Electronics Chapter Two:

Electronics Chapter Two:
Fifth , sixth & seveth weeks 12- 10/ 1-2/ 1439 هـ أ / سمر السلمي

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Chapter Two: 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.

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

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?

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

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 =

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 >> ===

following >> === Integration the two sides Therefore Finally, we get the relation connecting contact potential and concentrations =

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

charge density in the regain is and is With substitute charge density in Poisson's equation to obtain , than integrate =

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

From previous equations We find xn with substitute xn value in to contact potential equation, we obtain

= 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

Forward-biased and Reverse- biased Junction
Our discussion until now is about equilibrium condition (The absence of power supply (battery) between the parties of diode). But what about the condition (the existence of power supply between the parties of diode? How the power supply connect between the parties of the diode ?? What happens to contact potential increased or decreased? What will happened to diffusion and drift currents in the junction? In the condition of power supply existence, so that positive terminal of it connect with p-type side and negative terminal of it connect with n-type side; this condition called forward bias. however, If the two terminals reverse, the condition called reverse bias as in the figure blow. =

Forward-biased Junction (depletion region and applying bias)
In forward bias, we notice that the outer field (forward voltage) opposes built-in field created by the space charges in the depletion region. Therefore, the number of donor and acceptor ions reduces; thus depletion region width decreases. The potential barrier is lower in this condition than in equilibrium condition; in addition voltage difference across the diode decreases (( V0 –Vf )) as in the figure. Also we notice that Fermi level will not be at the same energy level in the two types as in equilibrium condition.

Forward-biased Junction (Diffusion and Drift)
This decreasing of voltage difference across the diode in forward bias affect the diffusion current ( due to injection of holes in p-type and electrons in n-type ) so, we obtain a huge flow of diffusion currents of the electrons and holes compared with it in equilibrium condition ; also due to climbing of electron easily to new voltage difference level . However, drift current is not effected by decreasing of voltage difference or length of potential barrier due to not effecting of minority carrier which remain pulled inside the diffusion current at the edge of the depletion region . Therefore, drift current in this condition remains as in equilibrium condition

Reverse-biased Junction (depletion region and applying bias)
In reverse bias, we notice that the outer field (reverse voltage ) in the same direction of built-in field created by the space charges in the depletion region. Therefore, the number of donor and acceptor ions rise; thus depletion region width increases. The potential barrier is higher in this condition than in equilibrium condition; in addition voltage difference across the diode increases ((V0 + Vr )) as in the figure. Also we notice that Fermi level will not be at the same energy level in the two types as in equilibrium condition =

Reverse-biased Junction (Diffusion and Drift)
In reverse bias which opposite of forward bias, the increasing of voltage difference across in the diode affect negatively to diffusion current . There shall be a few flow of diffusion currents of the electrons and holes transported to the other end compared with it in equilibrium condition ; also due to climbing of electron difficulty to new voltage difference level . Similar to forward bias , drift current is not effected by increasing of voltage difference or length of potential barrier. Therefore, drift current in this condition remains as in equilibrium condition =

Forward-biased and Reverse- biased Junction
Look at increases of direction of electron and hole diffusion in forward bias and decreases in reverse bias and drift as the same as in equilibrium condition.

Drive excess of minority carrier in Forward-biased and Reverse- biased Junction
In thermal equilibrium, we represent relations However, forward and reverse bias represent relations as follows The forward- biased voltage and reverse -biased voltage In the case of biased, the low-level injection or carriers minority concentration is very weak so will not affect the equilibrium majority carriers; thus we can consider

We start by holes in non equilibrium condition From relation By substitute We obtain By subtracting pn0 from both sides, we get a excess of minority -carriers of holes concentration in n-type

Similarly, electrons in non equilibrium condition From relation By substitute We obtain By subtracting np0 from both sides, we get a excess of minority -carriers of electrons concentration in p-type

Mathematical description of excess of minority carrier in Forward-biased and Reverse- biased Junction From previous lectures, we discussed about the equation for diffusion of minority carriers The solution for it is where A and B are constants which can be found from boundary conditions (from the following figure a semiconductor p-type variable concentration in one dimension) (In the case of bias {non- equilibrium} we will deal of low-level injection) Therefore, we assume that And Therefore, we obtain

Mathematical description of excess of minority carrier in Forward-biased and Reverse- biased Junction Similar, in case of bias in junction =

Mathematical description of excess of minority carrier in Forward-biased and Reverse- biased Junction to Drive diffusion current Similar, in case of bias in junction Thus, the equation for diffusion of minority carriers in n –type & p- type is By substituting to excess minority -carriers concentration from previous derivation In focusing of borders depletion region, we obtain

Drive diffusion current in Forward-biased and Reverse- biased Junction
We notice that diffusion current variables in equilibrium condition (or the absence of bias) while drift current does not change its status in equilibrium condition . To get the current density in the junction : first from p-type side (minority electrons current density) To focus in one dimension and to solving equations of diffusion of the minority carriers At borders of depletion region =

from n-type side diffusion current density (minority holes current density) To focus in one dimension and to solving equations of diffusion of the minority carriers At borders of depletion region To get the total current density of junction must collect two current densities for n-type and p-type, however, we should notice the direction of them

From the figure To calculate the current Where I0 the reverse saturation current is

Figure illustrates the distribution of the electron and hole currents in the junction in the forward bias

I – V Characteristic of Forward-biased and Reverse-biased Junction
You studied at electronic lab the relation between voltage and current of diode in forward-biased and reverse-biased depending on electrical conductivity to terminals of battery and diode as in the figure We conduct first in forward bias then reverse bias. In addition , a resistor is used to limit the forward current to a value that will not overheat the diode and cause damage. This resistor called dynamic resistance. To calculate resistance value is inverted slope dI/dV tangent to the curve. The value of resistance in forward bias less than the value in reverse bias Rf < Rb

At forward bias circuit, we notice when increasing forward voltage, there is a flow in the current (diffusion current here). After the voltage value reaches to (barrier potential), the forward current increase rapidly (barrier potential for Si is 0.7v and Ge is 0.3 v ) . From figure and explanation, we notice that diode is not linear which not depend on Ohm’s law due to first quarter of graph. =

At reverse bias circuit as fourth quarter of graph , there is small value of voltage and constant in relation between reverse voltage and diffusion current (which called Reverse Saturation Current). Keeping in mind, reverse voltage has large value compared with forward voltage to reached a particular voltage called Avalanche Voltage. This is shown clearly when we study a special type of diodes : Zener diode. The current increase very rapidly after Zener Breakdown or Breakdown voltage VBR

the relation between diffusion current and applied voltage Forward-biased Reverse-biased is an exponential relation Where I0 is reverse saturation current

Temperature Effect : for forward-biased diode as temperature is increased, the forward current increases for a given value of forward voltage. For a given value of forward current, the forward voltage decrease, as shown in figure. The blue curve is at room temperature and red curve is elevated temperature (300K + ΔT) notice that barrier potential decreases as temperature increases. For reverse-biased diode as temperature is increased, the reverse current increases. However, there is different between two curves. Keep in mind that the reverse current breakdown remains extremely small and can usually be neglected

Diode Models There are three models for diode 1-The Ideal Model:
This model is a simple switch. When the diode is forward-biased, it acts like a closed (on) switch. When it is reverse –biased, it acts like a closed (off) switch. the barrier potential, the forward dynamic resistance, and the reverse current are all neglected in this model.

Diode Models 2- The practical Model
The practical model adds the barrier potential to ideal model. When the diode is forward-biased, it is equivalent to a closed switch in series with a small equivalent voltage source equal to the barrier potential. When the diode is reverse-biased , it is equivalent to an open switch just as ideal model because barrier potential does not affect reverse bias. d

Diode Models 3- The Complete Model:
This model consists of the barrier potential, the small forward dynamic resistance, and the large internal reverse resistance.

Pn Junction’s mission Of the most important uses and benefits of the pn junction is the rectifier current from alternating current to direct current and you will study in detail that in Electronics Lab. where junction connect to AC source (which current shape wave or sine with time) to take advantage of the forward bias (passing current) reverse bias (not passage current) are rectifier the alternating current. There are two types of rectifier: half – wave rectifier and full wave rectifier. In addition to rectifier process, the filter process came next. By putting capacitor that will improve the current form of half wave or full wave into a form close to a straight line by putting a number of capacitors After rectifier and filter processes, regulator process came third (which we can put Zener diode) that give straight current. Three processes (rectifier , filter , regulator) to transformation from alternating current to direct current

Half-Wave Rectifiers =

Half-Wave Rectifiers To calculate average value of half-wave rectified output voltage However, this calculation for ideal diode. So in practical diode, we take in the account the barrier potential

Full-Wave Rectifiers There two circle for full –wave rectifiers
A center- tapped full –wave rectifiers A bridge full –wave rectifiers

center- tapped Full-Wave Rectifiers
=

The Bridge Full-Wave Rectifiers
=

Filters By using capacitor half or fall – wave is filtered. By putting
number of capacitors, the wave is filtered more than before

Regulator The final step to transformation from alternating current to direct current is regulator. There are numbers of regulator devices. One of them is Zener diode

Zener diode Zener diode differs from normal pn junction in its designed to work in reverse breakdown area without the occurrence of any problems. Difference in Zener diode, it has more impurities in one of side of diode than the other as (n+p ) or ( p+n). In equilibrium condition Fermi level is at the same energy level in n-type and p-type, but in non equilibrium condition is opposite as we knew. However, in the case of reverse bias aligns the conduction band in the n-type and valence band in the p-type as if they are at the same energy level and thus the electron from the conduction band in the n-type moves (or tunneling) to the valence band in p-type as shown in the figure. Among the benefits of this diode is used in regulate voltage.

Ohmic Contact The scientific method to create This contact
Previously, we discussed about pn junction (a semiconductor type of n-type next to the other type p-type) But what about another junction as semiconductor next to insulator or metal Ohmic Contact This contact consists metal and semiconductor (which follows Ohm's Law and does not have the ability to rectifier current and (current and voltage) characteristic is linear in both forward and reverse bias ) The scientific method to create This contact 1- by increasing impurities in a semiconductor at the contact area so that the charge carriers crosses barrier 2- by selecting a work function of metal (eΦ) close to work function of semiconductor work function: the required energy to remove an electron from Fermi level or required energy for ionization metal =

Ohmic Contact The left figure illustrates Fermi level and work function in the metal and semiconductor (n-type) before contact. The right figure illustrates them when configure junction of metal and semiconductor (n-type) in equilibrium condition and how Fermi level aligning between metal and semiconductor (when there are negative charges close to surface of metal, they attract positive charges in metal ) =

Ohmic Contact The left figure illustrates Fermi level and work function in the metal and semiconductor (p-type) before contact. The right figure illustrates them when configure junction of metal and semiconductor (p-type) in equilibrium condition and how Fermi level aligning between metal and semiconductor (when there are positive charges close to surface of metal, they attract negative charges in metal ) =

Metal–Oxide–Semiconductor Contact (MOS) structure
In this contact, a thin layer of oxide is put on the surface of a semiconductor n-type or p-type. Then, pole metallic (metal) is put above the surface of the oxide layer . We should choose a good electrical insulation of oxide which has a large energy gap and isolates the metal from the semiconductor which no passing electrical current between them. In thermal equilibrium condition In the absence of application of the electric field or the voltage, the Fermi and connection and valence levels are horizontal and flat. When applying an electric field, there is a bending in energy levels

Effect of voltage bias Metal–Oxide–Semiconductor Contact (MOS)
According to the applied voltage on this contact, it will consist three different situations such as what is shown in figure . 1- depletion inversion accumulation =

Metal–Oxide–Semiconductor Contact (MOS)
Effect of voltage bias (metal and n-type contact) 1 - Depletion : When applying negative bias voltage at the surface of metal, a small amount of negative charges is made. Then, the oxide layer prevent electric current from passage to semiconductor . However, the electrons in substrate of semiconductor n-type will be affected by these negative charges and moved away from the area located under oxide and created the depletion region in semiconductor similar to those that created in pn Junction =

Effect of voltage bias (metal and n-type contact) 2 - Inversion When increasing a negative bias voltage on surface of metal, Instead of expanding more of depletion region within the semiconductor, inversion status is formed which holes gather next to the surface of the oxide. Those holes is the minority carriers in the semiconductor n-type. 3 - Accumulation When applying positive bias voltage at the surface of metal, negative majority carriers attract and accumulate at the surface of the oxide in semiconductor n-type. =

Energy levels forms in MOS in different bias (metal and p-type contact) 1- Depletion : When applying positive bias voltage, Fermi level move down from its first location in thermal equilibrium condition. Also, straight line bend at the energy level in oxide and energy levels of the semiconductor p-type move down near the interface of oxide. In addition, electrons drop down in potential well. We notice that the distribution of carriers density of per unit area in semiconductor p-type equal in the metal =

Energy levels forms in MOS in different bias (metal and p-type contact) 2 - Inversion When increasing positive bias voltage more than threshold voltage VT ; the semiconductor inverse and electrons occupy inversion layer. Fermi level move more down from its first location in thermal equilibrium condition. Also, straight line bend at the energy level in oxide and energy levels of semiconductor p-type move more down near interface of oxide. In addition, electrons drop more down in potential well. We notice that the distribution of carriers density of per unit area in semiconductor p-type for maximum depletion region Wmax in addition to carrier of inversion layer Qn equal in the metal =

Energy levels forms in MOS in different bias (metal and p-type contact) 3- Accumulation When applying negative bias voltage, Fermi level move up from its first location in thermal equilibrium condition. Also, Straight line bend at the energy level in oxide and energy levels of the semiconductor p-type move up near the interface of oxide. In addition, holes climb up in potential well. We notice that the distribution of carriers density of per unit area in semiconductor p-type equal in the metal =

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