student654321 said:
i am writing a lab report into an experiment i did using a photodiode to detect alpha particles. I am curious however as to how a photodiode actually works. The alpha particles are emitted from the source, they hit the diode and then what happens? Its somehow changed into an electrical signal that comes up on the screen of my computer at a particular energy but how does it get there?
oh boy, semiconductor device physics! (warning, I'm not super good at it, but most electrical engineers had to learn some of it. but i take no responsibility if for inaccuracies.) here is a little hand-waving explanation:
first a little p-chem (from an EE's POV): for some physical reason that i have long forgotten (having to do with the quantum mechanics of the hydrogen atom and then hand-waving some results of that analysis to bigger atoms), the electrons in atoms exist in somewhat stable "shells" of discrete energy levels. kinda like a staircase as opposed to a ramp. the nature of these discrete energy shells, what potential energy electrons possesses in each shell and how many electrons each shell will hold essentially determine how these atoms will react chemically. i can only recall the first (or "bottom") two or three shells, the bottom shell holds at most 2 electrons, the next two holds at most 8 electrons each. given the number of electrons an atom has (it's "atomic number"), the bottom shell fills up first, then the next shell, and so on. this is what determines the form of the Periodic Table of Elements.
an atom that has these shells exactly filled is a very "happy" or "satisfied" atom and cannot be expected to react chemically with anything under normal circumstances. these are the inert gasses like Helium (He) or Neon (Ne) or Argon (Ar) at the extreme right of the Periodic Table. these are the only gasses or substances (that i know of) that are "monatomic", that is one atom per molecule. these atoms need no other atoms to grab electrons to fill an empty "hole" or to dump excess electrons on.
contrast this with Sodium (Na) and Clorine (Cl). Sodium, at the extreme left of the periodic table, has a complete shell
with one extra electron and Clorine, at the extreme right (except for the inert gasses), has almost a complete shell but is
missing one electron to complete the shell. call that a "hole" in the Cl atom. so suppose these two atoms happen to meet in a singles bar and that Na atom would like very much to plug that Cl atom's hole with his throbbing electron, and the Cl atom wouldn't mind it a bit. so they get together, the Na atom's extra electron jump over to the Cl shell, filling it, and spends nearly all of its time orbiting the Cl atom. fine, but these atoms were electrically neutral before doing the
dirty deed (because the number of protons were equal to the number of electrons) so when that electron jumps over from Na to Cl, that leaves the Na atom positively charged (one more proton than electrons) and the Cl atom negatively charged and those two atoms are going to stick together really tight because of electrostatic forces. think of that as the psychic bonding that happens to people (and many animals) after doing the horizontal bop.
(sorry for the anthropomorphizing, but it's the best way for me to imagine what is going on.)
okay, so you got the inert gasses (which are comparable to the celibates or those with removed hormones: "Who needs sex! I sure don't!") at the extreme right of the Periodic table. then there are the elements on the extreme left and extreme right (just to the left of the inert gasses) of the Periodic who you might consider to be the young, nubile, (and horny) heterosexuals. think of the diatomic gasses (\operatorname{Cl}_2 or \operatorname{O}_2) as hot b_itches locked up away from the men and resigned to lesbian relationships (but you better watch out if they get loose) in the meantime. \operatorname{N}_2 isn't so bad but if \operatorname{Cl}_2 gets out, you better turn around and run!
now another interesting group in this microcosm are the atoms, in the Group IV cloumn of the Periodic table, with outside shell exactly
half filled. that is 4 extra electrons (or is it 4 missing electrons??) in the outside shell. this would be Carbon (C) or Silicon (Si) or Germanium (Ge). they are the hermaphrodites. they don't know if they be the girls or if they be the boys, but they ain't celibate. they just kinda hook up the way you might imagine two hermaphrodites hooking up. the 4 "extra" electrons of one Si atom sort of fills the need of 4 "missing" electrons in the adjacent Si atom. that is what a pure, undoped, semiconductor is and, except for the occasional electron that thermal energy kicks up out of their satisfied shells, they're not much of a conductor of electricity as such (pure silicon, say).
so now what happens is that your local neighborhood semiconductor factory infuses into this lattice of hermaphrodite atoms, some slightly less hermaphrodite atoms such as Boron (B) or Aluminum (Al) or Gallium (Ga) with 3 electrons in the outer shell (Group III in the periodic table) or Phosphorus (P) or Arsenic (As) with 5 electrons in the outer shell (Group V in the periodic table but better thought of as missing 3 electrons in the outer shell). the material doped with B or Ga (called "P" type silicon) would be missing an electron here or there (wherever there is the occasional B or Ga atom in the Si lattice) and that missing electron would be called a "hole". the material doped with P or As (called "N" type silicon) would have an extra electron here or there (wherever there is the occasional P or As atom in the Si lattice) and that extra electron would be called an "electron". both holes and electrons act as particles. an electron acts as a particle of positive mass and negative charge. a hole acts as a particle of positive mass and positive charge. a hole and electron combine to be a satisfied shell.
now, by themselves, both the P type silicon and the N type silicon are electrically neutral (same number of protons as electrons, even if there are some extra electrons or missing electrons in the outer shells), but what do you think might happen if you stick some P type silicon next to some N type silicon with some suitable glue? (this is a semiconductor diode.) at least around the "PN junction", the boundary or contact surface between the two, some of those extra electrons will really want to jump over and fill those holes on the other side. and some of those holes will really want to jump over and satisfy those free electrons on their other side. when they do that, there will be a small voltage (called the "contact potential") because the P doped silicon will be more positively charged and the N doped silicon will be more negatively charged.
there is an equilibrium of forces because the positively charged P type material is trying to draw those electrons back, but the occasional empty shell "hole" is beckoning it to stay. or you can say the negatively charged N type material is trying to draw those holes back, but the occasional excess shell electron is beckoning it to stay. so if we did nothing, that situation, with an electrostatic charge pulling charges in one direction being opposed by the quantum mechanical physical chemsitry pulling the charges the opposite direction, could remain forever, if there were no other forces brought in from the outside. the number of electron/hole charges that have jumped over remains roughly constant. but it is a very precarious equilibrium.
so now we add another force, suppose we hook up a battery or some electrical source with the "+" terminal connected to the P silicon and the "-" terminal connected to the N silicon. then electrons in the N silicon will be given even more energy to jump over and combine with a hole in the P silicon (connected to the "+" terminal where that electron will eventually drain to). this resulting movement of charge is current and that diode is acting like a conductor of electrical current. now, suppose you hook the battery up the other way. those extra electrons will immediately drain into the "+" terminal (the "holes" will drain into the "-" terminal) and what will be left is silicon with satisfied shells. no extra electrons or holes in the shells. (it won't be electrically neutral since the N type silicon will have extra protons in each Phosphorus atom and the P type material will have one missing proton in each Al atom, but that's what you would expect with the N connected to "+" and the P connected to "-".) there will be no conduction of current that way.
this is why they are called "semiconductors" sometimes they conduct electricity and sometimes they don't. it depends on what you're doing to them.
now what you're doing is a little different. you have these diodes hooked up in a little circuit where you are likely not driving them with any external voltage. so you have that precarious equilibrium condition mentioned above. some electrons from the P material have jumped over to the N material and some holes from the N material have jumped over to the P (leaving the P material slightly positively charged and the N material slightly negatively charged and that charge keeps
more electrons or holes from doing the same).
so some positively charge alpha particle come into the N material and gives one of those holes (also positively charged but with a lot less mass) that would like to jump over to the P side but won't because the P side is positively charged, a little boot (positively charged particles repel each other and the alpha particle has a lot more mass/momentum than the hole, so guess who's going to have to move) and that hole now has enough energy to make the jump. i don't know what happens to the alpha particle, but i don't think it combines chemically with the silicon or phosphorus, so i think it continues on its merry way. that leaves the positive side even more positively charged and the negative side even more negatively charged than what it was at equilibrium. then some charge is going to want to move back to put that junction back to equilibrium. that is an electromotive force (or a voltage) and can be detected with an electronic circuit, just like the electric voltage coming out of a microphone (or your electric guitar) can be. that is what your pre-amp or amp is for.
(something similar might happen if a photon comes barrelling in and gives one of those extra electrons a kick up to a higher energy state.)
Also there's a pre-amp and an amplifier being used which i am not sure about either.. Any help would be greatly appreciated.
the electrical voltage generated by the photodiode is so small that, before the electronics used by your computer to read it (called an "analog-to-digital converter" or "A/D" or "ADC") can read it, that voltage has to be boosted. i can't explain how amplifiers work without explaining how the transistor (a PNP or NPN pair of junctions) works.
just pretend the pre-amp or amplifier is a magic box that makes little voltage fluctuations into bigger voltage fluctuations.
hope you had fun.
r b-j