Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
The temperature coefficient alpha tells you how resistance changes with temperature. It is defined so that R = R0 (1 + alpha x change in T). If alpha is negative, then when temperature T increases, resistance R decreases. So 'negative coefficient' simply means resistance falls as the material gets hotter. Semiconductors and insulators behave this way.
In a semiconductor or insulator, most electrons are locked in the valence band at low temperature, so few charge carriers exist. When you heat the material, thermal energy kicks many electrons across the energy gap into the conduction band, creating free electrons AND holes. More carriers means it conducts better, so resistance drops. The gain of extra carriers beats the small increase in scattering, so R goes down overall.
A metal already has a huge, fixed number of free electrons that does not change much with temperature. Heating cannot add many new carriers. What heating does is make the lattice atoms vibrate more, which scatters the electrons more often (relaxation time tau falls). More scattering means higher resistance. So for metals, R increases with T, giving a positive temperature coefficient.
Both do. An insulator is just a semiconductor with a very large energy gap (Eg above about 3 eV). Heat still frees some carriers across that big gap, so its resistance also decreases with temperature. That is why the correct NEET answer is 'insulators AND semiconductors,' not 'semiconductors only.' The effect is weaker in insulators because the gap is large, but the sign of the coefficient is still negative.
Silicon is a semiconductor, so it has a negative temperature coefficient (NTC). Heating silicon frees more electron-hole pairs, so its resistance drops. The same is true for germanium and carbon (in its semiconducting form). Practical NTC thermistors used in temperature sensors are made from semiconductor oxides for exactly this reason.
The solids which have the negative temperature coefficient of resistance are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Semiconductors and insulators. Their resistance decreases as temperature rises because heat frees more charge carriers across the energy gap.
Metals have a POSITIVE coefficient. Their resistance increases with temperature because the fixed free electrons are scattered more by the hotter, vibrating lattice.
NTC stands for Negative Temperature Coefficient. It describes a material whose resistance drops when it is heated. Semiconductors like silicon and germanium are common NTC materials.
Yes. NTC thermistors are made from semiconductor materials so their resistance falls smoothly with temperature, which lets them work as temperature sensors.
