A semiconductor is a material whose ability to carry current is in between a metal (good conductor) and an insulator (bad conductor). Its resistivity is roughly 10 to the power -5 to 10 to the power 6 ohm-metre, sitting between metals (about 10 to the power -8) and insulators (about 10 to the power 18 or more). Memory hook: "semi" means half, so a semiconductor is a "half-conductor" that conducts a little, and it conducts MORE when you heat it (unlike a metal, which conducts less when hot).
A semiconductor's resistivity (about 10^-5 to 10^6 ohm-metre) lies between metals and insulators, and unlike a metal its resistance falls when it is heated.
Your doubts, answered
Is a semiconductor a conductor or an insulator?
Neither fully. It is in between. A conductor (metal) has very low resistivity, about 10 to the power -8 to 10 to the power -6 ohm-metre. An insulator has very high resistivity, 10 to the power 18 ohm-metre or more. A semiconductor sits in the middle, about 10 to the power -5 to 10 to the power 6 ohm-metre, so it carries a small current, not zero and not a lot.
Why does a semiconductor conduct only a little at room temperature?
In a pure semiconductor like silicon, almost all electrons are locked in bonds. Only a few get enough thermal energy to break free and move. Few free charge carriers means a small current. Cooling it toward 0 K frees almost none, so it behaves like an insulator.
Does a semiconductor's resistance go up or down when heated?
It goes DOWN. This is the key test of a semiconductor. Heating gives more electrons enough energy to break free, so the number of free charge carriers rises and resistance falls. This is called a negative temperature coefficient of resistance. A metal does the opposite: heating raises its resistance.
Why is silicon a semiconductor but carbon (diamond) an insulator?
Both have four valence electrons and the same structure, but the energy gap (Eg) between the valence band and conduction band is different. Diamond has a large gap (about 5.4 eV) so electrons cannot jump across, making it an insulator. Silicon has a smaller gap (about 1.1 eV) so heat can push a few electrons across, making it a semiconductor.
Is a semiconductor the same as a superconductor?
No. A superconductor has ZERO resistance below a certain low temperature, so it conducts perfectly. A semiconductor always has some resistance, and that resistance simply decreases when heated. They are completely different ideas.
⚠️ The NEET trap ✗ Semiconductor resistance increases when temperature increases, just like a metal. ✓ Semiconductor resistance DECREASES when temperature increases, because heat generates more free charge carriers. Only conductors (metals) show rising resistance with temperature. 🧠 Metal and semiconductor react to heat in OPPOSITE ways. Metal: R goes up. Semiconductor: R goes down.
Real NEET questions
2022
As the temperature increases, the electrical resistance:
A · Increases for both conductors and semiconductors
B · Decreases for both conductors and semiconductors
C · Increases for conductors but decreases for semiconductors ✓
D · Decreases for conductors but increases for semiconductors
Solution: Step 1: In a conductor (metal), the number of free electrons stays almost fixed. Heating makes the lattice vibrate more, so electrons collide more often. More collisions means higher resistance. So R increases for conductors.
Step 2: In a semiconductor, heating gives more electrons enough energy to break free and become charge carriers. The big rise in carrier number lowers resistivity strongly. So R decreases for semiconductors.
Step 3: Match: increases for conductors, decreases for semiconductors. Correct option is C.
2020
The solids which have the negative temperature coefficient of resistance are:
A · Semiconductors only
B · Insulators and semiconductors ✓
C · Metals
D · Insulators only
Solution: Step 1: A negative temperature coefficient means resistance falls as temperature rises.
Step 2: In both insulators and semiconductors, heating thermally frees more charge carriers, so resistance drops. Both show this behaviour.
Step 3: Metals show the opposite (positive coefficient), so they are excluded. The answer includes insulators and semiconductors. Correct option is B.
A material that conducts electricity a little, in between a good conductor like copper and a poor conductor like glass. Common examples are silicon (Si) and germanium (Ge).
What is the resistivity range of a semiconductor?
About 10 to the power -5 to 10 to the power 6 ohm-metre. This lies between metals (10 to the power -8 to 10 to the power -6) and insulators (10 to the power 18 or more).
Why is a semiconductor important?
Its conductivity can be controlled by temperature, light, or by adding tiny amounts of impurity (doping). This control lets us build diodes, transistors, LEDs and solar cells.
Give two examples of semiconductors.
Silicon and germanium are the two elemental semiconductors used most in NEET. Compound semiconductors like gallium arsenide (GaAs) also exist.
What happens to a pure semiconductor at 0 K?
It behaves like a perfect insulator because no electrons have enough thermal energy to reach the conduction band, so there are no free charge carriers.
