Small-Signal Diodes

A small-signal diode is a two-terminal silicon PN junction that can rectify and clip signals. Rated to handle up to 1 W, these diodes are made by growing an N-type region on a P-type wafer so that there is a direct interface or junction between the two different materials. The wafer is then diced and packaged with terminals attached to both sides of the die. The P-type material is the anode and the N-type material is the cathode, as shown in the section view Fig.1a. The P-type anode contains a surplus of “holes,” or vacant sites that can be filled by electrons to conduct current, and the N-type cathode contains a surplus of electrons. The schematic symbol for a diode is shown in Fig.1b. The arrowhead indicates the direction of conventional current flow, but this is opposite to electron flow, indicated by the arrow pointed in the opposite direction.



Figure 1 PN diode: (a) functional diagram, and (b) schematic symbol.

If a positive voltage is applied to the anode and a negative voltage is applied to the cathode, or it is connected to ground, the diode is forward biased. Electrons flow from the cathode across the PN junction to the anode, but conventional current is considered to flow in the opposite direction. However, if a negative voltage is applied to the anode and a positive voltage is applied to the cathode, or it is connected to ground, the diode is reverse or back biased, as shown in Fig. 2. Under these conditions there will be little or no electron flow across the PN junction. A reverse-biased diode effectively becomes an insulator with resistance measurable in megohms because of the expansion of the highly resistive depletion region that forms around the PN junction.



Figure 2 Depletion region of PN junction diode.