Dear Victor,

When speaking about electric current, the motion of all charges must be considered not only the excess charges. In order to measure the current in a conductor or a semiconductor, one has to determine the charge flowing across its crosssection per second.

In semiconductors, there are three types of currents: the drift current, the diffusion current, and the displacement current. The drift current is driven by the electric field in the material,the diffusion current is caused by concentration gradients and the displacement current is caused the change of the electric displacement with time. So, in principle, the current component exists whenever its driver exists.

Therefore when injecting the semiconductor with majority carriers excess, the excess majority carriers will be repelled from the interior of semiconductor to its surface and the semiconductor will be charged. This process is very rapid and takes in the average the dielectric relaxation time. SO injecting majority excess leads to a transient charging capacitive current. Sustained excess of both charge carriers can only exist when injecting the material with minority carriers.In this case the bulk of the material will store excess charges and the building or the removal of such charges gives rise to a capacitive currents.

Apart from the displacement current,.the current in any crosssection of the semiconductor is the sum of four components, two for the minority and two for the majority. The two for each type is the drift and diffusion current. Under practical operating conditions with low injection condition, the majority carriers f low by mostly by drift while the minority carriers flows mostly by diffusion. I would like to stress here that all charge carriers contribute to the current and not only the excess.The excess constitutes the disturbance of the charge carriers on injection or extraction.

This topic is very important and it is discussed on the researchgate. Please

return to the links:

https://www.researchgate.net/post/How_long_does_the_material_take_to_revert_to_thermal_equilibrium_state_after_disturbing_it_with_majority

https://www.researchgate.net/post/How_would_a_semiconductor_disturbed_temporarily_with_minority_carrier_excess_return_to_thermal_equilibrium

https://www.researchgate.net/post/How_do_the_mobile_charge_carriers_flow_in_semiconductors_and_in_metals

My best wishes

Like charges repailes , unlike charges attracts!!!!!!!!!.

It drifting under small electric field across circuit to and spin motors etc. and to eventually recombine in same junction coming from other direction. But fields are small, therefore drift speed is about 1 mm/s and actually other electrons pushed away by newcomers recombine somewhere in middle of circuit with stream of holes (deficit of electrons) generated on other side of junction. If electron density gradient is too large (wire resistance is large) , this recombination results in excessive Joule heating.

Your question is: But once the charge is in the other quasi-neutral region, how does it move towards the contact?

Well, you can think the neutral region as a wire. ¿Did I answer your question? ¿Did I understand your question?

Thank you for your answers.

So I understand that excess majority carriers are purely conducting and won't recombine. However, a conducting wire which is biased conduct charges through an electric field (proportional to the potential bias). Here the region is "quasi-neutral" so charges are not drifting. I liked the idea of having electrons pushed towards the contact by other incoming electrons, is it a right way to see it?

Quasi-neutral mean the electric field is very small. But it never zero - otherwise no current will flow across circuit.

Electric field in say copper wire with 1A/mm2 current density is 0.018 V/m - compare it with ~10 MV/m fields in pn-junction

Very well, thank you for all these explanations.

A simulation results show that the holding current and voltage and the breakover point are strongly affected by varying the minority carrier lifetime at outer heterojunctions. Numerical results also indicate that shortening the minority carrier lifetime in the inner PN homojunction region only increases the OFF-state current.

from the paper of :Hamid Z. Fardi, (Department of Electrical Engineering University of Colorado at Denver Denver, CO 80217–336)

The key point: any excess charge of majority carriers would relax on the time scale of Maxwell dielectric relaxation, i.e. much faster that recombiantion processes.

When the semiconductor in equilibrium one can determined electron and hole concentrations in the conduction and valence bands, respectively. The net flow of the electrons and holes in a semiconductor generates current. The process by which these charged particles move is called transport. The two basic transport mechanisms in a semiconductor crystal is named the drift ,which is the movement of charge due to electric fields, and diffusion: the flow of charge due to density gradients. The ambi-polar transport equation describes the behavior of the excess electrons and holes. When holes flow from the p region across the space charge region into the n region, they become excess minority carrier holes and are subject to excess minority carrier diffusion, drift, and recombination. Likewise, when electrons from the n region flow across the space charge region into the p region, they become excess minority carrier electrons and are subject to these same processes.

Dear Victor,

When speaking about electric current, the motion of all charges must be considered not only the excess charges. In order to measure the current in a conductor or a semiconductor, one has to determine the charge flowing across its crosssection per second.

In semiconductors, there are three types of currents: the drift current, the diffusion current, and the displacement current. The drift current is driven by the electric field in the material,the diffusion current is caused by concentration gradients and the displacement current is caused the change of the electric displacement with time. So, in principle, the current component exists whenever its driver exists.

Therefore when injecting the semiconductor with majority carriers excess, the excess majority carriers will be repelled from the interior of semiconductor to its surface and the semiconductor will be charged. This process is very rapid and takes in the average the dielectric relaxation time. SO injecting majority excess leads to a transient charging capacitive current. Sustained excess of both charge carriers can only exist when injecting the material with minority carriers.In this case the bulk of the material will store excess charges and the building or the removal of such charges gives rise to a capacitive currents.

Apart from the displacement current,.the current in any crosssection of the semiconductor is the sum of four components, two for the minority and two for the majority. The two for each type is the drift and diffusion current. Under practical operating conditions with low injection condition, the majority carriers f low by mostly by drift while the minority carriers flows mostly by diffusion. I would like to stress here that all charge carriers contribute to the current and not only the excess.The excess constitutes the disturbance of the charge carriers on injection or extraction.

This topic is very important and it is discussed on the researchgate. Please

return to the links:

https://www.researchgate.net/post/How_long_does_the_material_take_to_revert_to_thermal_equilibrium_state_after_disturbing_it_with_majority

https://www.researchgate.net/post/How_would_a_semiconductor_disturbed_temporarily_with_minority_carrier_excess_return_to_thermal_equilibrium

https://www.researchgate.net/post/How_do_the_mobile_charge_carriers_flow_in_semiconductors_and_in_metals

My best wishes

Dear Dr. Abdelhalim,

Thank you for your reply. I definitely have my answer. I understand very well that I must consider all the components when considering the total current. But I needed to understand specially this component as the current generated by a solar cell is the one opposed to the diffusion current.

Thank you again, I very appreciate this explanation,

Regards

Yes.the question pointed out your deep understanding of current transport in solar cells.i just stressed some facts concerning current transport in semiconductors. I wish you success.

Dear Colleagues. I want to thank Prof.Dr. Zekry who teached me and thousands of PhD holders allover the world. Particularly, I thank him for his deep explanation of the meaning and underlying meachanisms of charge carrier transport in semiconductor. My dear Victor, I appreciate raising such questions, which show post-graduate students that although they develop devices and make use of recent simulation tools to investigate and develop existing and new devices, but they do need to review their basic knowledge about semiconductors and the theory of charge carrier transport. I addressed this theory in my Book (with the same title) and so many of my articles, e.g., "INVESTIGATION OF THE HOT CARRIERS REPELLING EFEECT IN SEMICONDUCTOR DEVICES.......", which was published IEEE Transactions, Electron Devices,, July 2006. You can download it here in Researchgate, from my publication list. https://www.researchgate.net/publication/3074995.

What I'd like to add to the thread here is that; charge carriers in semiconductor devices may be subjected to several transport mechanisms, in the same time time and same place. However, when there exist a dominant mechanism, it will control the carrier transport from one region to another. Also, the dominant mechanism may change from one region to another, during steady state transport (drift, diffusion, thermal diffusion)) or transient (acceleration and deceleration of hot and cold carriers, across space charge regions and quasi-neutral regions)).

Article Investigation about the hot-carrier repelling effect in semi...

As for the definition of the lifetime, I define it simply as follows:

The life time of charge carriers is the average time interval between their birth moment (generation by a certain generation mechansim, such as photogeneration) and their death moment (recombining with another charge carrier of opposite type).

In direct band gap :

If we now were to turn off the light, the minority carrier concentration will decrease (in the usual exponential fashion) to its thermal equilibrium value. The average time needed for a decrease of 1/e is theminority carrier life time . Life times in the case of direct semiconductors thus tend to be short - typically nanoseconds - and are dominated by the properties of the semiconductor itself.

In Indirect band gap :

IThe wave vectors now are different - direct recombination is not allowed; it would violate Braggs (generalized) law or the conservation of crystal momentum. Recombination needs a third partner (e.g. lattice defects), and life times will be large (typically microseconds if not milliseconds) and depends somehow on the concentration of suitable third partners.

A reasonable approximation for analysis diode p-n junction of a solar cell is to divide the diode up into depletion (capacitance effect) and quasi space charge neutral region. It has been found that the minority carrier concentration at the edge of the depletion region depends exponentially on the voltage applied to the diode. Morever it has been shown that when quasi neutral are uniformly doped and majority carriers currents small, minority carriers flow primarly by diffusion. Indeed carriers can flow by drift and diffusion when the minority carriers are not insignificant and the space –charge density is approximatly equal to zero.