Answer: Schottky diode: 

A Schottky diode, also known as a hot carrier diode, is a semiconductor diode which has a low forward voltage drop and a very fast switching action. There is a small voltage drop across the diode terminals when current flows through a diode. A normal diode will have a voltage drop between 0.6 to 1.7 volts, while a Schottky diode voltage drop is usually between 0.15 and 0.45 volts. This lower voltage drop provides better system efficiency and higher switching speed. In a Schottky diode, a semiconductor–metal junction is formed between a semiconductor and a metal, thus creating a Schottky barrier. The N-type semiconductor acts as the cathode and the metal side acts as the anode of the diode. 

The forward voltage drop ,reverse-recovery time , and junction capacitance  of Schottky diodes are closer to ideal than the average “rectifying” diode. This makes them well suited for high-frequency applications. Unfortunately, though, Schottky diodes typically have lower forward current  and reverse voltage  ratings than rectifying diodes and are thus unsuitable for applications involving substantial amounts of power. Though they are used in low voltage switching regulator power supplies. Schottky diodes have a shorter reverse recovery time.

 The Schottky diode is used in a wide variety of applications :

• It can naturally be used as a general-purpose rectifier.
• However, in terms of RF applications, it is particularly useful because of its high switching speed and high-frequency capability.
• Schottky diodes are similarly very good as RF detectors as their low capacitance and forward-voltage drop enable them to detect signals which an ordinary PN junction would not see.
• Schottky diode has a high-current density and low forward-voltage drop. As a result, Schottky diodes are widely used in power supplies.
• By using these diodes, less power is wasted, making the supply more efficient. Also this increase in efficiency means that less heat has to be dissipated, and smaller heatsinks may be able to be incorporated in the design.
• To produce a low output the transistor is driven hard on, and in this situation the base-collector junction of the diode is forward biased. When the Schottky diode is present, this takes most of the current and allows the turn-off time of the transistor to be greatly reduced, thereby improving the speed of the circuit.