ABSTRACT
We define wide bandgap semiconductors (WBGSs) as those semiconductors having a wider bandgap than Si at room temperature (1.12 eV). In all cases except elemental diamond © they are compounds. We briefly touched on the properties of the more commonly used compounds and WBGS in Chapter 1 (see Tables 1.1 through 1.4). There we compared the mobilitylifetime product μτ and dispersion (or ‘Fano noise’) of some materials, rather unfavourably with those of Si and Ge. Recall that the carrier drift length L (μτ product/unit applied field) appears in the Hecht equation to describe trapping and recombination. The higher bandgap results in higher average energy per electron-hole pair values, ω, and therefore potentially poorer energy resolutions. They are also mostly indirect bandgap semiconductors (see Section 1.7.3), so that direct recombination can take place and charge lifetime τ is short. So what are the attractions of the WBGSs? First, being wide bandgap, the thermal leakage current, being dominated by the term exp(–Eg/2kT) (see Section 1.11.1), is much reduced compared to Si at room temperature. This presents the possibility of x-ray spectroscopy without the inconvenience and cost of cooling to low temperatures. It has obvious attractions for portable radiation survey equipment and industrial energy dispersive x-ray fluorescence (EDXRF) applications. Other opportunities open up, for example, medical, security, and nonproliferation applications. Added to this, the stopping power, being largely determined by the highest atomic number (Z) element in the
material, is-with the exception of diamond and silicon carbide (SiC)—as good as, or better than Ge so that very thin detectors can be used for x-rays, thus reducing the bulk leakage current and partly compensating for a short carrier drift length. Higher Z not only increases the stopping power as mentioned, but gives a higher Peak/Compton scattering ratio for high energy x-rays and gamma-rays, which is where the main applications currently lie for most of them. However, the large escape peaks and fluorescence peaks from the high Z materials present can be problematic for spectroscopic analysis (see Section 1.14.1). Figure 10.1 [1] shows the detection efficiency of x-rays as a function of energy for 500-μm-thick detectors for a number of WBGSs (with Si for comparison).
