Basic Electronics Concepts
Diodes
Fundamentals
Lecture Delivered @ HITEC University By Atif M. Khokhar
Table of Contents
What are diodes made out of ……?
N-type material
P-type material
The pn junction
The biased pn junction
Lecture Delivered @ HITEC University By Atif M. Khokhar
Introduction to Electronics
(Semiconductor Devices)
Lecture Delivered @ HITEC University By Atif M. Khokhar
Crystalline Solids
• In a crystalline solid, the periodic arrangement
of atoms is repeated over the entire crystal
• Silicon crystal has a diamond lattice
Lecture Delivered @ HITEC University By Atif M. Khokhar
Crystalline Nature of Silicon
• As with all crystalline material, silicon consists
of a repeating basic unit structure called a unit
cell
• For silicon, the unit cell consists of an atom
surrounded by four equidistant nearest
neighbors which lie at the corners of the
tetrahedron
Lecture Delivered @ HITEC University By Atif M. Khokhar
What’s so special about Silicon?
Cheap and abundant
Amazing mechanical, chemical and
electronic properties
The material is very well-known to mankind
SiO2: sand, glass
Si is column IV of
the periodic table
Similar to the
carbon (C) and the
germanium (Ge)
Has 3s² and 3p²
valence electrons
Lecture Delivered @ HITEC University By Atif M. Khokhar
Lecture Delivered @ HITEC University By Atif M. Khokhar
N-Type Semiconductor
P-Type Semiconductor
Lecture Delivered @ HITEC University By Atif M. Khokhar
N-Type Material
N-Type Material: When extra valence electrons are introduced
into a material such as silicon an n-
n-type
material is produced. The extra valence
electrons are introduced by putting
+4 +4 +4 impurities or dopants into the silicon. The
dopants used to create an n-n-type material
are Group V elements. The most commonly
+4 +5 +4 used dopants from Group V are arsenic,
antimony and phosphorus.
+4 +4 +4
Lecture Delivered @ HITEC University By Atif M. Khokhar
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 dopants are
+4 +3 +4 aluminum, boron, and gallium.
gallium.
+4 +4 +4
Lecture Delivered @ HITEC University By Atif M. Khokhar
P-N Junction
• Also known as a diode
• One of the basics of semiconductor
technology -
• Created by placing n-type and p-type
material in close contact
Lecture Delivered @ HITEC University By Atif M. Khokhar
The P-N Junction
Lecture Delivered @ HITEC University By Atif M. Khokhar
Diffusion –movement due to difference in concentration, from
higher to lower concentration, mobile charges (holes) in p-type
combine with mobile charges (electrons) in n-type
Lecture Delivered @ HITEC University By Atif M. Khokhar
The PN Junction
Steady State
Metallurgical
Junction
Na Nd
- - - - - - + + + + + +
- - - - - - + + + + + +
P - - - - - - + + + + + + n
- - - - - - + + + + + +
- - - - - - + + + + + +
Space Charge
ionized Region ionized
acceptors donors
E-Field
_ _
+ +
h+ drift = h+ diffusion e- diffusion = e- drift
Lecture Delivered @ HITEC University By Atif M. Khokhar
• Formation of
depletion region
in pn-junction:
Lecture Delivered @ HITEC University By Atif M. Khokhar
Lecture Delivered @ HITEC University By Atif M. Khokhar
P-N Junction
• Region is totally depleted of mobile
charges - “depletion region”
– Electric field forms due to fixed
charges in the depletion region
– Depletion region has high resistance
due to lack of mobile charges
Lecture Delivered @ HITEC University By Atif M. Khokhar
The PN Junction
Metallurgical
Steady State
Na Junction Nd
- - - - - + + + + +
- - - - - + + + + +
P n
- - - - - + + + + +
- - - - - + + + + +
Space Charge
ionized Region ionized
acceptors donors
E-Field
_ _
+ +
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-
p- and n-
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.
Lecture Delivered @ HITEC University By Atif M. Khokhar
PN JUNCTION BIASING
Lecture Delivered @ HITEC University By Atif M. Khokhar
The Biased PN Junction
Metal
Contact
“Ohmic
Contact”
(Rs~0
(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.
These are described on then next slide.
Lecture Delivered @ HITEC University By Atif M. Khokhar
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.
Lecture Delivered @ HITEC University By Atif M. Khokhar
Forward biased pn-junction
Lecture Delivered @ HITEC University By Atif M. Khokhar
The Biased PN Junction
Reverse Bias: Under reverse bias the depletion region widens.
widens. This
causes the electric field produced by the ions to cancel
Vapplied < 0 out the applied reverse bias voltage. A small leakage
current, Is (saturation
(saturation current)
current) flows under reverse bias
conditions. This saturation current is made up of
electron--hole pairs being produced in the depletion
electron
region. Saturation current is sometimes referred to as
scale current because of it’s relationship to junction
temperature.
Lecture Delivered @ HITEC University By Atif M. Khokhar
Reversed biased pn-junction
Lecture Delivered @ HITEC University By Atif M. Khokhar
Diode Simulation
Lecture Delivered @ HITEC University By Atif M. Khokhar
PN junction under various bias
conditions
VA = 0 VA > 0 VA < 0
E E E
p n
p n
Hole diffusion Hole diffusion Hole diffusion
current current current
Hole drift current Hole drift Hole drift current
current
Electron diffusion current Electron diffusion current Electron diffusion current
Electron drift current Electron drift current Electron drift
Lecture Delivered @ HITEC University By Atif M. Khokhar current
26
Band diagram and carrier flow under bias
27
Lecture Delivered @ HITEC University By Atif M. Khokhar
Band diagram and carrier flow under bias
28
Lecture Delivered @ HITEC University By Atif M. Khokhar
Effect of bias on diffusion current
• When the diode forward-bias-voltage is
increased, the barrier for electron and hole
diffusion current decreases linearly. See the band
diagram.
• Since the carrier concentration decreases
exponentially with energy in both bands,
diffusion current increases exponentially as
the barrier is reduced.
• As the reverse-bias-voltage is increased, the
diffusion current decreases rapidly to zero,
since the fall-off in current is exponential.
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Lecture Delivered @ HITEC University By Atif M. Khokhar
Lecture Delivered @ HITEC University By Atif M. Khokhar
