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• W237308641 abstract "NOTICE Tkk itfon M l ptepued w an account of work •pouored by (he United Sum Government. Nrtthei the United Sutei not ft* United S U M Deptflment of Eneigy, nor My of theft emptoytti, nor my of iheti eantnclon, nibcoatricfon, o.- their employee], tukei •ny wtmnty, tiprtn or (replied, or utiana any lefaJ Ikbfliiy oi raponrtffity foi <heKcuncy,cttii|4ctene(( Of lacfulncM o( any information, apparatus, product or piODcm dfcetaed, o( reprMenb thil !u tnr would not infrinfc primely owned dtfiw. SEMICONDUCTOR DETECTORS - AN INTRODUCTION* F. S. Goulding Lawrence Berkeley Laboratory University of California Berkeley, California 94720 (i) LBL-7282 I. History Semiconductor detectors appeared on the scene' of Nuclear Physics in about I960, although McKay of Bell Laboratories had demonstrated the detection of alpha particles by semiconductor diodes several years earlier. The earliest work, just prior to 1960, focused on relatively thin surface barrier germanium detectors, used to detect short-range particles. These detectors, used at low temperature to reduce leakage current, demonstrated the potential of semi­ conductor detectors to realize energy resolutions substantially better than the ionization (gas) chambers used earlier. Furthermore, the a b i l i t y of a solid to stop particles in a very short distance and the rugged nature of solid state devices suggested that a small rugged detector might be feasible. Attention rapidly moved to silicon detectors following this earlier work, largely because silicon, haying a considerably larger band-gap than germanium, offered the prospect of good performance at room temperature. Silicon surface barrier detectors and, somewhat l a t e r , diffused junction detectors, both used at room temperature, therefore became the f i r s t types of semiconductor detectors used in nuclear spectroscopy and they rapidly proved their value as detectors of short-range particles. At this point, a l i t t l e must be said about the limitations of these detectors - later in the course, the details will be f i l l e d in. Nuclear Spectroscopy is largely a study of the energy level:; in the nucleus and the most profitable tool for the study of these levels is to observe the radiation emitted when transistions occur between allowed levels. This radiation, In general, is in the form of gamma rays, so detectors of gamra rays are clearly the most powerful tool in such investigations. Nal scintillation detectors had become important because of t h i s , hut their rather poor energy resolution, which results in an inability to separate closely spaced levels, was a major limitation. Silicon is a very poor material for gamma ray detection, since the gamma ray efficiency varies roughly as Z (where Z is the atomic number of the absorber) and Z is only 14 for silicon. Germanium would be a better material (Z = 28), but i t was some time before this potential was realized. To first order charge collection in a detector only occurs from regions where an electric f i e l d exists. Diffusion does occur from field-free regions, but i t is slow and its effects are usually undesirable. In junction or surface-barrier (sometimes called Schottky- barrler) detectors, this means that the de­ pletion layer is the sensitive region of a detector. The thickness of a depletion layer 1 conductor diode is proportional to a semi­ V if or Yv^P where V i s the applied voltage, N is the concentration of the dominant mpu- r i t y in the semiconductor and p is i t s r e s i s t i v i t y . Thus, 1n the case of a detector made from p-type silicon having a resistivity of 2000 ftcm, (which is very pure by normal commercial silicon standards), an applied voltage of 200 V yields a depletion layer thickness of only 200 urn (0.2 mm). This i: barely adequate to stop 15 MeV alpha oartfe-es or 4 MeV protons, so the use of such detec­ tors i s very limited even for particle experiments. Fortunately, this limitation was rapidly circum- ventpd due to the work of Pell at General Electric, who (for entirely unrelated purposes) demonstrated that lithium, an i n t e r s t i t i a l donor, could be drifted In an electric f i e l d at moderate temperatures to virtually completely compensate the acceptors in p-type silicon (and germanium). This work was rapidly exploited in semiconductor detectors to produce lithium-drifted detectors with sensitive regions up to several millimeters thick. By 1961, such detectors had become the primary spectroscopy tool in charged particle experiments. The next major step was obvious, but was delayed by the feeling on the part of many of us working in the f i e l d that large-scale use of liquid nitrogen was not acceptable. I t was clear that lithium- drifted germanium would provide the potential for high-resolution gamma-ray detection (albeit with much smaller efficiencies than Nal scintillation detectors). I t was equally clear that the small band-gap of germanium would require low temperature operation. Therefore, i t took some time for lithium-drifted germanium gamtia-ray detectors to appear (1964) but when they did, they were exploited very rapidly and they must be regarded as the dominant tool in the intensive period of nuclear spectroscopy In the mid to late 1960's. In 1965, the f i r s t non-nuclear application of semiconductor detectors started to develop. The availability of low-noise f i e l d effect transistors and their use with silicon detectors at low tempera­ tures realized adequate energy resolution to resolve the characteristic K x-rays of most elements. This made development of the f i e l d of energy-dispersive x-ray fluorescence possible and has led to a large commerical business which impacts on many areas of research far outside the nuclear area. The second major limitation in these earlier detectors and s t i l l a limitation, although less restrictive now, was the small thickness of the sensitive region of the detectors which limited their use to short-range particle detection. Of course, the small thickness also limits the gamma ray detection efficiency, but in these early days only charged particle detection was seriously considered. In the next part of this introduction, I will discuss detector physics in a l i t t l e more d e t a i l ; for the moment, two points should be noted: *This work was supported by the General Science and Basic Research Division of the Department of Energy under Contract No. K-7405-ENG-48." @default.
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• W237308641 title "SEMICONDUCTOR DETECTORS - AN INTRODUCTION" @default.
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