21-11-2012, 02:33 PM
Schottky diode
Schottky diode.docx (Size: 84.59 KB / Downloads: 29)
Reverse recovery time
The most important difference between p-n and Schottky diode is reverse recovery time, when the diode switches from non-conducting to conducting state and vice versa. Where in a p-n diode the reverse recovery time can be in the order of hundreds of nanoseconds and less than 100 ns for fast diodes, Schottky diodes do not have a recovery time, as there is nothing to recover from (i.e. no charge carrier depletion region at the junction). The switching time is ~100 ps for the small signal diodes, and up to tens of nanoseconds for special high-capacity power diodes. With p-n junction switching, there is also a reverse recovery current, which in high-power semiconductors brings increased EMI noise. With Schottky diodes switching essentially instantly with only slight capacitive loading, this is much less of a concern.
It is often said that the Schottky diode is a "majority carrier" semiconductor device. This means that if the semiconductor body is doped n-type, only the n-type carriers (mobile electrons) play a significant role in normal operation of the device. The majority carriers are quickly injected into the conduction band of the metal contact on the other side of the diode to become free moving electrons. Therefore no slow, random recombination of n- and p- type carriers is involved, so that this diode can cease conduction faster than an ordinary p-n rectifier diode. This property in turn allows a smaller device area, which also makes for a faster transition. This is another reason why Schottky diodes are useful in switch-mode power converters; the high speed of the diode means that the circuit can operate at frequencies in the range 200 kHz to 2 MHz, allowing the use of small inductors and capacitors with greater efficiency than would be possible with other diode types. Small-area Schottky diodes are the heart of RF detectors and mixers, which often operate up to 50 GHz.
Limitations
The most evident limitations of Schottky diodes are the relatively low reverse voltage rating for silicon-metal Schottky diodes, 50 V and below, and a relatively high reverse leakage current. Diode designs have been improving over time. Voltage ratings now can reach 200 V. Reverse leakage current, because it increases with temperature, leads to a thermal instability issue. This often limits the useful reverse voltage to well below the actual rating.
[edit] Silicon carbide Schottky diode
Since 2001 another important invention was presented by CREE (NC, USA): a silicon carbide (SiC) Schottky diode. SiC Schottky diodes have about 40 times lower reverse leakage current compared to silicon Schottky diodes. As of 2011, they are available from several manufacturers in variants up to 1700 V.
Silicon carbide has a high thermal conductivity and temperature has little influence on its switching and thermal characteristics. With special packaging it is possible to have operating junction temperatures of over 500 K, which allows passive radiation cooling in aerospace applications.
Applications
Voltage clamping
While standard silicon diodes have a forward voltage drop of about 0.6 volts and germanium diodes 0.3 volts, Schottky diodes' voltage drop at forward biases of around 1 mA is in the range 0.15 V to 0.46 V (see the 1N5817[2] and 1N5711 datasheets found online at manufacturer's websites), which makes them useful in voltage clamping applications and prevention of transistor saturation. This is due to the higher current density in the Schottky diode.
[edit] Reverse current / discharge protection
Schottky diodes are used in photovoltaic (PV) systems to prevent a reverse current flowing through the PV modules. For instance, they are used in stand-alone ("off-grid") systems to prevent batteries from discharging through the solar cells at night, and in grid-connected systems with multiple strings connected in parallel, in order to prevent reverse current flowing from adjacent strings through shaded strings if the bypass diodes have failed.
Power supply
They are also used as rectifiers in switched-mode power supplies; the low forward voltage and fast recovery time leads to increased efficiency.
Schottky diodes can be used in power supply "OR"ing circuits in products that have both an internal battery and a mains adapter input, or similar. However, the high reverse leakage current presents a problem in this case, as any high-impedance voltage sensing circuit (e.g. monitoring the battery voltage or detecting whether a mains adaptor is present) will see the voltage from the other power source through the diode leakage.
Designation
Commonly encountered Schottky diodes include the 1N5817 series (1 Ampere) rectifiers. Schottky metal-semiconductor junctions are featured in the successors to the 7400 TTL family of logic devices, the 74S, 74LS and 74ALS series, where they are employed as clamps in parallel with the collector-base junctions of the bipolar transistors to prevent their saturation, thereby greatly reducing their turn-off delays.
Small signal Schottky diodes like the 1N5711, 1N6263, 1SS106, 1SS108 or the BAT41–43, 45–49 series are widely used in high frequency applications as detectors, mixers and nonlinear elements, and have replaced germanium diodes, rendering them obsolete. They are also suitable for ESD protection of ESD sensitive devices like III-V-semiconductor devices, laser diodes and, to a lesser extent, exposed lines of CMOS circuitry.
Laser pumping is the act of energy transfer from an external source into the gain medium of a laser. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in the ground state or a less-excited state, population inversion is achieved. In this condition, the mechanism of stimulated emission can take place and the medium can act as a laser or an optical amplifier. The pump power must be higher than the lasing threshold of the laser.
The pump energy is usually provided in the form of light or electric current, but more exotic sources have been used, such as chemical or nuclear reactions.