Schottky diodes are unipolar components; only one type of charge carrier is responsible for current transmission. While the on-state current is flowing, no excess charge which could appear as storage charge when the diode is turned off (reversed polarity) is built up. This means that Schottky diodes have no reverse current IRRM, apart from a very low current for recharging the junction capacitance. A reverse recovery time is not defined.
Owing to their minimal switching losses, Schottky diodes are highly suitable for use in high frequency applications. Their blocking voltages, however, are limited due to the reverse currents which rise steeply when the temperature rises and their unipolar on-state character. Silicon based Schottky diodes are currently available with a blocking voltage of up to around 200 V. Those made of gallium arsenide (GaAs) are suitable for up to 300 V, while Schottky diodes made of silicon carbide (SiC) are available for up to 1200 V. The suitability of SiC for high blocking Schottky diodes is due to the material's breakdown field intensity, which is nine times higher than silicon.
Figure 1. Schottky diode. a) basic structure b) doping profile diagram
In Schottky diodes, the metal-semiconductor junction serves as a blocking junction. In on-state, only the small potential barrier between metal and semiconductor material must be overcome (around 0.3 V for silicon). There is no diffusion voltage at the pn-junction as is the case in PIN diodes (approx. 0.7 V for Si). This ensures a lower on-state voltage than occurs in any PIN diode, provided the n- region is thin. With n-doped material, only electrons participate in the current flow (unipolar). When diodes are operated close to the blocking voltage range, the off-state current will considerably increase. This must be taken into account for power loss ratings, otherwise thermal stability cannot be ensured.
When switching from conductive to blocking state, ideally only the low capacitance of the space charge region has to be charged. For this reason, the component storage charge is some powers of ten lower than for the PIN diode, thus causing very low switching losses. As a result, the Schottky diode comes very close to being an ideal diode. The Schottky diode is particularly well suited for use at very high frequencies and as a snubber diode with an extremely low on-state voltage.
For silicon, these advantages are limited to voltages < 100 V. When higher blocking voltages are to be applied, the n- region must be extended and the on-state voltage will increase considerably. In this voltage range, materials with a higher permissible electric field intensity such as GaAs (≥ 600 V) or SiC (≥1700 V) are used. They have similar on-state characteristics to PIN diodes, but the advantages they offer regarding switching properties are retained. The costs for the base material and the manufacture of diodes made of the materials mentioned last are so high, however, that it only makes sense to use them in applications that require particularly high efficiency, switching frequencies, or temperatures.
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