Show MeNo, not all metalloids are semiconductors. Of the six elements commonly classified as metalloids, only silicon and germanium are true semiconductors in their pure, bulk form. Boron is a semiconductor only at elevated temperatures, while arsenic, antimony, and tellurium behave as semimetals or weak conductors. Polonium, sometimes grouped with metalloids, is a metal. Conductivity across the group spans roughly twelve orders of magnitude, so calling the whole class "semiconductors" is a category error carried over from textbook shorthand.
What Makes a Metalloid a Metalloid
The metalloid label is not a formal IUPAC category. It is a semantic convention that clusters elements sitting near the diagonal staircase on the periodic table, between the metals to the left and nonmetals to the right. That position gives them intermediate properties: partial metallic luster, moderate electronegativity, and an ionization energy between the two extremes.
Class membership itself varies by source. The six-element list (boron, silicon, germanium, arsenic, antimony, tellurium) is standard in most English-language textbooks. Some references add polonium, astatine, or even selenium and carbon. This inconsistency matters because it determines whether the answer to "are all metalloids semiconductors" is "two of six" or "two of nine."
Conductivity is the property that decides the semiconductor question. Metals conduct via a half-filled conduction band. Insulators have a large band gap. Semiconductors sit in between, with a band gap small enough that thermal energy or doping can promote electrons across it.
The Band Gap Mechanism Behind Semiconductor Behavior
A semiconductor is defined by its band gap: the energy difference between the valence band (where electrons are bound) and the conduction band (where they flow). Silicon's gap is 1.12 electron volts at room temperature. Germanium's is 0.66 eV. These values are small enough that roughly one electron in a trillion is thermally excited at 25°C, giving a modest intrinsic conductivity that rises steeply with temperature and can be amplified enormously by doping.
Metalloids fail this test for two distinct reasons. Some have band gaps so small the bands effectively overlap, producing semimetal behavior rather than clean semiconductor switching. Others have gaps on the order of 1.5 to 2 eV or more, which makes them behave closer to insulators at room temperature.
Graphene, a purely carbon allotrope, has zero band gap yet is studied as a semiconductor candidate—showing that "metalloid" and "semiconductor" are overlapping but not identical labels. In plain terms: the periodic table label describes where an element sits; the band gap describes how it conducts, and the two do not always agree.
Quantified Comparison of the Six Metalloids
The table below reconciles band gap, room-temperature conductivity, and dominant conduction type. Conductivity is given in siemens per centimeter (S/cm), a common metric in materials science. Silicon's intrinsic value is about 4 × 10⁻⁶ S/cm; germanium's about 0.02 S/cm.
| Element | Band gap (eV, 300 K) | Intrinsic conductivity (S/cm) | Semiconductor behavior | Dominant conduction |
|---|---|---|---|---|
| Boron | ~1.5 (indirect) | ~10⁻⁶ at 25°C; rises sharply above 300°C | Only at high temperature | p-type (hole) |
| Silicon | 1.12 | ~4 × 10⁻⁶ | Yes, textbook intrinsic | Electrons and holes |
| Germanium | 0.66 | ~0.02 | Yes, but leaky | Electrons and holes |
| Arsenic | ~1.0 (semimetal overlap) | ~10³ (metallic-like) | No—semimetal | Electrons |
| Antimony | ~0.7 (semimetal overlap) | ~10⁴ | No—semimetal | Electrons |
| Tellurium | 0.33 | ~0.1 (p-type) | Weak; narrow-gap, not commercial | Holes |
Two rows in this table are the crux. Arsenic and antimony have band gaps that look semiconductor-sized in isolation, but their band structure includes a semimetal point: the conduction band minimum and valence band maximum overlap in momentum space, so no gap exists at absolute zero. That is why they conduct roughly a thousand to a million times better than silicon at room temperature.
Why Silicon and Germanium Dominate
Silicon and germanium both crystallize in the diamond cubic structure, in which each atom bonds tetrahedrally to four neighbors. That lattice produces a clean, direct-to-indirect band gap that can be engineered with dopants at parts-per-million concentrations. A silicon wafer doped with one phosphorus atom per million conducts roughly 10,000 times better than intrinsic silicon.
The commercial split between them comes down to temperature tolerance. Germanium's narrow 0.66 eV gap means its intrinsic carrier concentration at 100°C is roughly 10,000 times that of silicon at the same temperature—enough leakage to break most circuits. Silicon's wider gap keeps leakage manageable up to about 150°C junction temperature, which is why the entire logic-chip industry standardized on silicon after 1960.
Silicon also wins economically. Purified semiconductor-grade silicon costs roughly USD 30 to 60 per kilogram as of 2025, versus USD 1,000 or more for electronic-grade germanium. Germanium still appears in high-speed photodetectors and infrared optics, where its narrower gap is an advantage rather than a defect.
The Counterintuitive Case of Boron and Tellurium
Boron is often called a semiconductor, but the label holds only above room temperature. Its resistivity at 25°C is roughly 10⁶ ohm-centimeters—closer to an insulator than a semiconductor. Conductivity becomes measurable only above about 300°C, and even then the mechanism is hopping conduction between defect states rather than clean band transport.
Tellurium is the opposite edge case. It is a narrow-gap p-type semiconductor with a 0.33 eV gap, and it actually conducts better than silicon at room temperature. But tellurium is brittle, rare (annual production under 1,000 metric tons globally), and toxic in vapor form, so it has never become a commercial semiconductor in bulk despite functioning as one in the physics sense.
This is the non-obvious correction to textbook shorthand: the phrase "metalloids are semiconductors" survives because silicon and germanium are the two most economically important elements in the group. The other four are grouped with them by periodic-table position, not by device behavior.
Timeline: How the Classification Was Built
| Year | Event | Why it matters |
|---|---|---|
| 1826 | Jöns Jacob Berzelius isolates silicon | First metalloid semiconductor known, though unrecognized as such |
| 1869 | Mendeleev's periodic table published | The diagonal staircase takes shape |
| 1886 | Clemens Winkler isolates germanium | Second true semiconductor metalloid found |
| 1947 | Bardeen and Brattain demonstrate the point-contact transistor | Proves silicon and germanium are useful semiconductors |
| 1954 | First commercial silicon transistor (Texas Instruments) | Cemented silicon as the industry standard |
| 1980s | "Metalloid" formalized as a teaching category | The six-element list becomes standard |
| 2000s | Two-dimensional materials research expands | Questions the boundary between metalloid and semiconductor |
The timeline shows the category was assembled backward: the semiconductor identity of two members drove the group's reputation, and the rest were swept in by proximity on the table.
Common Misconceptions About Metalloids
The most persistent misconception is that "metalloid" means "semiconductor." It does not. Metalloid means "intermediate in properties"; semiconductor means "narrow band gap with controllable conductivity." An element can be one without the other, as arsenic and antimony show.
A second error is assuming all semiconductors are metalloids. Gallium arsenide, indium phosphide, and gallium nitride are compound semiconductors made from a metal plus a nonmetal or metalloid. The compounds, not the elements, power LEDs, laser diodes, and high-frequency electronics.
A third mistake: treating doping as a property of the element. Undoped silicon is a poor conductor at 4 × 10⁻⁶ S/cm. The same silicon, doped, exceeds 100 S/cm. Metalloid-ness is an atomic property; semiconductor performance is an engineered one.
What to Actually Do With This Information
If you are a student sorting elements for an exam, memorize the six-element list and then annotate: silicon and germanium are the true semiconductors; boron needs heat; arsenic and antimony are semimetals; tellurium is a narrow-gap oddity.
If you are evaluating a material for a project, check the band gap first. A gap between roughly 0.5 and 1.5 eV in a stable crystal is a workable semiconductor candidate. Anything above 2 eV behaves as an insulator at room temperature; anything with band overlap behaves as a semimetal.
If you are reading a manufacturing datasheet, look for the dopant concentration, not the element label. A spec sheet saying "metalloid semiconductor" tells you nothing about performance; the doping profile, carrier mobility, and temperature rating tell you everything.
If you are teaching, correct the shorthand early. Say "silicon and germanium are semiconductors among the metalloids" rather than "metalloids are semiconductors." The distinction takes one sentence and prevents years of confusion.
Limits of the Metalloid Category
The metalloid label is pedagogically useful but scientifically loose. IUPAC has never adopted it as an official category, and the membership list varies by textbook and by language. Some references exclude tellurium; others include selenium. The boundary between metalloid and semimetal is not sharp, and the boundary between semiconductor and semimetal depends on band structure details that vary with temperature and pressure.
Thermoelectric materials complicate the picture further. Bismuth telluride, for example, combines a semimetal (bismuth) with a metalloid (tellurium) to produce a narrow-gap semiconductor used in Peltier coolers. The compound is a semiconductor even though only one of its constituents is a metalloid, which shows that element classification and material function operate at different levels.
What remains certain: of the six classic metalloids, two are dependable semiconductors, one is a high-temperature semiconductor, two are semimetals, and one is a narrow-gap outlier. The phrase "are all metalloids semiconductors" has a clean answer—no, only a minority are—and the exceptions are as instructive as the rule.