Mechanisms generating only electrons or holes

In a pure semiconductor, breaking a covalent bond (band-to-band generation) always creates an electron–hole pair. To create only electrons or only holes, the carrier must come from a localized energy level inside the band gap (an impurity or defect level), or be injected from outside. The main mechanisms are:

1. Ionization of donor impurities (only electrons)

A pentavalent atom (P, As, Sb) in Si uses four electrons for bonding; the fifth is loosely bound at the donor level $E_d$, about 0.05 eV below $E_c$. A small energy (thermal energy at room temperature is enough) frees it into the conduction band. The donor becomes a fixed positive ion $N_d^+$; no hole is formed in the valence band.

2. Ionization of acceptor impurities (only holes)

A trivalent atom (B, Al, Ga) has one incomplete bond. An electron from the valence band jumps to the acceptor level $E_a$ (about 0.05 eV above $E_v$) to complete the bond. A hole is left in the valence band and the acceptor becomes a fixed negative ion $N_a^-$; no free electron is formed in the conduction band.

3. Optical (photo) excitation of impurity levels

A photon with energy $h\nu \ge E_c - E_d$ (but less than $E_g$) can excite a donor electron into the conduction band, giving only electrons. Similarly $h\nu \ge E_a - E_v$ creates only holes. This is extrinsic photoconductivity, used in long-wavelength infrared detectors (e.g. Ge:Hg, Si:As).

4. Emission from traps / defect centres

Crystal defects and deep-level impurities (e.g. Au in Si) can hold an electron or a hole. When a filled trap releases its electron to the conduction band (or an empty trap captures an electron from the valence band), only one type of carrier is generated.

5. Carrier injection

Electrons or holes can be injected from outside: an ohmic metal contact or the n-side of a forward-biased p–n junction injects electrons into the p-region (minority carrier injection); the p-side injects holes into the n-region. Only one type of excess carrier is added to that region.

   Ec ----------------------   e- (only electrons)
   Ed  - - - + - - - - - - -   donor ionized (+)

   Ea  - - - - - - - - - -     acceptor ionized (-)
   Ev ----------- o --------   hole (only holes)

These mechanisms are the basis of doping: they make $n \ne p$, which gives n-type and p-type extrinsic semiconductors.