Through the researches in the new field of solid state Physics scientists discovered that the atomic structure of certain materials like Germanium and Silicon, contains few free electrons that could scape leaving holes of vacancies of charges in the crystal lattice.
Principle of the semiconductor diode where:
a) Crystal structure
b) P-N junction
c) Curve showing operational functions
In this way, the so called boundary layer of P-N junction shaped allows current to flow in just one direction upon application of an alternating voltage between the two contacts points on the crystal.
Fig.249 - Cross section of a point contact diode.

Basically, this kind of man-made diode consists in handling the Germanium atomic structure by adding into its crystal lattice small quantities of impurities known as DOPE.
Those impurities are atoms of several chemical elements, generally metals comprising their own electronic configuration. For instance, Phosphor, Arsenium and Antimony have 5 electrons in their outer orbit while Aluminium, Indium and Galium or Boron only 3.
Fig. 250 - The first thermionic diode invented by Fleming in 1904.
Those atoms are called DONORS and ACCEPTORS respectively due to their capacity o donating or receiving electrons.
When adding those impurities into the Germanium crystal lattice, two kinds of crystalline structure are formed: amixture containing surplus of free electrons, also called NEGATIVE GERMANIUM; another one with shortage of electrons known as POSITIVE GERMANIUM.
By joinning together a P and N Germanium, an electrical charge transfer occurs at the boundary layer of the P-N junction. Thus, P and N crystals have a surplus and shortage of electrons respectively. Those charges repel each other preventing further diffusion of electons and holes in the
Fig. 251- Pictorial evolution of the crystal diode where:
a) Lead Sulphide or Galena
b) Carborundum detector
c) Early type of Germanium diode
crystal; therefore a potential difference is generated at the same time. In reallity this type of crystalline arrangement operates as a rectifier, i.e. when a positive voltage refereing to P crystal is applied to N crystal, the voltge is increased without any flow of current. Therefore, if a negative voltage is applied to the N crystal, it is neutralized providing a surge of small voltage in such away that it starts the electrons and holes movements all over again. Fig 248
In principle the first point contact diode was almost identical to that earliest types of crystal detectors. Basically it comprises a pellet of Germanium with a flat polished surface was where it was soldered soldered to one of the connecting wires; the other wire is provided with a sharply point springy Tungsten end, similar to the cat'whisker principle invented by Pickard. Fig 249

The Germanium diode had several advantages when compared with the diode valve such as: no heater, low cost, small volume and it could operate either as a detection as well as a switching device. In the beguinning its main application was in the manufacturing of televisers and computers. Fig 250
As aforementined, the point-contact diodes were an improved version of the original crystal detector. Fig 251
In this way, soon they were replaced by whiskerless diodes whose operational principle was related with the uniform thickness configuration of the junctions in the semi-conductor material.

Fig. 251a - Early types of Germanium diodes used in Radar high frequency circuits, circa 1940
Fig. 251c - Close up of Germanium diode type 1N38A.

Fig. 251b - Germanium diode type 1N38A for general application purpose.