is that their threshold is much reduced compared to ordinary enhancement-mode nMOS. Indeed, it's negative. So they are always on, given that the gate voltage on a chip doesn't normally go negative. In normal configuration, they act more like resistors than switches - and that's enough to figure out their logical behaviour.

Cheers
Ed

Hi Ed,

Are you saying that depletion-mode nMOS transistor's n-channel is ALWAYS open so that the current comes down from the voltage toward "drain" before it goes through the n-channel toward source until it reaches V-output to another nMOS transistor's gate?

It looks like that depletion-mode nMOS transistor is not necessarily to be turned ON always, but its gate ignores above 1 voltage up to 5 voltage while its n-channel is ALWAYS open. In other words, depletion-mode and enhancement-mode nMOS transistors are identical, but the difference is that the n-channel of depletion-mode nMOS transistor allows the current to go through its n-channel at unlimited time and the n-channel of enhancement-mode nMOS transistor's n-channel can be manipulated by the gate to open or close its n-channel as it acts like a switch.

Are you positive sure if there is NO current to be flowed toward the gate of depletion-mode nMOS transistor? If its gate receives below 0 V into negative, then any negative voltage has NO current present.

Take care,
Bryan

You seem to hold a distinction between current flowing and the transistor being on - but these two things are the same thing. Perhaps I misread you.

The gate of a FET is a capacitor structure. No current flows into or out of the gate, except for transient current which charges or discharges the capacitor.

To turn off a depletion-mode nMOS transistor, you'd have to take the gate voltage negative, and that's not normally something which happens. So yes, in nMOS logic, you can think of the depletion-mode transistors as always on. As I said before, logically they behave as a resistor. Only if you are analysing at a lower level, to determine the speed of a logic gate, would you need to model them more subtly.

Cheers
Ed