Well, it seems that Ilias Katsouras refers to organic materials (I don't know a lot about this kind of semiconductors). In the case of inorganic materials, direct optical band gap is ALWAYS greater than the electrical band gap. Electrical band gap is simply the difference between the top of the valence band, and the bottom of the conduction band. For the optical band gap, you need to know where the higher energetic electrons are in the valence band, and determine the energy they need to reach the lowest energy level AVAILABLE on the conduction band. As you can expect, usually the higher energetic electrons on the valence band are the top of this band. But, for the lowest energy level AVAILABLE on the conduction band, the history is different. Depending on the doping level and the band structure, you fill the bottom of the conduction band until a limit. This filling of the lowest energy levels at the conduction band is a constant for your doping level. Thus, if you supply any external energy (electric field or ligth illumination mainly), the excited electrons that could reach an unoccupied level at the conduction band need an energy HIGHER than the electrical band gap, because of the already occupied levels due to doping. This is the so called OPTICAL BAND GAP (because of the main effect related to this phenomena is the hopping of electrons due to the illumination). And there is a lot of theory behind the behavior of the optical band gap, and here it is not the place to discuss about. Dear Sriram Subramanian, as you're working on a subject that I'm working on more than a 25 years ago, I'll be glad to discuss on it if you like. The same goes on to Chandra Shekhar Prajapati (I've see that you're working on ZnO thin films).

the band structure is the basic thing which makes a difference between the EBG and OBG...thats it...still we have many things like diffraction of electrical particles comes to OBG and the energy transfer relations comes for EBG

Can you explain it more clearly

Yes but let me know where is the application u r using...since different way of application will have different difference...sorry to say that this is actually a research website

yes, they are different. They can not be connected. Just think about optical bandgap and go on.

Thanks all

Thanks Dheeraj . I know this portal is dedicated for research .

yes thanks fr ur understandings...u may ask about the application and the drawbacks ....we are ready...coming with basic makes lot of time to say...if u want to know from basic u can cumon to my [email protected]

Regards

Dheeraj

We refer to the electrical bandgap in cases when you reduce (add an electron to the Lowest Unoccupied Molecular Orbital, LUMO) or oxidize (add a hole=remove an electron from the Highest Occupied Molecular Orbital, HOMO) in a molecular species. In a photoexcitation, where you probe the optical bandgap, you create a strongly bound electron-hole pair by transfering an electron from the HOMO to the LUMO (or from the ground state to an excited state if you like). This Coulombic interaction is what makes the optical bandgap in general smaller than the electrical one. Keep in mind that the real picture is more complicated, since the energy of an unoccupied state is changed upon occupation.

Thanks llias

Actually i'm trying to find out the methods to find the band gap theoretically for thin films.

To Sriram,

Electrical band gap can be found out by using four probe or resistivity measurement techniques, It gives the information about band gap Eg...while, Optical measurement like luminescence, Raman use to distinguish the band gap Eg into direct or indirect band gap materials. Between two methods, optical method gives appropriate measurement of band gap. for which material you want to know the values?

Great . I'm now preparing thin films of semiconducting oxide materials using spray pyrolysis.

And is there any theoretical method available to find these things

@llins: is it correct that the optical band gap is smaller than the electrical one? May be equal or higher.

@llias: Can you please explain the difference between Apparant bandgap and optical bandgap...?

Well, it seems that Ilias Katsouras refers to organic materials (I don't know a lot about this kind of semiconductors). In the case of inorganic materials, direct optical band gap is ALWAYS greater than the electrical band gap. Electrical band gap is simply the difference between the top of the valence band, and the bottom of the conduction band. For the optical band gap, you need to know where the higher energetic electrons are in the valence band, and determine the energy they need to reach the lowest energy level AVAILABLE on the conduction band. As you can expect, usually the higher energetic electrons on the valence band are the top of this band. But, for the lowest energy level AVAILABLE on the conduction band, the history is different. Depending on the doping level and the band structure, you fill the bottom of the conduction band until a limit. This filling of the lowest energy levels at the conduction band is a constant for your doping level. Thus, if you supply any external energy (electric field or ligth illumination mainly), the excited electrons that could reach an unoccupied level at the conduction band need an energy HIGHER than the electrical band gap, because of the already occupied levels due to doping. This is the so called OPTICAL BAND GAP (because of the main effect related to this phenomena is the hopping of electrons due to the illumination). And there is a lot of theory behind the behavior of the optical band gap, and here it is not the place to discuss about. Dear Sriram Subramanian, as you're working on a subject that I'm working on more than a 25 years ago, I'll be glad to discuss on it if you like. The same goes on to Chandra Shekhar Prajapati (I've see that you're working on ZnO thin films).

@Jeyadheepan Karu: there is no apparent band gap. May be what you're meaning as "apparent band gap" is the electrical band gap.

@Tiburico-Silver: Well. I exactly mean the electrical bandgap... Normally in transition metal oxides and some heavily doped defect semiconductors (please note not for all the semiconductor), where the optical bandgap is always greater than the electrical bandgap (ie. Apparant bandgap), the electrical bandgap is not mentioned as it is, but as apparent bandgap...

Ieally thank you for making the people understanding the topic, but please remove the word ALWAYS from the sentence "direct optical band gap is ALWAYS greater than the electrical band gap", because of the above said reason.

Dear Tiburcio-Silver

I'm very glad for you answer. I'm ready to discuss about it. Fortunately i'm also now working on ZnO thin films.

@Tiburico-Silver and Jeyadheepan Karu: From above discussion, it can be conclude that in case of inorganic semiconductor, the optical band gap may be equal or greater than electrical band gap, but never be less. and in case of organic semiconductor, optical bang gap is less than the electrical one ( according to Ilias Katsouras).

@Jeyadheepan Karu: Even if it's possible, in fact it is unnecessary to edit my comment. I've said that "In the case of INORGANIC MATERIALS, direct optical band gap is ALWAYS greater than the electrical band gap", not including then the organic ones where the picture is not the same in terms of band structure. Greets.

Dear Chandra Shekhar Prajapati: As already mentioned, I don't know a lot about organic semiconductors. Thus, I can't said anything about the behavior of the optical band gap in this kind of materials. For this matter I'm unable to give you any answer. Sorry.

@Tiburico-Silver: Thanks a lot for your valuable reply.

@Tiburico-Silver: Thank you for your clarification... and Sorry to say this that i haven't noticed the word "Direct" in your reply... thank you once again...

@Tiburico-Silver : nicely written. Can I say, in optical absorption measurements, what I measure is more close to OBG than EBG and PL measurements, the measured value is more close to EBG? Also when we fabricate devices such as solar cells, LEDs etc, then the bandgap we mention is the EBG, is it right?

@Tiburico-Silver, @@Jeyadheepan Karu, Good discussion. I have learnt something today. I am also working on ZnO. No surprises!! It is the material of @! century..

@Jeyadheepan Karu: you can measure the direct optical band gap by tauc plot.

@ A Tuburcio-Silver : Optical band gap arises only in case of doped systems?

Nice question. The optical and electrical band-gapes should be same. The transport and relaxation of charges could be different as optical gap could be excited by absorption or PL while to probe the electrical gap you will luckily use electrodes. Addition of electrodes to organic thin film could completely change the transport mechanism ( e.g. Temperature Dependent Barrier Crossover Regime in Tunneling Single Molecular Devices Based on the Matrix of Isolated Molecules by Andrei V. Pakoulev, Vladimir Burtman, JPC 2009.), however these changes rather "hide" the bandgape behind other barriers rather than change the bandgape itself

Most of the people are measuring electrical band gap of extrinsic semiconductor by taking the intrinsic part of the log sigma vs 1/T curve. Is it worth to do so ?

I agree with Vladimir with all its answer.

@Vladimir Burtman / @Tiburcio-Silver / @Iias Katsouras

Could you indicate some paper explaining this conceptual difference that all of you explained in your respective answers?  

Thanks for the answers for everybody in this question, so far I've learned a lot from this page.

Hi Alexandre, this  is my reference:

emperature Dependent Barrier Crossover Regime in Tunneling Single Molecular Devices Based on the Matrix of Isolated Molecules by Andrei V. Pakoulev, Vladimir Burtman, JPC 2009.

Please note that experimental details are extremely important in proper responce to your question. 

I also recommend you to review the whole  2013 (Summer) issue of Natural Materials on Nano-electronics. Good luck! V

Optical bandgap means bandgap estimated using optical means or characterization. In simple words, let a light of different energies incident on the material. The material absorbs some energies and transmits some energies (the detector measures this). The threshold energy at which the material starts absorbing light is "Optical Bandgap".

In a similar manner, electrical bandgap means bandgap estimated using electrical means or characterization. Here instead of measuring what light is observed of transmitted. You make electrical contacts for the material and measure the current instead of optical absorption. The incident light is of course absorbed, and carriers (electrons and holes) are generated in proportion to absorption. We measure current (nothing but charge which is proportional to absorption) and this current too shoots up at a threshold energy i.e. electrical bandgap.

Probably it affects the exciton binding energy. As far as my understanding is concerned optical gap is the electronic excitation from electronic ground state  to excited state. It need not to be  between the first vibrational level of ground state to the first vibrational level of the excited state. It can be between any vibrational levels whereas in transport gap it should be between first vibrational levels.

Hi. We are very familiar that when we want to measure optical band gap, we are using UV-Vis spec. Then from there we get optical band gap. From previous discussion that I missed, I understood that there is difference between these two. Then how we can measure electrical band gap especially for dielectric samples such as polymer, glass and semiconductor? I look forward for your reply. Thanks.

Dear Mohd Hamzah, can you explain how we can calculate optical band gap from UV-Vis spectroscopy, please.

What is the difference between band gap and effective band gap?