The forward voltage is the voltage across a diode that occurs when it is conducting in the forward direction. The current and voltage characteristics of a diode are determined by its diffusion barrier (diffusion potential). The forward voltage, especially in the small current region, is proportional to the diffusion potential. (The forward voltage in the high-current region is also affected by series resistance as described in Section 3.6.) In the case of a pn junction, the diffusion potential is equal to a difference in potential between the lower edges of the conduction bands of the n-type and p-type semiconductors. In the case of a metal-semiconductor junction, the diffusion potential is equal to a difference between the work functions of the n-type (or p-type) semiconductor and the metal. Figure 3-5 shows unbiased pn and metal-semiconductor junctions. The diffusion potential of the pn junction changes with the dopant concentration, but only slightly. In contrast, the diffusion potential of a metal-semiconductor junction depends on the metal joined with the n-type semiconductor. Generally, the diffusion potential of the metal-semiconductor junction is lower than that of the pn junction. Figure 3.6 shows the VF–IF curves of metal-semiconductor junctions with different metals.
Figure 3-7 compares the VF–IF curves of the 1SS427 switching diode (VR = 80 V) and the 1SS416 SBD (VR = 30 V). The SBD provides a larger forward current than the switching diode at the same forward voltage.