There are two types of semiconductors, distinguished by their electrical properties: n (negative) and p (positive). The junction of these two materials (pn junction) is called a diode. A pn junction for the LED chip is formed by growing epitaxial layers (thin films) of n- and p-type semiconductors on substrates through liquid phases or gaseous phases (Figure 2). n-type semiconductors contain a high number of negatively charged "free electrons," whereas p-type semiconductors contain a high number of positively charged "free holes" (positive holes). Each type of semiconductor has a specific energy profile (bandgap), which varies among individual semiconductors. When forward-biased, the energy difference between the n- and p-type semiconductors in LED-chips drops, and thus the electrons of the n-type will move to bind to the free (positive) holes of the p-type, causing the emission of light energy, i.e., the LED's luminous phenomenon (Figure 3).