P-N Junction Diode Construction and Biasing

P–N Junction Diode Fundamentals

A p–n junction diode is constructed from a semiconductor crystal, typically silicon, although germanium and gallium arsenide are also utilized. Impurities are added to create two distinct regions. One side, called the n-type semiconductor, contains negative charge carriers (electrons). The other side, the p-type semiconductor, contains positive charge carriers (holes).

When these two materials (n-type and p-type) are joined, a momentary flow of electrons occurs from the n-side to the p-side. This results in a third region where no mobile charge carriers are present. This area is known as the depletion region due to the absence of free electrons and holes.

The diode's terminals are connected to the n-type and p-type regions. The boundary between these two regions is the p–n junction, which is where the diode's primary action occurs.

Formation of the PN Junction

When p-type and n-type semiconductors are placed in contact, the diffusion of majority charge carriers across the junction begins. This arrangement of p-type and n-type semiconductor material is called the PN junction. In this setup, the P region acts as one electrode, and the N region acts as the other. Thus, the PN junction is also referred to as a junction diode, semiconductor diode, or crystal diode.

Potential Barrier

Due to the immobile charge carriers in the depletion layer, a potential difference is established. This potential acts as a barrier to the diffusion of charge carriers across the junction and is called the potential barrier (V_b).

When the p-type and n-type semiconductors are in contact, charge carrier diffusion starts. On both sides of the junction, a layer of immobile charge carriers (electrons in the P-region and holes in the N-region) is formed. This layer is the depletion layer.

P-N Junction Biasing Conditions

P-N Junction as Forward Biased

When a battery is connected to the diode such that the p-section is connected to the positive pole and the n-section to the negative pole, the junction diode is said to be forward biased (as shown in the figure). If the forward bias voltage is greater than the potential barrier, the majority carriers move toward the junction and across it. The current that flows due to these majority carriers is called the forward current. A forward-biased p-n diode acts like a closed circuit.

Reverse Biased Condition

When the battery is connected to the junction diode with the p-section connected to the negative pole and the n-section to the positive pole, the junction is said to be reverse biased.

When reverse bias is applied, the majority charge carriers do not cross the junction. However, a small amount of current flows due to the motion of minority carriers. This current is called the reverse current, which increases with temperature.

Figure illustrating both forward and reverse diode biasing conditions would typically be inserted here.