Minority-carrier traps

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... 

It is convenient to adopt the terminology of Miller et al. (1977) and define a center to be an electron trap if e e p and a hole trap if the reverse is true. In addition, a minority-carrier trap is one for which the emission rate of minority carriers emin is greater than that of majority carriers emaj, whereas for majority-carrier traps emaj emin. By these definitions, an electron trap is a majority-carrier trap in an n-type region and a minority-carrier trap in p-type. Note that these definitions are independent of whether the trap is a donor or an acceptor, terms that imply a specific charge state of the center (Pantelides, 1978). Because the emission rates are thermally activated [Eq. (9)], an electron trap usually lies in the upper half of the band gap and a hole trap in the lower. 

Consider condition 2 when (4.46) is satisfied. From the generation-recombination theorem for a two-carrier semiconductor without minority carrier trapping [4.17],... 

If the field is strong enough a significant fraction of the holes can be swept out of the detector material [4.18, 19], an effect which can be important in the absence of minority carrier trapping strong trapping prevents the holes from being swept out. 

Another application of these detectors is to detect small signals when background radiation is very dim. It can be shown that in feeble background radiation and at low detector temperatures the minority carrier trapping effect can be important, so that from (4.45)... 

Point defects often result in states in the energy gap of a semiconductor and therefore can cause doping, formation of tails of states around the band edges, minority carrier traps, and favored centers for recombination of minority with majority carriers. Therefore, they represent a critical factor determining the behavior of semiconductors. Understanding the formation, charge-state, and interaction of... 

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. 

Frequently it has been observed with n-type as well as with p-type electrodes in aqueous solutions that the onset potential of the pure photocurrent differs considerably from the flatband potential. The latter can be determined by capacity measurements in the dark as illustrated by the dashed line in the ij — Ub curve in Fig. 8 a. This effect is usually explained by recombination and trapping of minority carriers created by light excitation at the surface. It is obvious that these effects have a negative effect... 

The occurrence and deactivation of excited states of the first type are schematically shown in Fig. 35. Let the minority carriers (holes) be injected into the semiconductor in the course of an electrode reaction (reduction of substance A). The holes recombine with the majority carriers (electrons). The energy, which is released in the direct band-to-band recombination, is equal to the energy gap, so that we have the relation ha> = Eg for the emitted light quantum (case I). More probable, however, is recombination through surface or bulk levels, lying in the forbidden band, which successively trap the electrons and holes. In this case the excess energy of recombined carriers is released in smaller amounts, so that hco < Eg (case II in Fig. 35). Both these types of recombination are revealed in luminescence spectra recorded with n-type semiconductor electrodes under electrochemical generation of holes (Fig. 

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)... 

In many PEC systems the chemical kinetics for the primary charge transfer process at the interface are not observed at the light intensities of interest for practical devices and the interface can be modeled as a Schottky barrier. This is true because the inherent overpotential, the energy difference between where minority carriers are trapped at the band edge and the location of the appropriate redox potential in the electrolyte, drives the reaction of interest. The Schottky barrier assumption breaks down near zero bias where the effects of interface states or surface recombination become more important.(13)... 

Consideration of the rates of arrival, charge transfer, trapping and recombination of minority carriers leads to expressions for the time dependent surface concentrations (cm-2) of free ps,(Tee and trapped pJ>trap holes ... 

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