2-57
With forward bias, the resistance of the point-contact diode is higher than that of the junction diode.
With reverse bias, the current flow through a point-contact diode is not as independent of the voltage
applied to the crystal as it is in the junction diode. The point-contact diode has an advantage over the
junction diode because the capacitance between the catwhisker and the crystal is less than the capacitance
between the two sides of the junction diode. As such, the capacitive reactance existing across the point-
contact diode is higher and the capacitive current that will flow in the circuit at high frequencies is
smaller. A cutaway view of the entire point-contact diode is shown in figure 2-51C. The schematic
symbol of a point-contact diode is shown in figure 2-51D.
Schottky Barrier Diode
The SCHOTTKY BARRIER DIODE is actually a variation of the point-contact diode in which the
metal semiconductor junction is a surface rather than a point contact. The large contact area, or barrier,
between the metal and the semiconductor in the Schottky barrier diode provides some advantages over the
point-contact diode. Lower forward resistance and lower noise generation are the most important
advantages of the Schottky barrier diode. The applications of the Schottky barrier diode are the same as
those of the point-contact diode. The low noise level generated by Schottky diodes makes them especially
suitable as microwave receiver detectors and mixers.
The Schottky barrier diode is sometimes called the HOT-ELECTRON or HOT-CARRIER DIODE
because the electrons flowing from the semiconductor to the metal have a higher energy level than the
electrons in the metal. The effect is the same as it would be if the metal were heated to a higher
temperature than normal. Figure 2-52 is an illustration of the construction of a Schottky barrier diode.
Figure 2-52.—Schottky-barrier diode.
PIN Diodes
The pin diode consists of two narrow, but highly doped, semiconductor regions separated by a
thicker, lightly-doped material called the intrinsic region. As suggested in the name, pin, one of the
heavily doped regions is p-type material and the other is n-type. The same semiconductor material,
usually silicon, is used for all three areas. Silicon is used most often for its power-handling capability and
because it provides a highly resistive intrinsic (i) region. The pin diode acts as an ordinary diode at
frequencies up to about 100 megahertz, but above this frequency the operational characteristics change.
The large intrinsic region increases the transit time of electrons crossing the region. Above 100
megahertz, electrons begin to accumulate in the intrinsic region. The carrier storage in the intrinsic region
causes the diode to stop acting as a rectifier and begin acting as a variable resistance. The equivalent
