Does a bit in Flash memory store net charge

Question

This is with regards to the discussion in the comments in my answer to this question with regards to the charge stored in a Flash.

The way a Flash memory works is that it either stores a small charge (signifying logical 0) on the gate of a floating gate transistor, or doesn't store it (signifying logical 1).

Capacitors, batteries etc are often said to store charge, but in reality they store energy but no net charge since there is always the same number of positive and negative charges on opposite plates.

Now, the question is: is there ever net electric charge stored on any transistor in a Flash chip, such that Kirchoff's current law is temporarily breached? (Presumably charge conservation holds, so electrons from the programming current trapped on the gates are later balanced when erasing that memory cell). Or are the charges of the trapped electrons balanced internally to the transistor by, for example, holes (in the sense of semiconductor theory, an absence of an electron in the valence band) in the substrate?

References to credible sources on the matter are highly appreciated.

Answer 1

In the whole transistor, no. In the floating gate, yes. In the body of the semiconductor, yes.

Explanation time: Your understanding of capacitors is incorrect. Capacitors don't move charge. They have no field producing components and are completely passive in every way shape and form. They do store charge, of an equal and opposite kind, on each plate. Just like this, Flash also stores charge, however it does so on a floating plate by tunneling electrons onto the plate. The total charge in the universe is conserved because an equal amount of charge develops in the semiconductive material allowing for either a channel to be formed or destroyed.

That "net charge" does exist, but only if you look at one plate at a time. And just as was discussed in the other question, eventually those charges will leak and the "net charge" on each plate will be gone, obeying Kirkoff's laws, which at its most simple form is really just conservation of matter (you didn't make any electrons from nothingness to charge the plate, you took them from somewhere).

In the end it's just a matter of perspective: Do you include both plates in your arbitrary geometry used to measure field? If so, then there is no net charge. Do you include only one plate in your arbitrary geometry used to measure filed? If so, then there is a net charge creating a field going somewhere outside of your geometry.

Edit: The question was changed and so the answer has changed somewhat too.

Now, the question is: is there net electric charge stored on the Flash chip in the form of electrons from the programming current trapped on the gates, such that Kirchoff's current law is temporarily breached (to be balanced when erasing the chip: charge conservation of course always holds), or are the charges of the trapped electrons balanced internally to the chip by, for example, holes (in the sense of semiconductor theory, an absence of an electron in the valence band) in the substrate?

The answer to which is the second option. Electrons trapped on the floating plate induce an accumulation of holes in the semiconductor. This accumulation layer acts like a break in the transistor circuit which is why it acts like a logical zero. As a reference, I pointed to a MOS capacitor, which is the basis of most field effect transistors in production today.

Answer 2

The other answers are excellent, but here's some resources, and A LOT of hand waving.

If you want the details of all of this, I would highly suggest chapter 2 in the thesis by Paul Hasler: http://thesis.library.caltech.edu/2477/
I would also check for the course by Brad Minch, who teaches at Olin, or the book called "analog vlsi" by Shih-Chii Liu. All of us are either directly under Carver Mead, or one degree away.

If you don't want to die with quantum mechanics, here's the short version. I can throw charge onto a gate by making a hot electron in the channel that "throws" charge up into the oxide, and some makes it to the floating gate. I then then apply a large voltage to effectively "thin" the barrier made by oxide to make the gate more positive.

Now, I use pFETs for this because I make analog floating gates, so here are some images from my work: The image above is a floating-gate pFET. If I set VDS to be high, and then put the gate, Vg, to be in subthreshold, I can create a field that is high enough to cause impact-ionization of the holes that puts off a hot electron that goes up into "somewhere", and if you are lucky, it's the oxide.

To make the node negative, I present the dreaded band diagram. If "1" creates enough energy because the of the VDS, you get impact ionization at "2". If "2" has enough energy, it might jump to the oxide "3". It is important to note that this behavior is optimal in subthreshold because of the high field seen at the drain edge.

To make the node more positive, I set all of the terminals to 0 volts, but raise the Vtun terminal high to "thin" the tox barrier. (a) is with 0v everywhere. (b) is with Vtun high, so that you get a "thin" barrier, which means there's a probability that you will see Fowler-Nordheim tunneling.

In summary, tunneling is quantum (make the node positive), and injection is classical physics (make the node negative).

Answer 3

There is no net charge on the IC. If there was, then the whole IC would be charged, and would attract the opposite charge.

In the transistors in flash memory, some electrons are trapped in the gate oxide. This creates an electric field, and this field beings in other (opposite) charges around it. These mobile charges can't cross the oxide barrier, but remain just across from the gate in the silicon. It is actually the presence (or absence) of these charges that makes the transistor conductive or not.