Walter Schottky was one of the first researches to study the vacancies in the structure of the semi-conductor materials.
Fig. 252 - The barrier diode invented by Dr. Schottky.

In reality they are holes in the crystal lattice due to the movement of electrical charges.
Fig. 253 - The four-element Thyratron valve provided with shield grid where:
a) Anode
b) Cathode
c) Control grid
d) Shield grid
e) Insulator
f) Insulator

Such a kind of strucuture was known later as the "Schottky defect". These studies led him to postulate 1938 a theory that explained the rectifying behaviour of a metal-semiconducting contact as dependent on a barrier layer at the surface of contact between the two materials. The metal-semiconductor diodes later built based in this theory is known as Schottky barrier diodes.
Basically this type of diode has a metal-semiconductor contact such as an Aluminum layer in intimate contact with an N-type Silicon substrate. Thus the diode behaves electrically similar to a P-N junction, in such way the current flow is caused originally to majority carriers having an inherently fast response.
The barrier diode is used in several kinds of circuit topology such as: low noise mixers, microwave or high frequency as well as comutators. Fig 252
In the path of its technological development soon led to a new type of diode, whose operation principle was similar to a gaseous triode valve as known as the Thyratron. Fig 253