The Schottky Diode

The Schottky diode uses the same metal-semiconductor junction already introduced in the chapter on the metal-semiconductor FET (MESFET) as a gate contact, but here as a standalone two-terminal device for rectification rather than a controllable channel contact. A metal contact – commonly platinum silicide, chromium, molybdenum, aluminum, or titanium, depending on the desired barrier height and process compatibility – is deposited directly onto lightly doped n-type silicon or n-type GaAs. Unlike a classic p-n diode, the rectifying barrier here forms directly at the metal-semiconductor interface itself, without needing a second, oppositely doped semiconductor region to be implanted or diffused.

At the edge of the metal contact, the electric field is strongly enhanced compared to the flat contact area, which would otherwise undesirably increase leakage current and reduce breakdown voltage; a so-called guard ring – a shallow p-n junction implanted in a ring around the contact edge with opposite curvature – specifically mitigates this field enhancement and is standard in practically all modern Schottky diode designs.

Because the Schottky barrier height is typically lower than the built-in diffusion voltage of a comparable p-n junction, the forward voltage of a silicon Schottky diode is usually only 0.2 to 0.45 V, compared to about 0.6 to 0.7 V for an ordinary silicon p-n diode. This several-hundred-millivolt reduction in forward voltage significantly lowers power dissipation at high currents, which brings a noticeable efficiency gain particularly in applications with low output voltage and high current, such as rectifier circuits in switching power supplies.

Of even greater practical importance, however, is the switching behavior: because the Schottky diode is a pure majority-carrier device – unlike the p-n diode, no injection and storage of minority carriers occurs in the respective other region –, the so-called reverse recovery charge is almost entirely eliminated. In an ordinary p-n diode, switching from forward to reverse conduction first requires depleting the minority carrier charge stored in the device, which can take anywhere from a few tens to several hundred nanoseconds depending on the device and causes unwanted current spikes during that time; Schottky diodes, by contrast, can switch within a few nanoseconds or even less, since essentially only the comparatively small junction capacitance needs to be recharged.