![]() So to sum up - on the surfaces where two semiconductors meet is a distribution of charges due to the doping. ![]() For an unbiased transistor this is still incredibly small, but if you bias the transistor, you draw away charges from the surface, and you reduce the barrier the electron has to cross, you increase the probability that electrons cross. So now you have the possibility that electrons simply pop over to the other side if the barrier, btw this effect is called quantum tunneling. However for electrons trying to pass through a potential barrier the numbers work out much better. Actually there is but for the standard case the probability is something like one in one billion times one billion times one billion, effectively zero. If that hill were small enough, and the marble were small enough there would be some probability that the marble appears on the other side of the hill. If the marble could get to the other side, it would could exist perfectly fine, but the hill is in the way. That marble can't get to the other side ( well assume that it can't go around the hill ). It doesn't have enough energy to go higher. That marble can move around a lot, but it can only go about halfway up the hill. Imagine a marble on a smooth surface with a hill. Now we need a little trick from quantum mechanics. But after it goes through one barrier and then encounters another barrier of the opposite type. A n-p barrier does not offer an obstacle. Well ( like a diode) through a p-n barrier. ![]() Electrons will need energy to go from one side to the other. The surface of the two form something similar to the charged plates above. So now imagine two oppositely doped semiconductors sitting in a vacuum. But at the surface of the material, ions collect to form a net charge. The electrons are not constrained to go anywhere. So how does this apply to transistors? Inside a doped semiconductor there is a soup opf atoms, ions, electrons. Instead of using ergs or joules to describe these sort of energies, they use a new unit the electron-volt or eV for short ( as well as the KeV-Kilo-eV, the Mev Million-eV, GeV, Giga-eV, TeV Tera-eV etc. We see this primarily in two areas electrons being pushed around between plates in a vacuum tube, and particle accelerators. The difference between potential energy per unit charge at each plate due to electric field is called the potential difference or voltage. The electric field created by the charges on the plate, contributes to the potential energy of the system. To cross the plates an electric charge ( eg an electron ) has to have certain amount of kinetic energy. Basic electromagnetism says there is a constant electric field between the plates. Imagine ( like in the old vacuum tube days ) an electron moving between two charged plates. When physicists talk of "potential" they mean energy (actually potential energy per unit charge ). My knowledge of transistor physics is outdated but let me try. Once you understand how a diode works, transistors become easy! Why do I refer you to diodes, it's because a transistor can be considered as device with two back-to-back connected diodes. When you apply an external voltage, this barrier potential reduces as electrons are able to flow more freely, thus turning the diode on. This would then result in the formation of a charge neural depletion region, and any new electron which now needs to cross-over to the p-type substrate would require an additional external voltage. Now if you don't apply any external voltage, there will be an equilibrium state where the electrons at the border of the n-type material will cross over to the p-type substrate. have extra free electrons) and the substrate might be p type material (i.e. Now, the source may be of n type material(i.e. Think of each connection in the transistor as a diode. For the first answer, this barrier is caused due to the different materials used to construct the transistor.
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