Electron flow in transistor circuits

[vital989]

I would appreciate explanations on how do electrons flow in these circuits:

https://www.quora.com/How-do-electrons-flow-in-these-circuits

 I'm interested in physics here


P.S.  Why mosfets are used in microchips? I heard it's because you can make them so tiny, so what's the major reason you can't make bipolar transistors so tiny? What are the physics? 

[IanB]

Electrons flow if they can move and there is an electric field (potential gradient) present.

MOSFETs are used in microchips because they use less power than bipolar devices.

[T3sl4co1l]

Hmm, I forget if BJTs are scale limited.

FYI, electrons and holes "flow" just fine on their own, without any applied force: diffusion allows them to spread out without any motive force at all.  Indeed, this is the dominant means of current flow in BJTs, and the reason they work so well!

You do tend to have the problem, with BJTs, that, specifically because of diffusion, you have to carefully isolate them from each other.  Otherwise one transistor conducting nearby will release charge carriers into the silicon substrate and anything touching -- the diffusion length is several microns, so a sensible leakage current can be found at several transistors' width away, even for bulky ~2um construction typical of the 70s (e.g., LM741, etc.).

I believe those problems were usually solved by making a N-well within a P-substrate; this forms the collector, and the collector-substrate junction shunts the leakage (as collector current proper!), keeping it away from others.  Subsequent P and N diffusions formed the base and emitter (of increasing doping level, as necessary to more than overcome the existing doping from previous steps; which is why the B-E junction usually has a very low breakdown voltage).

MOSFETs nowadays still have to deal with similar problems; this was solved by etching deep vertical gaps between transistors, so that the diffusion path must be vertical as well (down, into the substrate, over, and back up).  Such a solution would work just as well with BJTs.

BJTs haven't at all disappeared; with the introduction of strained SiGe:C processes, HBTs (heterojunction BJTs -- even better than the regular kind) have been used in ICs in great quantity already.  The higher fT/Ic figure means higher speed devices on the cutting edge, or even further reduced current consumption for conventional-spec parts.

Tim

[hamster_nz]

Why mosfets are used in microchips? I heard it's because you can make them so tiny, so what's the major reason you can't make bipolar transistors so tiny? What are the physics? 

Massive oversimplification time...

One reason is that transistors are current driven devices they require electrons to be flowing (and therefore using power) to act as a digital switch. Because of this you can't integrate billions of transistors on the same small bit of silicon - on average 50% of the transistors will have a current flowing through them, so it would get too hot even when doing nothing.

MOSFETs are voltage driven devices - apart from the current used to move the charge stored in the MOSFET's gate no current is required to switch a MOSFET. This enables designs that use MOSFETs uses very little power, as when not actively switching states the power usage digital logic built using  MOSFETs is essentially zero.

[IanB]

FYI, electrons and holes "flow" just fine on their own, without any applied force: diffusion allows them to spread out without any motive force at all.  Indeed, this is the dominant means of current flow in BJTs, and the reason they work so well!

Just to be a tiny bit pedantic here, a standard law of diffusion certainly states that particles will tend to move from a region of higher concentration to a region of lower concentration. However, if the particles carry a charge like electrons do, then the region of higher concentration also will have a greater electrical potential than the region of lower concentration. So not only is there a concentration gradient, there is also a potential gradient assisting the movement.

If there were a uniform concentration of electrons everywhere and no external fields then there would be no net drift velocity in any direction and no spontaneous aggregation. The region of uniform concentration would remain uniform indefinitely.

[T3sl4co1l]

Yes, there will be a small potential associated with a local excess of electrons; in vacuum tubes, this is the space charge effect.  (A valid analogy, because vacuum tubes are essentially semiconductors but with the charge carriers being entirely electrons, and promoted to "vacuum state" (free, >work potential) levels, rather than the bound but mobile conduction band levels.)

As far as I know, semiconductors are always (or almost always) very damned near electrically neutral, mainly because n_A (number of atoms/ions per cm^3) is in the 10^23 cm^-3 range, while electrons/holes/doping is in the 10^15 cm^-3 range.

Strangely, I forget if electrons and holes always accompany each other in minority carrier situations, or if one or the other is dominant.  I should read a book

Tim

[rfeecs]

Just to be a tiny bit pedantic here, a standard law of diffusion certainly states that particles will tend to move from a region of higher concentration to a region of lower concentration. However, if the particles carry a charge like electrons do, then the region of higher concentration also will have a greater electrical potential than the region of lower concentration. So not only is there a concentration gradient, there is also a potential gradient assisting the movement.

Sort of sounds like a PN junction .

If all the carriers were to stay put, the N-doped side would have an excess of negative carriers (electrons) and the P-doped side would have an excess of positive carriers (holes).  Both sides would be electrically neutral, because the mobile charges are balanced by the fixed charges in the nuclei of the dopant atoms.

Because of diffusion, the carriers spread out and electrons and holes annihilate each other by re-combining.  This leaves the fixed charges behind on each side of the junction which causes an electric field in the depletion region that opposes the diffusion.  The drift from the electric field, and the diffusion from the concentration gradient balance each other.  This leads to an equilibrium.