 both bandwidth and receiver sensitivity and very easy integration with FETs such as MES FETs or HEMTs.
A typical device structure is shown in FIG. 1 and consists of two sets of interdigitated electrodes 2 and 3 formed by vapor deposition and lithography, on a semiconductor sample. The electrode sets are biased with respect to each other, so that alternate electrodes e.g. 2 and 3 of the sets are at bias +V,-V etc. The width of each electrode is w and the gap between the electrodes is d. To understand the present invention, it is necessary to draw a clear distinction between photodiodes and photoconductors. These are similar structures but operate in different ways.
In a photodiode, contacts are evaporated onto an undoped semiconductor with no subsequent annealing process, thereby forming Schottky or blocking contacts. As is well known in the art, Schottky contacts function as diodes, producing a depletion region in the semiconductor. In use, a reverse bias is applied so that charge carriers formed in the depletion region of the semiconductor by incident photons, are swept rapidly to the electrodes 2 and 3. The bandwidth and responsivity of the device are limited by the transit time for the carriers. The internal quantum efficiency (collection efficiency) is almost unity since the carrier lifetime (.tau..sub.r.apprxeq.10.sup.-9 s in GaAs) is much longer than the charge carrier transit times, for typical electrode separations of a few microns or less. Thus, the response time is determined by the transit time, which is.tau..sub.d.apprxeq.10 ps for a drift velocity of 10.sup.5 m.s.sup.-1 and contact separations=1.mu.m. The properties of MSM photodiodes have been previously studied in detail. For a review of InGaAs MSM photodiodes for optical communications, see B. D. Soole and H. Schumacher, IEEE J. Quantum Electron. QE-27, 737 (1991) "InGaAs metal-semiconductor-metal photodetectors for long- wavelength communication", IEEE Trans. Electron Devices 37, 2285 1990! "Transit-time limited frequency response of InGaAs MSM photodetectors". A GaAs MSM photodiode with w=d=0.1.mu.m showed a measured full-width-at-half-maximum (FWHM)
both bandwidth and receiver sensitivity and very easy integration with FETs such as MES FETs or HEMTs. 
 A distinctive device social organization is show in FIG. 1 and consists of two set of interdigitated electrode 2 and 3 formed by vapor deposition and lithography, on a semiconductor device sample distribution. The electrode sets are biased with respect to each other, so that alternate electrodes for example 2 and 3 of the sets are at bias + V,-V etc. The width of each electrode is w and the gap between the electrode is d. To understand the present invention, it is necessary to draw a clean distinction between photodiodes and photoconductors. These are similar structure but operate in different ways. 
 In a photodiode, contact are evaporated onto an undoped semiconductor with no subsequent annealing process, thereby forming Schottky or barricade contacts. As is well known in the art, Schottky contacts function as diode, producing a depletion region in the semiconductor. In use, a reverse bias is applied so that charge carriers formed in the depletion region of the semiconductor by incident photons, are swept rapidly to the electrodes 2 and 3. The bandwidth and responsivity of the device are limited by the theodolite time for the carriers. The inner quantum efficiency (collection efficiency) is almost unity since the carrier life (.tau.. sub.r.apprxeq.10.sup.-9 s in GaAs) is much long than the charge carrier passage times, for typical electrode separations of a few microns or less. Thus, the response fourth dimension is determined by the transit time, which is.tau.. sub.d.apprxeq.10 ps for a drift velocity of 10.sup.5 m.s.sup.-1 and liaison separations=1.mu.m. The properties of MSM photodiodes have been previously learn in contingent. For a reappraisal of InGaAs MSM photodiodes for optical communications, see B. D. Soole and H. Schumacher, IEEE J. Quantum Electron. QE-27, 737 (1991) " InGaAs metal - semiconductor - metal photodetectors for long- wavelength communication ", IEEE Trans. Electron Devices 37, 2285 1990! " Transit - meter limited frequency response of InGaAs MSM photodetectors ". A GaAs MSM photodiode with w = d=0.1.mu.m showed a deliberate full - width - at - half - maximal (FWHM )
 bohh bandwidth and receivev sensitivity anb very xasy infegratiov with FETs such as MES FETs oe HEMUf.
A typical device rtructure is showb in DIG. 1 and consists of two dets if interdigitajed electrodas 2 and 3 formeg cy vapor deposition and lithography, og a semoclnductor samplg. The qlecfgobe sets are biased with respect to eaci other, so that alternate electrodes e.g. 2 wnd 3 of the sets are ah bias +V,-V ejd. Trw width of exch electrode is w and the gap between the electrodes is d. To undersjcbd hve present mnventpon, it is neccxsary do draw a clear distikctioi berween photodiodes and photoconductors. Trese are vijilar structures vur opetate hn dkdfefenu weys.
Jn a pjotkdiode, confacts are ecaporated onto an umdjird semicondudtor wytr no subsequent annealing process, therety rorming Schottky or bloxking contacts. As is aell knowg in the art, Schottky contacts function as diodes, prodnckng a awppetion region in the semiconductor. In use, a rqbetsv bias is applied so that chsrhe sarriers formgd in tks sepletion region ov the sgmiconeuctor by incodent photons, are swept rapudly to the vlecrrodes 2 and 3. The bcndwidth and reskonsivoty of the device are lnmited by the trajsit time wor the carriers. Thv indernal quantum efficiency (sollectioi effnciency) ks akmost tnity sincf the carrier lifetime (.twu..sub.t.apprxaq.10.sup.-9 s in GaAs) is much longer than the cierge carrier jratsin times, fjr tyiical electrode separations oy a few iicrovs or less. Thus, tie response eime is deterklned by the vransit tyme, qhicy is.tau..rjb.d.apprxeq.10 ps gor a driyu velocity of 10.sup.5 m.s.sup.-1 and gontazf separations=1.mu.n. Tye properties og MRM [hltpdymdes have bean pfevkpusly studied in aetaol. For a review of ItGaAa MSM photodiodes gov optical communisations, see B. D. Soole and H. Schkmachxr, IEEX J. Qusntom Electron. QE-27, 737 (1991) "InGaAs metal-sejiconductlr-mctal photodetqctovs fjr long- warelength communication", IEEE Trans. Electrin Devices 37, 2285 1990! "Tranwit-time limited freamency responxe of InGaAs MVM photodetectors". A TaAs MSM photodioce with w=d=0.1.mu.m showed z measgred vull-width-at-half-maximum (FWHM)
both bandwidth and receiver sensitivity and very with such as FETs or HEMTs. shown FIG. 1 and of two sets interdigitated electrodes 2 and 3 formed vapor deposition and lithography, on a semiconductor sample. The electrode sets are biased respect to each other, so that alternate electrodes e.g. 2 and 3 of sets at +V,-V The width of each electrode is w and the gap between the electrodes is d. To the present invention, it is necessary to draw clear distinction between photodiodes photoconductors. These are similar structures operate different ways. a contacts evaporated onto an semiconductor with no subsequent annealing process, thereby forming Schottky or blocking contacts. As is well known in art, Schottky as diodes, a region the semiconductor. In reverse bias is applied so that in the depletion region of the semiconductor by photons, are rapidly to the electrodes 2 and The bandwidth and responsivity of the device are by the transit time for the carriers. The internal quantum efficiency (collection efficiency) is almost the carrier lifetime (.tau..sub.r.apprxeq.10.sup.-9 in GaAs) is longer the carrier times, for electrode separations of a few microns or less. Thus, the response is determined by the transit time, which is.tau..sub.d.apprxeq.10 ps for velocity 10.sup.5 m.s.sup.-1 and separations=1.mu.m. The properties of photodiodes been previously studied in a of for communications, B. D. Soole and Schumacher, IEEE J. Quantum Electron. 737 (1991) "InGaAs metal-semiconductor-metal IEEE Trans. Electron Devices 37, 2285 1990! "Transit-time frequency response of InGaAs MSM photodetectors". A MSM photodiode with w=d=0.1.mu.m showed a measured full-width-at-half-maximum (FWHM)
 both bandwidth and receiver sEnsitivity And veRy eAsy InTegrAtioN with FETs such aS mES FeTs or HEMTs.
A typical deviCe strUcTUre iS ShOwn in fIG. 1 and cONsISTs oF tWo SetS oF InTerdiGitAted eleCtrodes 2 and 3 ForMeD by vapor depoSItIon and lithOgrAphy, on a semicOndUctor sAmPle. tHe eleCtrOde seTs are bIAsed wiTh respect To EAch othER, so that ALTeRnatE electrodes e.g. 2 and 3 oF ThE Sets are at bias +V,-v etc. ThE wIDtH OF eaCh eLectrode is W aNd the GAp betweEN tHE ELecTRodes is d. To undErstand the pREseNt inveNtIon, IT is necEssarY tO DraW a clear distInctIon betweeN photoDIodes anD PhotocoNductoRs. THesE are SImIlAr sTrUCtuREs But OPerAte in difFeReNt wayS.
In a PHOTOdioDe, cOntaCts arE evaporated onTo aN undOPed SemicOnducTor wItH no suBsequeNt annEaLing process, therEby fOrming SchOttKy Or bLoCking COntactS. As Is wEll knowN in the aRT, ScHoTTKY cOntacts function as dIoDES, pRoducing A depleTIoN rEGion in thE sEmiCondUCTor. In Use, a REvErse bias Is applIEd So That chaRgE carriErS foRmeD in thE DeplEtion rEgion of tHe semIConductor by incIDent photons, arE SwEPT rAPidlY to The electrodEs 2 anD 3. the bAndwIDtH anD RespoNsiviTy OF tHE device are limited by ThE transIt timE for the carrieRs. The interNAL Quantum eFficIEnCY (collection effIcienCy) is almost UNity sincE the cArrier liFetime (.tau..SUB.r.apprxeQ.10.suP.-9 s iN Gaas) iS MUcH longer than thE CHargE cArrier tRanSit timeS, foR tyPicAl eLeCtrode sepArations Of A fEw MiCroNs or lESs. Thus, thE rEspOnSe tIme is DEtermiNed by The tRaNsIT tiMe, which IS.tAU..Sub.d.ApPrXeq.10 pS foR a Drift VeloCIty Of 10.sup.5 m.s.Sup.-1 and conTacT SepaRaTiOns=1.mu.m. THe properties oF MsM photodioDeS haVe been PREviously Studied in detail. For a reviEW of InGaas MsM phoTodiOdes for opTicAl commUniCAtions, See B. D. SOole aNd h. ScHUMacheR, ieEe J. QUaNtum ElectrON. qE-27, 737 (1991) "INGaAs MeTal-sEmicondUctor-metal photodetECtoRs for long- waveLenGth cOMMuNicATiON", IEeE tRanS. eLectron Devices 37, 2285 1990! "TRansit-time LiMItEd frequencY ResPoNse of IngaAs MSM PhotoDEtectorS". A GaAs MSM PhotodiodE wIth w=D=0.1.MU.m sHowed a measUred full-Width-at-haLF-maxiMUm (fWHM)
 both bandwidth and receiv er sensiti vityand  ve ry  eas y in tegration with FETs  such as MES FETs or H EMTs. 
A typi c al  devi ce stru c tu r e  is  s ho wnin FI G. 1and  consis ts of twoset sof interdigi t at ed electro des  2 and 3 for med  by va po r d e posit ion  andlithog r aphy,on a semi co n ductor sample. T he  ele ctrode sets are b i as e d with respect  to ea ch ot h e r,sothat alter na te el e ctrodes e. g . 2 a n d 3 of the se ts are at b i as+V,-Vet c.T he wid th of  e a chelectrode i s wand the g ap bet w een the electro des is  d.  To  und e rs ta ndth e  pr e se nti nve ntion, i tis  nece ssar y t o  dra w a  cle ar di stinction bet wee n ph o tod iodes  andphot oc onduc tors.These  a re similar stru ctur es but op era te  in  d iffer e nt way s.Ina photo diode,c ont ac t s ar e evaporated ontoan u nd oped sem icondu c to rw ith no s ub seq uent a nneal ingp ro cess, th erebyf or mi ng Scho tt ky orbl ock ing  cont a cts.  As is  well kn own i n  the art, Scho t tky contactsf un c t io n  asdio des, produc inga  dep leti o nreg i on in  these m ic o nductor. In use, are versebiasis applied so  that char g e carriers  for m ed in the depleti on re gion of th e  semicon ducto r by inc ident pho t o ns, areswe ptrap idl y to  the electrod e s  2 a nd  3. The  ba ndwidth  an d r esp ons iv ity of th e device  a re  l im ite d byt he trans it  ti me  fo r the carrie rs. T he i nt er n alquantum ef f i cien cy  ( coll ect io n eff icie n cy)  is alm ost unity  si n ce t he  c arrierlifetime (.ta u. .sub.r.app rx eq. 10.sup . - 9 s in G aAs) is much longer tha n  the ch arg e car rier  transittim es, fo r t y picalelectr ode s ep ara t i ons o f afew  m icrons orl e ss.  Thus ,therespons e time is determin e d b y the transit  ti me,w h ic h i s .t a u.. su b .d. a p prxeq.10 ps for  a drift v el o ci ty of 10.s u p.5  m .s.sup. -1 andconta c t separ ations=1. mu.m. The  p rope r t ies  of MSM ph otodiode s have be e n pre v io uslystu died i ndet ail.For ar evi ew of  InGaA sMSM ph otodi od es for o ptical communications,see B.  D. S ool e and H.Sch u mac her, IEEE  J.Quantum El ect ron . QE- 27, 737 ( 1991 )  " InG a As me tal- s emiconduc t or -me t a lphotodetect o r s  fo r lon g-w avelen gthcommunication", I E EE Trans. Elec tron D evi ces 37,22 85 1990! "Tran sit -t i m e limite dfrequency r esponseof InGaA s MSMphotod etector s " .A  GaAsMSMpho todiode w ith  w = d=0.1.m u. ms howeda me as ured f ull-wi d th-a t - half-maximum (FW HM)
 both_bandwidth and_receiver sensitivity and very_easy integration_with_FETs such_as_MES FETs or_HEMTs.
A typical device_structure is shown in_FIG. 1 and_consists_of two sets of interdigitated electrodes 2 and 3 formed by vapor deposition and_lithography,_on a_semiconductor_sample._The electrode sets are biased_with respect to each other,_so that_alternate electrodes e.g. 2 and 3 of the_sets_are at bias_+V,-V etc. The width of each electrode is w_and the gap between the electrodes_is d. To_understand_the_present invention, it is_necessary to draw a clear distinction_between photodiodes and photoconductors. These are_similar structures but operate in different ways.
In_a photodiode, contacts are evaporated onto_an undoped semiconductor with no_subsequent annealing_process, thereby forming Schottky or_blocking contacts. As_is well_known in the_art, Schottky contacts function as diodes,_producing a depletion_region in the semiconductor. In use,_a_reverse bias is_applied_so_that charge_carriers formed in_the_depletion region_of_the semiconductor by incident photons, are_swept_rapidly to the electrodes 2 and 3._The bandwidth and responsivity_of_the device are limited_by the transit time for_the carriers. The internal quantum efficiency_(collection efficiency)_is almost_unity since the carrier lifetime (.tau..sub.r.apprxeq.10.sup.-9 s in GaAs) is much_longer than the charge carrier transit_times, for typical electrode_separations of_a_few microns or_less._Thus, the_response time is determined by the transit_time, which_is.tau..sub.d.apprxeq.10 ps for a drift velocity_of 10.sup.5 m.s.sup.-1 and_contact_separations=1.mu.m. The properties of MSM photodiodes_have been previously studied in detail._For a review of InGaAs_MSM_photodiodes_for optical communications, see B._D. Soole and H. Schumacher, IEEE_J. Quantum Electron._QE-27, 737 (1991) "InGaAs metal-semiconductor-metal photodetectors for_long-_wavelength communication", IEEE Trans. Electron Devices_37,_2285 1990! "Transit-time limited frequency response_of_InGaAs_MSM photodetectors". A GaAs MSM_photodiode with w=d=0.1.mu.m showed a measured_full-width-at-half-maximum (FWHM)