easa-part-66-module-4 Schottky Diode

It is also called Schottky barrier diode or hot-carrier diode. It is mainly used as a rectifier at signal frequencies exceeding 300 MHz. It has more uniform junction region and is more rugged than PIN diode - its main rival.

(a) Construction

It is a metal-semiconductor junction diode with no depletion layer. It uses a metal (like gold, silver, platinum, tungsten etc.) on the side of the junction and usually an N-type doped silicon semiconductor on the other side. The diode and its schematic symbol are shown in Fig. 4.1.1.23.

(b) Operation

When the diode is unbiased, electrons on the N-side have lower energy levels than electrons in the metal. Hence, they cannot surmount the junction barrier (called Schottky barrier) for going over to the metal.

When the diode is forward-biased, conduction electrons on N-side gain enough energy to cross the junction and enter the metal. Since these electrons plunge into the metal with very large energy, they are commonly called ‘hot-carriers’. That is why this diode is often referred to as hotcarrier diode.

(c) Applications

This diode possesses two unique features as compared to an ordinary P-N junction diode:

1. It is a unipolar device because it has electrons as majority carriers on both sides of the

junction. An ordinary P-N junction diode is a bipolar device because it has both electrons and

holes as majority carriers;

2. Since no holes are available in metal, there is no depletion layer or stored charges to worry

about. Hence, Schottky diode can switch OFF faster than a bipolar diode.

Because of these qualities, Schottky diode can easily rectify signals of frequencies exceeding 300 MHz. As shown in Fig. 4.1.1.24, it can produce an almost perfect half-wave rectified output.

The present maximum current rating of the device is about 100 A. It is commonly used in switching power supplies that operate at frequencies of 20 GHz. Another big advantage of this diode is its low noise figure which is extremely important in communication receivers and radar units etc.

It is also used in clipping and clamping circuits, computer gating, mixing and detecting networks used is communication systems.
4.1.1.7 Diodes Symbols

Figure 4.1.1.25 represents the symbols for different type of diodes.

Figure 4.1.1.25: Some diode symbols.