PN Junction Diode

PN JUNCTION DIODE

^Ø   A p–n junction is formed by joining P-type and N-type semiconductors together in very close contact.

^Ø   The term junction refers to the boundary interface where the two regions of the semiconductor meet.

^Ø   Diode is a two-terminal electronic component that conducts electric current in only one direction.

^Ø   The crystal conducts conventional current in a direction from the p-type side (called the anode) to the n-type side (called the cathode), but not in the opposite direction.

Symbol of PN junction diode

1Biasing

“Biasing” is providing minimum external voltage and current to activate the device to study its characteristics.

There are two operating regions and two "biasing" conditions for the standard Junction Diode and they are:

^v Zero Bias:

When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is developed across a depletion layer.

(i) Forward Bias:

^Ø   When the positive terminal of a battery is connected to P-type semiconductor and negative terminal to N-type is known as forward bias of PN junction.

^Ø   The applied forward potential establishes an electric field opposite to the potential barrier. Therefore the potential barrier is reduced at the junction. As the potential barrier is very small (0.3V for Ge and 0.7V for Si),a small forward voltage is sufficient to completely eliminate the barrier potential, thus the junction resistance becomes zero.

^Ø   In otherwords, the applied positive potential repels the holes in the ‘P’ region so that the holes moves towards the junction and applied negative potential repels the electrons in the ‘N’ region towards the junction results in depletion region starts decreasing. When the applied potential is more than the internal barrier potential then the depletion region completely disappear, thus the junction resistance becomes zero.

^Ø   Once the potential barrier is eliminated by a forward voltage, j unction establishes the low resistance path for the entire circuit, thus a current flows in the circuit, it is called as forward current.

(ii)Reverse Bias:

^Ø   For reverse bias, the negative terminal is connected to P-type semiconductor and positive terminal to N type semiconductor.

^Ø   When reverse bias voltage is applied to the junction, all the majority carriers of ‘P’ region are attracted towards the negative terminal of the battery and the majority carriers of the N region attracted towards the positive terminal of the battery, hence the depletion region increases.

^Ø   The applied reverse voltage establishes an electric field which acts in the same direction of the potential barrier. Therefore, the resultant field at the junction is strengthened and the barrier width is increased. This increased potential barrier prevents the flow of charge carriers across the junction, results in a high resistance path.

^Ø   This process cannot continue indefinitely because after certain extent the junction break down occurs. As a result a small amount of current flows through it due to minority carriers. This current is known as “reverse saturation current”.

V-I characteristics of PN junction diode

Forward Bias:

^Ø   The application of a forward biasing voltage on the junction diode results in the depletion layer becoming very thin and narrow which represents a low impedance path through the junction thereby allowing high currents to flow.

^Ø   The point at which this sudden increase in current takes place is represented on the static I-V characteristics curve above as the "knee" point.

Reverse Bias:

^Ø   In Reverse biasing voltage a high resistance value to the PN junction and practically zero current flows through the junction diode with an increase in bias voltage.

^Ø   However, a very small leakage current does flow through the junction which can be measured in microamperes, (μA).

^Ø   One final point, if the reverse bias voltage Vr applied to the diode is increased to a

sufficiently  high enough value, it will cause the PN junction to overheat and fail due to

the avalanche effect around the junction.

^Ø   This may cause the diode to become shorted and will result in the flow of maximum circuit current, and this shown as a step downward slope in the reverse static characteristics curve below.