The behavior of electrons in solids were quantified by Erwin Shchrodinger's famous quantum-mechanical equation in 1926.While the theoretician were unraveling the concepts of electrons, band theories, and valences, practical mind oriented scientists like: Mevil F. Mott in England, Alexander Davidov in Russia
Grounded base or grounded grid circuit compared with a grounded grid in a triode valve.
Grounded emitter or grounded cathode circuit compared with a grounded cathode in a triode valve.

and as aforementioned Walter Schottky in Germany, were experimenting with various types of crystals to explain metal-semiconductor rectification.
The rectification mechanism was generally agreed upon: the semiconductor material becomes depleted of current carriers at the junction, which creates an effective barrier to equilibrium electron flow across the junction. Application of an electric field that reduces the barrier allows electron flow, while reversal of that field further deplets the semiconductor carriers, thus heightening the barrier to electron flow.
As aforeseen over many years, since the end of XIX century, the search for a reliable solid-state amplification device was the saga of many scientists' engineers and inventors.
Therefore it happened only in 1948 when a team of physicists, William Shcockley, John Bardeen and Walter
Close up view of the solid-state amplifer showing its three contacts points: EMITTER (E), COLLECTOR (C), and BASE (B).

H. Brattain, working at Bell Laboratories, USA, established on a theorical basis that in
Diagram describing the reversal polarities of the TRANSISTORS N-P-N and P-N-P.

such semiconducting device the flow of a acurrent sent through the two contacts could be controlled by means of a current sent by a third contact similar to the grid behavior found in a thermionic valve or tiode. Fig 259
This device was christenned as TRANSISTOR, an acronym for: TRANSFER AND RESISTOR. Fig 260
In terms of an electrical circuit, the TRANSISTOR is a set of two diodes operating in opposition to each other, the N-P-N crystalline strucuture of which is provided with three contact points or electrodes denominated: EMITTER and COLLECTOR which are connected to the two N crystals, while a third one or BASE is connected to the positive portion or P crystal. Fig 261
The TRANSISTOR or semiconductor device provide with three contact points compared with an early Germanium diode

This semiconducting device is also called bipolar TRANSISTOR.
In its normal operation, the emitter to base junction is biased in the forward direction and the collector to base function in the reverse direction. Fig 262
Both types, N-P-N and P-N-P TRANSISTORS operate identically. However,
TRANSISTOR curve tracer showing in its front panel the polarity reversal switch for testing P-N-P and N-P-N TRANSISTORS.

considering those two kinds of crystalline structurer, they have reverse polarities. Fig 263 Thus, while in the N-P-N the electrons flow from the emitter to the collector, in the P-N-P they flow from the collector to the emitter. Fig 264

In spite of the enormous applications unfortunately the first Germanium TRANSISTOR was almost unable to maintain narrow tolerances regarding its electrical characteristics. However, by improving manufacturing techniques this situation was solved and TRANSISTORS as a solid-state amplifer were able to achieve an equally reliable performance as in the termionic valve. Thus, around 1958, to overcome the evolution of the solid-state the American company RCA launched in the market a miniaturized thermionic device the NUVISTOR.

Principle of TRANSISTOR N-P-N considering the crystal inner strucuture. It consists of an N-layer connected to the EMITTER (E); the P-layer connected to the BASE (B) and finally to a second N-layer to which the COLLECTOR (C) is connected. In the reality this arrangement comprises two diodes in opposition with each other. (b) In the neutral condition there is a shortage of electrons left of junction (EB), wherea on its right there is a shortage of vancancies.
If a positive voltage with respect to (E) is applied to (B), the potential difference will be overcome, and the diode E-B becomes conductive in that free electrons flow toward the base in such way this current increases with the voltage. Therefore, considering the P-Layer has intentially been given far less dope than the N-layer, resulting that the electrons fail to find enough vancancies to be neutralized in the base, and will be accumulate there. If a positive voltage with respect to (E) is applied to (C), de diode B-C will be blocked. I.e. there will be no flow of current. If E-B is conducting, and provided a voltage high enough, the elctrons accumulated in B will be attracted by this external voltage. In this way a small part of the electrons supplied by (E) will flow to (B), and a large part will flow to (C), but the number of electons supplied by (E) will depend on the voltage between (B) and (C). Since there is amplification, the current toward the COLLECTOR is bigger that toward the BASE, but itdepends on the voltage or current through the base.