Device Physics - Science topic

Aparna, thank you for reply.

The discussion is about 1! device which serves two functions simultaneously. Combining 2 devices on 1 substrate does not count)

Dear Subham Paramanik ,

The crystallographic defects produce allowed energy levels in energy band. These allowed electronic states can act as traps or recombination centers. The states near either the conduction band or valence band may act as traps as they prefer to interact with one the nearest band.

The defects near the mid gap interact with both bands and therefore act as recombination canters.

The traps affect the carrier majority carrier concentration while the recombination centers affect both type charges and thereby affect much the conduction process in the semiconductors.

Defects may be geometrical such as vacancies, dislocations, grain boundaries or they may be impurities such as heavy metals in silicon.

In some crystals ultraviolet may prenatally cause the fracturing of the bonds and therefore create crystallographic defects. This is in the material with unstable chemical bonds that can undergo chemical changes with the energetic UV photons.

Best wishes

Henri Cloetens Thanks so much for answering. But I didn't quite get your idea. Indeed, the "go back" happens at the point of breakdown voltage, and you describe the process of avalanche breakdown in detail，but for normal sistuations, as the breakdown happens, the I-V cure should just goes up sharply, right? I still don't understand why it goes back. Thanks.

You can try Silicon.

You distinguish a normal classical Hall efect from a Quantum Hall effect.

Normal size devices exhibit the first, contain considerable number of electrons.

The magetic field acting on the current pushes electrons to one side of the device

and is counteracted by the Hall voltage set up from charge accumulation. Proportionality between magnetic field and Hall voltage for steady current.

Quantum devices contain fewer electrons in narrow or small devices (Nanostructures) . The magnetic field provokes the equivalent of Landau levels that contain the states for electrons. These pass at regular intervals as the magnetic field increases. Thus there are regular jumps

in the electron conductance as magnetic induction increases.(In single electron conductance, or normal quantum hall effect

The fractional quantum Hall effect is believed to be the consequence of electron interactions and quasi particle formation. This is an extremly complicated phenomena, and not nearly as well understood as many would have you believe.

Sorry outside of my field.Best regards

Dear Jitesh Pandya , you can monitor the PL intensity variations of a solid sample like a perovskite film using Front-Face photoluminescence spectroscopy. It is usually carried out with a conventional spectrofluorimeter equiped with a Front-Face accessory. In this way, you can scan the emission spectrum in succession and investigate the effect of moisture on PL emission over time. Moreover, if you do not have access to a front-face accessory, you can also take advantage of a plate reader in order to scan the PL emission of the surface. If still do not have access to any of these, you can use a UV cabinet and take several photos in succession. By extracting the RGB values you may investigate the PL variations over time. The following question has thoroughly discussed the front-face PL spectroscopy. Attached you may also find a picture illustrating how the front-face accessory works.

Best,

As a preliminary question:

1. which sort of material is your device made of? (metal, semiconductor, organic, other?),

2. what is the topology of the conducing layer (3D, 2D, 1D, powder-like, etc...)?

3. and what is the size of the active layer (cm or mm or µm or less)?

As a matter of fact, in a material with a low number of electrons, even a moderate number of electronic traps can capture electrons and greatly affect the conductivity. This is however unlikely in a metallic device.

i am interested too

In general there are several things which can affect the series resistance such as transport properties of the bulk semiconductor and the height of barrier. Check the thermionic emission and the possible tramps which can affect the transport properties. Moreover, the length distance between the Schottky barrier and the Ohmic contact should be the low to avoid serial resistance. In addition, if your specific contact resistance of your Ohmic contact is high it is going to increase the series resistance of your diode. For more details, it is important to know what is your specific Schottky structure. What are your Metals and semiconductor?

Best regards

While simulating the quantum tunneling, you should specify qtx and qty with reference to the gate electrode. If you take the reference as source (since tunneling occurs from source to channel) you will face the problem of Error - Code 2 in GetTransmissionProbability function of CTunnelCurrent". Check this in your code.

All the best.

Well, so far i was using plain silicon wafer.But my concern is that under 1 sun illumination condition i.e( 100 mW/cm2).Is it reasonable to expect high rise in temperature over silicon wafer?

So far my reaction chamber is closed rectangular steel box(which is not insulator).And i place silicon wafer direct under sun simulator.

FOr mine case conditions are not ideal(black body). SO how much temperature should i expect?

Thanks.

Dear CAolleagues,

This topic is interesting.

Any switch including the electronic devices working as switches has two sates the off sate and the on state. The ideal switch is taken as reference for to compare switches. The ideal switch has the following performance:

The off resistance is infinity, or the off current is equal to zero.

The on resistance is eqaul to zero

and the switching times ton =toff=0

Such switch is not existing in the reality,

real switches have normally very small off current which some times called the leakage current with analogy to mechanical valves for fluids where when off they must cut off completely but it may firmly tight and leak fluids. Therefore it is called a leakage current. Leakage current may have different mechanisms in electronic devices. It may be due to volume conduction or surface conduction.

The on current is a unique terminology for the current flowing in the switch when it is on. Normally as the switch has very small on resistance, it must be limited by external circuit connected to the switch mostly the load resistance.

Best wishes

By varying temperature for fix voltage and extracting current out of the diode and plotting Richardson graph can give barrier height.

You need to sense the current through the capacitor and integrate this current with time. Or you need to read the voltage across the capacitor and multiply it by C. So you can your the operators in your simulator to get Q. Naturally the second method is simpler since you need only a multiplication process.

Best wishes

Dear Shashikant,

welcome,

This answer may come late. To determine the cut in voltage of the pn-diode you make a linear plot of the I-V curve in the current range of interest.

Then, it appears as broken line with two pieces; one horizontal pieca and one inclined piece with the twp pieces are the best linear fit to the real I-V curve. The intersection point of the two pieces will be the cut-in point.

So, it is practical to find different values of the cut in voltage depending on the fitted current range. So, one has to give the cut in voltage with its valid current range.

For more information please see the link:

Best wishes

Arjun: Some other facts wanted to share since I believe your question is with regard to NVM memory programming and erasure speed enhancement by operating at higher substrate temperature, that is what I am guessing. Although programming and erasure both are done by currents from gate to channel, if channel hot carrier method is used, carrier carrier scattering at higher temperature will randomize the mometumn and the carrier velocity will not be as high as 300 K value to surmount the oxide barrier. As band gap narrows with higher temperature, conduction band oxide barrier is reduced and this will enhance F-N tunneling based programming and erasure. So at higher temperature F-N tunneling might give faster programming and erasure speed than channel hot carrier programming which is strictly inefficient at higher temperature. Problem is when charge retention needs to be longer and since conduction band barrier offset to oxide is reduced for narrowed band gap, charge loss and leakage for sustained memory set operation cannot be avoided. I do not know whether you contemplated your thought at this aspect of analysis.

Sincerely

Dr Nabil Shovon Ashraf.r

Dear Mr. Amit,

I favor to use less 1% as a maximum load ed, because the very high value of doped is caused high defect, random properties and low efficiency of nano compound in any reaction.

Regards,

Dr. Luma M. Ahmed

I tried to driving the device with a low frequency (10 Hz) square pulse and emission pulses as well.  But when I increased the frequency, the emission pulse becomes narrower and eventually stops emitting or at least cannot be perceived with naked eye. When the frequency is decreased the emission pulse is longer and more perceivable.

As, interface states in CdS/ CIGS in hetero junction solar cells, reduces the minority carrier life time, it can be modeled by SRH recombination mechanism. 

I think it in this way.

Surface states ---- increasing recombination---- lower minority carrier life time--- reduces the collection probability of minority carriers (practical situation) 

If we reduce SRH life time, it will reduce the collection probability mathematically. (modeling) 

You can solve drift diffusion and Poisson's equation simultaneously to solve this kind of problems.

The reference given below can help you to model this kind of solar cell numerically. 

I modeled CZTS/CdS in that way. I am attaching my publication as well :) 

Dear colleagues,

As some of you mentioned, the energy balance tells us that the heat source in a solar cell equals the absorbed power minus the electrical power generated (which is nul in short circuit (SC) and in open circuit (OC) conditions, note that it is maximum at MPP) minus the photoluminescence (PL) from the cell. The PL intensity is proportional to the excess carrier density so it is much larger in OC than in SC. Thus, the heat source in a solar cell is lower in open circuit than in short circuit. However, the difference is significative only if the cell efficiency is sufficiently close to the radiative limit (because in this case the fraction of radiative to non-radiative recombination is non negligible). To calculate limiting scenarios, the concept of External Radiative Efficiency (ERE) is interesting. An example can be found in the paragraph "Dependence on Voltage of the heat source and the cell temperature" in the book "Thermal Behavior of Photovoltaic Devices. Physics and Engineering".

Olivier Dupré

Junctions less transistor means , how tunneling is possible.  source to channel are same doping or not ?

You can connect two solar inverter outputs (AC) together in parallel, but they must be frequency and phase locked and made to be grid-tie if they are to also be paralleled with the larger power grid. There are many ways of doing this, but a master-slave approach works quite well with one master and up to several slave inverters, all synchronized. It can be done with either analog or digital control.

Two more comprehensive analysis on TFET devices that if you haven't read already, should peruse to aid in your research. There are two accomplished faculties in device modeling area working in ECE department of Georgia Institute of Technology, USA. They are Dr. Arijit RayChowdhury and Dr. Saibal Mukhopadhaye. They are ex-graduates of Jadavpur University. See if you contact them, whether they give some clues for your problem.

Sincerely,

Dr. Nabil Shovon Ashraf.

Dear Vudumula,

spatial lifetime profiles can be added with the help of SSE (Synopsys Structure Editor). Details about incooperation are described in chapter 16 Generation-Recombination of SDevice manual

(formerly inside of ISE TCAD suite this could be done very gently with help of MDRAW, maybe it is also available inside SSE in MDRAW emulation mode)

If you are simply measuring the forward characteristics it should make no difference what you call it.   If you are fitting it to a SPICE model you will need to change the energy gap - try 1.39V initially.  Another other common feature (due to short carrier lifetime) is that the ideality can be significantly lower at small currents than in the main operating range; fortunately, even the standard level 1 diode model caters for this using the recombination current (ISR) and its ideality (NR).

Dear Seeraz,

I did not work with this instrument. However, it may helpful to as you some questions which may support in solving the problem. What is the excitation used to measure C-t ?

I expect it is a ramp voltage. What is the speed of this ramp. I see that your tine scale in seconds. It is so that if we a capacitor and then we apply a ramp on it , a current = CdV/dt will pass through it and one senses this current amplifies it and it then represents the capacitor just by dividing it the slope of the ramp dV/dt. I think The slope of the voltage ramp must be fast enough to an appreciable current that can be sensed and above the noise level.

I thin these thoughts may lighten the the problem.

Best wishes

Thanks sir for your worthy response

Dear RAM , 

Can you describe the construction of your device in more details? From the conceptual point of view devices can be constructed vertically and laterally like the vertical mos transistor and the lateral mos transistors where the channel is vertical in the first structure and lateral in the second structure. There is also the vertical bipolar transistor and the lateral bipolar transistor where the stack of the three transistor layers is vertical or lateral. So, from the principle point of view devices can be made vertical or lateral according to the stacking of their constituting layers. Switching devices base on metal insulator metal devices can be stacked in a planar manner. In this case one has to control the distance between the two electrodes by the photolithography. In case of vertical stacking the active device thickness is controlled by the I-layer thickness.

This is the major difference.

wish you success

Organic semiconductors are materials with very specific properties. In contrast with inorganic semiconductors, organic semiconductors have low dielectric constant, low effective mobility, no doping, and almost zero intrinsic concentration of free charges. As a result, only injected charges contribute to the current flow and the screening effect is almost negligible. Please note that the energy band bending originates in the screening effect of the space-charge field of accumulated charges. In other words, charge carriers move in the material to compensate the external electric field.

In metals we have almost infinity concentration of free carriers and the charge mobility. Therefore the electric field does not penetrate into the conductive metals and charges are accumulated at surface only (Dirac pulse).

On the other hand, the insulators have no charges and the electric field penetrates them totally. As a result, the energy bands follow linear tendency (linear potential means constant electric field since the field is the gradient of potential).

Inorganic semiconductors are somewhere in between. The carrier concentration is high enough to compensate electric field totally in the bulk of the material; however, close tot he surface is still partially present the energy band bending.

Organic semiconductors are almost like insulators. Actually, we should say that they are dielectrics, the big family of materials which includes insulators, semiconductors, as well as continuous transition between them. Because of above-mentioned properties of organic semiconductors, they behave more like imperfect insulators and the field penetrates them totally. It should be mentioned here that in organic semiconductor devices we use thin film technology. The organic film thickness is usually about 100 nm or less, which is greatly smaller than any other thickness used in silicon devices. This means that even though the energy band bending can be present in organic semiconductors, it should be greatly larger than the film thickness (e.g. the depletion layer thickness will be one micron) and therefore it can be approximated as a linear dependence across the thickness direction.

In conclusion, the energy band bending can be totally neglected because of low carrier concentration, low mobility, no doping and low film thickness. Nevertheless, the device can still exhibit the same properties as the one with energy band bending. The main reason is that many device properties do not require band bending, but it depends on charge accumulation and transport.

Thanks all for your insightful responses. I am particularly concerned about the Au/P3HT interface, as Yun Li mentioned. I will probably be moving to a top contact architecture soon to alleviate that issue.

I actually do use a piece of copper tape on the stage, although it is not a vacuum chuck so the contact is not very strong. I have not been successful in creating an ohmic contact with copper tape.

A coating with Al or In or other metal may be the best approach - I will try that.

Advantages: it has liquid form even at 15-20 deg. Celsius. As result - you can use it for  contact anywhere without any preparation.

Disadvantages: it is not solid at room temperatures. As result - you can create contact but you can't define it's area. Any random additional friction, pressure and so on may change contact area. Also remember that InGa is toxic.

So, if you want to create temporary contact for test measure - use it. If you need in stable high-quality contact - use something else.

say for 180nm technology VDD=1.8v, and i mean how to get full swing in GDI Design style

Overcurrent protection is good for such situation.

Kevin thank you for your answer

That is somewhat to simple, maybe the question wasn't that clear enough. I also need to know what the nodal displacements of the material are when subjected to a force F while the connection to these two bar imposes a displacement constraint for the underside of the material. So im trying to go much deeping into the dynamics as to what you propose. 

If the right bar would be there this would be something like a beam problem even, very simple, but due to the added constraint the problem complicates.

what kind of blades are required for Dicing of Sapphire wafer ?

I think vacuum evaporators means thermal evaporators. If I am right, then you can use Cr coated tungsten rod  for Cr evaporation. you can easily evaporate Cr with 20 A current.

Did you fabricate your transistors on a common gate substrate, e.g. Si/SiO2? If so, your whole substrate leaks once you start to operate your devices. In order to reduce the leakage you have to pattern your semiconductor. The easiest way is to scratch it around your source/drain electrodes with a cocktail stick. Obviously, you have to be careful not to destroy the electrodes and your substrate (it is safe to scratch on Si/SiO2 substrates, though).

Dear UV Kiran,

a good starting point could be the European Directive EN 301 549 V1.1.1 (2014-02) which is open and publicly available at ETSI website (http://www.etsi.org/technologies-clusters/technologies/human-factors?tab=2). There you will find also related standards which may be of interest too.

Hope this helps,

Mireia

I've written one to simulate organic solar cells. You can download it here https://www.gpvdm.com. The model solves the electron/hole drift diffusion equations to account for transport, the Shockley-Read-Hall equations to account for carrier trapping/recombination and Poisson's equation to calculate  the internal potential distribution.  It's also got an optical solver in it, so you can calculate the photon distribution within the device.

In terms of flexibility and ability " Matlab" is  first and last word in my mind for a wide range of applications such as : Optoelectronic devices and so on ...

It must be remembered that of the three named parameters, only one - Beta - is a model, a virtual setting. The remaining ones are determined by measuring the experimental values - currents. Beta is introduced to configure model law Fowler-Nordheim to describe the experimental data. Beta depends also on the geometry of the emitter surface, and the properties of the composite material.

Thank you sir, for your valuable suggestion.........

In a semiconductor the mobility of electrons (referring to ‘conduction electrons’ or ‘free-electrons’) is greater than that of a holes (indirectly referring to ‘valence electrons’) because of different band structure and scattering mechanisms of these two carrier types.

Conduction electrons (free-electrons) travel in the conduction band and valence electrons (holes) travel in the valence band. In an applied electric field, valence electrons cannot move as freely as the free electrons because their movement is restricted. The mobility of a particle in a semiconductor is larger if its effective mass is smaller and the time between scattering events is larger.

Holes are created by the elevation of electrons from innermost shells to higher shells or shells with higher energy levels. Since holes are subjected to the stronger atomic force pulled by the nucleus than the electrons residing in the higher shells or farther shells, holes have a lower mobility.

In an intrinsic silicon, at temperature 300 K:

Electron mobility = 1500 cm2/(V∙s)

Hole mobility = 475 cm2/(V∙s)

 

Dear Ashish, I meant to say the same. We usually plot the measured barrier height vs Work function of the metal. From this data we extract the slope by fitting a straight line. Now this slope will be less than 1 indicating the presence of pinning. So the presence of interface states and their effect on the barrier height has been measured. If there was no pinning then the barrier height of the M-S interface would vary linearly phi_b=phi_m-phi_s and the plot of barrier height vs. Work function will give slope of 1 which would be proof that there is no pinning. Refer to the below paper. The figure 2(b) shows extracted S values for Ge, Si and ZnO. All are less than 1 indicating pinning in these materials.

I am confused about the meaning of source/drain doping density. Do you mean the value or the unit? There are three ways to specify the doping density, (1) fractional value, that is doping per atom, (2) doping density per cm^2, or (3) directly use density per cm^3. In the first case, the typical value would be 10^(-3), and convert this value to density by dividing it by the volume of an atom. In the second case, the typical value might be 10^(14) /cm^2, and then convert it to density by dividing this value by the grid size.

@Alfonso...Thanks a lot...

Thank you Mr.Farid. It 's a good answer. But in fact , I mean, is there any way for integrating on energy to obtain a moderate amount for mean free time(for all energy range)?

For MSM structure I hope you are able to get the small signal current characteristics, actually by integrating the small signal current alone with respect to time gives the excess charge developed across the device due to your small signal voltage, the excess charge divided by the your small signal peak to peak voltage swing gives you the capacitance.

@Ramesh:

I also see the temperature increase (by ~5-10K) under forward bias, which I guess is not avoidable with 2V*1A/cm^2 = 20kW/m^2.

The vacuum we use is between 1e-4 to 1e-6mbar, depending on the experiment. 1e-4 is usually fine for our experiments.

For traditional solar cells, higher illumination intensity should result in slightly higher efficiency. The current increases proportional to illumination, if this were the only effect the efficiency would stay the same. However, the open circuit voltage also increases (with the natural logarithm of illumination), and consequently the fill factor increases a little bit, too. In summary the efficiency is higher. There are two things which can counteract: if there is significant series resistance the fill factor may decrease at higher illumination. Also, if the cell temperature rises the open circuit voltage goes down. I don't know whether there are additional effects in quantum dot cells.

i am not satisfied with the diagram

if we have the P-I-N type of the tunnel FET, to accumulate the channel we provide the +ve voltage to the gate

fermi level of t he device should be in the same level

now assume P as source and N as drain

1, in source fermi level lies near the valence band

2, in gate (I) fermi level at center

3, drain (N) fermi level near conduction band

to create the channel we apply +ve voltage at the gate, so the channel created hence fermi level at the gate shifts near the conduction band,

but in the physical design of the device some part of the drain and the source is overlapped by the gate,

means as the fermi level at the gate shifts near the conduction band we get a smooth shift of the conduction band of the drain below the valance band of the source, hence by applying the voltage the electron from the valance band of the source tunnel through the conduction through the conduction band of the gate and pass to the conduction band of the drain.

NOTE : Proper biasing required.

Thank you ma'm. I will go through this article and contact you if I have any doubts.

Charge transport in an electron-hole plasma driven by high-field terahertz (THz) pulses is strongly influenced by electron-hole interactions, as has been shown in a recent publication [P. Bowlan , Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.107.256602 107, 256602 (2011)]. We introduce a picture of high-field THz transport which accounts for the roles of both types of carriers including their interactions. While holes make a negligible contribution to the current, they are heated by absorbing energy from the driving THz field and introduce a friction force for the electrons over a period of about 500 fs. Our model uses an extended version of the loss-function concept to calculate the time-dependent friction. The local field that drives the electrons differs from the incident THz field by screening due to Coulomb correlations in the plasma. We illustrate how spatial correlations between charged particles (electrons, holes, impurities) create a significant local-field correction to the THz driving field. The dominant contribution stems from Coulomb-correlated heavy-hole wave packets, which are strongly polarizable via inter-valence-band transitions.

Bowlan, P.; Kuehn, W.; Reimann, K.; Woerner, M.; Elsaesser, T.; Hey, R.; Flytzanis, C.

2012-04-01

I advise that you check that TE is modelled correctly. Check the position of the Fermi-level up to the interface. If TE is modelled correctly, it should be flat up to the interface. If not, it will droop down and join the metal Fermi-level (diffusion mechanism).

Nanofibers with a very small diameter and high aspect ratio one dimensional material. Please clarify the word "device". If used in making a composite material then the characteristics of the composite depends upon the volume fraction of nanofibers present in the composite, the dimensions of the composite and the method of fabrication of the composite (alligned or not alligned).

Hi Ashish,

The effect of temperature on 2DEG is not very straightforward. The origin of 2DEG is not only from the polarization charge, but also from the AlGaN surface trap density that is dependent on the fabrication conditions. The location and level of the traps would have different effect on the 2DEG density. Eq (15) in Ambacher's paper is for the ideal case which the trapping effect is not considered. Different trap level would affect the effective Schottky barrier height thereby having different thermionic effect.

To my knowledge, the polarization charge is not very sensitive to temperature compared to the schottky barrier height, Fermi level, and delta Ec. However, the link you posted regarding the Fermi level is not applicable to 2DEG characterization, because the charge concentration is too high that Boltzmann approximation used in the link is not applicable anymore. You should use Fermi-Dirac for Fermi level instead.

You can read some paper by prof. H. Bassler, if you are interested in the carrier transport (hopping) in the disordered (off- and diagonal disorder) system with a Gaussian distribution. Prof. Bassler mainly focused on the doped conducting polymers.

Dear Professor Mallik, Thank you very much for the references.

Use polyfit in matlab

For making the electrodes one can use basic silver paint and copper wires contacting them. These electrodes can sustain up to 400 C for measurements. It is beneficial to define the area of film exposed to gases. For measurement, depending on the resistance of the film one can use regular multimeters (ranges from 50 Mohm to 1 ohm) with RS232 probe connection to computer for recording data (R vs time). I think this much is sufficient for first trial.

You need to have enough thin gate insulating layer. In other case the threshold voltage will be higher than you can apply. Don't go for more than 100 nm of the insulating layer if possible.

Of course, there are ways how to manipulate the threshold voltage (using self-assembled monolayers on the gate insulator), but it is quite complicated.

You at least need two layers of graphene to build a field-effect tunneling transistor... Normally due to the very small band gap, graphene can not be used solely, you need some other materials as transport barriers between the layers, and the materials you use will determine the work function, from that point conventional rules apply.

First of all, it is necessary to determine the concept of ideal hypothetical semiconductor. It may be that this semiconductor should have a perfect crystalline structure, where completely absent both the intrinsic defects (vacancies, interstitials) and residual impurity atoms. In this case, in the band gap will be missing any energy level, caused by defects. Generation of the carriers will take place only through band-to-band transitions. Thus, there are no capture centers for carriers. Therefore, in such semiconductors will have a scattering of carriers only by vibrations of the crystal lattice. This initial idea of ​​such semiconductors. Then the properties of these semiconductors can be developed further. Of course there may be other options above semiconductors. I present one of them (in terms of crystal structure).

Yes, I agree. However, BHJ has very unusual active layer morphology and writing a generic tool for that is difficult. You can always start with the effective media approach as described by Koster et al. (http://prb.aps.org/abstract/PRB/v72/i8/e085205).