Semiconductor Diodes
Lecture 1
EEME-2752 Electronics Engineering
Type of Materials
 Conductors
 Materials that offer low resistance to the flow of current
 Semi conductors
 Materials that have a value of resistance between that of a
conductor and an insulator
 Insulators
 Material that offers high resistance to the flow of current
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Semiconductor Materials
 Three semiconductor materials are commonly used in the
construction of semiconductor devices
 Germanium (Ge)
 Silicon (Si)
 Gallium arsenide (GaAs)
 Semiconductors can be in intrinsic or extrinsic form depending
upon the level of impurity present
 Intrinsic semiconductors (without impurity)
 Extrinsic semiconductors (with impurity)
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Intrinsic Materials
 Semiconductor materials have four valance electrons
 Pure Silicon or Germanium crystal have four valance electrons of
one atom bound in covalent bond with four neighbouring atoms
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Intrinsic Materials
 Covalent bonding of Silicon atoms
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Energy Levels
 The farther the electron is from the nucleus, the higher the energy
state
•
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The electron that has left
the parent atom has
higher energy than any
electron in the atomic
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Energy Levels
 Conductivity of conductors decreases with the increase in temperature and
conductors have positive temperature coefficient
 Conductivity of semiconductors increases with the increase in temperature and
semiconductors have negative temperature coefficient
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Extrinsic Materials
 A semiconductor material that has been subjected to the process
of doping is called extrinsic material
 Extrinsic materials can be classified into p-type materials and n-
type materials depending on the impurity used for doping
 Pentavalent impurity atoms are used to create n-type materials
and trivalent impurity atoms are used to create p-type materials
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N-Type Materials
 Pentavalent impurity atoms ( Arsenic, Phosphorous, Antimony) are added to
increase the number of conduction band electrons
 The fifth electron loosely bound is free to move within the n-type material
 The impurity atom are called donor atoms
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P-Type Materials
 Trivalent impurity atoms( Boron, Indium, Gallium) are added to increase the
number of holes
 The vacancy of one electron creates a hole which is treated as a positive carrier
that can move through p-type material
 The impurity atoms are called acceptor atoms
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Majority and Minority Carriers
 In p-type material holes are majority carriers and electrons are minority
carriers
 In n-type materials electrons are majority carriers and holes are minority
carriers
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Semiconductor Diode
 p-n junction diode is formed by joining p-type and n-type materials together
 The electrons and holes across the junction combine to form a region free of
carriers
 This region around the junction is called depletion region
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Semiconductor Diode
 In case there is no voltage applied across the diode no current will flow through
the diode
 Biasing is the process of application of external voltage across the two terminals
of the device to extract a response
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Semiconductor Diode
REVERSE BIAS (VD < 0)
 Current will flow through the diode due to minority carriers
 Current that exists in reverse bias condition is called reverse saturation current
(IS)
 The reverse saturation is very small usually in microamperes or nanoamperes
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Semiconductor Diode
FORWARD BIAS (VD > 0)
 There will be the flow of current due to majority carriers across the junction
 The increase in bias voltage will result in an exponential rise the current
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Forward Bias Voltage
 The point at which the diode changes from no-bias condition to forward bias
condition knee point
 This occurs when holes and electrons are provided with sufficient energy to
cross the junction
 The energy comes from external voltage source
 The forward bias voltage required for
 Gallium arsenide diode is 1.2 V
 Silicone diode is 0.7 V
 Germanium diode is 0.3 V
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Diode Resistance
 Due to the nonlinear characteristic curve of the diode its resistance vary with
the operating point
 The type of applied voltage or signal will determine the resistance level of the
diode
 There are three types of diode resistance
 DC or Static Resistance
 AC or Dynamic Resistance
 Average AC resistance
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Diode Resistance
DC (Static) Resistance
 Operating point will not change with time if an dc voltage is applied across the
diode
 Diode resistance can be found by
VD
RD =
ID
 Lower the current through the diode higher
will be the resistance
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Diode Resistance
AC (Dynamic) Resistance
 Application of a time varying signal will move the operating point
 The straight line drawn tangent to the curve at
Q-point will define dynamic resistance
Vd
rd =
I d
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Diode Resistance
Average AC Resistance
 Resistance determined by the straight
line between two intersections
established by maximum and
minimum value of the input voltage
rav
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Vd
=
I d
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 Determine the ac resistance at ID
= 2mA
 Determine the ac resistance at ID
= 25mA
 Determine the DC resistance at
both points
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Diode Models
 Ideally the diode conducts current in only one direction
 and acts like an open in the opposite direction
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Diode Models
Piecewise Linear model
 Approximate the non-linear characteristics of a diode with two linear segments
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Diode Models
Simplified model
 Average resistance is small enough and can be ignored to simplify the diode
model
 When the VD exceeds VK the diode will start to conduct
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Diode Models
Ideal diode model
 If VK is ignored in comparison the applied voltage the diode model can be
reduced to ideal model
 An ideal diode behaves as a switch and conducts in one direction
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Diode Models
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