Free-floating electron "holes" recombining when a charge flows through a transistor

Asked May 21, 2017, 3:18 PM EDT

As I understand it, doped silicon has the N and P type junction with a neutral zone in between occurring naturally due to recombination of the electrons from the N-type zone and the positively charged electron "holes" from the P-type zone. The P zone is full of free-floating electron "holes" which are positively charged. I am wondering why, when a voltage is applied to the transistor, the electrons passing through it are not attracted to the holes, why they don't recombine with the holes (giving off a photon) like the electrons from the N type do. And if the electrons from the charge passing through the transistor DO recombine with the free-floating holes, how does the transistor return to its original state, instead of the P type silicon now being neutral when the voltage is removed? How would the electrons that filled the holes be removed again (since they came from an outside source, not from the N type)?

Fayette County Kentucky

5 Responses

Thank you for your question. I am currently seeking someone with more specific expertise in this area than I have and will try to find you an answer as soon as possible.

Thank you, i appreciate it! I know it's a pretty specific and in-depth question, probably even for an electrical engineer.

Good morning,

I consulted with Dr. Janet Lumpp, Professor of Electrical Engineering with the University of Kentucky College of Engineering in Lexington. Her response is below:

"You are right about the n-type and p-type having extra electrons (- charge) and holes (+ charge) respectively. The extra electrons and holes come from dopant atoms that have one more or one less electron than the atom they are replacing in the crystal structure. The holes are not free-floating themselves. A hole is left when an electron moves. When we play musical chairs we usually have more players (electrons) than chairs mimicking an n-type material. The extra player is a free floating electron. If we played with more chairs than players like a p-type material, the empty chair would be a hole. Imagine the players hopping from chair to chair so that the empty chair appears to move in the opposite direction. That is how holes move.

Recombination is a different reaction than the movement of excess charges. Semiconductors can absorb light or heat to excite an electron to a higher energy level called the conduction band which leaves a hole behind in the valence band. When an electron fills in the hole, that is recombination which releases either heat or light to balance the energy lost by the electron.

Electrons also fill in holes when p-type and n-type semiconductors are joined (or deposited). The interface is called the depletion region because the free-floating electrons and holes are depleted. A potential (voltage) appears across the depletion region because of the dopant atoms. Applying an external voltage causes the depletion region to extend farther into the p- and n-type materials or if the applied voltage is reversed, then the depletion region shrinks and it is easier to get the current to flow through the depletion region as electrons hop in one direction and holes hop in the other direction. When the voltage is turned off, the charges redistribute."

I hope this provides you some help with your question.

It does! Thanks so much to you and to Dr. Lumpp!