PN Junction Diode

PN JUNCTION DIODE

A PN Junction diode or semiconductor diode is formed when n‒type and p‒type semiconductor crystals are joined together. However mechanically joining two pieces of semiconductor creates all sorts of problems structurally, electrically and chemically at the interface, making it unsuitable for device application.

In practice, adding acceptor impurities to an wafer or donor to a p‒type wafer forms a p‒n junction.

As shows in fig 1.26, the N‒type material has high concentration of free electrons while P‒type material has high concentration of holes. Therefore at the junction there is a tendency for free electrons to diffuse over the P‒side and holes to N‒side. This process is called diffusion. Electrons coming into P‒material combine with the holes of the acceptor atoms and creating a negatively charged layer of unneutralized acceptor ions fixed in space. Similarly the holes moving into the N‒material combine with free electrons of the donor atoms and a positively charged layer of unneutralised donor ions is formed in the N‒side near the junction. Thus the narrow width of fixed positive charges on the n‒side of the junction and fixed negative charges on the p‒side of the junction appear as shown in fig 1.26. This region is known as the depletion region (or space charge region or transition region). It creates a built‒in‒potential or barrier potential v_b across the junction. The barrier potential v_b is 0.3 V for germanium and 0.7 V for silicon.

Diode Under Forward Bias Condition

A forward bias of PN Junction diode is established by applying positive potential of a battery (voltage v) to P‒side and its negative is connected to N‒side, as shown in fig 1.27.

Operation

Under the forward bias condition the applied positive potential repels the holes in P‒type region so that the hole move towards the junction and the applied negative potential repels the electrons in the N‒type region and the electrons move towards the junction. Hence the effective barrier height decreases from V_b to (V_b‒V) with consequent reduction in the width of the depletion region. There is a great enhancement in the probability of the majority carriers moving across the junction resulting in an appreciable current flow. The conventional current flows from the positive terminal of the battery across the P to N‒junction and to the negative terminal of the battery and is called the forward current (I_f).

V‒I characteristic under forward bias

Under forward bias condition, the V‒I characteristics of a PN Junction diode as shown in fig. 1.28. As the forward voltage (V_F) is increased, for V_F< V_b, the forward current I_F is almost zero because the potential barrier prevents the electrons from N‒region and holes from P‒region to flow across the depletion region in the opposite direction.

For PN Junction diode, the forward current is very small upto V_F=V_γ=0.7 V for silicon and 0.3 V for Germanium called cut‒in or transition or threshold voltage, beyond which it increases rapidly with diode voltage.

For V_F> V_b, the potential barrier at the junction completely disappears and hence the holes across the junction from P‒type to N‒type and the electrons cross the junction in the opposite direction, resulting in relatively large current flow in the external circuit.

Diode Under Reverse Bias Condition

If an external voltage is applied to P‒N junction such that positive terminal of the battery is connected to N‒type region and the negative terminal of battery is connected to the P‒type region, a reverse bias condition is established.

Operation

Under reverse bias condition, the majority electrons which move away from the junction are attracted by the positive potential at the N‒side and similarly the majority holes are attracted towards the negative polarity of the battery connected to the P‒side.

As a result, the number of uncovered positive ions in the depletion region of the N‒type material and uncovered negative ions in the P‒type material will increase. The net‒effect is widening of the depletion region and the barrier potential rises. The majority carriers cannot overcome this barrier energy and their flow is reduced to zero. The minority carriers however will cross the junction at reverse bias voltage and contribute to reverse current. The minority carrier current reaches its saturation value at even very low reverse bias voltage because of the low concentration of minority carriers. The current that exists under reverse‒bias condition is called the reverse saturation current. The magnitude of reverse saturation current mainly depends upon junction temperature because the major source of minority carriers is thermally broken covalent bonds.

VI Characteristic under Reverse Bias

For large applied revere bias voltage, the free electrons from the N‒type region moving towards the positive terminal of the battery acquire sufficient energy to mover with high velocity to dislodge valence electrons from semiconductor atoms in the crystal. These newly liberated electrons, in turn acquires sufficient energy to dislodge other parent electrons. Thus a large number of free electrons are formed which is called as an avalanche of free electrons. This leads to the breakdown of the junction leading to very large reverse current. The reverse voltage at which the junction breakdown occurs is known as breakdown voltage.

Effect on capacitance of a PN junction diode

Forward Biased

Diffusion or storage capacitance, It arises due to the arrangement of minority carrier density. Its value is much higher than the depletion layer capacitance. The typical value of diffusion capacitance C_D is 0.02 μF which is 5000 times more than the depletion layer capacitance. The value of C_D is a function of frequency. It is negligible for a reverse biased PN junction.

Reverse Biased

Depletion layer capacitance. Its is value is very high for a reverse biased PN junction. The value of C_T depletion layer capacitance can be controlled by varying the applied reverse

voltage, because C_T depends upon the nature of a PN junction.