Electronics The fifth and Sixth Lectures fourth week 1 -29 / 5 - 6/ 1438 هـ أ / سمر السلمي

Outline for today Chapter Two: Junction Diode Physical Electronics Definition. Its symbol. Types pn Junction structure and fabrication What happing inside pn Junction The Contact Potential and Energy Level in pn Junction at Equilibrium Conditions Diffusion and Drift in pn Junction at Equilibrium Conditions Derive Contact Potential at Equilibrium Conditions from Current Density in pn Junction Mathematical description at Equilibrium Conditions in pn Junction to Contact Potential and Carrier Concentrations Derive Contact Potential and depletion region width at Equilibrium Conditions in pn Junction from Poisson’s equation

Time of Periodic Exams The First Homework The second homework The first periodic exam in / 6 / 1438 هـ 27 The First Homework I put the first homework in my website in the university homework Due Monday 30 / 5/ 1438 هـ in my mailbox in Faculty of Physics Department , I will not accept any homework after that , but if you could not come to university you should sent it to me by email in the same day The second homework I put the second homework in my website in the university homework Due Thursday 17 / 6/ 1438 H in my mailbox in Faculty of Physics Department , I will not accept any homework after that , but if you could not come to university you should sent it to me by email in the same day

Junction Diode Physical Electronics We studied in previous lectures about semiconductor properties and mentioned in the last slide about the subject (the existence of two different types of semiconductor next to each other) or (the existence of two different materials next to each other) In the second chapter, we will examine the two cases the first case is diode or pn Junction If a piece of intrinsic semiconductor is doped so one part is n-type and the other part is p-type, and pn junction forms at the boundary between the two regions.

pn Junction’s Symbol pn Junctions’ types Symbol of diode as in the first figure, triangle base is p-type and triangle head is n-type. pn Junctions’ types Light Emitting Diode LED Photodiode Zener Diode. Avalanche Diode Tunnel Diode Scottky Diode Varactor Diode Laser Diode PIN Diode…etc =

pn Junction structure and fabrication The pn junction do not made up simply by surface adhesion of two types n-type & p-type as in the figure of previous slide (slid 4) Due to the irregular surfaces and Failure harmonization of covalent bonds at surfaces etc. However, manufacturing will be by putting one of extrinsic semiconductor type in the top center of other type as in the next figure. This happened by long steps of oxidize, expose, implant , diffusion and etc. to reach the final figure (some of manufacturing steps of diode)

pn Junction structure and fabrication the pn junction do not made up simply by surface adhesion of two types n-type & p-type as in the figure of previous slide (slid 4) Due to the irregular surfaces and Failure harmonization of covalent bonds at surfaces etc. However, manufacturing will be by putting one of extrinsic semiconductor type in the top center of other type as in the next figure. This happened by long steps of oxidize, expose, implant , etch, diffusion and etc. to reach the final figure.

What happing inside pn Junction when the n-type and p-type join next to each other in the diode, we obtain one side of semiconductor has plenty electron and few holes (n-type) next to other side that has plenty holes and few electron (p-type) . Therefore, there will be diffusion between two sides. Electron diffuse from n-type to p-type leaving behind positive ions ND+ in region called (depletion region). The opposite, holes diffuse from p-type to n-type leaving behind negative ions NA_ in depletion region.

What happing inside pn Junction Diffusion will not continue to infinity. Due to the two types of ions trying to pull charge carriers which trying to diffusing far away. donors seek of keeping electrons and acceptors seek of keeping holes; therefore, electric field is created from ions and works to slow of diffusion process and reaches out to the state of stability; therefore, far from depletion region, the semiconductor will be intrinsic neutral

What happing inside pn Junction Depletion region : It is the contact area between n-type and p-type and contains of positive space charge of n – side and negative space charge of p – side , also it not contains charge carriers. The symbol for it is W . In some book it called space charge region or transition region .

The Contact Potential and Energy Level in pn Junction at Equilibrium Conditions Before the n-type and p-type join next to each other in the diode, we know that Fermi level is near conduction band in n -type and Fermi level is near valence band in p-type as in figure (a) but how will be Fermi level in the case of n-type and p-type join next to each other in the diode?? Will Fermi level be in the same place to two types or will be separated at adhesion point? will conduction and valence bands in the diode at the same place?

The Contact Potential and Energy Level in pn Junction at Equilibrium Conditions In fact at energy bands in pn junction, Fermi level must be at the same energy level in the two types at equilibrium condition. Therefore, conduction or valence bands must be at the different energy level in the two types as in the figure below. So, we notice that bending the levels of conduction or valence bands; therefore, bending intrinsic level in the region between xn and –xp which is depletion region. This bending and difference of energy levels, therefore potential difference between n-type and p-type is called contact potential

The Contact Potential and Energy Level in pn Junction at Equilibrium Conditions Contact potential; the potential difference between n-type region and p-type region in diode which prevents more electrons flow from n-type to p-type, and more holes flow from p-type to n-type. The symbol for it is V0 =

The Contact Potential and Energy Level in pn Junction at Equilibrium Conditions from the figure, we can calculate contact voltage by a number of equations =

Diffusion and Drift in pn Junction at Equilibrium Conditions As we discussed earlier, we expect diffusion in junction due to the large carrier concentration gradients . Thus , as we notice, electrons diffuse from n side into p side and holes diffuse from p to n. Also, we mentioned an opposing electric field is created at junction due to pulling of positive and negative ions to charge carriers. Their directions are opposite to directions of carrier diffusion. Therefore, electrons drift from p- type to n- type and holes drift from n- type to p- type as in the figure below at equilibrium conditions.

Diffusion and Drift in pn Junction at Equilibrium Conditions notice to electron and hole diffusion direction, also to electron and hole drift direction. Therefore, the sum of total current density to electron and hole are zero at equilibrium conditions =

Derive Contact Potential at Equilibrium Conditions from Current Density in pn Junction We start from the total current density of holes in diode By assuming to deal in one dimension (x) and the use of Einstein relation and by substitute with relation voltage and electric field therefore >> ===

Derive Contact Potential at Equilibrium Conditions from Current Density in pn Junction following >> === Integration the two sides Therefore Finally, we get the relation connecting contact potential and concentrations =

Derive Contact Potential at Equilibrium Conditions from Current Density in pn Junction Similar if we start from the total current density of electron in diode By assuming to deal in one dimension (x) and the use of Einstein relation And by substitute with relation potential and electric field therefore >> === =

Derive Contact Potential at Equilibrium Conditions in pn Junction following Integration the two sides Therefore Finally, we get the relation connecting contact potential and concentrations =

Mathematical description at Equilibrium Conditions in pn Junction to Contact Potential and Carrier Concentrations From last equation, we can write the equation as following We represent the equation as ratio of majority carrier concentration to minority carrier concentration because we deal with equilibrium conditions, the best to write it as Therefore, electrons and holes concentrations are =

Derive Contact Potential and depletion region width at Equilibrium Conditions in pn Junction from Poisson’s equation =

Derive Contact Potential and depletion region width at Equilibrium Conditions in pn Junction from Poisson’s equation In the beginning, we know from Maxwell equation Where ρ charge density and ϵ0 permittivity in a vacuum Since the electric field connects with Potential by relation We get Poisson's equation =

Derive Contact Potential and depletion region width from the figure, we notice that When focusing at one dimension (x), Also we study material not vacuum is relative permittivity = ϵr

Derive Contact Potential and depletion region width charge density in the regain is and is With substitute charge density in Poisson's equation to obtain , than integrate =

Derive Contact Potential and depletion region width The maxim value of electric field is To calculate contact potential from equation Integrate the electric filed along depletion region, therefore We can easily find the integration from triangle area in the previous figure of the relation between electric field and X-axis With substitute of electric field value, we obtain

Derive Contact Potential and depletion region width From previous equations We find xn with substitute xn value in to contact potential equation, we obtain

Derive Contact Potential and depletion region width = Derive Contact Potential and depletion region width From contact potential last relation, we can find depletion region width W or Also, xn & xp xn is depletion region width From n-type side xp is depletion region width From p-type side

Solving first Homework