Doped Semiconductor and Electron Flow - Page 1
Forum section: electronics / beginners

author: maelh
author_rank: Regular Contributor
timestamp: July 18, 2021, 08:16:54 pm
content: Hello,Assuming you have an n-type semiconductor, such as described here:https://toshiba.semicon-storage.com/ap-en/semiconductor/knowledge/e-learning/discrete/chap1/chap1-3.htmlThe free electron (I assume it is free because it does not form a covalent bond with Si?) will be able to move around the n-type semiconductor crystal, and will do so if there is an electric field applied to it.Now usually the field is only applied at two opposing ends of that crystal (assuming a long rectangular shaped crystal), for example by connecting a battery. So I assume the field only has a direct effect at the ends, and not throughout the whole crystal. The propagation of the field or the field effect, would by achieved by the electron flow.Is that correct so far?My question now is: Assume you have another type of voltage source, which releases ions or other negatively charged particles, that are too large in diameter, to be able to easily flow in a copper wire or a (doped) semiconductor crystal. In that case, I assume the electric field from that voltage source would really just have an effect on the electrons at both ends of the crystal (where the voltage source is in direct contact with the crystal).And once all the free electrons are "sucked out" of the crystal, the electron flow would stop, since no new electrons enter the crystal (as the negatively charged particles provided by the voltage source are too large in diameter, and are not electrons).That is assuming the electric field at the two ends of the crystal is strong enough to push out all the electrons, otherwise only some might leave, and some might stay.Is that correct?

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2021, 08:27:56 pm
content: Not correct. The dopant atoms remain in place, while the loosely bound electrons can move in the obvious direction, but the crystal remains electrically neutral (in net charge) as conduction electrons flow into the other end. There is no circuit that can inject atoms or ions into the crystal. Ion implantation (from an ion accelerator beam) can be used to jam foreign atoms into the crystal.When a crystal has both electrons and holes (vacancies that behave as positively charged particles), the relative importance of the two can be measured using the Hall effect (look up the details), especially as a function of temperature.

author: IanB
author_rank: Super Contributor
timestamp: July 18, 2021, 08:35:17 pm
content: You are sort of correct, but you need some more detail in your understanding.Firstly, "voltage source" is a poor choice of words, since voltage does not flow, and therefore voltage can have neither sources nor sinks.When we talk about "voltage", we mean a potential difference between two points, and about electric fields that exist where potential differences exist.When we create a potential difference, we have the potential for current to flow, but only if the apparatus that generates the electric field has the ability to source and sink current. "Current source" is a good term, since electric current can flow between two points.Let's suppose we place insulating barriers on each end of the semiconductor so that no current can flow, and then suppose we impose a large potential difference on the outside of the insulators, so that an electric field appears across the material.In this case, the free electrons in the semiconductor will tend to move under the influence of the electric field, and they will redistribute themselves towards one end of the crystal. Eventually the movement of the electrons will create an opposing field that balances the external one, and then movement will stop.This is what you suggested in your question, and it is correct in this kind of situation. But note that the electrons do not leave the material, they just get bunched up at one end.

author: maelh
author_rank: Regular Contributor
timestamp: July 18, 2021, 08:37:22 pm
content: Quote from: TimFox on July 18, 2021, 08:27:56 pmThat's the usual explanation (and yes, it's clear the atoms stay in place). But if the electrons flow only in one direction, why don't they get "jammed up" at one end of the crystal? If there is a continuous flow of electrons, they would have to leave the crystal at some point, as is shown in many animations, no?If not, how is such an animation to be understood?

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2021, 08:41:36 pm
content: In a different device, a gas-filled (but not discharge) rectifier tube, the volume is filled with xenon or mercury vapor (both heavy atoms) and a hot cathode emits electrons towards a positive anode. On their way, these electrons knock outer electrons from the gas atoms, leaving a positive ion. The lighter electrons quickly head to the anode, while the heavy ions lumber slowly to the cathode. The cloud of positive charge greatly reduces the space charge near the cathode (compared with a vacuum rectifier), thus allowing current flow at lower anode-cathode voltage than in a comparable vacuum diode.

author: maelh
author_rank: Regular Contributor
timestamp: July 18, 2021, 08:44:31 pm
content: Quote from: IanB on July 18, 2021, 08:35:17 pmOk, that makes sense. But they will actually leave the crystal if you have no insulation, and a traditional current source like a battery, right?

author: IanB
author_rank: Super Contributor
timestamp: July 18, 2021, 08:45:15 pm
content: Quote from: maelh on July 18, 2021, 08:16:54 pmBy the way, you cannot remove electrons from consideration. The only way to make charged particles, ions or anything else, is to add or remove electrons from those particles. And by doing this, electrons have to move, or "flow". So electrons are always involved in electricity. (Unless you were in a parallel universe where positrons existed, but that does not apply to our universe.)

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2021, 08:46:05 pm
content: In your example, the field on a crystal results from the voltage across it due to a wired (“ohmic”) connection at each end.If put the crystal between two plates without contact, you make a capacitor where you get polarization due to separation of charge, but the finite conductivity of the “regular” electrons will tend to neutralize the dipole moment.

author: IanB
author_rank: Super Contributor
timestamp: July 18, 2021, 08:46:53 pm
content: Quote from: maelh on July 18, 2021, 08:44:31 pmYes, if current can flow (if there is no insulation), then electrons can enter or leave the crystal.

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2021, 08:48:04 pm
content: The current flow is a real effect, but it does not change the zero net charge of the crystal.

author: IanB
author_rank: Super Contributor
timestamp: July 18, 2021, 08:48:55 pm
content: Quote from: TimFox on July 18, 2021, 08:46:05 pmBut the electrons will neutralize the dipole moment by migrating spacially, right? This is more or less like static electricity, where spacial charge density is altered by an imposed field.

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2021, 08:50:28 pm
content: Static electricity is a charge distribution on an insulator. Silicon at room temperature is a mediocre conductor.

author: IanB
author_rank: Super Contributor
timestamp: July 18, 2021, 08:53:57 pm
content: Quote from: TimFox on July 18, 2021, 08:48:04 pmIf I take an insulated metal rod and apply an electric charge to it, it will no longer have zero net charge. Therefore, in principle, I could take an insulated crystal of semiconductor and apply a charge to that, too?

author: maelh
author_rank: Regular Contributor
timestamp: July 18, 2021, 08:59:13 pm
content: Quote from: IanB on July 18, 2021, 08:45:15 pmAs far as I am awarepositronsdo existnaturally, just that they usually get canceled out by electrons, and are a lot less common to observe:QuoteAlso besides positive ions, protons are positive charged particles, and can move, for example if solved in a fluid.Though, that's not what I was aiming at.Regarding electrons in ions etc. I assumed they would be tightly bound and therefore not act in isolation, just how molecules act as a unit (and therefore their diameter matters). Similar to how there are free and bound electrons in the crystal.

author: maelh
author_rank: Regular Contributor
timestamp: July 18, 2021, 09:05:05 pm
content: All of that leads to a follow up question:So when a potential is applied across a long (say a couple meter long) resistor, is there really an electric field crossing the whole resistor? Or is it rather a local electric field that propagates from one end to another, similar to a pulse across a transmission line?If so, what propagates this electric field, since electrons are slow to react/flow in comparison?I would just be surprised if you really had a large electric field, like across a capacitor.

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2021, 09:16:48 pm
content: Again, an undoped semiconductor crystal is a mediocre conductor (0.1 to 60 ohm-cm) at room temperature, hence the term semiconductor”. If you take a P-N junction, a depletion region (insulating) forms at the junction with zero applied voltage, but that’s different.If you take a uniform long cylinder of a high-resistivity material and apply an external voltage between the two ends, the voltage gradient will take a finite time to reach equilibrium (due to stray capacitance and the high resistance), but the uniform voltage gradient is an “E-field”. This field affects charge carriers in the conductor, but Ohm’s Law results from other effects in the solid like electrons scattering from stationary atoms, and you get a flow of current. Nothing like a propagating pulse: that would be described as transmission-line effects with non-constant applied voltage.

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2021, 09:25:26 pm
content: To IanB:Your insulated metal rod is one “plate” of a capacitor, and a semiconductor will do the same. The other plate is everything else in sight.If you wave the rod about, the capacitance changes and therefore the voltage and field change, but the charge remains the same. (To be pedantic, accelerating the charge will radiate, so move it slowly.)

author: maelh
author_rank: Regular Contributor
timestamp: July 18, 2021, 09:27:14 pm
content: Quote from: TimFox on July 18, 2021, 09:16:48 pmI am aware of the bad conduction properties of undoped semiconductors and how it is temperature dependent (and how any material can become a conductor given high enough voltage). Not sure how it applies to my question, though? I was talking about a doped semiconductor.QuoteOk, but can you really visualize it as field lines spanning from one end of the resistor to the other end? I am having trouble visualizing how the voltage actually looks across a circuit, when thinking in terms of field lines.I would appreciate a visualization of this kind, most of the visualizations focus on the current flow.

author: TimFox
author_rank: Super Contributor
timestamp: July 18, 2021, 09:41:05 pm
content: The silicon atoms have a finite contribution of “conduction” electrons even without the dopant.Look at your carbon rod as a long narrow cylinder. After equilibrium is reached, you have a uniform voltage gradient along the cylinder axis: dV/dx=E, whereEis the (constant) field strength component along the x-axis of the cylinder. If you apply a reasonable voltage along a very long rod, the result is a small field.The physics form of Ohm’s Law isJ= sE, where the vectorJis the current density (A/m2), s is the scalar conductivity, and the vectorEis the electric field, which equals the gradient of the scalar potentialV. With a uniform resistance along the rod, the voltage gradient is uniform, and the direction of the current density vectorJis parallel toE, which is along the axis. The currentIis a scalar (polarity but no direction arrow) that results from integrating the density over a relevant surface (like the cross-section of the rod).

author: maelh
author_rank: Regular Contributor
timestamp: July 18, 2021, 10:09:58 pm
content: Thanks.I found this also useful, including the linked book excerpt:https://www.reddit.com/r/askscience/comments/1v50sv/is_it_possible_to_visualize_electric_current_more/http://books.google.com/books?id=8oyNPd5QbYgC&pg=PA761&lpg=PA761&dq=surface+charge+circuit&source=bl&ots=60cKWib_cn&sig=Fg2kUN-niRmqFlMCSLvlbzRLQAw&hl=en&sa=X&ei=UX3UUrCRCPbNsQSVwYHYDw&ved=0CCwQ6AEwAA

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 19, 2021, 02:06:29 am
content: What kind of ionic source would you envision?There are materials and systems where that happens. Electrochemistry is such a case. The anode and cathode participate in half-cell reactions respectively, which may lead to them donating or accepting whole ions to the solution; current flow in solution is partly by ionic motion alone (very slow, varies with concentration -- hence why electroplating or etching baths need to be well stirred, to maintain consistent concentration), or in combination with charge transfer (for example, protons can skip relatively quickly between water molecules, H2O + H3O+ <--> H3O+ + H2O; even single electrons can hop around, for example the interchange of Fe(II) and Fe(III) which differ only by an electron, I mean, plus whatever's stuck to them (ligands) but that doesn't necessarily have to change).There are solids with ionic conduction as well. Particularly ceramics at elevated temperature, like ZrO2 for oxygen ions. Indeed this property is used to sense stoichiometric combustion conditions in automotive exhaust ("O2") sensors, and also for fuel cells of similar type (i.e. 2 H2 + O2 <--> 2 H2O, or I'm not sure if it works with carbon species as well?).Other than that, what you're missing I think is electrode reactions. In a semiconductor, you have either a highly doped buffer region, or direct contact to a metal. In either case, the semiconductor-metal (Schottky) junction, by nature, neutralizes any ions, or fills any holes, in its vicinity. The heavily doped region does a similar thing, basically by saturation (strong doping --> high concentration of carriers, fast recombination). The combination of the two is typically used to prevent contact rectification (ohmic Schottky junction); it's also practical to some extent (e.g. aluminum metal on top of Al-doped Si, extra diffusion doesn't change anything). And, conversely, that contact is an unlimited source of the same charge carriers (metals have on the order of 10^26 electrons/cm^3 vs. heavily doped semiconductors, Idunno, 10^20 or whatever it is; in turn compared with say, intrinsic Si ~ 10^14 pairs/cm^3).So, while electric field causes carriers to drift in the bulk, sooner or later those carriers either need to be removed to an electrode, or supplied from one. And this happens with great ease at ohmic contacts, hence why they're so handy.And yeah, ions don't move in Si crystals; not at operating temperatures. They can move at some hundreds of °C, I think usually closer to 500 depending on species, concentration and depth; and annealing of defects (say from epitaxy) a bit lower than that (not so much atoms moving through the bulk, as just snapping into the right places).Although some impurities like sodium move at troubling rates through SiO2, at room temperature -- hence why the purity needs to be so high to make MOS devices, otherwise the moving charges cause history-dependent change in threshold voltage, and probably other weird stuff with surface states, Idunno.Tim

author: jenniferkim
author_rank: Contributor
timestamp: July 19, 2021, 05:53:43 am
content: Check this image:Read more details onsemiconductors.