P. N.

JUNCTION
DIODES

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 N-Type Material

N-Type Material: When extra valence electrons are introduced

into a material such as silicon an n-type
material is produced. The extra valence
electrons are introduced by putting
+4 +4 +4 impurities or dopant into the silicon. The
dopant used to create an n-type material are
Group V elements. The most commonly
+4 +5 +4 used dopant from Group V are arsenic,
antimony and phosphorus.
The 2D diagram to the left shows the extra
+4 +4 +4 electron that will be present when a Group V
dopant is introduced to a material such as
silicon. This extra electron is very mobile.

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 P-Type Material

P-Type Material: P-type material is produced when the dopant

that is introduced is from Group III. Group
III elements have only 3 valence electrons
and therefore there is an electron missing.
+4 +4 +4 This creates a hole (h+), or a positive charge
that can move around in the material.
Commonly used Group III dopant are
+4 +3 +4 aluminum, boron, and gallium.
The 2D diagram to the left shows the hole
that will be present when a Group III dopant
+4 +4 +4 is introduced to a material such as silicon.
This hole is quite mobile in the same way the
extra electron is mobile in a n-type material.

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 The PN Junction
Metallurgical
Steady State
Na Junction Nd

- - - - - + + + + +
When no external source
P
+ + + + + is connected to the pn
- - - - - n
+ + + + +
junction, diffusion and
+ + + + +
- - - - -
Space Charge
drift balance each other
ionized
acceptors
- - - - - Region ionized out for both the holes
donors
E-Field and electrons
_ _
+ +
h+ drift == h+ diffusion e- diffusion == e- drift

Space Charge Region: Also called the depletion region. This region
includes the net positively and negatively charged regions. The space
charge region does not have any free carriers. The width of the space charge
region is denoted by W in pn junction formula’s.

Metallurgical Junction: The interface where the p- and n-type materials meet.

Na & Nd: Represent the amount of negative and positive doping in number of
carriers per centimeter cubed. Usually in the range of 1015 to 1020.
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 The Biased PN Junction
Metal
Contact
“Ohmic
_
Contact”
+
(Rs~0)
Applied
P Electric Field n

_
+
Vapplied
The pn junction is considered biased when an external voltage is applied.
There are two types of biasing: Forward bias and Reverse bias.
06/24/2025 These are described on then next slide. 5
 The Biased PN Junction

Forward Bias: In forward bias the depletion region shrinks slightly in

width. With this shrinking the energy required for
charge carriers to cross the depletion region decreases
Vapplied > 0 exponentially. Therefore, as the applied voltage
increases, current starts to flow across the junction.
The barrier potential of the diode is the voltage at
which appreciable current starts to flow through the
diode. The barrier potential varies for different
materials.
Reverse Bias: Under reverse bias the depletion region widens. This
causes the electric field produced by the ions to cancel
out the applied reverse bias voltage. A small leakage
Vapplied < 0 current, Is (saturation current) flows under reverse bias
conditions. This saturation current is made up of
electron-hole pairs being produced in the depletion
region. Saturation current is sometimes referred to as
scale current because of it’s relationship to junction
06/24/2025 temperature. 6
 Properties of Diodes
Figure 1.10 – The Diode Transconductance Curve 2
ID (mA) • VD = Bias Voltage
• ID = Current through
Diode. ID is Negative
for Reverse Bias and
Positive for Forward
IS Bias
VBR • IS = Saturation
Current
~V VD
• VBR = Breakdown
Voltage
• V = Barrier Potential
Voltage

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06/24/2025 8
06/24/2025 9
 Properties of Diodes
The Shockley Equation
• The transconductance curve on the previous slide is characterized by
the following equation:

ID = IS(eVD/VT – 1)
• As described in the last slide, ID is the current through the diode, IS is
the saturation current and VD is the applied biasing voltage.
• VT is the thermal equivalent voltage and is approximately 26 mV at room
temperature. The equation to find VT at various temperatures is:
VT = kT
q
k = 1.38 x 10-23 J/K T = temperature in Kelvin q = 1.6 x 10-19 C
•  is the emission coefficient for the diode. It is determined by the way
the diode is constructed. It somewhat varies with diode current. For a
silicon diode  is around 2 for low currents and goes down to about 1 at
higher currents
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PN junction diode definitions:

1. Forward biasing: When an external D.C. voltage is applied across the

junction in such a direction that it cancels the potential barrier, thus
permitting current flow, it is called forward biasing.

2. Reverse biasing: When an external D.C. voltage is applied across the

junction in such a direction that it widens the potential barrier, it is
called reverse biasing.

3. Breakdown voltage: It is the reverse voltage at which pn junction breaks

down with sudden rise in reverse current.

4. Knee voltage or cut-in voltage: It is the forward voltage at which the

current through the junction starts to increase rapidly.

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5. Maximum forward current: It is the highest instantaneous forward
current that a pn junction can conduct without damage to the junction.

6. Peak inverse voltage (PIV): It is the maximum reverse voltage that can
be applied to the pn junction without damage to the junction.

7. Maximum power rating: It is the maximum power that can be dissipated

at the junction without damaging it. The power dissipated at the junction
is equal to the product of junction current and the voltage across the
junction.

8. Reverse current or leakage current or saturation current: It is the

current that flows through a reverse biased diode. This current is due to
minority carriers. Under normal operating voltages, the reverse current
is quite small. Its value is extremely small (<1μA) for Silicon diodes but it
is appreciable (approx. 100μA) for Germanium diodes.
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1. The diffusion current can be defined as the flow of charge carriers within a
semiconductor travels from a higher concentration region to a lower
concentration region. A higher concentration region is nothing but where
the number of electrons present in the semiconductor. Similarly, a lower
concentration region is where the less number of electrons present in the
semiconductor. The process of diffusion mainly occurs when a
semiconductor is doped non-uniformly.

2. Drift current can be defined as the charge carrier’s moves in a

semiconductor because of the electric field. There are two kinds of charge
carriers in a semiconductor like holes and electrons. Once the voltage is
applied to a semiconductor, then electrons move toward the +Ve terminal
of a battery whereas the holes travel toward the –Ve terminal of a battery.
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 Drift Current Diffusion Current

The movement of charge carriers is because

The diffusion current can be occurred because
of the applied electric field is known as drift
of the diffusion in charge carriers.
current.
It requires electrical energy for the process Some amount of external energy is enough for
of drift current. the process of diffusion current.

This current obeys Ohm’s Law. This current obeys Fick’s Law.

The direction of charge carriers in the For charge carriers, the densities of diffusion
semiconductor is reverse to each other. are reverse in symbol to each other.

The direction of the drift current, as well as The direction of this current can be decided
the electric field, will be the same. by the concentration of the carrier slope.

It is independent of permittivity
It depends on the permittivity

The direction of this current mainly depends

The direction of this current mainly depends
on the charge within the concentrations of
on the polarity of the applied electric field.
carrier

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 Types of Diodes and Their Uses

PN Junction Are used to allow current to flow in one direction

Diodes: while blocking current flow in the opposite
direction. The pn junction diode is the typical diode
that has been used in the previous circuits.

A K P n
Schematic Symbol for a PN Representative Structure for
Junction Diode a PN Junction Diode

Zener Diodes: Are specifically designed to operate under reverse

breakdown conditions. These diodes have a very
accurate and specific reverse breakdown voltage.

A K
Schematic Symbol for a
06/24/2025 Zener Diode 15
 Types of Diodes and Their Uses

Light-Emitting Light-emitting diodes are designed with a very large

Diodes: band gap so movement of carriers across their
depletion region emits photons of light energy.
Lower band gap LEDs (Light-Emitting Diodes) emit
infrared radiation, while LEDs with higher band gap
energy emit visible light. Many stop lights are now
starting to use LEDs because they are extremely
bright and last longer than regular bulbs for a
relatively low cost.

The arrows in the LED

A K representation indicate
emitted light.
Schematic Symbol for a
Light-Emitting Diode

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 Types of Diodes and Their Uses

Photodiodes: While LEDs emit light, Photodiodes are sensitive to

received light. They are constructed so their pn
junction can be exposed to the outside through a
clear window or lens.
A K In Photoconductive mode the saturation current
increases in proportion to the intensity of the
received light. This type of diode is used in CD
 players.
A K In Photovoltaic mode, when the pn junction is
exposed to a certain wavelength of light, the diode
Schematic Symbols for
generates voltage and can be used as an energy
Photodiodes source. This type of diode is used in the
production of solar power.

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