Minority carrier injection

The net carrier concentration, shown in Fig. 7.8, was obtained at a frequency of 100 kHz. DLTS spectra were recorded using reverse- and forward-bias modes in the temperature range of 80-350 K. In the re verse-bias mode, the devices were reverse biased from -1.2V to -0.2V, with a pulse width of 1 ms. Two hole (majority-carrier) trap levels were found in all the devices. These levels were designated as Hi at I iv+0.26 and H2, for which an activation energy could not be resolved. Upon minority-carrier injection (forward-bias mode), DLTS showed two additional electron (minority-carrier) traps, which are labeled Ei (Ec-0.1eV) and E2 (Ec-0.83eV) in Table 7.1. The spectra were measured at an emission time of 465.2 s and the width of the... 

The existence of two types of mobile charge carriers in semiconductors enables us to distinguish between a majority charge carrier transferred from the electrode into the electrolyte and a minority charge carrier injected from the electrolyte into the electrode. Minority carrier injection causes significant reverse currents, but may also contribute to the total current under forward conditions. 

A high gain transistor requires a nearly equal to 1. In the absence of collector junction breakdown, a is the product of the base transport factor and emitter efficiency. The base transport factor, aT, is the fraction of the minority current (electrons for an n-p—n transistor) that reaches the collector. ocT 1 — W2 /2L, where W is the base width, is the distance between emitter and collector junctions and Lg is the minority carrier diffusion length in the base. High gain transistors require a thin base as well as a long minority carrier lifetime for a large Lg. Because aT is >0.995 in modem transistors, there is little room for improvement. The emitter efficiency, the fraction of emitter current due to minority carriers injected into the base instead of the emitter,... 

As indicated in Figure 1, if a semiconductor is biased to depletion in contact with an electrolyte, a photocurrent can be generated upon illumination. This occurs because the photo-excited majority carriers are driven by the electric field in the depletion layer to the counter electrode and minority carriers migrate to the interface where they are trapped at the band edge. Nozik has recently speculated that hot minority carrier injection may play a role in supra-band edge reactions.(19)... 

What about the minority carrier injection process depicted in Figure 14 Here, contrasting with the process considered above, the hole injection step itself is usually very fast (see below). Then the current is limited by diffusion/recombination described by the Shockley equation [201] ... 

The back-electron-transfer step is identical to the band-to-molecular state charge-transfer process involving minority-carrier injection (Section 2.3.5). This step is sensitive to defects that would act as donors with smaller activation barriers than a single regeneration step involving band-to-molecular acceptor transitions. Optimally the surface should have low defect densities and the cation level should lie as high above the valence band as possible (electrochemical determinations of the redox potential of the cation radical are needed). 

In-situ luminescence measurements have been used to study the semiconductor/ electrolyte interface for many years (e.g. Petermann et al., 1972). Luminescence may result from optical excitation of electron/hole pairs that subsequently combine with the emission of light (photoluminescence). Alternatively, minority carriers injected from redox species in the electrolyte can recombine with majority carriers and give rise to electroluminescence. The review by Kelly et al. (1999) summarises the main features of photoluminescence (PL) and electroluminescence (EL) at semiconductor electrodes. The experimental arrangements for luminescence measurements are relatively straightforward. Suitable detectors include a silicon photodiode placed close to the sample, a conventional photomultiplier or a cooled charge-coupled silicon detector (CCD). The CCD system is used with a grating spectrograph to obtain luminescence spectra. 

In the case of minority carrier injection, the interfacial current is not only determined by the electric field but also by diffusion of the carriers. The hole current (p is then... 

Measurements of Surface Recombination and Minority Carrier Injection... 

It should be mentioned that a somewhat different method was introduced by Pleskov [12]. Here, the semiconductor is in contact with a second liquid at the rear instead of a p-n junction being used. The semiconductor is under reverse bias with respect to this liquid. Any minority carrier injection at the front then leads to an increase of current at the rear contact. This method is of interest for semiconductors with which a p-n junction cannot be made. 

These are very good examples from the fundamental point of view which could never be realized with pure solid state devices. In addition it should be emphasized that electroluminescence is a very useful in situ tool for the detection of minority carrier injection. In principle, the same type of information can be obtained by using the... 

Solid solns of InP can cover the energy gap continuously from 0.3 to 1.3 ev. Rectification has been observed in InP although It is more characteristic of a Schottky type barrier than the minority carrier injection phenomenon observed in germanium. 

Heller, Miller and coworkers have previously shown that the p-Si surface is capable of sustaining minority carrier injection into the solution and stabilizes reduction of the redox couple [19]. However,... 

Contacts must supply a reservoir of carriers ready to enter the semiconductor at the rate required by the ohmic currents. At low fields, the balance is maintained by dielectric relaxation which quickly compensates any deviation from equilibrium by a charge supply from the interior and from the other contact. At high fields departures from this balance occur. When the contact region has an abundance of carriers the ohmic extrapolation is exceeded by space charge supplied currents. These are called by the misnomer space charge limited currents . When, on the other hand, the demand for carriers exceeds the supply, the current becomes emission limited. A very comprehensive account of injection in insulators has been given recently by Lampert and Mark (1970), Lampert and Schilling (1970), and by Baron and Meyer (1970). The injection processes in insulators and amorphous semiconductors differ markedy from minority carrier injection in crystalline semiconductors where the majority carrier density adjusts... 

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