Electron/hole pairs

The size of the electron-hole pair can be estimated from the Bolu radius for this system ... 

In n type semiconductors, electrons are tire majority carriers. Holes will also be present tlirough accidental incoriioration of acceptor impurities or, more importantly, tlirough tlie intentional creation of electron-hole pairs. Holes in n type and electrons in p type semiconductors are minority carriers. 

There are many ways of increasing tlie equilibrium carrier population of a semiconductor. Most often tliis is done by generating electron-hole pairs as, for instance, in tlie process of absorjition of a photon witli h E. Under reasonable levels of illumination and doping, tlie generation of electron-hole pairs affects primarily the minority carrier density. However, tlie excess population of minority carriers is not stable it gradually disappears tlirough a variety of recombination processes in which an electron in tlie CB fills a hole in a VB. The excess energy E is released as a photon or phonons. The foniier case corresponds to a radiative recombination process, tlie latter to a non-radiative one. The radiative processes only rarely involve direct recombination across tlie gap. Usually, tliis type of process is assisted by shallow defects (impurities). Non-radiative recombination involves a defect-related deep level at which a carrier is trapped first, and a second transition is needed to complete tlie process. 

Photovoltaic Devices. For many inorganic semiconductors, absorption of light can be used to create free electrons and holes. In an organic semiconducting soHd, however, absorption of a photon leads to the formation of a bound electron—hole pair. Separation of this pair in an electric field can... 

The primary photochemical act, subsequent to near-uv light (wavelengths <400 nm) absorption by Ti02 particles, is generation of electron—hole pairs where the separation (eq. 3) into conduction band electrons (e g ) and valence band holes (/lyB ) faciUtated by the electric field gradient in the space charge region. Chemically, the hole associated with valence band levels is constrained at... 

Fig. 8. The photodiode detector (a) band model where the photon generates electron—hole pairs that are separated by the built-in potential setting up a photocurrent (b) physical model for a planar diode. The passivation is typically Si02 for Si diodes, an In oxide for InSb diodes, and CdTe for HgCdTe...

A schematic representation of a PR apparatus is shown in Figure 2. In PR a pump beam (laser or other light source) chopped at frequency 2 creates photo-injected electron-hole pairs that modulate the built-in electric field of the semiconductor. The photon energy of the pump beam must be larger than the lowest energy gap of the material. A typical pump beam for measurements at or below room temperature is a 5-mW He-Ne laser. (At elevated temperatures a more powerful pump must be employed.)... 

The most complicated form of Modulation Spectroscopy is electromodulation, since in certain cases it can accelerate the electron-hole pairs created by the light. If the electric field is not too large the quantity AR/Rcan be written as ... 

These modulation methods do not accelerate the electron-hole pairs and hence produce only a first-derivative Modulation Spectroscopy. Their line shapes are given by Equation (1), with m = 2. 

Fig. 4.7. A semiconductor detector operated as a pin diode with a reverse voltage or bias. An incident X-ray photon ultimately produces a series of electron-hole pairs. They are "swept out" by the bias field of-500 V- electrons in the direction ofthe n-layer holes in the direction ofthe p-layer. Thus, a small charge pulse is produced after [4.21],...

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