UNIT - I

Semiconductor Diodes
Semiconductors
• Semiconductors are materials whose electrical properties lie between
Conductors and Insulators
• Three semiconductors most frequently used in the construction of
electronic devices are Ge, Si, and GaAs
• Covalent Bonding
The bonding of atoms, strengthened by the sharing of electrons, is
called covalent bonding.
• Intrinsic Semiconductor
extremely pure semiconductor
• Extrinsic Semiconductor
impurities added to make the semiconductor more conductive
Semiconductors

• Doping
process by which an impurity is added to a semiconductor
Two types of doping - Donor doping and Acceptor doping
• Depending upon the type of impurity added the extrinsic
semiconductor may be classified as :
n type semiconductor and p type semiconductor
n- type and p-type semiconductors

• n-type
electrons are majority charge carriers - negatively charged particles
• p-type
holes are majority charge carriers - positively charged particles
 Introduction to
PN Junction Diode
PN Junction Diode

The defined direction of conventional current for the positive voltage

region matches the arrowhead in the diode symbol
A PN Junction Diode is one of the simplest semiconductor devices

around, which has the characteristic of passing current in one

direction only

Unlike a resistor, a diode does not behave linearly with respect to the

applied voltage

Diode has an exponential current-voltage relationship and therefore

cannot be described by simply using an equation such as Ohm’s law

Zero bias / Unbiased p-n Junction
 Zero bias / Unbiased p-n Junction

• The electrons and the holes in the region of the junction will combine, resulting in a lack of
free carriers in the region near the junction. This region near the junction which is empty of
free charge carriers is called Depletion Region.
• Barrier voltage or Barrier potential
Barrier Voltage

The size of the barrier voltage depends on:

• the amount of doping
• junction temperature
• type of material used
Depletion/Space Charge Region
Forward Biased p-n Junction

The current that exists under forward-bias conditions is called the

forward current and is represented by ID
Bias : Application of an external voltage across a device to extract a response
 Forward Biased p-n Junction

• Reduces the width of the Depletion region

• The current that exists under forward-bias conditions is called the
forward current and is represented by ID
 Reverse Biased p-n Junction

The current that exists under reverse-bias conditions is called the reverse
saturation current and is represented by Is
 Reverse Biased p-n Junction

• The Depletion region width increases

• The current that exists under reverse-bias conditions is called the
reverse saturation current and is represented by Is
Shockley’s Equation

Is is the reverse saturation current

VD is the applied voltage across the diode
n is an ideality factor, ranges between 1 and 2
VT is called the thermal voltage and is determined by

k is Boltzmann’s constant 1.38 x 10-23 J/K

TK is the absolute temperature in kelvins= 273 + the temperature in °C
q is the magnitude of electronic charge 1.6 x 10-19 C
Shockley’s Equation

For positive values of VD the first term of the above equation will grow
very quickly and totally overpower the effect of the second term

For negative values of VD the exponential term drops very quickly

below the level of I

At V = 0 V
Semiconductor diode characteristics
Breakdown region

Zener
Breakdown

and

Avalanche
Breakdown

Theoretically, Zener breakdown occurs at a lower voltage level then

avalanche breakdown in a diode.
Avalanche breakdown
• Reverse bias increases the electrical field across the depletion region

• Increases velocity of minority charge carrier

• Collision of charge carrier takes out the electrons from the atom and

further collide with the other atoms

• The process is continuous, and the electric field becomes so much

higher that the reverse current starts flowing in the PN junction

• After the breakdown, the junction cannot regain its original position

because the diode is completely burnt off

Zener breakdown
• Width of depletion region doping of semiconductor material

• Heavily doped material very thin depletion region

• Zener breakdown occurs in the very thin depletion region

• Strong electric field in the region of the junction disrupts the bonding

forces within the atom and “generate” carriers

• The depletion region regains it original position after the removal of

the reverse voltage

Peak inverse voltage
The maximum reverse-bias potential that can be applied

before entering the breakdown region is called the peak

inverse voltage (referred to simply as the PIV rating) or the

peak reverse voltage (denoted the PRV rating)

Modeling the semiconductor diode
Ideal Diode
An ideal diode is a diode that acts like a perfect conductor when voltage
is applied forward biased and like a perfect insulator when voltage is
applied reverse biased.
Ideal Diode Circuit Representation
Equivalent Circuit
Equivalent circuit is a combination of elements
properly chosen to best represent the actual
terminal characteristics of a device or system in a
particular operating region

Device symbol can be removed from a schematic

and the equivalent circuit inserted in its place
without severely affecting the actual behavior of
the system
What is Diode Approximation?
Diode approximation is a mathematical method used

to approximate the nonlinear behavior of real diodes to

enable calculations and circuit analysis

Diode Equivalent Models
• Shockley’s equation gives the exponential relationship
between current and voltage through the diode

• However every time while using diodes in a circuit, we do

not need to apply the exponential formula to find the values
of voltage or current

• Instead we can approximate the characteristic of diode by

replacing the diode in the circuit with its equivalent circuit
Diode Modeling
Diode Modeling
Although a diode is a non linear device, yet for practical
applications it is approximated to be a linear device

This approximation model in which a non linear device

behaves as a linear one is called the piecewise linear
model of a non linear device

This method is used to approximate the diode characteristic

curve as a series of linear segments
Piecewise-Linear Equivalent Circuit
Piecewise-Linear Equivalent Circuit

Real diode is modeled as 3 components in

series:
• an ideal diode
• a voltage source
• a resistor
Piecewise-Linear Equivalent Circuit
Piecewise-Linear Equivalent Circuit

• The battery Vk specifies that the voltage across the device must be
greater that the battery voltage (0.7 V for Si diode, 0.3 V for Ge diode)
before conduction through the device in the direction dictated by the
ideal diode is established
• When the diode is into conduction the resistance offered by the diode
is specified by the resistance rav
Simplified Equivalent Circuit
For most applications, the resistance rav is sufficiently small to be
ignored in comparison to the other elements of the network
Simplified Equivalent Circuit

• When the diode is forward-biased, it is equivalent to a closed switch in

series with a small equivalent voltage source equal to the barrier
potential (0.7 V for Si diode, 0.3 V for Ge diode)
• This equivalent voltage source represents the barrier potential that
must be exceeded by the bias voltage before the diode conducts
• When conducting, a voltage drop of 0.7 V appears across the Si diode
or 0.3 V appears across the Ge diodes
Ideal Equivalent Circuit
Ideal Semiconductor Diode
Practical vs ideal diode
 Difference between Ideal diodes and Practical diodes
Ideal diodes Practical diodes
Act as perfect conductor and Cannot act as perfect conductor and
perfect insulator perfect insulator
Draws no current when reverse Draws very low current when
biased reverse biased
Offers infinite resistance when Offers very high resistance when
reverse biased reverse biased
Cannot be manufactured Can be manufactured
Has zero cut-in voltage Has very low cut-in voltage
Has zero voltage drops across its Has very low voltage drop across it,
junction when forward biased when forward biased
 DC Load Line Analysis

Applying Kirchhoff’s voltage law in the

clockwise direction,

Simple Series Diode Configuration

Solving the circuit of the Fig.is all about finding the current and
voltage levels that will satisfy both the characteristics of the diode
and the chosen network parameters at the same time.
The two variables of the above Eq. VD and ID , are the same as the
diode characteristics axis variables.
This similarity permits plotting this Eq. on the same graph as the
diode characteristics.
With VD = 0 V in the circuit Eq.

With ID = 0 V in the circuit Eq.

• A straight line drawn between the two points will define the load line
as depicted in Fig.
• Change the level of R (the load) and the intersection on the vertical
axis will change.
• The result will be a change in the slope of the load line and a different
point of intersection between the load line and the device
characteristics.
 Drawing the load line and finding the point
of operation.

• We now have a load line defined by the network and a characteristic curve
defined by the device.
• The point of intersection between the two is the point of operation for this
circuit.
• We can determine the diode voltage VDQ and diode current IDQ as shown.
• The point of operation is usually called the quiescent point
(abbreviated as “ Q -point”).
The solution obtained at the intersection of the two curves is the same as
would be obtained by a simultaneous mathematical solution of

and

• Since the curve for a diode has nonlinear characteristics, the

mathematics involved would require the use of nonlinear techniques
• The load-line analysis described above provides a solution with a
minimum of effort and a “pictorial” description of why the levels of
solution for VDQ and IDQ were obtained.
Solving diode circuits using modeling
1. An ideal diode and a 5 Ω resistor are connected in series with a 15 V
power supply as shown in figure below. Calculate the current that
flows through the diode.

• The diode is forward biased and it is an ideal one

• Hence, it acts like a closed switch with no barrier voltage
• Current flowing through the diode can be calculated using Ohm’s law
V = IR
I =V/R = 15/5 = 3 A
2. Consider an ideal junction diode. Find the value of current flowing
through AB is

• The barrier potential of the diode is neglected as it is an ideal diode

• Current flowing through AB can be obtained by using Ohm’s law
I = V/R = 3 - (-7) / 1× 103
= 10mA
3. Assume the voltage source is 12 volts and the resistor is 2 kΩ. Obtain
the current in the circuit assuming ideal diode, silicon diode with zero
forward resistance and silicon diode with forward resistance of 10 Ω.

Using ideal diode approximation:

• Here we assume the diode is a closed switch. Consequently all of the
source voltage must drop across the single resistor
• I = E/R
• I = 12V/2kΩ
• I = 6mA
Using silicon diode with zero forward resistance:
• We include the knee/cutin voltage or diode barrier voltage
• I = (E − Vcutin)/R
• I = (12V − 0.7V)/2kΩ
• I = 5.65mA

Using silicon diode with forward resistance of 10Ω:

• We include both the knee voltage and forward resistance.
• I = (E − Vcutin)/(R + Rforward)
• I = (12V − 0.7V)/(2kΩ + 10Ω)
• I = 5.622mA
4. Determine the current and resistor voltage. Assume the power supply
is 20 volts, the diode is silicon and the resistor is 2 kΩ.

• Diode is reverse-biased
• The model for a reverse-biased diode is an open switch and the
circulating current in an open circuit is zero
• Therefore, the resistor voltage must also be zero
5. Determine the current and resistor voltages. Assume the power supply
is 9 volts, the diodes are silicon and R1 = 1 kΩ, R2 = 2 kΩ

• Both diodes are forward-biased

• I = (E − Vcutin1 − Vcutin2)/(R1 + R2)
• I = (9V − 0.7V − 0.7V)/(1kΩ + 2kΩ)
• I = 2.533mA
6. Determine the source current and resistor voltages for the circuit. Also
find the resistor voltages if the diode polarity is reversed. Assume the
power supply is 10 volts, the diode is silicon and the resistors are 1 kΩ
each.

• Diode is forward-biased
• Voltage across R2 = Vcutin = 0.7 V
• Voltage across R1 = E – Vcutin = E – voltage across R2 = 9.3 V
• Source current = I = (E − Vcutin)/R1 = (10V − 0.7V)/1kΩ = 9.3mA
• If the diode polarity is reversed it behaves as an open switch
• The circuit reduces to a simple 1:1 voltage divider, each resistor
dropping half of the supply, or 5 volts each