Diode fundamentals
Diodes are electronic components that conduct current in only one direction. In function, they are similar to one-way valves. Diodes are manufactured using semiconductor materials such as silicon, germanium or selenium and are used as voltage regulators, signal rectifiers, oscillators and signal modulators / demodulators. Semiconductor diodes consist of a PN junction having two terminals, an anode (+) and a cathode (-). Current flows from the anode to the cathode within the diode. Diodes can be used as voltage regulators, tuning devices in RF tuned circuits, frequency multiplying devices in RF circuits, mixers in RF circuits, switching applications, or can be used to make logic decisions in digital circuits.
Diodes can be sub-divided into six major categories: general purpose diodes (or PN junction diodes), PIN diodes, Schottky barrier diodes, step-recovery diodes, varactor diodes and Zener diodes. PIN diodes are three-layer semiconductor diodes consisting of an intrinsic layer separating heavily doped P and N layers. The charge stored in the intrinsic layer in conjunction with other diode parameters determines the resistance of the diode at RF and microwave frequencies. This resistance typically ranges from kilohms to less than 1 ohm for a given diode. PIN diodes are typically used as switches or attenuator elements.
Schottky diodes in their simplest form consist of a metal layer that contacts a semiconductor element. The metal / semiconductor junctions exhibit rectifying behavior (i.e., the current passes through the structure more readily with one polarity than the other). Schottky diodes are used primarily in high frequency, fast-switching applications, and in many digital circuits to decrease switching times.
Step-recovery diodes employ graded doping where the doping level of the semiconductive materials is reduced as the PN junction is approached. This produces an abrupt turn-off time by allowing a very fast release of stored charge when switching from forward to reverse bias. It also allows a rapid re-establishment of forward current when switching from reverse to forward bias. These diodes are used in very high frequency (VHF) and fast switching applications.
Varactor diodes are p-n junction diodes that are designed to act as a voltage-controlled capacitance when operated under reverse bias. One characteristic of PN junctions is inherent capacitance. When the junction is reverse biased, increasing the applied voltage will cause the depletion region to widen, thus increasing the effective distance between the two “plates” of the capacitor and decreasing the effective capacitance. By adjusting the doping gradient and junction width, the capacitance range can be controlled and changes can be applied using reverse voltage. A four-to-one capacitance range is not unusual; a typical varactor diode (sometimes called a “varicap diode”) might vary from 60 picofarads (pf) at zero bias down to 15 pf at 20 volts. Very careful manufacturing can get a capacitance range of up to ten-to-one, although this seems at present to be a practical limit. Varactor diodes are used in electronic tuning systems, to eliminate the use of and need for moving parts. Zener diodes act like normal rectifiers until the voltage applied to them reaches a certain set point. At this point (known as the Zener voltage or avalanche voltage), the Zener diode begins conducting.