Questions before the Midterm

[amr alaa]

السلام عليكم و رحمة الله و بركاته

How are you all. I hope you are preparing for your midterms
In order for us to communicate we decided to allocate this topic for any questions regarding the electronics subject. You can ask whatever questions you want. However there are some rules
1- NO THANKS
2-Please be specific in your question. We will not be able to answer open ended questions such as (how to study this subject) or (I do not understand the PN junction physics).

isA, we will try to answer your questions as fast as possible (either me or Eng.Hesham or Eng. Mohammed hafiz). Finally, I would be grateful if someone of the moderators keep this topic as organized as possible by keeping the answers closer to their respective questions


بالتوفيق ان شاء الله
عمرو علاء

[amr alaa]

http://store1.up-00.com/Apr10/wLY33239.jpg

هو ده مكتوب فى الكتاب ان اسمه shematic diagram و انا مش فاهمه هما ليه ال states شكلهم كده حسب ما انا فاهمه المفروض ان عدد ال states اللى تحت بيبقى كبير و بعد كده بتقل عشان تشغل تحت الالكترونات الاقل فى الطاقه فمش فاهمه ليه ال states بتبقى قليله تحت و بتزيد و بعد كده تقل تانى
الاجابة :
اعتبري الاول مثلا اوربيتال S فده بيكون أقل أليكترونات و التاني P الكتروناته اكتر و الاول اصلا كله بيكون مكتمل و التاني اسمه (valence ) و بيتحرك منه للأخير اللي أسمه (condacturs)


إجابة بشمهندس عمرو :

According to the equations we have concerning the density of states , mainly(Eq.25 page 20 and Eq.27 page 21), The density of states in the valence band should have a parabolic behavior. Meaning that it should be zero at the Ev and increases as the energy decreases (the opposite occurs in the conduction band).. However, in page 19, the shape of the valence band is weird somehow. But the noticeable part is that it takes parabolic form near Ev. Thus we expect this form and our Eq.27 to give same results ( p in the valence band). Because, if you noticed, what we are interested in mainly is the density of states in the conduction band NEAR Ec and density of states in the valence band NEAR Ev. The rest of the band is not important (even though it is large ) because it gets multiplied with fermi distribution function which is nearly zero away from the band edges.

[amr alaa]

هو احنا ليه شغلنا كله على T= 0 K يعنى اما بنقول AT 0 K مفيش انتقال الكترونات من VB TO CB يعنى معنى كده ادام مبيحصلش انتقال ان الذرة مستقرة ازاى تبقى الذرة مستقرة فى 0K و خصوصا ان احنا كنا بناخد فى ثانوى ان الصفر المطلق ده مينفعش نوصله و انه حاجة مش PRACTICAL

الاجابة :



اللى انا فاهمه .. ان فكرة ال 0 k زى لما نتكلم على infinity كدا ..

يعنى بنقول عن عند ال صفر المطلق (اقل درجة حرارة ممكنة) .. يا يمكن اى الكترون يبقا عنده طاقة حرارية اصلا .. فا مش ممكن يحصل انتقال طبعا .. و عشان كدا يبقا عدد ال n ب زيرو و احنا مطمنين .. او يعنى يبقا ال fermi function بتدينا 0 فى ال cb ..

فا ايه المشكلة هنا.. ايه علاقتها بال practical case ... ده شرح نظرى للموضوع ..

إجابة بشمهندس عمرو علاء


I think you can reach easily the 0k practically (but I may be wrong). However, what happens at 0k is that the electrons do not have any energy so that they can not acquire enough energy to overcome the energy gap and reach the conduction band and become free. This is reflected on the fermi distribution by making the probability to find an electron in conduction band equal to zero. This is why at 0k the material behaves like an insulator.

[amr alaa]

المفروض ان Nc : effective density of states in CB يعنى ايه و ايه الفرق بينه و بين S(E): density of states

The difference is
S(E): represents the number of STATES per unit volume that exists between E and E+delta_E and it is function of the energy E
Nc: is just a constant called the effective density of states in the conduction band and I like to think of it as the number of ELECTRONS per unit volume in the conduction band when Ef=Ec. Look at the eq.26 page 20

[Amanda]

في الشيت الاول السؤال التاني

هي المسائله تعويض مباشر بس لما باجي اعمل بالاله h^3 بتدي 0 فمش عارفه اعمل ايه

والكتاب الي فيه الحلول صفحه 2 حللها عادي ومطلع ارقام؟؟؟؟؟؟؟؟
الاجابة :
أحسبيها على مرتين مرة h^2 مضروبة في h

There is a simple solution
deal with h=6.626*10^-34 as being h=6.626 and then adjust the powers in your final answer
Thus, if you need to multiply by h^2 in your equations then multiply only by 6.626^2 in your calculator but remember to multiply your final answer by 10^-68

[amr alaa]

فى pn junction
انا فاهمه ان ال thermal equilibrium بيحصل لما ميبقاش فى drift current لام مبيبقاش فى applied external voltage
هل ده صح و لا غلط
و امتى بيحصل ان Ef is aligned

Thermal equilibrium is a condition at which no net current flow through the PN junction. This is reflected on the energy band diagram as the fermi-level Ef is aligned through the whole material (both n & p). This condition is achieved when there is no EXTERNAL applied potential on the PN junction (even though you have an internally built in potential across the depletion region). Thus we suspect that in case of forward biasing or reverse biasing (at which current flow through the PN junction in the forward or reverse direction respectively) the fermi level will not be aligned anymore.

[amr alaa]

ربنا يتقبل منك يا بشمهندس المجهود ده (:

فيه سؤال لذيذ حسسنى انى مفهمتش اى حاجة من اول السنة !!


فى ch 1 .. لما جينا نتكلم على ال n type مثلا .. و قلنا ان عدد ال n بيزيد فى ال cb

و جبنا القانون اللى بيحسبه ..

اكيد ال n بيزيد تمام ..


لكن ال p فى ال vb .. ليه بتتأثر بزيادة ال n و تقل ؟!

هى مالها ال p ..


يعنى كدا كدا الشغل اللى بيحصل بعيد عنها .. فا انا مش فاهم دى .. و اللى حاس بيه ان المفروض كانت ال p = ال p intrinsic ..

:D

مش عارف بقا .. ممكن توضيح ؟!


(او اى حد فاهم حتى )

This is a really an interesting question. We can understand it from mathematics because when we derived the expression for n (or p) we derived it in terms of Ef in the general position ( i.e. we did not assume that Ef is in the middle of the energy gap as in the case of the intrinsic semi-conductor). Therefor, the derivation was general for any position of the fermi level (remember that for n-type material Ef was near Ec and for p-type material Ef is near Ev). Thus the derivation for n and p was valid for any semi-conductor under thermal equilibrium wither it was intrinsic or extrinsic.
Physically what happens, I do not want to go in deep details. But all I can say for now that adding impurity atoms (either donor or acceptor) has the effect of shifting the whole fermi distribution (either up or down). Thus, BOTH the probability of finding the electron f(E) or the probability of finding the hole (1-f(E)) is affected.
I will be happy to elaborate more on this issue with you after the exam isA

[amr alaa]

يا باشمهندس انا مش فاهمه يا يعني ايه الstatic and daynamic resistance؟؟؟
مش فاهمه برضو الPIV تماما؟؟؟:011:
الاجابة
static resistance is v(dc)/I(dc)..
dynamic resistance is v(ac)/I(ac)
PIV is always = V of kathode- V of Anode


static resistance= V_dc/I_dc
dynamic resistance = delta_V/delta_I --> this is the 1/slope of the I-V characteristic at a specific operating point (i.e. at certain V_dc and I_dc
PIV "peak inverse voltage" = maximum voltage occurring on the diode in the REVERSE direction

[Yusuf]

شكرا لحضرتك كتير يا بشمهندس و نتمنى ان الامتحان يكون سهل و ربنا يجازي حضرتك خير

[engineer 1]

excuse me engineer, I have a question in sheet 3 problem 2, how to get Vo?
Thanks a lot