Electron Devices

1. PN Junction Diode

2. Reverse Breakdown
• As the magnitude of the reverse-bias voltage V is increased, a value is reached at which a
very large reverse current flows.
• When V reaches the value VZ , the dramatic increase in reverse current is accompanied by
a very small increase in the reverse voltage; that is, the reverse voltage across the junction
remains very close to the value VZ .
• The phenomenon that occurs at V = VZ is known as junction breakdown.
• There are two possible mechanisms for PN junction breakdown:
• the Zener effect and the Avalanche effect.
• If a PN junction breaks down with a breakdown voltage VZ < 5 V, the breakdown mechanism
is usually the Zener effect.
• Avalanche breakdown occurs when VZ is greater than approximately 7 V.
• For junctions that break down between 5 V and 7 V, the breakdown mechanism can be
either the zener or the avalanche effect or a combination of the two.

3. Reverse Breakdown
• Zener breakdown occurs when the electric field in the depletion layer increases to
the point of breaking covalent bonds and generating electron–hole pairs.
• The electrons generated in this way will be swept by the electric field into the n side
and the holes into the p side.
• Thus, these electrons and holes constitute a reverse current across the junction.
Once the zener effect starts, a large number of carriers can be generated, with a
negligible increase in the junction voltage
• The other breakdown mechanism, avalanche breakdown, occurs when the minority
carriers that cross the depletion region under the influence of the electric field gain
sufficient kinetic energy to be able to break covalent bonds in atoms with which they
collide.
• The carriers liberated by this process may have sufficiently high energy to be able to
cause other carriers to be liberated in another ionizing collision.
• This process keeps repeating in the fashion of an avalanche, with the result that
many carriers are created to increase the current flow.

4. Reverse Breakdown
• The reverse breakdown voltage of a semiconductor diode will increase or
decrease with temperature.
• However, if the initial breakdown voltage is less than 5 V, the breakdown
voltage may decrease with temperature.
• The breakdown voltage for a diode depends on the doping level, which the
manufacturer sets, depending on the type of diode.
• A typical rectifier diode (the most widely used type) has a breakdown voltage
of greater than 50 V.
• Some specialized diodes have a breakdown voltage that is only 5 V.
• The Zener breakdown voltage decreases as the temperature increases,
creating a negative temperature coefficient (TC). The avalanche
breakdown voltage increases with temperature (positive TC)

5. Reverse Breakdown in Diode
Zener Breakdown Avalanche Breakdown

6. MODE OF OPERATION OF IDEAL DIODE

7. MODE OF OPERATION OF IDEAL DIODE

8. MODE OF OPERATION OF PRACTICAL DIODE

9. COMPLETE MODEL OF PRACTICAL DIODE

11. Resistance in Diode
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 the diode at the operating point can be found simply by finding the
corresponding levels of VD and ID as shown in Figure and applying the following equation:
• The dc resistance of a diode is independent of the shape of the characteristic in the region
surrounding the point of interest.

12. Resistance in Diode
AC or Dynamic Resistance
• The varying input will move the instantaneous operating point up and
down a region of the characteristics and thus defines a specific change in
current and voltage as shown in Figure.
• The designation Q-point is derived from the word quiescent , which
means “still or unvarying.”

13. Resistance in Diode
AC or Dynamic Resistance
• A straight line drawn tangent to the curve through the Q -point as shown in
Figure 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.
• In equation form,
• In general, therefore, the lower the Q-point of operation (smaller current or
lower voltage), the higher is the ac resistance.
• The derivative of a function at a point is equal to the slope of the tangent line
drawn at that point.

14. Resistance in Diode

15. • Average AC Resistance
• If the input signal is sufficiently large to produce a broad swing such as indicated in Figure,
the resistance associated with the device for this region is called the average ac resistance.
• The average ac resistance is, by definition, the resistance determined by a straight line drawn
between the two intersections established by the maximum and minimum values of input
voltage. In equation form
Resistance in Diode

16. Diode equivalent circuit
• An 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.
Piecewise-Linear Equivalent Circuit
• One technique for obtaining an equivalent circuit
for a diode is to approximate the characteristics of
the device by straight-line segments, as shown in
Figure.
• The resulting equivalent circuit is called a
piecewise-linear equivalent circuit.
• It should be obvious from Figure that the straight-
line segments do not result in an exact duplication
of the actual characteristics, especially in the knee
region

18. Theory of PN Junction Diode

19. Total Current in a PN junction Diode