The predecessors to solid state devices were vacuum tubes. There, the electrons were travelling in the vacuum between the cathode and the plates, and being modulated in various ways by the electric fields from voltages on various plate arrangements.
The term "solid state" refers to the fact that the electrons in semiconductor devices are travelling through the solid matrix of the atomic structure of the material. As already stated, the diode and transistor (BJT, JFET, MOSFET) are the basic structures. But even exotic stuff like superconductivity can be considered a solid state phenomenon.
Basicaly solid state electronics is related to the property of semi-conductors like silicon, germanium in pure as well as doped form.
Metals show linear voltage-current characteristics in accordance with the Ohm's law, whereas semi-conductors with different type of doping have different voltage-current characteristics. These can be used to generate specific responses to given stimuli, ie, to implement logic.
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