basic electonics and electical, Summaries of Electrical and Electronics Engineering

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PN Junction Diode and its

Characteristics

Presented By:

Dr. Pravin R. Prajapati Associate Professor Electronics & Communication Department A. D. Patel Institute of Technology New V V Nagar

PN JUNCTIONDIODE

The formation of depletion layer in PN

junction.

Principle

• If the p-region (anode) is connected to the positive terminal of the external DC source and n-side (cathode) is connected to the negative terminal of the DC source then the biasing is said to be “forward biasing”.
• Due to the negative terminal of external source connected to the n-region, free electrons from n-side are pushed towards the p-side. Similarly the positive end of the supply will push holes from p-side towards the n-side.

With increase in the external supply voltage V, more and more number of holes (p- side) and electrons (n-side) start travelling towards the junction as shown in figure.

• The holes will start converting the negative ions into neutral atoms and the electrons will convert the positive ions into neutral atoms. As a result of this, the width of depletion region will reduce.
• Due to reduction in the depletion region width, the barrier potential will also reduce. Eventually at a particular value of V the depletion region will collapse. Now there is absolutely no opposition to the flow of electrons and holes. Hence a large number of electrons and holes (majority carriers) can cross the junction under the influence of externally connected DC voltage.

PN Junction Diode

7

i)Reverse saturation current: In reverse bias condition there will be negligible amount of current that will flow through the device due to minority carrier which is called as reverse saturation current. ii. Knee voltage: The applied forward voltage at which the PN junctions start conducting is called the cut-in voltage. It is also known as knee voltage (Vk or Vz). The value of cut-in voltage is 0.7 V for Silicon and 0.3 V for Germanium PN junction diodes. iii. Depletion layer: The region in PN junction which comprises of immobile ions is called depletion region. iv. Static resistance of diode: The resistance of a diode at the operating point can be obtained by taking the ratio of VF and IF. The resistance offered by the diode to the forward DC operating conditions is called as “DC or static resistance”.

Avalanche break down in P- N junction

diode.

• They knock out the electrons from the covalent bonds of a PN junction.
• These additional carriers pick up the energy from the applied voltage and generate still more carriers. As a result of this, the reverse current through the PN junction increase.
• The cumulative process of carrier generation (i.e. multiplication) is known as avalanche breakdown.
• Under breakdown condition, the reverse current is no longer limited by the PN junction. But it can be limited only by the circuit components.
• The value of breakdown voltage depends upon the degree of doping in PN junction.

The forward and reverse characteristics of PN junction diode

Region B to C:

• As soon as the forward voltage equals the cut in voltage, current through the diode increase suddenly. The nature of this current is exponential. The large forward current in the region B-C of the forward characteristics is limited by connecting a resistor R in series with the diode. Forward current is of the order of a few mA.
• The forward current is a conventional current that flows from anode to cathode. Therefore it is considered to be a positive current, and the forward characteristics appears in the first quadrant.

Reverse characteristics of a Diode.

• Current flowing through a diode in the reverse biased state is the

reverse saturation current which flows due to the minority carriers.

• Therefore it is treated as a negative current. Hence the reverse

characteristics appears equal to Io if the temperature is constant.

• As the reverse voltage is increased, the reverse saturation current

remains constant equal to 10 if the temperature is constant. This is

because, reverse saturation current does not depend on reverse

voltage but it depends only on temperature.

• But as the reverse voltage reaches the breakdown voltage value. A

large current flows through the diode, due to the reasons discussed

earlier.

• Thus we define the reverse breakdown voltage of a pn junction

diode as the reverse voltage at which breakdown takes place and

a large reverse current starts flowing through the diode.

• Operation in the breakdown region should be avoided because

the diode may be damaged due to excessive power dissipation.

• Typically the reverse breakdown voltage for p-n junction diode is

in range of 50 to 100 volts.