Generation of eh pairs:

The generation is classified according to the energy supplied to the valence electronsto raise them to the conduction band and creating eh pairs available for coeducation into:

- Thermal generation,

-photo generation,

-impact ionization, and

-Field emission.

-The thermal generation is always present whenever the temperature of the material is greater than zero kelvin. It sets the intrinsic concentration of the mobile carriers in the material. It affects the reverse saturation currents in the junctions and sets the maximum operating temperature of devices. Thermistors as a temperature sensors are based on the thermal generation.

-The phtogeneration is the basis of the photocoductors,photodiodes , photo transisrors and photothyristor an addition to the solar cells and more applications.

-Impact ionization sets the breakdown voltage in the devices and it is utilised in the operation of the zener diode and the avalanche photo diode.

-The field emission occurs when the internal electric field is so high that it torn out valence electrons. Examples are tunnel diodes.

Thank you

I'm not sure if I understand your question correctly, but I think the relevant physical processes depend on the type of device you are interested in. Typically the following association can be made:

*) Unipolar devices: FETs

Most relevant parameters: mobility, carrier densities

*) Bipolar devices: LEDs, Solarcells

Most relevant parameter: excess carrier lifetime or mobility-lifetime-product

(more general: recombination kinetics, which might be nonlinear)

Of course this is oversimplified, but indicates that generation-recombination is most important in diode structures such as LEDs or solar cells. In particular for the later, device-performance is strongly linked to the generation-recombination kinetics.

Find a nice overview over the most important processes in Sze (Physics of Semiconductor Devices) - Section 1.5.4

Best regards,

Deniz Bozyigit

It affects a lot in LED, solar cell, photo-detector. How to reduce them or enhance them depends on the application you want.

Adding to Deniz answer:

Bipolar devices like BJTs have very strong dependence on recombination at the interfaces that directly affects the gain of the device. Also the lifetime in different regions of the device affect the performance of the device.

Similar to that in IGBTs, lifetime is a key parameter in device characteristics and it strongly affects the performance of the device specifically the on-state resistance.

Thank you Deniz for your contribution. Let us divide the devices into Devices based on the eh pairs generation phenomenon and devices affected by the recombination of eh pairs and devices make full use of the recombination of eh pairs.

In principle recombination exists always in a semiconductor to bring it to steady state when its minority carrier concentration is disturbed.

The electronic devices are divided into unipolar and bipolar devices. In bipolar devices both electrons and holes are involved in the the current conduction in the device.

Bipolar devices are like: pn junction diodes, bipolar transistors,thyristors

In unipolar devices only one type of carriers is more or less involved in current conduction like Field effect devices and MS contacts.

As net recombination requires the presence of excess of electrons and holes, which is case in bipolar devices, it plays a vital role in determining the electrical performance of these devices. While it does not play such role in unipolar devices. This makes the great difference between the current conduction in the two classes of the devices.

Since the minority carrier life time is material property describing the recombination rate,we find that this parameter is one of the main parameters of a semiconductor material. It affects much the static and the dynamic performance of the bipolar devices. This is an outline about the recombination and which devices it affects.

I will complete with the other colleagues in the coming comments.

A sidenote to the notion of the minority carrier lifetime:

Talking about the minority carrier lifetime implies that the decay kinetics of excess carriers is governed by a linear differential equation:

dn/dt = \tau^{-1} n(t)

Looking at a wide variety of semiconductors - it turns out this actually is not even very often the case - it is correct though in the most studies system: the doped single crystal semiconductor in the low injection regime.

In other cases, other kinetics become important - example:

- intrinsic semiconductor, or SRH-recombination or high injection regime:

dn/dt =krec n(t)^2

- organic semiconductors:

dn/dt = krec n(t)^alpha with 1

Yes, the minority carrier lifetime will be constant independent of the excess minority carrier concentration in case of low injection condition.in this case it is a material parameter. Other wise it is the recombination constant which is more general for describing the recombination rate. The recombination rate constant depends on the specific recombination mechanism. However,one can still define a minority carrier lifetime for a semiconductor material provided that the injection level to measure it is low. Could you please give a reference for recombination rate of your second and third equation of the recombination rate?

Thank you

I want to echo YR Wu's comment that one has to decide what kind of devices one is interested in before one can address the question as to what are the roles of generation and recombination mechanisms in determining the device's performance. In LED, as an example, the carriers are injected electrically and their recombination via photon emission produces the light one wants to create with LED. Clearly in this case eh generation is through a pn junction and the efficiency depends on the band structure and minority carrier lifetime. ON the other end, the efficiency of emission of photon from recombination of eh pairs also depends on the band structure (eg indirect gap is bad for efficiency of radiative recombination) but also on non-radiative recombination which competes with radiative recombination. Non-radiative recombination via deep traps is a big problems with many poor quality semiconductors. It is even difficulty to say that non-radiative recombination is a material property since in most cases the device fabricator does not know exactly what kind or quantity of deep traps there are in the sample.

Thank you Peter for your detailed comment on LEDs. I agree with you and Wu that one has to specify the device and i expressed this explicitly in my question and my previous comment. Yes , there are two types of recombination concerning whether the recombination energy is transferred to heat or light, the nonradiative and radiative recombination.

Could you tell us how far can we reduce the unwanted nonradiative recombination to get the highest possible radiation?

Thank you again

Typically the non-radiative recombination centers are defects which produce a very localized potential, such as vacancies and transition metal impurities, which will trap an e or h. Once either an e or h is trapped the defect becomes charged and it will automatically exert a strong Coulomb attraction for the remaining carrier. What makes these traps very efficient non-radiative recombination centers is the strong coupling between the carriers and the atoms forming the defect center as a result of the strong localization of the carriers around the center. This strong coupling results in the eh being able to recombine by emitting many phonons efficiently. A discussion of how the strong e-phonon coupling necessitates the use of a configuration-coordinate diagram approach to treat the coupled electron-phonon state can be found in a discussion on the DX center in the 4th edition of my book: Fundamentals of Semiconductors. The seminal paper on this approach and its effect on non-radiative recombination in crystals was published by Huang and Rhys in 1950! Their paper published in Prof. of Roy Soc. A 240, 406 is a "must read" for any one interested in the theory of non-radiative recombination.

Thank you Dr Peter,

I am very pleased with your contribution in this forum. These scientists are the pioneers of solid state electronics. I personally owe them for my knowledge in this science. Full respect and love to them.

I asked you if you kindly show how one increased the nonradioactive recombination relative to the radiative recombination in the advanced LEDS.

Best regards.

Following my previous comment on the effect of minority Carriers on the bipolar devices. The minority carrier life time affects much the forward voltage drop, namely it normally decreases the on voltage drop when it increases at the same current density.Concerning the dynamic performance, it decreases the on time and increases the the off time when it is increased.

Lifetime can be partially technologically controlled by introducing or extracting (gettering)the deep lying impurities impurities and defects acting as recombination centers.

The recombination mechanisms:

The recombination mechanisms as the colleagues noted before can be classified into radiative and nonradiative recombination. In the radiative recombination the free energy of recombination will be converted into photons of light with a photon energy equals the energy gap. While in the nonradiative recombination the recombination energy will be converted int heat. The radiative recombination is utilized in light emitting diodes.

Concerning the major recombination processes, they are:

- Band to band radiative recombination

-Band trap band recombination nonradiative recombination

- Auger band band recombination. nonradiative recombination.

All these transitions are competing and act simultaneously and the measured minority carrier life time is the resultant of them.

Band band radiative recombination will be appreciable only in case of direct band gap material like galliumarsenide. Silicon is indirect band gap material and therefore could not be used in LEDs.

My coming comment will be on generation of electron hole pairs, types utilization and effects on performance.

I wish more shares

Thank you.

Generation of eh pairs:

The generation is classified according to the energy supplied to the valence electronsto raise them to the conduction band and creating eh pairs available for coeducation into:

- Thermal generation,

-photo generation,

-impact ionization, and

-Field emission.

-The thermal generation is always present whenever the temperature of the material is greater than zero kelvin. It sets the intrinsic concentration of the mobile carriers in the material. It affects the reverse saturation currents in the junctions and sets the maximum operating temperature of devices. Thermistors as a temperature sensors are based on the thermal generation.

-The phtogeneration is the basis of the photocoductors,photodiodes , photo transisrors and photothyristor an addition to the solar cells and more applications.

-Impact ionization sets the breakdown voltage in the devices and it is utilised in the operation of the zener diode and the avalanche photo diode.

-The field emission occurs when the internal electric field is so high that it torn out valence electrons. Examples are tunnel diodes.

Thank you

Hello,

Thanks for your helpful information. However, I do have some concerns.

Could you please explain what do you mean with: Recombination rate is controlled by the minority carrier lifetime as well as The energy of recombination will be emerged as a photon of light. Is it accurate to say that after recombination we will have a photon of light rather than losses of energy as heat?

Kind Regards,

Fabricio:

You must have one electron and one hole for recombination to take place.  If, for example, this is an n-type semiconductor so that holes are the minority carriers, you have a lot of electrons around but they can only recombine with one of the relatively few holes.  So the recombination rate is controlled by the lifetime of holes in that case---they are the bottleneck.

As to whether you will get a photon or a phonon (unit of lattice vibrational energy, which is what translates to heat) from a recombination event, that depends on a lot of things.  This page lists the possible mechanisms and explains their outcomes:

http://www.iue.tuwien.ac.at/phd/entner/node11.html

http://www.iue.tuwien.ac.at/phd/entner/node11.html

Welcome Fabricio

Thank you Zynep,

The answer of the Zeynep is very satisfactory concerning the first part of the question.

But i want to add that when you have  say an n-type semiconductor and introduce in it majority carriers, electrons, the excess electrons will be displaced and repelled to the surface of the semiconductor exactly as in the case of metals. So, this process is called dielectric relaxation where the interior of the semiconductor will be field free.

In case of injecting holes, they will be firstly neutralized by pulling electrons to them and the interior of the semiconductor will be will be field free but has an equal amount of excess holes and electrons. The holes will recombine with electrons  with a recombination rate determined by  the excess hole concentration divided by the life time of holes which are minorities.

The second part is explained in my posts, the free energy of the recombination may be converted into light or heat. In case of band band radiative recombination the free energy will be converted to photons. Please you have to tae care of the context of the text.

I would like to advise you to revert to the book in the link:https://www.researchgate.net/publication/236003006_Electronic_Devices

Book Electronic Devices

Dear Professor Abdelhalim and Zynep,

Thanks for your answers. I need to study about this topic and I will let you know if I have more questions.

Regards,

Dear professor, could you please explain how recombination energy converted in to photon energy? 

Dear Saravanakkumar,

you are welcome.

The emission of a photon when an excited electron falls from its excited state to its ground state is a natural phenomenon where it is the inverse of photo absorption where an electron in the ground state absorbs photon and gets transferred into an excited state at higher energy level. It is affected by the interaction of photons and electrons in the atoms. When an electron falls from an orbital with higher energy to orbital with lower energy in atoms , the only way to release its excess energy is by emission of photons.

In the classical emission theory of electromagnetic radiation: an electromagnetic radiation will be released when electrons gets decelerated. Considering that the transition of electron from the excited higher energy level to the lower ground state energy level is a form of time varying current. So, it is plausible to understand the emission of light in the light of electromagnetic radiation theory.  May be the main difference here is the quantization of emitted radiation in form of photons.

Best wishes

Dear all, this is interesting talking, I really enjoyed your conversation,

I would like to add the recombination phenomenon in semiconductor laser is a results from collision of an electron with a hole, the process is essentially the return of free conduction electron to the valance band and is accompanied by the emission of energy, in a semiconductor thermal generation of e-h pairs occures continuously. Therefore, ther is net recombination rate given by the difference between the recombination and generation rates

As one side remark, in the "Generation of eh pairs" list given by Prof. Zekry above, the mechanism mentioned by Prof. Yu is missing: electrical injection (via interfaces, e.g. in a p--n junction device under forward bias, where minorities are injected at the junction and majorities are supplied by the external current). Although this is not a "local creation mechanism" as are the others (thermal generation, photo generation, impact ionization, and field emission), it nevertheless also influences (and sometimes dominates) the device behavior (i.e., its electrical and optical performance).

As an additional aspect I'd just like to mention that for a complete picture of device physics one has to take into consideration not only the energy transfer processes but also entropy effects, so basically the full set of thermodynamical aspects of the device. As an example, the Peltier coefficient of a semiconductor (i.e., the heat energy per charge carrier accompanying an electrical current, contributing to heat transport according to the generalized Fourier law) divided by the temperature (i.e., the thermopower = Seebeck coefficient) can be interpreted as a measure for the entropy per charge carrier.

As an additional side remark I'd like to point out that the device structure may lead to a changed net radiative recombination rate -- cf., e.g., the recent paper "Manipulating the Net Radiative Recombination Rate in Lead Halide Perovskite Films by Modification of Light Outcoupling" (https://www.researchgate.net/publication/320218893).

I would at first like to thank Jan- Martin Wagner for introducing additional information about the generation and the recombination of mobile charge carriers in semiconductors and their effect on the device performance. Peter Yu, Peter Y. Yu Added an answer, in answering this question in this forum pointed out the same effect,that the injection of minority carriers in a semiconductor and their neutralization by an equal excess of majority carrier is a type of generation of excess electron hole pairs. Yes is is true.

I would like to thank Ankush also for bringing the effect of the recombination rate on the the solar cells. Yes the recombination rate is a dominant factor in determining the photo current and the dark current. Specifically it reduces the photcurrent and increases the dark current leading to reducing the photoconversion efficeincy.

In fact, the story of the recombination generation of mobile charge carriers is that of the electronic materials and devices in addition to the current conduction.

For more details about the basic principles of this subject please follow the link:https://www.researchgate.net/publication/236003006_Electronic_Devices

Best wishes

Jan-Martin excited the recombination and generation in an important class of materials; namely the organic semiconductors and the hybrid one.

I would like to add a comment about the recombination mechanisms in organic materials.

From the conceptual point of view, the recombination mechanisms occurring in the the metallic semiconductor also occur in the organic semiconductors.These mechanisms can be classified into radiative and nonradative types. Radiative recombination is a consequence of the direct fall of electrons from the conduction band to the valence band band while the nonradiative one when the fall of the electrons occurs through trap levels in the bandgap. These trap levels are called recombination centers.

Every recombination mechanism has its specific dependence on the hole and electron concentrations and its rate constants.

There is a recombination mechanism which exists in the organic semiconductors.

It is the Langevin mechanism which occurs as direct consequence of an electron and holes comes in the field of the other.  They get this chance when they meet while moving with low mobility.  Its recombination rate r lv can be expressed by the expression:

r lv=q (mun+mup) pn/ epsilon,

where mun, mup are the electron and hole mobilities, p,n are the hole and electron concentrations, epsilon is the permitivity and q is the electron charge.

It is found that this mechanism is applicable in case of the organic LEDs, where it must be reduced by an appreciable factor when applied to organic solar cells.

Generally the rate can be expressed by r lv= kr pn, where kr is the recombination rate constant that can be determined experimentally.

Best wishes

link :https://www.researchgate.net/publication/236003006_Electronic_Devices

Dear Professor, In genral case, during decelartion of electrons, photons are emitted. But

1. When photons are released, what is the nature of electrons, whether it is particle or wave nature?

2. If it is particle, whether it is sphere or some other shape?

Considering that in case of sphere, photons are generated or delivered from the electron, at which geometrical point?

3. If the electrons are as wave nature, what about the photo generation mechanism?

I'm quite sure that nobody knows the shape of an electron.

Dear Mr. Saravanakkumar,

welcome

Your question concerns the nature of electrons and photons and their interactions. This topic has been treated intensively in the basic particle physics.

As far as i know the electron is a very tiny particle has a rest mass me and carries an electric charge q. When it moves, it moves like a wave. So, it is not a wave but moves like a wave. This wave motion is described by the laws of the quantum mechanics based on the Schroedinger equation.

In solving Schroedinger equation for the electron motion one considers that the electron has a mass and charge which are concentrated in a point.

When it is moving in the atom, it can be considered as affecting very tiny electric currents. The atomic magnetic moments are the consequences of the electron orbital currents and electron spin currents.

Therefore, based on the classical electromagnetic theory where an electromagnetic radiation will be emitted when time varying electric current exists.

I assumed that when the electron is falling from an orbit with higher energy to an orbital with lower energy it can be modeled by an impulse current. giving a flash of light which is a photon. This is just a model to bring plausibility explanation of the photo emission process.

As for the exact geometrical shape of the electron, i can not till you its exact shape in agreement with Jan-Martin.

Best wishes

Dear prof.Abdelhalim Zerkey,

Thank you for your answer sir,

So when tiny current (impulse current) which is due to electron orbital currents and electron spin currents perturbed by moving electron in an atom, light will be flashed.

"So when tiny current (impulse current) which is due to electron orbital currents and electron spin currents perturbed by moving electron in an atom, light will be flashed." -- Wait: As Abdelhalim Zekry explicitly wrote, this is just a plausibility explanation, and it is based entirely on classical electromagnetic theory.

On the atomic level, however, quantum mechanics rules over classical physics, so one needs to have a good reason if one sticks to the classical picture. In the quantum world, things work in a fundamentally different way, and only in exceptional cases the behavior is not so much different from the classical one.

For example, in the quantum world there is a phenomenon known as "tunneling" -- and this cannot be understood on purely classical grounds; in classical physics, this is a miracle similar to the stable existence of atoms.

So, now to your other questions:

"1. When photons are released, what is the nature of electrons, whether it is particle or wave nature?" -- The release of photons by electrons just happens, and this is independent of the particle or wave nature of the electron.

"3. If the electrons are as wave nature, what about the photo generation mechanism?" -- Sorry, I'm not sure that I understand this question correctly: What process do you have in mind? The excitation of an electron as a consequence of having absorbed a photon, or something else?

Dear Professors,

I would like to know that can 'Recombination Generation Heat' be negative in AlGaN/GaN HEMT system.

Nice and useful discussions.

Thank you

Dear Sushanta

welcome!

An interesting phenomenon that recombination process absorbs heat from the material and reduce its temperature. Will the recombination energy emitted as photons? This means that heat can be converted to light by generating electron hole pairs that can recombine to emit light!!

Please shoe the evidence!!

Best wishes

Yes, there is an entropic contribution to light emitted by a semiconductor, coming exactly from the recombination process (quite similarly as in a thermoelectric cooling process). Look at an LED: The energy of the emitted photons is higher than the voltage applied, which can be nicely seen from the band structure of a p--n junction under forward bias (cf., e.g., Research Semiconductor thermodynamics: Peltier effect at a p-n junction

Article The chemical potential of radiation

Article Light with nonzero chemical potential

Additionally, the following publications could be of interest in that respect: "Bias-dependent Peltier coefficient and internal cooling in bipolar devices" (Phys. Rev. B; 2002) and "Internal cooling in a semiconductor laser diode" (IEEE Photonics Technology Letters; 2002); the links are given below.

Article Bias-Dependent Peltier Coefficient and Internal Cooling in B...

Article Internal Cooling in a Semiconductor Laser Diode

Thank you respected Dr. Wagner for your comment about the possibility of cooling the material accompanying material during generation recombination process. My question concerned the mechanism of cooling process how it comes to effect. I think the thermal generation process or the thermal assisted generation process may convert thermal energy into energy stored in electron hole pairs by generation and then the recombination process will be of the type radiative such that in this way thermal energy can be converted to light and cooling occurs.

Best wushes

Really, I enjoyed reading this series of questions and answers pairs.

does recombination give us always a photon of light ?

Abdelhalim Zekry

Dear Beddair,

Welcome,

There are different recombination mechanisms of how the fee energy of the electron is lost when it is recombines with hole. These mechanisms can be either heat emitting in form thermal vibrations called phonons or can be light emitting in form of photons. All recombination mechanisms are running simultaneously with the dominate one which has the highest rate. Light emission by recombination requires that the material has direct bandgap as those used in light emitting diodes LEDs.

I invite you to refer to the basic book in the link: https://www.researchgate.net/publication/236003006_Electronic_Devices

Best wishes

There is a third recombination mechanism: Auger recombination, which is a three-particle process. For further reading, have a look at http://hyperphysics.phy-astr.gsu.edu/hbase/Atomic/auger.html or https://eng.libretexts.org/Bookshelves/Materials_Science/Supplemental_Modules_%28Materials_Science%29/The_Science_of_Solar/Solar_Basics/C._Semiconductors_and_Solar_Interactions/IV._Recombination_of_Charge_Carriers/3._Auger_Recombination

Dear Tarek,

welcome!

The piezoelectric materials are materials when stressed they will be charged. They are normally insulators such as Quartz. And so they have a large band gap.

They will be only affected by high energy photons. So, from the conceptual point of view piezoelectric materials are subjected to generation recombination processes as they are similar to the semiconductors but their band gap is much larger.

But why do you ask this question?

Best wishes

Useful discussion. Following..

Very useful discussion. Thank you all