Download Document

Current in Semiconductors
Drift Current, Mobility & Diffusion Current
Engr. Fazal Ur Rehman
P-Type Semiconductor
Lack of one
electron creates a
hole
Si
Si
“Al” atom has
three
electrons in its
valence shell
and is called
“Trivalent
atom”
Al
Si
Si
The holes created make
the material P-type
semiconductor which has
more holes than its free
electrons
P-Type Semiconductor
•
•
•
•
•
•
Impurity atoms from 3rd group of the periodic table can enhance the amount
of positive carriers (holes) in intrinsic semiconductors. These impurities are
commonly known as acceptor.
A small amount of impurity 1 part in 106 to 108 is added in P-type
semiconductor.
The impurity atoms form covalent bonds, resulting bond will lack one
electron.
Deficiency of electron means a creation of hole. The valance band is highly
conductive to allow transfer of electrons from hole to hole to constitute an
electric current.
The Majority carriers are holes that are almost independent of temperature.
The minority carriers are electrons that are independent of temperature.
Critical Temperature
• The temperature at which the minority carriers
concentration become 10% of majority carrier
concentration is called critical temperature.
• At this high temperature these materials become intrinsic
materials.
• For all practical purpose high temperature should be
avoided.
Few concepts
• The Famous Mass Action Law
• Density of Holes in N-Type semicoductor
• Example
Drift Current
Silicon Bar containing holes and free electrons
Battery
-
Hole Current
Electron Current
+
The sum of electron current and hole
current is called “Drift Current”
Electric Field Intensity (E)
The potential difference per unit length is called “Electric Field Intensity E”. It is
measured in Volts/Meter
Length of bar = l
V
E
l
(Volts/m)
V = 10V
l = 10cm
Battery
-
10 V
+
E=?
Current Density (J)
The current per unit cross sectional area (A) is called “Current
Density J”. It is measured in Amperes/m2
I
J
A
A=hXw
(A/m2)
l
I = 10 mA
h
I
w
h = 10 cm
w = 1 cm
J=?
Charge Carrier Density (n, p)
•
•
•
•
Number of electrons per unit volume is called electron density. It is denoted
by “n” and measured in “electrons/m3”
Number of holes per unit volume is called hole density. It is denoted by “p”
and measured in “holes/m3”
The electron density (n) and hole density (p) are collectively known as
“Charge Carrier Densities”
For pure (intrinsic) semiconductor: n = p
l = 20 cm
l
h
h = 10 cm
w = 1 cm
Total Free Electrons = 2 X 1016
w
n=?p=?
Charge Density (Qn, Qp)
• Unit electron charge is denoted by “qn” and its value is 1.6 X 10-19C.
It is negative charge
• Unit hole charge is denoted by “qp” and its value is 1.6 X 10-19C. It is
positive charge
• Charge per unit volume is called “Charge Density”. It is denoted by
Qn or Qp and is measured in C/m3
• Qn = nqn
&
Qp = pqp
n = 1.5 X 1016 electrons/m3
Qn = ?
Qp = ?
Drift Mobility (μn , μp)
• The velocity of charge carrier per unit electric field intensity is called
drift mobility of charge carrier.
• It is the measure of the ability of charge carrier to move through a
particular material
• It is denoted by μn for electron mobility and μp for hole mobility
• It is measured in m2/ V.s
Few concepts
• Conductors are primarily uni-polar devices, means they conduct
current by means of electrons only. Semiconductor devices are
bipolar devices, current flow due to electrons & holes.
Drift Mobility for Si and Ge
Material
μn
(m2/ V.s)
μp
(m2/ V.s)
Silicon
0.14
0.05
Germanium
0.38
0.18
Let’s Look Back!
Quantity
Symbol
Unit
Electric Field Intensity
E
V/m
Current Density
J
A/m2
Hole Current Density
Jp
A/m2
Electron Current
Density
Jn
A/m2
Hole Density
p
Holes/m3
Electron Density
n
Electrons/m3
Hole Charge Density
Qp
C/m3
Electron Charge
Density
Qn
C/m3
Hole Mobility
μp
m2/ V.s
Electron Mobility
μn
m2/ V.s
Putting it together!
• J = Jn + Jp = Qnvn + Qpvp
= (nqn)(μnE) + (pqp)(μpE)
= nqnvn + pqpvp
• σ = nqnμn + pqpμp
J=?
Qn = ?
n=?
Jn = ?
vn = ?
qn = ?
Jp = ?
Qp = ? vp = ?
μn = ? E = ?
p=?
qp = ?
μp = ?
σ=?
Diffusion Current
When carriers diffuse in the material the current caused is
termed as “ Diffusion Current”
Home Work
• Examples: 2.2, 2.3
• End Problems: 2.6  2.15
Thank Yu
Any Questions?