INTRODUCTION TO DIODES
The p-n Junction The p-n junction is a homojunction between a p-type and an n-type semiconductor. It acts as a diode, which can serve in electronics as a rectifier, logic gate, voltage regulator (Zener diode), switching or tuner (varactor diode); and in optoelectronics as a light-emitting diode (LED), laser diode, photodetector, or solar cell. In a relatively simplified view of semiconductor materials, we can envision a semiconductor as having two types of charge carriers-holes and free electrons which travel in opposite directions when the semiconductor is subject to an external electric field, giving rise to a net flow of current in the direction of the electric field. Figure 1 illustrates the concept.
A p-n junction consists of a p-type and n-type section of the same semiconductor materials in metallurgical contact. The p-type region has an abundance of holes (majority carriers) and a few mobile electrons (minority carriers); the n-type region has an abundance of mobile electrons and a few holes (Fig. 2). Both charge carriers are in continuous random thermal motion in all directions.
Fig. 2. Energy levels and carrier concentrations for a p-type and n-type semiconductor before contact.
When a section of p-type material and a section of n-type material are brought in contact to form a pn junction, a number of interesting properties arise. The pn junction forms the basis of the semiconductor diode. Electrons and holes diffuse from areas of high concentration toward areas of low concentration. Thus, electrons diffuse from the n-region to the p-region., leaving behind positively charged ionized donor atoms. In the p-region the electrons recombine with the abundant holes. Similarly, holes diffuse from the p-region into the n-region, leaving behind negatively charged ionized acceptor atoms. In the n-region the holes recombine with the abundant mobile electrons. This diffusion process does not continue indefinitely, however, because it causes a disruption of the charge balance in the two regions. As a result, a narrow region on both sides of the junction becomes nearly depleted of the mobile charge carriers. This region is called the depletion layer. It contains only the fixed charges (positive ions on the n-side and negative ions on the p-side). The thickness of the depletion layer in each region is inversely proportional to the concentration of dopants in the region. The net effect is that, the depletion region sees a separation of charge, giving rise to an electric field pointing from the n side to the p side. The fixed charges create an electric field in the depletion layer that points from the n-side towards the p-side of the junction. The charge separation therefore causes a contact potential (also known as built-in potential) to exist at the junction. This built-in field obstructs the diffusion of further mobile carriers through the junction region. An equilibrium condition is established that results in a net contact potential difference Vo between the two sides of the depletion layer, with the n-side exhibiting a higher potential than the p-side. This contact potential is typically on the order of a few tenths of a volt and depends on the material (about 0.5 to 0.7 V for silicon). The built-in potential provides a lower potential energy for an electron on the n-side relative to the p-side. As a result, the energy bands bend as shown in Fig. 3. In thermal equilibrium there is only a single Fermi function for the entire structure so that the Fermi levels in the p- and the n-regions must align. No net current flows across the junction. The currents associated with the diffusion and built-in field (drift current) cancel for both the electrons and holes.
Fig. 3. A p-n junction in the Thermal equilibrium at T > 0º K. The depletion-layer, energy-band diagram, and concentrations (on a logarithmic scale) of the mobile electrons n(x) and holes p(x) are shown...