A semiconductor whose conduction band minimum and valence band maximum occur at different crystal momenta. Radiative recombination requires a phonon, so emission efficiency is low. Silicon and germanium are examples.
Japanese name: 間接遷移型半導体
Indirect-bandgap semiconductor appears in 2 practice questions across 2 topics, including Electronic, magnetic and optical materials, Glasses and amorphous materials.
The energy of electrons in a crystal varies with crystal momentum (the wave vector k). In an indirect-bandgap semiconductor, the lowest point of the conduction band (the valley) and the highest point of the valence band (the peak) lie at different k. In silicon the peak is at the Γ point (k = 0), and the valleys lie about 85% of the way from Γ toward the X point. In germanium the valleys are at the L point. Room-temperature band gaps are about 1.12 eV for Si and about 0.66 eV for Ge, and GaP (about 2.26 eV), AlAs and diamond are also indirect.
The momentum of a photon is very small compared with the crystal momentum of an electron, so k hardly changes when light is absorbed or emitted. To absorb light near the band gap in an indirect semiconductor, a phonon must make up the momentum difference at the same time, which lowers the probability. Silicon therefore has a small absorption coefficient near the band edge, and solar cells need substrates 100 µm or more thick, or surface texturing to trap light. When finding the band gap from measured absorption coefficients, (αhν)^(1/2) is plotted against hν for indirect semiconductors.
The probability that electrons and holes recombine by emitting light is also very small in indirect semiconductors. The radiative recombination coefficient of Si is about four orders of magnitude smaller than that of GaAs, and most injected electrons and holes turn into heat through nonradiative recombination via defect levels. This is why it is hard to make LEDs and lasers from silicon. The green GaP light-emitting diodes of the past made light by localizing electrons at levels created by adding nitrogen, relaxing the momentum constraint. Conversely, long carrier lifetimes are an advantage in solar cells and photodetectors.
Although its absorption is weak, silicon detects light well from the visible to a wavelength of about 1.1 µm, so it is the mainstay of image sensors and solar cells. Ways to make it emit are also being studied: in germanium the direct gap (about 0.80 eV) is only about 0.14 eV above the indirect gap, so there are attempts to make it emit using tensile strain and heavy n-type doping. In practical silicon photonics, it is common to bond direct-gap semiconductors such as InP for light sources and to use Ge grown on Si for detection.
Answer: C. It is an indirect-gap semiconductor, so radiative recombination requires phonons, and the emission efficiency is low
Silicon is an indirect-gap semiconductor, with the conduction band minimum and the valence band maximum at different crystal momenta. For an electron and hole to recombine by emitting light, a phonon must participate at the same time to make up the momentum difference, so the probability of radiative recombination is very low and most recombination is nonradiative through defect levels, producing heat. Direct-gap semiconductors such as GaAs and InGaN emit light efficiently and are used in LEDs and lasers. Being direct-gap is in fact favorable for light emission. About 3.4 eV is the band gap of GaN, not silicon; silicon's is about 1.12 eV, so any light it emits is infrared at a wavelength of about 1.1 µm.
Answer: B. It absorbs visible light more strongly than crystalline silicon, so films under 1 µm suffice, but its electron mobility is 2 to 3 orders lower
Crystalline silicon is an indirect-gap semiconductor that absorbs the longer wavelengths of visible light weakly, so solar cells usually use substrates 100 µm thick or more. In a-Si:H there is no long-range periodicity, so the constraint of conserving momentum in a transition is relaxed and its visible absorption coefficient is larger than that of the crystal, letting a film 1 µm thick or less absorb the light. On the other hand, carriers are trapped in band-tail states created by the disordered network, so the electron mobility is about 1–10 cm²/(V·s), 2 to 3 orders of magnitude lower than the roughly 1400 cm²/(V·s) of crystalline silicon. It is therefore unsuited to high-speed logic circuits, but taking advantage of large-area deposition at about 300 °C, it has been used for pixel TFTs in liquid crystal displays and for thin-film solar cells. With a band gap of about 1.7 eV it absorbs visible light strongly and cannot be a transparent electrode.
Besides band structure calculations and angle-resolved photoemission spectroscopy, they are judged from how the absorption coefficient rises. Direct semiconductors absorb strongly right at the absorption edge and emit strongly. In indirect semiconductors absorption increases gradually and emission is weak.
Near-infrared light with a wavelength of about 1.1 µm, corresponding to the band gap of about 1.12 eV. It is invisible and the efficiency is extremely low, so it is not used for lighting or displays.
Although its absorption is weak, its band gap suits sunlight, the element is abundant, and high-quality crystals can be made cheaply in large volumes. Its long carrier lifetimes, which let electrons and holes generated deep in a thick substrate reach the electrodes, are another advantage.
Silicon is an indirect-gap semiconductor, with the conduction band minimum and the valence band maximum at different crystal momenta.
What is the main reason silicon is hardly ever used as the …Crystalline silicon is an indirect-gap semiconductor that absorbs the longer wavelengths of visible light weakly, so solar cells usually use substrates 100 µm thick or more.
Which correctly compares the properties of hydrogenated amo…Phonon Amorphous silicon Coercivity Curie temperature Matthiessen's rule Soft magnetic material