Choose the right diode (Zener, Schottky, PIN, varactor, LED) for a job and recognize each symbol.
Syllabus: Diodes
Compendium articles on the ideas in this lesson, beyond what the exam asks.
Practise the 11 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".
Every diode symbol is a triangle plus a bar. Current flows with the triangle. The bar's shape (or what surrounds it) changes the meaning: a curve for a varactor, a Z for a Zener, an S for a Schottky, light arrows for an LED, a circle with a gate lead for thyristors.
Memory hookZ-bend = Zener. S-bend = Schottky. Curved = varactor.
A conducting diode drops a fixed voltage, its knee. For an LED the drop is set by the semiconductor's band gap. A Zener is built to break down in reverse at a set voltage, and the voltage stays nearly constant while current varies: a voltage reference. Too much current heats the junction, and excess junction temperature is what destroys a diode.
RememberZener: constant voltage with varying current. LED drop: band gap. Overcurrent kills by heat.
A Schottky barrier diode is a metal-semiconductor junction. It has a lower forward drop than silicon, so a power-supply rectifier wastes less power. It switches fast, so it works as a VHF/UHF mixer or detector. A point-contact diode (a fine wire on a crystal) has tiny capacitance: an RF detector.
RememberSchottky: metal-semiconductor, lower drop, fast. Point contact: RF detector.
A PIN diode has a wide undoped (intrinsic) layer between P and N, so its junction capacitance is low. Forward DC bias current sets its RF resistance: lots of current passes RF, none blocks it. Vary the bias and you control RF attenuation. Switch it fully and it is an RF switch.
RememberPIN: low capacitance. Forward DC bias current controls RF attenuation.
A varactor is a diode used with reverse bias. More reverse voltage widens the depletion region, which is like pulling capacitor plates apart: capacitance falls. Use it to tune oscillators and filters (for example in automatic frequency control).
RememberVaractor = voltage-controlled capacitor.