1
\$\begingroup\$

So I finally Dived into understanding the P-N Junction and Field Effect Transistors. (this may almost be teetering on the "Physics portion" of electronics, but it's still mainly for understanding the Field Effect....

This video (Which I hope was correct) really made it feel so simple or easier to understand. http://www.youtube.com/watch?v=CLLcRRBph90 (Just for reference, don't really have to watch it)

Anyways from What I understood, 2 Semiconducting Material (P being Positively charged and containing "holes" and N being Negatively charged (Extra Electrons) from whatever they were "doped" with. When a Positive current is applied it makes the Electrons "Jump" into the Holes, this causes current to flow (The free electrons in the N junction, jump into the P Junction).

Anyways I have a few questions.

  1. What exactly "causes" the electrons to jump into the Holes? and What exactly is happening when the electrons attach to the holes? are they immediatly unseated...or do new ones take their place or what? I understand the "holes" concept but im not sure I understand what happens when current is flowing. Do these electrons from the N Region go into the hole of a P region and then continue to flow, do they stay there? do they discharge someone else?

The Depletion Region

  1. So if you reverse bias a PN junction it causes electrons to go from the P region to the N region, causing a Depletion Region. First of all, does the P region have ANY electrons in it at all.... or is it doped to have absolutely zero (and therefore any free electrons are coming from your ground/- source (Battery/etc...) and therefore just filled with Holes?

And what exactly is "In" the depletion region? Just nothing...floating electrons? I understand the concept of a Depletion region, but im not sure I understand "Why" one forms when reverse Biasing a PN junction. The more voltage you apply the bigger the depletion region becomes, but is this because more electrons are getting shoved into the N region where they have no "holes" so they just "float around"? and cause a Neutral zone of sorts (or an insulator)? How comes they don't just bypass the N region....since it's technically still a conductive material? (Or do "some" actually still do..like a Reverse Biased diode will leak a little bit won't it?)

Also sidequestion: The PN Junction is obviously pretty important..but besides the fact were able to control the flow of electrons using the Depletion Region, and reverse biasing and such...is there any other "special" qualities that aren't talked about? Since Even Transistors almost seem like a Diode, and that seems to be the basis of the PN junction.

Sorry if this is borderlining Physics im just trying to nail down the PN junction into terms I can understand. (Although if anyone knows of any Physics books that go into detail on the PN Junction and Semiconductors/Electronics Physics I would appreciate it)

\$\endgroup\$
2
  • \$\begingroup\$ A semiconductor being P- / N-doped does NOT mean it is positively / negatively charged. It just means it has free charges, either positive ones (holes) or negative ones (electrons), but they are compensated by opposite charges in the nuclei, keeping the whole material itself still neutral. \$\endgroup\$ – Curd Jan 22 '13 at 9:57
  • \$\begingroup\$ @Curd Ah ok, I guess I thought free charges/holes added/subtracted to the overall molecules charge. \$\endgroup\$ – user3073 Jan 22 '13 at 22:19
1
\$\begingroup\$

First of all, it is wrong(misleading) to refer P type as positively charge and N type as negatively charged, both P type and N type are neutral in nature, however it is right to say that P type contains free charge carriers in form of holes and N type contains carriers in form of electrons.

Secondly, a depletion region/layer is already in picture from the beginning i.e. while fabricating P-N Junction, due to abrupt change in concentration of electrons/holes in two types of materials, electrons from N type material and holes from P type material diffuses to P type and N type materials respectively. This leads to formation of depletion region/layer which contains ions (Positive and negative ions), not electrons or holes. These ions are generally immobile in nature. And in this way, region nearby p-n interface loose its neutrality and becomes charged.Since space charges in depletion region leads to an electric field which opposes further movement of electrons and holes due to process of diffusion, P-N junction reach to a state of equilibrium.

Next thing is, again, applying a positive current is somehow misleading, we apply positive voltage to P material and negative voltage (zero voltage) to N material, and when battery connected this way, its Forward Bias/Biasing.With a battery connected this way, the holes in the P-type region and the electrons in the N-type region are pushed toward the junction. This reduces the width of the depletion zone. The positive charge applied to the P-type material repels the holes, while the negative charge applied to the N-type material repels the electrons. As electrons and holes are pushed toward the junction, the distance between them decreases.Only majority carriers (electrons in N-type material or holes in P-type) can flow through a semiconductor for a macroscopic length. With this in mind, consider the flow of electrons across the junction. The forward bias causes a force on the electrons pushing them from the N side toward the P side. With forward bias, the depletion region is narrow enough that electrons can cross the junction and inject into the P-type material. However, they do not continue to flow through the P-type material indefinitely, because it is energetically favorable for them to recombine with holes. Although the electrons penetrate only a short distance into the P-type material, the electric current continues uninterrupted, because holes (the majority carriers) begin to flow in the opposite direction. The total current (the sum of the electron and hole currents) is constant in space, because any variation would cause charge buildup over time

Therefore, the current flow through the diode involves electrons flowing through the N-type region toward the junction, holes flowing through the P-type region in the opposite direction toward the junction, and the two species of carriers constantly recombining in the vicinity of the junction. The electrons and holes travel in opposite directions, but they also have opposite charges, so the overall current is in the same direction on both sides of the diode, as required.

Same analogy can be obtained/derived for Reverse Bias situation as well.

I think i answered most of the questions of yours, rest you can answer by yourself.

Though, i will also suggest you to go through some standard book (Streetman and Banerjee is good) to understand concepts fully, once you understand them, there will be no doubt in future as well, but its really difficult to understand P-N junction or physics concepts through a 1/2 hour video.

\$\endgroup\$
2
  • \$\begingroup\$ Can you explain why something having more Holes (+ Carriers) or Electrons (- Carriers) doesn't make the whole thing +/or - biased? \$\endgroup\$ – user3073 Jan 22 '13 at 19:18
  • \$\begingroup\$ Again, explaination is same, there is no doubt about the fact that it has more holes, but it also has same number of electrons as well, difference is, holes are free to move(mobile carriers) and electrons are not free to move as free. Voltage will develop when there is only positive charges or holes, or accumulation of charges, but since material is neutral, there will be no voltage or +/- biasing. \$\endgroup\$ – Nishu Jan 23 '13 at 7:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy