298 Philips tech. Rev. 39, 298-300, 1980, No. 11

Semiconductor detectors

w. K. Hofker

Between 1958 and 1968 we in the Philips group atIKO were working on the development of semicon-ductor detectors. This kind of detector is much moresatisfactory for measuring various types of radiationthan counters such as the Geiger-Müller counter. AGeiger-Müller counter can count the number of par-ticles, but it cannot determine the energy of the par-ticles [1]. This is because the charge it supplies to theelectronic circuit is not in general proportional to theabsorbed radiation energy. In the semiconductor de-tector this proportionality does exist, because thematerial is a solid. Furthermore, the absorptioncapacity of a solid is much greater than that of thecounter gas. The volume of a solid-state detector cantherefore be much smaller.

The first solid-state detectors used the scintillationscaused by radiation in certain substances - for in-stance sodium iodide activated with a little thallium.The scintillations were converted into an electricalsignal by a photomultiplier tube. Another type of de-tector, the conduction counter or 'crystal counter' [2]

made its appearance when it became possible to makepurer crystals and more sensitive amplifiers. In thistype of detector the solid (e.g. semiconductor mat-erial) is provided with conducting contacts. Thecharge released in the solid by the radiation is meas-ured directly.

The ability to make semiconductor detectors thathad a diode characteristic (a P-N junction) led to aconsiderable improvement in the energy resolution.In addition, these semiconductor detectors are oftenfaster than the ones mentioned above. If a sufficientlyhigh voltage is applied to the semiconductor diode inthe reverse direction, a region is created that has ahigh electric field-strength and hence few free chargecarriers: the depletion layer. An ionizing particle thatpenetrates into this region forms a track of electron-hole pairs. Under the influence of the field the elec-trons and holes move in opposite directions towardsthe electrodes, and a current pulse is produced in theexternal circuit. The electric charge in the pulse is ameasure of the energy that the particle has given up inthe depletion region.The semiconductor detector was introduced by

K. G. McKay [3] as early as 1949, but not until 1956did it attain practical importance in the form of the

Dr Ir W. K. Hojker is with Philips Research Laboratories, Amster-dam Department.

surface-barrier detector [4]. A detector of this kindconsists of a wafer of silicon that has a high resistanceand has a thin layer of metal applied to it (fig. 1). ASchottky barrier is formed between the silicon and themetal layer. When a reverse voltage is applied a de-tection-sensitive layer about 0.5 mm thick is formed;this is sufficient for measuring alpha radiation. How-ever, the layer is too thin to detect gamma radiationor X-radiation.

vFig. 1. Diagram of a surface-barrier detector with a voltage appliedto it in the reverse direction. In metal electrode, usuallya thin goldlayer that has been deposited by evaporation. n N-type substrate.d thickness of the layer in which there is a strong electric field. Thethickness of this layer is proportional to VëVwhere {! is the resist-ivity of the basic material and V is the voltage across the detector.The electrons and holes released by an ionizing particle r in thelayer of thickness d produce a current pulse that is applied to anamplifier connected to VI and V2•

Further developments enabled silicon crystals ofeven higher resistance to be made. This was achievedby diffusing lithium into the material, creating adetection-sensitive layer 5 to 10 mm thick. This wassufficient for measuring X-radiation and high-energyparticles, e.g. deuterons at about 25 MeV [5]. If ger-manium is used instead of silicon the same technologycan be used for making detectors for measuringgamma radiation. These germanium detectors gave apreviously unattainable energy resolution for gammaradiation (fig. 2).As well as being used to detect radiation and par-

ticles for nuclear research, semiconductor detectorsare also employed in many other kinds of investiga-tion, such as materials analysis - with neutronbeams, ion beams and X-radiation - and medicalresearch - with gamma-ray cameras and for dosi-metry.The studies made by the Philips group have mainly

been concerned with the manufacture of detectorswith highly reproducible characteristics. We shall nowdiscuss some of the results in more detail.

Philips tech. Rev. 39, No. 11 SEMICONDUCTOR DETECTORS 299

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Fig. 2. Part of a gamma spectrum - of holmium 166 m - recorded by a germanium detector.Each dot represents the number of pulses N in an energy range of 2 keY as a function of theenergy E. A very large number of separate lines can be seen: the quantum energy is given for mostof them. The dashed line at the top shows the sarne spectrum, measured with an NaI(TI) seintil-lation detector. (For clarity this spectrum has been shifted upwards in the figure.) The data weremade available by Or Ir J. Konijn of IKO.

Results from our investigations

Surface-barrier detector

For surface-barrier detectors [6] we have developeda method for treating a surface of a few square centi-metres. The detectors produced were unaffected by areverse voltage of more than 100 V. The reverse cur-rent - which is responsible for the noise - was keptto below 1 ~A. Unlike most of the detectors that wereavailable at the time, surface-barrier detectors weresuitable for use in a high vacuum. (in most experi-ments the particles have to be observed in a vacuum.)Our technology also enabled a high output to be ob-tained in quantity production.

Energy-loss detector

In a surface-barrier detector only part of the thick-ness between the front and the back of the detector issensitive enough to detect a particle. In the remainderof the thickness the particle does in fact lose energy,

[1] See for example w. K. Hofker, GeigerMuller counters, thisissue, page 296.

[2] P. J. van Heerden, The crystal counter, Thesis, Utrecht 1945.P. J. van Heerden, Physica 16, 505, 1950.P. J. van Heerden and J. M. W. Milatz, Physica 16, 517,1950.

[3] K. G. McKay, Phys. Rev. 76,1537, 1949.[4] J. Mayer and B. Gossick, Rev. sci. Instr. 27,407,1956.[5] More information on Si(Li) and Ge(Li) detectors can be found

for example in W. K. Hofker, Semiconductor detectors forionizing radiation, Philips tech. Rev. 27, 323-336, 1966.

[6] See for example W. K. Hofker, K. Nienhuis and J. C. Post,a-particle speetrometry with semiconductor detectors, Philipstech. Rev. 30, 13-22, 1969.

Fig. 3. Checkerboard detector, with a set of electrodes in the formof strips on the front and back. The strips at the front are at rightangles to those at the back (see mirror image).

300 SEMICONDUCTOR DETECTORS Philips tech. Rev. 39, No. II

but it is impossible to convert this energy loss into anelectrical signal. In certain experiments, however, it isnecessary to place a 'thick' detector behind this 'thin'detector. The identity of the particle can then bedetermined. A particle loses part of its energy in thethin detector, and the remainder in the thick detector,and the product of the two differs for different typesof particle. However, it is then necessary to be able tomeasure the entire energy loss in the thin detector,which is then called an energy-loss detector.

With the same technology that we had used formaking surface-barrier detectors we have also madeenergy-loss detectors. In these detectors an aluminiumcontact layer is applied to the thin oxide layer at therear. In spite of the high field-strength at this contactthe reverse current, and hence the noise, have beenfound to be very small.

Checkerboard detector

The principle of the energy-loss detector was usedto develop the 'checkerboard detector' [7]. Instead ofcontinuous contact surfaces this detector has contactstrips on both sides. The strips on one side are per-pendicular to those on the other side, hence the name(fig. 3). A captured particle induces an electricalsignal in one of the strips on either side of the detec-tor, so that the position where the particle entersthe detector is within the' field' where the activatedstrips intersect. A checkerboard detector thereforeeffectively consists of about 90 fields. In the BOL [8]

detector telescopes (fig. 4) the check er board detector- thickness about 0.3 mm - was used as an energy-loss detector. Compared with a conventional energy-loss detector a checkerboard detector has ten timesthe angular resolution for the same value of the totalarea, i.e. for the same solid angle.

Reliability

In a BOL detector telescope [8] the checkerboarddetector was combined with one or more Si(Li) de-tectors [5] - each about 5 mm thick - which werealso developed by the Philips group at IKO. A total ofabout 100 semiconductor detectors were used in theBOL instrument. For the proper functioning of BOLit was therefore extremely important that the detec-tors should be very reliable.

Even stricter requirements for reliability are therule in space studies. Several space laboratories haveselected Elcoma surface-barrier detectors for theirprojects. These detectors were manufactured with thetechnology developed by the Philips group at theInstitute. One of the users is the Utrecht Laboratoryfor Space Research, who are using the detectors tomeasure the energies of protons in the NASA satellite

ISEE-3, launchedon 12th August 1978. These satellitestudies are being made to try to track down sources ofproton radiation in interplanetary space to find out ifthere is any correlation with solar flares. The satelliteexperiment includes three telescopes, each equippedwith a Philips detector (fig. 5) [9]. The experiment isexpected to continue for several years.

Fig. 4. Detector telescope for BOL. The shiny patch behind thecheckerboard detector is the surface of an Si(Li) detector. The wiresprovide the electrical connections from the check erboard strips toamplifiers (nor shown).

Fig. 5. Flight model of the proton spectrometer, now mounted inthe NASA satellite ISEE-3. The instrument has three telescopes,each with detection equipment for measuring the energy and direc-tion distribution of proton fluxes in interplanetary space. Each ofthe relescopes has a Philips surface-barrier detector for determiningthe proton energy.

[7] W. K. Hofker el ai., IEEE Trans. NS-I3, No. 3, 208,1966.[s] See for example R. van Dantzig, BOL, this issue, page 302.[9] J. J. van Rooijen, G. J. van Dijen, H. Th. Latleur and

P. Lowes, Space Sci. Instr. 4, 373, 1979.

Philips tech. Rev. 39, No. 11 301

The inner ring of the BOL scattering chamber. The hemispheres were attached to the ring,which also carried a mechanism for bringing different targets into the external beam of thesynchrocyclotron.