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Sunday, March 12, 2017

#366 Velocity saturation

A brief comment related to previous blog on the mobility of charge carriers. Velocity of charge carriers moving in semiconductor under the influence of electric field increases with the increasing electric field, but then saturates at certain maximum value. Saturation occurs because of the excessive scattering of charge carriers drifting in semiconductor lattice with very high velocity enforced by the very high electric field.

 

Saturation velocity and electric field at which it is reached are material parameters which are different in different single-crystal semiconductors due to the different spatial distribution of atoms in the crystal lattice in different semiconductors. Interestingly, high saturation velocity does not have to coincide with high electron mobility featured by any given semiconductor. For instance, silicon (Si), featuring significantly lower electron mobility than gallium arsenide (GaAs), displays higher saturation velocity than the latter.

 

In general, values of saturation velocity and values of the electric field at which it saturates are very good predictors of the ability of semiconductor material to operate under the very high electric field. Keep in mind that the very high electric field in ultra-small geometry devices may occur at relatively low bias voltages.

 

Posted by Jerzy Ruzyllo at 10:49 AM | Semiconductors | Link


Sunday, March 5, 2017

#365 Charge carriers mobility

Mobility of electrons and holes is a key material parameter which pre-determines performance of any given semiconductor material in high-speed device applications. Sometimes the fact that the electron mobility is a material specific parameter, and hence, is different in different semiconductors does not seem to be fully recognized.

 

Free electrons and holes carrying an electric charge and moving in semiconductor material are subject to severe scattering resulting from collisions and electrostatic interactions with host and dopant atoms in the lattice. All these interactions come down to the material specific adjustments of the charge carriers movement in semiconductors which are collectively expressed by a single material parameter µ known as mobility (unit [cm2/Vs]) of electrons, µn, and mobility of holes, µp, in semiconductors. Depending on the composition of material and its crystal structure electron mobility may vary by orders of magnitude.  For instance, electron mobility at room temperature in silicon, Si, is 1500 cm2/Vs while in indium antimonide, InSb, about 80000 cm2/Vs.

 

Posted by Jerzy Ruzyllo at 11:26 AM | Semiconductors | Link


Sunday, February 19, 2017

#364 Why ternary and quaternary compounds?

The question is why bother with ternary (three elements) and quaternary (four elements) semiconductor compounds if binary III-V and II-VI compounds cover a brad range of bandgaps and lattice constants? The answer is that  by mixing and matching various binary  alloys continuous and  independent modifications of the energy band width and lattice constants of the end compound can be achieved.

Taking aluminum gallium arsenide (AlGaAs) as an example we can see that by changing Al fraction x in AlxGa1-xAs, material transitions from gallium arsenide GaAs (x = 0) to aluminum arsenide AlAs (x = 1) is taking place. In the process the bandgap of the compound changes from Eg = 1.42 eV to  Eg = 2.16 eV for AlAs with negligible changes in the lattice constant.  

 

In the case of II-VI compounds by alloying for instance CdTe and ZnTe into CdxZn(1-x)Te, or CZT, and changing its composition gradually by changing x the bandgap of the ternary alloy can be varied from 1.5 eV for x =1 to 2.2 eV for x = 0. Even finer tuning of the bandgap characteristics within similar range, but at the expense of more complex processing, can be accomplished by alloying two binary compounds into quaternary compound, e.g ZnSe and CdTe into quaternary compound Zn1-yCdySe1-xTex.

 

Posted by Jerzy Ruzyllo at 09:54 AM | Semiconductors | Link


Wednesday, February 8, 2017

#363 Semiconductor Glossary

In the case you would be interested in my "Semiconductor Glossary" in other than the hard copy version you may want to check its ebook and Kindle editions.

Posted by Jerzy Ruzyllo at 07:03 PM | Semiconductors | Link


Sunday, February 5, 2017

#362 More on tin...

Continuing on the comments on tin from two weeks ago here is a short verison of what I already said in blog #306.

 

 First, simulation and early experiments indicate that by forming a germanium-tin alloy, GeSn, with 3% of Sn added to Ge, a semiconductor which converts indirect bandgap of Ge into direct bandgap and increases hole mobility of Ge is formed.

 

Second, a single layer of tin atoms called stanene (from stannum and graphene) should feature, according to theoretical physicists, outstanding electrical conductivity around room temperature potentially making it a great interconnect material in future generation ICs.

Posted by Jerzy Ruzyllo at 04:00 PM | Semiconductors | Link


Sunday, January 22, 2017

#361 What’s wrong/good with tin?

Tin (Sn) is an element in group IV of the Periodic Table often referred to as “semiconductor group” for the reason that it includes elements displaying semiconductor properties. However, unlike other elements in group IV, notably carbon, (C), silicon (Si), and germanium (Ge), characteristics of tin are not conducive with the needs of semiconductor device technology. The two key reasons are an extremely narrow energy gap (effectively close to 0 eV) and its low melting point of 223 oC making tin hardly compatible with mainstream semiconductor device manufacturing practices. As the results tin is primarily used as solder.

 

While elemental thin in either bulk or thin-film form is not of use in semiconductor device technology, tin combined with other elements to form compounds as well tin in 2D (2-dimensional) form seem to display a number of potentially attractive characteristics. More about it next time…  

 

Posted by Jerzy Ruzyllo at 06:27 PM | Semiconductors | Link


Sunday, January 8, 2017

#360 "Vertical" is it.

Did you take note of the term “vertical” being recently a buzz word in semiconductors related discussions? After decades of dealing with  needed to keep  improving device/circuit performance downsizing of “horizontal” (or “lateral” if you want, or “planar” as some would say) geometries of integrated devices, we are getting serious about going vertical. Vertical transistor, vertical channel, vertical interconnect, vertical 3D integration are just few examples of this well-defined trend.

 

All this as a way to work around obvious constraints involving continued scaling down of horizontal dimensions. More importantly, however, as a way to keep on improving device/circuit performance without having to cope with technically challenging and very expensive issues involved in scaling lateral geometries to the single digit nanometers.

Posted by Jerzy Ruzyllo at 07:47 PM | Semiconductors | Link


Tuesday, December 20, 2016

#359 Holiday Sale

In the case you would be interested, Semiconductor Glossary, 2nd Edition is on sale (35% off) until January 15, 2017.

Posted by Jerzy Ruzyllo at 03:43 PM | Semiconductors | Link


Sunday, December 18, 2016

#358 IEDM 2016

As usaul in December, I would like to attract your attention to the International Electron Device Meeting (IDEM) which was concluded over a week ago in San Francisco. To check on what's going on in the world of semiconductors go to the program of IEDM 2016.

Posted by Jerzy Ruzyllo at 04:47 PM | Semiconductors | Link


Sunday, December 11, 2016

#357 Inorganic semiconductors, cont.

In the Periodic Table of the Elements  a section concerned witrh groups II to Vi   is often referred to as a “semiconductor periodic table”. All inorganic semiconductors either elemental in group IV (carbon, silicon and germanium), or in the form of compounds synthesized from the elements from the group IV (IV-IV compounds, e.g., silicon carbide, SiC), groups III and V (III-V compounds, e.g., gallium arsenide, GaAs), as well as groups II and VI (II-VI compounds, e.g., cadmium selenide, CdSe) originate from this section of the periodic table.

Posted by Jerzy Ruzyllo at 05:18 PM | Semiconductors | Link


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Semi1source.com/blog is the personal blog of Jerzy Ruzyllo. With over 35 years of experience in academic research and teaching in the area of semiconductor engineering (currently holding position of a Distinguished Professor of Electrical Engineering and Professor of Materials Science and Engineering at Penn State University), he has a unique perspective on the developments in this progress driving technical domain and enjoys blogging about it.



With over 2000 terms defined and explained, Semiconductor Glossary is the most complete reference in the field of semiconductors on the market today.






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