---
title: "Mechanisms generating only electrons or holes"
subject: "Electrical Engineering Material"
chapter: 6
key: "semester_3/electrical_engineering_material:generation-electrons-holes-mechanisms"
source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/generation-electrons-holes-mechanisms
site: BCT Engineering
---

# Mechanisms generating only electrons or holes

Electrical Engineering Material · Chapter 6: Semiconductors

- Asked in: 2069 Asar, 6 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/generation-electrons-holes-mechanisms
- Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-6

## Question

What are the mechanisms for generation of only electrons or holes in semi-conductor? Explain in brief.

## Short answer

To generate only electrons, ionize donor impurities or optically excite donor levels; to generate only holes, ionize acceptor impurities or excite acceptor levels. Injection from contacts or emission from traps also produce a single carrier type. These mechanisms enable extrinsic doping.

## Answer

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.

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

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

| Mechanism | Level used | Carrier produced |
|---|---|---|
| Band-to-band (thermal/optical) | $E_v \to E_c$ | electron + hole |
| Donor ionization | $E_d \to E_c$ | electron only |
| Acceptor ionization | $E_v \to E_a$ | hole only |
| Trap emission | deep level | one type |
| Injection | external source | one type |

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

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does breaking a covalent bond in a pure semiconductor create?
   A. An electron only
   B. A hole only
   C. A fixed ion
   D. An electron-hole pair
   Answer: D. Band-to-band generation always yields a pair. (Key point: Band-to-band generation)

2. Why does donor ionisation produce only electrons?
   A. The donor electron goes to the conduction band and leaves a fixed positive ion, not a valence-band hole
   B. The valence band is empty
   C. Donors absorb holes
   D. Donors have zero mass
   Answer: A. The electron comes from a localised level, so no valence-band hole is formed. (Key point: Donor ionisation)

3. What happens when an acceptor ionises?
   A. It gives a conduction electron
   B. It takes a valence electron, leaving a hole and a fixed negative ion
   C. It creates an electron-hole pair
   D. It releases a photon
   Answer: B. A valence electron fills the acceptor level, leaving a hole. (Key point: Acceptor ionisation)

4. Which photon energy creates only electrons from donors?
   A. hv >= Eg
   B. hv < Ec - Ed
   C. Ec - Ed <= hv < Eg
   D. hv > 2Eg
   Answer: C. A photon with energy at least Ec - Ed but below Eg excites donor electrons only. (Key point: Extrinsic photoconductivity)

## Related in Electrical Engineering Material

- [Generation of carriers and conductivity](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Ageneration-conduction-conductivity) — Electrical Engineering Material, chapter 6
- [Transmission through CuO layer (7 eV electrons)](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Atunneling-cuo-layer-7ev) — Electrical Engineering Material, chapter 1

## Related in other subjects

- [Drift velocity of electrons in copper](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Adrift-velocity-copper-two-hundred-amp) — Engineering Physics, chapter 8
- [Breakdown mechanisms in commercial liquids](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Acommercial-liquid-breakdown) — High Voltage Engineering, chapter 6, asked 3×
- [Kyoto Protocol flexibility mechanisms](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Akyoto-flexibility-mechanisms) — Energy Environment and Society, chapter 2
- [T/F: span choice depends on voltage only](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-span-depends-voltage-not-conductor) — Transmission and Distribution System Design, chapter 4