1. Electronics The Seventh and Eighth and Lectures
fifth week
6 - 8 / 6/ 1438 هـ
أ / سمر السلمي

2. Outline for today Chapter Two: Junction Diode Physical Electronics
Forward-biased and Reverse- biased Junction
depletion region and applying bias, and Diffusion and Drift currents
Drive excess of minority carrier in Forward-biased and Reverse- biased Junction
Mathematical description of excess of minority carrier in Forward-biased and Reverse- biased Junction
Drive diffusion current in forward-biased and reverse- biased Junction
I – V characteristic of forward-biased and reverse-biased Junction
Diode Models
Pn Junction’s mission(half and full wave rectifiers, filters, regulator)
Zener diode

3. The first periodic exam in / 6 / 1438 هـ 27
Time of Periodic Exams
The first periodic exam in / 6 / 1438 هـ 27
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 in the same day

4. 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. =

5. 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.

6. 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

7. 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 =

8. 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 =

9. 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.

10. 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

11. Drive excess of minority carrier in Forward-biased and Reverse- biased Junction
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

12. Drive excess of minority carrier in Forward-biased and Reverse- biased Junction
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

13. 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

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

15. 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

16. 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 =

17. Drive diffusion current in Forward-biased and Reverse- biased Junction
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

18. Drive diffusion current in Forward-biased and Reverse- biased Junction
From the figure
To calculate the current
Where I0 the reverse saturation current is

19. Drive diffusion current in Forward-biased and Reverse- biased Junction
Figure illustrates the distribution of the electron and hole currents in the junction in the forward bias

20. 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

21. I – V Characteristic of Forward-biased and Reverse-biased Junction
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. =

22. I – V Characteristic of Forward-biased and Reverse-biased Junction
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

23. I – V Characteristic of Forward-biased and Reverse-biased Junction
the relation between diffusion
current and applied voltage
Forward-biased
Reverse-biased
is an exponential relation
Where I0 is reverse saturation current

24. I – V Characteristic of Forward-biased and Reverse-biased Junction
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

25. 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.

26. 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

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

28. 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

29. Half-Wave Rectifiers =

30. 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

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

32. center- tapped Full-Wave Rectifiers=

33. The Bridge Full-Wave Rectifiers=

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

35. 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

36. 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.