Semi-conductors. Semi-Conductors. R. C. Tailor, Asso Prof. Recall Semiconductors. UT, M.D. Anderson Cancer Center

Transcription

1 Semicoductors v2k13ov R. C. Tailor, Asso Prof. UT, M.D. Aderso Cacer Ceter Refereces: Joh P. McKelvey, Solid State Physics, chapter8, Krieger Publishig Co, Malabar, FL. (1993). A.F. Mckilay, Thermol. Dosimetry. Med. Phys. Hadbook5, P123, Adam Hilger Ltd. Bristal & Hosp. Phys. Associatio. F.H. Attix, Itrod. to Radiol. Phys. & Rad. Dosim, Chapter15, P456, Publisher: Joh Willey & Sos (1986). F.H. Attix, W.C. Roesch, Eugee Tochili, Rad. Dosimetry, volii, Chapter14, P294, Publisher: Joh Willey & Sos? (1966). Gle F. Koll, Radiatio Detectio ad Measuremets, Chapter12 (P414) & Chapter13 (P492), Publisher: Joh Willey & Sos (1979). MOSFET: GS Ibbott, Ge Practice of Rad Oc Phys i 21 st Cetury, AAPM moograph # 26, P332. M Soubra et al., Evaluatio of a dualbias dual metaloxide lico, Med.Phys.21, P567 (1994). TP Ma et.al., Ioizig radiatio effect i MOS devices & circuits, Chapter3, P87, Publisher: Joh Willey & Sos (1979). Recall Semicoductors Next vaccat bad Gap ~ 1 ev Fully occupied bad Gap ~ 1/10 th of isulators. Charge carriers ~ less by (compared to metals). Resistivity ~ 1/T (ulike metals). Charge desity with T. This domiates the process meafreepath with T. Table133 i Gle Material Z Gap: ev ev per eh Commo: Ge Compouds: GaAs CdTe HgI Bi 2 S 3? 1.3? GaSe? 2.03? etc. Diamod (isulator): C 6 5.6? SemiCoductors Itrisic Crystal of tetravalet atoms like or Ge Extrisic (Doped) Small impurity (1 i ) has a large cotrollig effect (due to charge carrier # beig small). Impurity sites produce discrete eergy levels i the forbidde gap. type p type

2 type Petavalet impurity like P provides a extra e after establishig covalet bods with the eighbors. Majority carriers are e. P e p type Trivalet impurity like Al creates a deficit of e after establishig covalet bods with the eighbors. Majority carriers are h. Al h Cod. bad Cod. bad E ev E Valece bad Valece bad Diode (p juctio) Depletio regio acts as dielectric Ope circuit: I juctio regio, maj. charge carriers (e, h) combie. Regio becomes devoid of maj. charge carriers ad therefore acts as a dielectric. ptype regio becomes deficit i charge, ad type deficit i ve charge. Results i et ve ad ve charge across the juctio. Cotact pot. diff. grows to equil. value V 0, which prevets further diffusio of maj. charge carriers. I diff due to maj. Carriers = I drift due to mi. carriers. Forward Bias: Barrier height goes to V = V 0 Vext Width of depletio layer also goes. I diff goes, but I drift remais uchaged. As a result I et goes (ma to A). Reverse Bias: Barrier height goes to V = V 0 Vext Width of depletio layer also goes. I diff goes, but I drift remais uchaged. As a result I et goes (~ A). p i df p p V 0 ~ 0.5V ~ 500 m ( gree) i dr Diode i dosimetry: Radiatio causes io pairs, which i tur geerates curret. This radio curret ca be measured with a lowimpedace circuit (opamp). Geerates curret i A rage for therapy dose rates. Bias: NO Bias is advatageous because it elimiates the leakage curret which is strogly T depedet. As bias is decreased, leakage curret drops faster tha the radiatioiduced curret. (sigal / oise better without bias). Radiatio i Op. Output Useful for high spatialresolutio dosimetry (small fields, steep dose gradiets). Disadvatages iclude Radiatio damage, Directioal ad T depedece. E depedece is sigificat especially for xrays. This is miimized by use of highz pottig aroud the diode. Over all, it is ot suitable for absolute dosimetry.

3 Fast dosimetry: Fast cause permaet defects i lattice. This results i effective icrease i resistace. A costatcurret (~25 ma), passed i forward directio, produces ~1V drop across the juctio. The Vdrop icreases with dose. doses 0.1 to 0.5 rads have bee detected with a diode of base thickess 1.9 mm, maitaied at 0 0 C.Ed regios are heavily doped. The lightly doped pregio ear the juctio is called as pbase regio. Sesitivity icreases with icreasig base thickess. Diodes are composed of 0.05 mm thick p regio, the regio beig half the thickess. E respose ~flat over o.3 to 14 MeV. Below 0.2 MeV, respose decreases rapidly. Costat curret source ~25 ma P base 1V for NO dose 4V for dose 0.5 rad Reversedbias diode as io chamber: The diode, uder reverse bias, behaves much like ioizatio chambers. The reverse bias icreases the width of depletio regio (up to ~ 3mm). The type is extremely thi (typically 0.1 ). Radiatio produces (e, h) pairs i the depletio regio, which are collected by the pot. diff. across the layer as i a gaseous ioizatio chamber. Each particle track therefore produces a pulse of charge. For eergy spectroscopy, particle must loose etire eergy withi the depletio regio. Depletio width of ~3mm is adequate for ad shortrage charged particles. For spectroscopy, thicker depletio regio is eeded. PType Bias 10 to 500V Type gal to Semicoductors for spectroscopy Large depletio depths, as required for spectroscopy, are achieved i Ge() & () detectors (ext slides). Notice remarkable eergy resolutio of Ge () ad () detectors compared to NaI. E resolutio is better by a factor 5 to 10, although at the cost of lower coutigefficiecy (due to smaller size ad lower Z) compared to NaI. Spectrum from NaI Spectrum from Ge()

4 Ge() ad () detectors (Also called pi diodes (regios: ptype, itrisic, type) These detectors were made i 1960s. Later i 1970s, HPGe replaced them. Whether i use or ot, Ge() detector eeded to be stored at liquid N 2 temperature all the time. I (), mobility of ios is low eough at room T to permit temporary storage without coolig. Lower Z of () offers advatage for detectio of electros ad loweergy x rays. E resolutio of () is poorer tha Ge(). A process of driftig to icrease depletio depth beyod 3 mm is briefly show (ext slide). Makig Ge() or () After purificatio, Ge or crystal teds to be Ptype. Ge (p) thium is diffused thru oe surface. A atom takes a iterstitial site, doates a e to coductio bad, ad turs ito io. Near exposed surface, doated e outumber holes. ad the regio turs ito type. Uder reverse bias ad elevated T (~60 0 C), ios diffuse to deeper depths. Bias causes drift, ad T ehaces mobility of. Diffusio cotiues for days/weeks, evetually resultig i exact compesatio over depth up to ~1cm. Type ptype HPGe (High Purity Ge or Itrisic Ge) Developed to elimiate problem of storig Ge() at liquid N 2 temperature. However liquid N2 is still required durig operatio as leakage curret is excessive at room temperature. Large depletio thickess has bee achieved by combiatio of : High purity: 10 6 ppm (perhaps the mostpurified material ever produced). High reverse bias (~1000V) ve Bias ~ 1000V Eresolutio is similar to Ge() gal to Cofiguratio: ptype High Purity Type Ge NOTE: These days Ge() is o more i use, because equivalet large depletio depth is achievable with HPGe.

5 Trasistors: MOSFET (Metal Oxide Semicoductors FieldEffect Trasistors Marketed by Thomso & Nielse (ow BEST) as replacemet of diode dosimeters. Ref: Med.Phys.18, P542 (1991)Med.Phys. 21, P567 (1994), Med.Phys. 21, P1721 (1994) Ioizig Radiatio Effects i MOS devices & Circuits, by TP Ma, PV Dressedorfer Applicatios: DD, Profiles, Ivivo: Skidose, *Iteralorga dose * Use of MOSFET (with sigalrelay circuit i glasscapsule) was ivestigated aroud Specs: Active area 0.2 x 0.2 mm (Body 2 x 3 x 1mm thick) Retais sigal for up to ~ 6 moths (Fade~3% / 15 mi) gal reproducibility ~ 12 % Sesitivity ~ 3mV / cgy fe ~ 70 / 200 Gy, Shorter uder applied bias (High sesitivity) Price ~ $ 70 Loger uder zero bias (Low sesitivity) Advatages: High spatial resolutio, V. small Edep & Aisotropy, o Drate depedece. Miimal itrusio o patiet ski (small /lightweight). Trasistors: MOSFET (cot d) Gate ve I some desigs, Source & Substrate are coected together iterally. I others, geerally the two are coected together exterally. Cost. Curret ~100 A Metal O 2 p Chael p Source Drai Substrate: type Irradiatio produces (e,h) pairs i O 2. Durig irradiatio (zero bias to G ), holes get trapped ear O 2 iterface. Trapped Charge Dose A ve bias to G forms coductio chael, allowig curret flow (S to D). The bias voltage for a desired curret is called V TH (threshold bias). Durig readout, V TH eeds to be more ve to obtai the same curret. Shift i V TH Dose Shortcomigs: Aisotropy, sigificat T depedece (miimized by use of matchedpair), reproducibility ot better tha TLD, suffers from drift / fadig due to (e,h) recombiatio. V gs