Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
A hole is not a real particle. It is a vacancy - a missing electron in the valence band. But we treat it as if it were a positive charge carrier because it moves and carries current. When a valence electron jumps to fill a hole, the hole appears to move to the electron's old spot. So the empty space seems to travel, and we model it as a positive charge of magnitude equal to the electron charge (1.6 x 10^-19 C).
No, they move in opposite directions inside the material, but both add to the current in the same direction. When an electric field is applied, free electrons drift toward the positive terminal (opposite to the field). Holes drift toward the negative terminal (along the field). Since electrons are negative and holes are positive, their opposite motions both produce current in the same direction. Total current I = current due to electrons + current due to holes.
Free electrons move in the conduction band (the upper energy band). Holes move in the valence band (the lower energy band). At room temperature, thermal energy pushes some electrons across the energy gap from valence band to conduction band. Each electron that jumps up leaves one hole behind. So electrons conduct at the top, holes conduct at the bottom.
An electron moves freely in the conduction band. A hole moves only because a bound valence electron hops from atom to atom to fill it - this hopping is slower and faces more resistance. So the electron drifts faster for the same electric field. In numbers, electron mobility is greater than hole mobility (for silicon, roughly 1350 vs 480 cm^2/V-s). This is why an n-type sample conducts better than a p-type sample with the same carrier count.
No. Current also depends on mobility, not just carrier number. Current density is J = n e (mu_e + mu_h) E for a material with both, and for a single-type material the current is proportional to n e mu E. Even with the same number of carriers, higher electron mobility means more current. This is exactly what NEET 2021 tested: an n-type and a p-type with equal carrier concentration give different currents because electron mobility beats hole mobility.
The electron concentration in an n-type semiconductor is the same as the hole concentration in a p-type semiconductor. An external electric field is applied across each of them. Compare the currents in them.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Free electrons and holes. Free electrons carry negative charge in the conduction band, and holes carry positive charge in the valence band. In an intrinsic (pure) semiconductor their numbers are equal: n_e = n_h = n_i.
A hole carries a positive charge equal in size to the electron charge, +1.6 x 10^-19 C. It represents a missing electron in the valence band and behaves like a mobile positive charge.
Yes. In a pure semiconductor, every electron that jumps to the conduction band leaves one hole behind, so electrons and holes are always created in pairs and are equal in number: n_e = n_h = n_i.
The total current is the sum of the electron current and the hole current: I = I_electrons + I_holes. Both carriers move in opposite directions but contribute to current in the same direction.
Because electron mobility is greater than hole mobility. For the same number of carriers and the same electric field, the faster electrons produce a larger current, so n-type conducts better.
