Crystal diodes have been popular before the thermionic diode, thermionic and solid state diodes developed in parallel. The principle of thermionic diodes was discovered by Frederick Guthrie in 1873.
Semiconductor Diode
Diodes which are currently based on semiconductor pn junction. In a pn diode, conventional current can flow from p-type side (the anode) to the n-type side (the cathode), but can not flow in reverse. Another type of semiconductor diodes, Schottky diodes, formed from the relationship between metals and semiconductors with a pn junction.
Diode Pn is a connection material that serves primarily as a rectifier. P-type material into the anode side while the n-type material into cathode. Depending on the polarity of the voltage given to him, the diode can act as a switch is closed (if the anode have a positive voltage while the cathode get a negative voltage) and is valid as a switch is open (when the anode have a negative voltage while the cathode get a positive voltage). These conditions occur only at the conceptual ideal diode. In diodes factual (real), it is necessary voltage greater than 0.7 V (for diodes made of silicon material) at the anode to the cathode to the diode can deliver an electrical current.Voltage of 0.7 V is called a voltage barrier (barrier voltage). Diodes made from Germanium has a beam voltage of about 0.3 V.
• Light-emitting diode or LED is a diode that emits light when a given forward bias. LEDs are made from semiconductors such as gallium arsenide mixed phosphide (GaAsP), gallium phosphide (GaP), gallium indium phosphide (GaInP), gallium arsenide aluminum (GaAlAs), etc..
• Photo diode (fotovoltaic) is used to convert light energy into electrical energy in the same direction
• Diode laser is used to generate low-level laser light, the way it works is similar LED
• Zenner diodes used for voltage regulation.
Diode light, or better known as LED (light-emitting diode) is a semiconductor that emits incoherent monochromatic light when given a forward voltage.
Color produced depends on the semiconducting material used, and can be near ultraviolet or near infrared.
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Photoresistors made of semiconductors that have very high resistance. If the light is about to have a high enough frequency, photons absorbed by the semiconductor will cause the electrons have enough energy to jump into the conduction band. 
