P-N junction diode

1. Chapter 1:
Semiconductor Diodes
Module-1
Basic Electronics

2. Diodes
Simplest Semiconductor Device
It is a 2-terminal device

3. Basic operation
Ideally it conducts current in only one direction
and acts like an open in the opposite direction

4. Characteristics of an ideal diode: Conduction Region
Look at the vertical line!
In the conduction region, ideally
• the voltage across the diode is 0V,
• the current is ID,
• the forward resistance (RF) is defined as RF = VF/IF,
• the diode acts like a short.

5. Characteristics of an ideal diode: Non-Conduction Region
Look at the horizontal line!
In the non-conduction region, ideally
• all of the voltage is across the diode,
• the current is 0A,
• the reverse resistance (RR) is defined as RR = VR/IR,
• the diode acts like open.

6. Semiconductor Materials
Common materials used in the development of semiconductor
devices:
• Silicon (Si)
• Germanium (Ge)
The term conductor is applied to any material that will support a
generous flow of charge when a voltage source of limited
magnitude is applied across its terminals.
An insulator is a material that offers a very low level of
conductivity under pressure from an applied voltage source.
A semiconductor, therefore, is a material that has a conductivity
level some- where between the extremes of an insulator and a
conductor.

7. Semiconductor Materials
Inversely related to the conductivity of a material is its
resistance to the flow of charge, or current.
That is, the higher the conductivity level, the lower the
resistance level. In tables, the term resistivity (𝜌, Greek letter
rho) is often used when comparing the resistance levels of
materials.

8. Semiconductor Materials
One very important consideration is the fact that semiconductor materials
can be manufactured to a very high purity level. In fact, recent advances
have reduced impurity levels in the pure material to 1 part in 10 billion (1 :
10,000,000,000).
Addition of one part impurity (of the proper type) per million in a wafer of
silicon material can change that material from a relatively poor conductor to
a good conductor of electricity.
Their characteristics can be altered significantly through the application of
heat or light—an important consideration in the development of heat- and
light-sensitive devices.

9. Semiconductor Materials

10. Semiconductor Materials

11. Semiconductor Materials
it is still possible for the valence electrons to absorb sufficient kinetic energy from natural
causes to break the covalent bond and assume the “free” state.
The free electrons in the material due only to natural causes are referred to as
intrinsic carriers.
At room temperature there are approximately 1.5*1010 free carriers in a cubic centimeter of
intrinsic silicon material.
At the same temperature, intrinsic germanium material will have
approximately 2.5 *1013 free carriers per cubic centimeter.
The ratio of the number of carriers in germanium to that of silicon is greater than 103 and
would indicate that germanium is a better conductor at room temperature.
An increase in temperature of a semiconductor can result in a substantial increase in
the number of free electrons in the material.
Semiconductor materials such as Ge and Si that show a reduction in resistance with
increase in temperature are said to have a negative temperature coefficient.

12. Doping
The electrical characteristics of Silicon and Germanium are improved by adding
materials in a process called doping.
A semiconductor material that has been subjected to the doping process is called
an extrinsic material.
The additional materials are in two types:
• n-type
• p-type

13. Doping
The electrical characteristics of Silicon and Germanium are improved by adding
materials in a process called doping.
A semiconductor material that has been subjected to the doping process is called
an extrinsic material.
The additional materials are in two types:
• n-type
• p-type

14. N-type material
The n-type is created by introducing those
impurity elements that have five valence
electrons (pentavalent), such as antimony,
arsenic, and phosphorus.
This remaining electron, loosely bound to its
parent (antimony) atom, is relatively free to
move within the newly formed n-type
material.
Since the inserted impurity atom has
donated a relatively “free” electron to the
structure:
Diffused impurities with five valence
electrons are called donor atoms.

15. P-type material
The p-type is created by introducing those
impurity elements that have three valence
electrons , such as boron, gallium, and
indium.
Note that there is now an insufficient number
of electrons to complete the covalent bonds
of the newly formed lattice.
The resulting vacancy is called a hole and is
represented by a small circle or positive sign
due to the absence of a negative charge.
Since the resulting vacancy will readily
accept a “free” electron:
The diffused impurities with three valence
electrons are called acceptor atoms.

16. Electron vs Hole flow
If a valence electron acquires sufficient kinetic energy to break its covalent bond
and fills the void created by a hole, then a vacancy, or hole,
will be created in the covalent bond that released the electron.
There is, therefore, a transfer of holes to the left and electrons to the right,

17. n-type materials make the Silicon (or Germanium) atoms more negative.
p-type materials make the Silicon (or Germanium) atoms more positive.
Join n-type and p-type doped Silicon (or Germanium) to form a p-n junction
In an n-type material the electron is called the majority carrier and the hole the
minority carrier.
In a p-type material the hole is the majority carrier and the electron is the minority
carrier.
The n- and p-type materials represent the basic building blocks of semiconductor
devices.
.
n-type versus p-type

18. p-n junction
When the materials are joined, the negatively charged atoms of the n-type doped
side are attracted to the positively charged atoms of the p-type doped side.
The electrons in the n-type material migrate across the junction to the p-type
material (electron flow).
Or you could say the ‘holes’ in the p-type material migrate across the junction to the
n-type material (conventional current flow).
The result is the formation of a depletion layer around the junction.
depletion
layer
p n

19. P-N junction diode

20. A diode, as any semiconductor device is not perfect!
There are two sets of currents:
• Majority Carriers
The electrons in the n-type and ‘holes’ in the p-type material
are the source of the majority of the current flow in a diode.
• Minority Carriers
Electrons in the p-type and ‘holes’ in the n-type material
are rebel currents. They produce a small amount of opposing current.
Majority and Minority Carriers in Diode

21. Operating Conditions
• No Bias
• Forward Bias
• Reverse Bias

22. No external voltage is applied: VD = 0V and no current is flowing ID = 0A.
Only a modest depletion layer exists.
No Bias Condition

23. No bias Condition
At n-type semiconductor large number of free electrons is present
while at p-type semiconductor small number of free electrons is
present.
Due to this high concentration of electrons at n-side, they get
repelled from each other. Hence they try to move towards the low
concentration region.
Hence, the free electrons from the n-side are attracted towards the
holes at the p-side. Thus, the free electrons move from n-region
(high concentration region) to p-region (low concentration region).
At p-type semiconductor large number of holes is present while at n-
type semiconductor small number of holes is present.
Hence, the concentration of holes at p-type semiconductor is high
while the concentration of holes at n-type semiconductor is low.
Hence, the holes from the p-side are attracted towards the free
electrons at the n-side. Thus, the holes move from p-region (high
concentration region) to n-region (low concentration region).

24. Forward Bias Condition
External voltage is applied across the
p-n junction in the same polarity of
the p- and n-type materials.
The depletion layer is narrow. The
electrons from the n-type material and
‘holes’ from the p-type material have
sufficient energy to cross the junction.

25. Reverse Bias Condition
External voltage is applied across the p-n junction
in the opposite polarity of the p- and n-type materials.
This causes the depletion layer to widen.
The electrons in the n-type material are
attracted towards the positive terminal and
the ‘holes’ in the p-type material are attracted
towards the negative terminal.

26. Actual Diode Characteristics
Note the regions for No Bias, Reverse Bias, and Forward Bias conditions.
Look closely at the scale for each of these conditions!

27. Another detail about the diode is the useful Zener region.
The diode is in the reverse bias condition.
At some point the reverse bias voltage is so large the diode breaks down.
The reverse current increases dramatically.
This maximum voltage is called avalanche breakdown voltage and the current
is called avalanche current.
Zener Region

28. The point at which the diode changes from No Bias condition to
Forward Bias condition happens when the electron and ‘holes’
are given sufficient energy to cross the p-n junction. This energy
comes from the external voltage applied across the diode.
The Forward bias voltage required for a
• Silicon diode VT = 0.7V
• Germanium diode VT = 0.3V
Forward Bias Voltage

29. As temperature increases it adds energy to the diode.
It reduces the required Forward bias voltage in Forward Bias condition.
It increases the amount of Reverse current in Reverse Bias condition.
It increases maximum Reverse Bias Avalanche Voltage.
Germanium diodes are more sensitive to temperature variations than Silicon
Diodes.
Temperature Effects

30. Semiconductors act differently to DC and AC currents. There are 3 types of
resistances.
• DC or Static Resistance
• AC or Dynamic Resistance
• Average AC Resistance
Resistance Levels

31. RD = VD/ID
For a specific applied DC voltage VD, the diode will have a specific current ID,
and a specific resistance RD.
The amount of resistance RD, depends on the applied DC voltage.
DC or Static Resistance

32. Forward Bias region:
• The resistance depends on the amount of current (ID) in the diode.
• The voltage across the diode is fairly constant (26mV for 25 C).
• rB ranges from a typical 0.1 for high power devices to 2 for low power, general
purpose diodes. In some cases rB can be ignored.
Reverse Bias region:
The resistance is essentially infinite. The diode acts like an open.
AC or Dynamic Resistance
[Formula 1.8]

33. AC resistance can be determined by picking 2 points on the characteristic curve
developed for a particular circuit.
[Formula 1.9]
Average AC Resistance

34. Data about a diode is presented uniformly for many different diodes. This makes
cross-matching of diodes for replacement or design easier.
1. VF, forward voltage at a specific current and temperature
2. IF, maximum forward current at a specific temperature
3. IR, maximum reverse current at a specific temperature
4. PIV or PRV or V(BR), maximum reverse voltage at a specific temperature
5. Power Dissipation, maximum power dissipated at a specific temperature
6. C, Capacitance levels in reverse bias
7. trr, reverse recovery time
8. Temperatures, operating and storage temperature ranges
Diode Specification Sheets

35. In Reverse Bias the depletion layer is very large. The diode’s strong positive and
negative polarities create capacitance, CT. The amount of capacitance depends on
the reverse voltage applied.
In Forward Bias storage capacitance or diffusion capacitance (CD) exists as the
diode voltage increases.
Capacitance

36. This is the amount of time it takes for the diode to stop conducting once the diode
is switched from Forward Bias to Reverse Bias.
Reverse Recovery Time (trr)

37. Anode is abbreviated – A
Cathode is abbreviated – K
(because the Cathode end of the diode symbol looks like a
backwards K)
Diode Symbol and Notation

38. A. Diode Checker
B. Ohmmeter
C. Curve Tracer
Diode Testing

39. A. Diode Checker
Many DMM’s have a diode checking function.
A normal diode will exhibit its Forward Bias voltage (VF).
The diode should be tested out of circuit.
Silicon diode 0.7V
Germanium diode 0.3V

40. An ohmmeter set on a low ohms scale can be used to test a diode.
A normal diode will have the following readings.
The diode should be tested out of circuit.
B. Ohmmeter

41. C. Curve Tracer
A curve tracer is a specialized type of test equipment. It will display the
characteristic curve of the diode in the test circuit. This curve can be compared
to the specifications of the diode from a data sheet.

42. Other Types of Diodes
1. Zener Diode
2. Light Emitting Diode
3. Diode Arrays

43. A Zener is a diode operated in reverse bias at the Peak Inverse Voltage (PIV)
called the Zener Voltage (VZ).
Symbol
Common Zener Voltages: 1.8V to 200V
1. Zener Diode

44. 2. Light Emitting Diode (LED)
This diode when forward biased emits photons. These can be in the visible
spectrum.
Symbol
The forward bias voltage is higher, usually around 2-3V.

45. Multiple diodes can be packaged together in an integrated circuit (IC).
A variety of combinations exist.
Example of an array:
3. Diode Arrays