Metalloid
A metalloid is an element with properties between metals and nonmetals, found along the staircase line of the periodic table; silicon is the best known.
Definition
A metalloid is an element that is part metal and part nonmetal: it might look shiny like a metal but break like glass. Metalloids (or semimetals) are the elements along the zigzag line separating metals from nonmetals, boron, silicon, germanium, arsenic, antimony, tellurium, and sometimes polonium and astatine; they have a metallic appearance but are brittle and are semiconductors, conducting better than nonmetals but worse than metals, and their conductivity increases with temperature, the opposite of metals. They have intermediate ionization energies and electronegativities (silicon is $1.90$ on the Pauling scale) and form network covalent solids with a narrow band gap between the valence and conduction bands, roughly $1.1\text{ eV}$ for silicon and $0.67\text{ eV}$ for germanium. Thermal excitation or doping populates the conduction band, so conductivity can be controlled by adding group $13$ (p-type) or group $15$ (n-type) impurities, and metalloid oxides are amphoteric or weakly acidic.
Example
Silicon is the most famous metalloid: shiny and gray but brittle, and the main ingredient in computer chips. Silicon ($Si$, atomic number $14$) sits directly below carbon and forms four covalent bonds like carbon does, and while pure silicon barely conducts, a trace of phosphorus or boron turns it into the semiconductor at the heart of every transistor. Doping with phosphorus, which has five valence electrons, adds a free electron per dopant atom (n-type), doping with boron, which has three, creates electron vacancies called holes (p-type), and joining the two produces a p-n junction, the basic diode from which solar cells and transistors are built.
Key Insight
Metalloids sit on a staircase line that runs down the right side of the periodic table, right between the metals and the nonmetals. The ability to tune a metalloid between conducting and insulating is exactly what makes a computer possible, since metals always conduct and nonmetals never do, and only a semiconductor can be switched. The classification is fuzzy by design: there is no sharp line where metallic bonding gives way to covalent bonding, elements like antimony and arsenic are argued over, and chemistry is continuous even when tables draw borders.