Semiconductor If a valence Electron acquires sufficient kinetic energy to break its covalent bond and fills the void created by a hole then a vacancy, or hole will be created in the covalent bond that released the electron Hence there is a transfer of holes to the left and electrons to the right

1. Semiconductor Diodes

2. Zener Diode
 The Zener diode is a heavily doped diode which, as a result of doping, has
a very narrow depletion region. This allows the diode to be operated in the
reverse biased region of the characteristic curve without damaging the PN
junction.
 “Zener Effect”: The area of Zener diode operation (<5V) where the Diode
maintains a constant voltage output while operating reverse biased.
 “Avalanche Effect”: >5V applied to the diode while reverse biased which
tends to cause the diode to eventually breakdown due to heat generation
within the lattice structure of the crystal.
 Because of its higher temperature and current capability, silicon is usually
preferred in manufacture of Zener Diodes
 Zener Diodes provide a stable reference voltage for use in power supplies,
voltmeter & other instruments, voltage regulators.

3. Zener Diode
Zener diode equivalent circuit models and the characteristic curve illustrating ZZ

4. Comparison of Semiconductors
 The general shape is similar
 Point of vertical rise is different
 Germanium is closest to the vertical axis
 Center of the Knee of the curve is 0.3 v for Ge and
0.7 for Si
 Reverse Saturation Current is 10pA for Si and 1 µA
for Ge
 The breakdown voltage for Si is typically between
50v and 1kV with maximum around 20 kV
 The breakdown voltage for Ge is typically less than
100V with maximum around 400 V
Comparison of Si and Ge diodes

5. Practical Diode (Si Vs Ge)
Narrow temperature range
(lower than 1000C)
Wider temperature range
(up to 2000C)
Lower current ratingHigher current rating
Lower PIV ( 400V)Higher PIV ( 1000V)
Lower forward-bias voltage
(0.3V)
Higher forward-bias
voltage (0.7V)
GermaniumSilicon

6. Temperature Effects
 In the forward bias region the
characteristics of a Si diode shift to the left
at a rate of 2.5 mv per centigrade degree
increase in temperature
 The reverse saturation current Is will just
about double in magnitude for every 10°C
increase in temperature.
 The reverse breakdown voltage of a
semiconductor diode will increase or
decrease with temperature depending on
the zener potential
 At room temperature Si and GaAs have
relatively small reverse saturation current
Variation in Si diode characteristics
with temperature change

7. Ideal VS Practical
Ideal Diode
Characteristics of Ideal Diode Characteristics of Practical Diode
Practical Diode

8. Ideal VS Practical
 Semiconductor diode behaves in a manner similar to a
mechanical switch where it can control whether current will flow
between it’s two terminals.
 However, the diode is different from a mechanical switch in the
sense that when switch is closed it will only permit current to
flow in one direction.
 From the characteristics curve it is evident that there is a
resistance associated with the diode which is greater than 0 Ω
 However, if the resistance is small enough compared to other
resistors of the network, it’s assumed that resistance of the diode
is 0 Ω

9. •Semiconductors act differently to DC and AC currents.
•There are 3 types of resistances:
•DC or Static Resistance
• AC or Dynamic Resistance
• Average AC Resistance
Resistance Levels

10. DC or Static Resistance
The application of a dc voltage to a circuit containing a
semiconductor diode will result in an operating point on the
characteristic curve that will not change with time. The
resistance of a diode at a particular operating point is called the
dc or static resistance diode. It can be determined using equation
(1.1):
(1.1)RD = VD/ID
In general, the lower the current through a diode the higher the
dc resistance level.

11. DC or Static Resistance
a) At ID = 2 mA , VD = 0.5 V and thus
RD = 250 Ω
b) At ID = 20 mA , VD = 0.8 V and thus
RD = 40 Ω
c) At VD = - 10 V , ID = - IS = -1 µA
and thus RD = 10 MΩ
Determine the DC Resistance levels for the diode of following figure at ID = 2 mA,
ID = 20 mA, VD = - 10 V

12. AC or Dynamic Resistance
• Static resistance is using dc input. If the input is sinusoidal the
scenario will be changed.
• The varying input will move instantaneous operating point UP
and DOWN of a region.
• Thus the specific changes in current and voltage is obtained. It
can be determined using equation
rd = ∆VD/ ∆ID

13. Continued...
Defining the dynamic or ac resistance. Determining the ac resistance at a Q-point.
D
D
D
V
r
I



• With no applied varying signal, the point of operation would be the Q-point
•A straight line drawn tangent to the curve through the Q-point will define a particular
change in voltage and current that can be used to determine the ac or dynamic resistance
for this region of the diode characteristics
•An effort should be made to keep the change in voltage and current as small as
possible and equidistant to either side of the Q-point

14. Continued...
 In general, the lower the Q-point of operation (smaller current or lower
voltage) the higher the ac resistance
 For the characteristics of Figure below:
 (a) Determine the ac resistance at ID = 2 mA.
 (b) Determine the ac resistance at ID = 25 mA.
 (c) Compare the results of parts (a) and (b) to the dc resistances at each current level
(a) rd =27.5Ω
(b) rd = 2 Ω
(c) RD = 350 Ω >> 27.5 Ω
31.62 Ω >> 2 Ω

15. Continued...
 The derivative of a function at a point is equal to the slope of the tangent
line drawn at that point
26
D
D
mV
r
I

• The dynamic resistance can be found simply by substituting the quiescent value of
the diode current into the equation.
• There is no need to have the characteristics available or to worry about sketching
tangent lines
• However, this equation is accurate only for values of ID in the vertical-rise section
of the curve. For small values of ID below the knee of the curve, this becomes
inappropriate
• We sometimes consider the resistance of the semiconductor material itself (Refer
to the book of Boylestad !!!)

16. Average AC Resistance
pt a pt
D
av
D
V
r
I



• If the input signal is sufficiently large to produce a broad swing, the resistance
associated with the device for this region is called the average ac resistance.
• It can be determined by a straight line drawn between the two intersections
established by the maximum and minimum values of input voltage.

17. Fig: Defining the piecewise-linear equivalent
circuit using straight-line segments to
approximate the characteristic curve.
Diode Equivalent Circuits
Components of the piecewise-linear equivalent circuit.
• The ideal diode establishes that there is only one
direction of conduction through the device and a
reverse bias condition will result in the open circuit
state
• The average ac resistance rav defines the resistance
level of the device when it is in the ‘on’ state
• The battery VT which opposes the conduction
direction appears in the circuit to establish that the Si
semiconductor does not reach the conduction state until
VD reaches 0.7 with a forward bias

18. Continued...
Simplified equivalent circuit for the silicon semiconductor diode.
Ideal diode and its characteristics.

19. Transition and diffusion capacitance versus applied bias for a
silicon diode.
Including the effect of the transition or diffusion
capacitance on the diode.
Transition and Diffusion Capacitance
Electronic devices are inherently sensitive to very high
frequencies.
Most shunt capacitive effects can be ignored at lower
frequencies because the reactance XC =1/2πfC is very
large
This, however, cannot be ignored at very high
frequencies. XC will become sufficiently small due to the
high value of f to introduce a low-reactance “shorting”
path.
In the p-n semiconductor diode, there are two capacitive
effects to be considered. Both types of capacitance are
present in the forward- and reverse-bias regions, but one
so outweighs the other in each region that we consider
the effects of only one in each region.
In the reverse-bias region we have the transition- or
depletion-region capacitance (CT), while in the
forward-bias region we have the diffusion (CD) or
storage capacitance.

20. Continued...
Capacitance of a parallel plate capacitor, C = εA/d
Here, ε = permittivity of the dielectric (insulator) between the plates of
area A separated by a distance d
• In the reverse-bias region there is a depletion region (free of carriers) that
behaves essentially like an insulator between the layers of opposite charge.
• Since the depletion width (d) will increase with increased reverse-bias
potential, the resulting transition capacitance will decrease.
• Although the transition capacitance effect will also be present in the
forward-bias region, it is overshadowed by a capacitance effect directly
dependent on the rate at which charge is injected into the regions just outside
the depletion region.
• In this case the charge carriers will reduce the depletion region and hence
will result in an increased levels of diffusion capacitance.

21. Reverse Recovery Time
Reverse Recovery Time
ts = Storage time
tt = Transition Interval
Reverse Recovery Time, trr = ts + tt

22. Diode Specification Sheet
 Data on specific semiconductor devices are normally provided by the manufacturer
in one of two forms. Most frequently, it is a very brief description limited to perhaps
one page. It includes:
1. The forward voltage VF (at a specified current and temperature)
2. The maximum forward current IF (at a specified temperature)
3. The reverse saturation current IR (at a specified voltage and temperature)
4. The reverse-voltage rating [PIV or PRV or V(BR), where BR comes from the term
“breakdown” (at a specified temperature)]
5. The maximum power dissipation level at a particular temperature
6. Capacitance levels
7. Reverse recovery time trr
8. Operating temperature range

23. Continued...

24. Light Emitting Diode (LED)
(a) Process of electroluminescence in the LED
(b) graphic symbol.
The light-emitting diode (LED) is a diode that will give off
visible light when it is energized
In any forward-biased p-n junction there is, a recombination of
holes and electrons, within the structure and primarily close to
the junction
This recombination requires that the energy possessed by the
unbound free electron be transferred to another state.
In all semiconductor p-n junctions some of this energy will be
given off as heat and some in the form of photons.
In silicon and germanium the greater percentage is given up in
the form of heat and the emitted light is insignificant.
In other materials, such as gallium arsenide phosphide
(GaAsP) or gallium phosphide (GaP), the number of photons
of light energy emitted is sufficient to create a very visible
light source.
The process of giving off light by applying an electrical source
of energy is called electroluminescence.

25. Light Emitting Diode (LED)
Litronix segment display
Since the LED is a p-n junction device, it will
have a forward-biased characteristic similar
to the diode response curves
LED displays are available today in many
different sizes and shapes. The light emitting
region is available in lengths from 0.1 to 1 in.
Numbers can be created by segments. By
applying a forward bias to the proper p-type
material segment, any number from 0 to 9
can be displayed.

26. End of Chapter