BASE CAPACITOR COUPLED PHOTOSENSOR WITH EMITTER TUNNEL OXIDE FOR VERY WIDE DYNAMIC RANGE IN A CONTACTLESS IMAGING ARRAY

Abstract: A technique for decreasing the effective gain of a bipolar phototransistor at high light levels makes the image usable over a greatly extended range of illumination conditions. The effective current gain at high light levels is reduced by fabricating a "non-ideal" emitter, such as by inserting a thin 20 ANGSTROM tunnel oxide between the emitter and base junction. The tunnel oxide between the emitter and base serves as a variable resistor as well as a good junction for carrier injection from the emitter. The total base voltage is the sum of the oxide voltage and the intrinsic base voltage. At high image intensity, the bipolar phototransistor will gradually enter into the saturation mode, i.e., the base to collector junction is forward biased. The beta is thus reduced. The bias of the collector should be about 0.3 - 0.8V higher than the emitter at the 20 ANGSTROM tunnel oxide thickness for optimum operation.

Description: BASE CAPACITOR COUPLED PHOTOSENSOR WITH EMITTER TUNNEL OXIDE FOR VERY WIDE DYNAMIC RANGE EN A CONTACTLESS IMAGING ARRAY BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates to photosensing elements and. in particular, to a base capacitor coupled photosensor that incoφorates a thin tunnel oxide between the photosensor's emitter and base to reduce beta at high current levels, thereby greatly increasing the dynamic range of the imager. 2. Discussion of Related Art Eric Fossum, "Active-Pixel Sensors Challenge CCDs", Laser Focus World, pp. 83-87. June 1993, discusses emerging active-pixel sensor technology that is poised to replace charge coupled device (CCD) technology in many imaging applications. As discussed by Fossum. a CCD relies on charge shifting to rod out an image Since it is very difficult to achieve 1009c charge transfer efficiency in a CCD structure, performance is sometimes degraded below acceptable levels. In contrast to CCD technology, an active-pixel sensor operates similarly to a random access memory (RAM), wherein each pixel coniains ns own selection and readout transistors. The signal readout then takes place over conductive .v ires rather than by shifting charge. Thus, active pixel sensors improve upon CCD technology by providing advantages such as random access, nondestructive readout and intergrability with on-chip electronics In U.S Pat No 5.289.023. issued February 22. 1994, Carver Λ Mead discloses a photosensing pixel element that uses a bipolar phototransistor as both an integrating photosensor and a select device In Mead ' s preferred embodiment, ihe phototransistor is a vertical structure having its collector disposed in a substrate oi N-type silicon. The base terminal of the bipolar phototransistor, which compπses a p- type doped region disposed within the collector region, is utilized as the select node for the pixel. Conventional field oxide regions are employed to isolate the base regions of adioining phototransistors An n-doped polvsilicon line is disposed over the surface of the substrate and is insulated therefrom except in regions where it is in contact with the p-doped region Where the n-doped polysilicon is in contact with the surtace of the p-iype base region, it forms an n+ epitaxial region that serves as the emitter of the phototransistor The polysilicon line provides the emitter contact As further disclosed in (he '023 patent, a plurality of the Mead phototransistors may be arranged in an array of rows and columns The bases of ail phototransistors in a row of the arrav arc capacitivelv coupled together to a common row-select line, and the emitters ol .ill phototransistors in a column are integral with a column sense line. The input ol a sense amplifier is connected to the sense line ol each column of integrating photosensors The sense line is connected to the ins erting input of an amplifying element of an integrating sense amplifier. A capacitor, preferably a varactor. is also connected between the inverting input and the output of the amplifying element Exponential feedback is provided in the sense amplifiers tor signal compression at high light levels The outputs of the sense amplifiers are connected to sample/hold circuits The rows of the array are selected one at a time and the outputs of the sample/hold circuits tor each row are scanned out of the array while the pixel data for the next row are sampled. U.S. Patent No. 5.289.023 is hereby incorporated by reference in its entirety Bipolar phototransistors typically have a high current gain at high light level, where it is not wanted, and a low current gam at low current levels, where it is most needed. The photosen.sor disclosed in the '023 patent addresses the low-intensity problem by using pulsed addressing of the capacitively- coupied base Howev er, the problem at high intensity is not addressed SUMMARY OF THE INVENTION The present invention provides a technique for decreasing the effective gain of a bipolar phototransistor at high light levels, thereby making the imager usable over a greatly extended range of illumination conditions. The effective current gain at high light levels can be reduced by a "non-ideal" emitter, such as by inserting a thin 20A tunnel oxide between the emitter and base junction. The tunnel oxide between the emitter and base serves as a variable resistor as well as a good junction for earner injection from the emitter. The total base voltage is the sum of the oxide voltage and the intrinsic base voltage. Λt high image intensity, the bipolar phototransistor will gradually enter into the saturation mode, i.e., the base to collector junction is forward biased. The beta is thus reduced. The bias of the collector should be about 0.3 - 0.8V higher than the emitter at the 2θA tunnel oxide thickness for optimum operation. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings w hich set forth an illustrative embodiment in which the principles of the invention are utilized. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-section drawing illustrating a prior art base capacitor coupled bipolar phototransistor. Fig. 2 is a cross-section drawing illustrating a base capacitor coupled bipolar phototransistor in accordance with the present invention. Fig. 3 is a graph showing the I-V characteristics across the tunnel oxide used in the Fig. 2 structure. Fig. 4 is a graph showing base voltage versus tunnel oxide voltage in the Fig. 2 structure. Fig. 5 is a graph showing current saturation in the sense line ol the Fig. 2 structure. Figs. 6a-6c are simple graphs showing minority carrier density as a function of position in the base region of the Fig. 2 structure for three base voltages. DETAILED DESCRIPTION OF THE INVENTION Fig. 1 shows a cross-section of the contactless bipolar phototransistor 100 disclosed by Mead in the above-cited '023 patent. The phototransistor 100 is intended to be exemplary of an array of rows and columns of such devices separated by conventionally formed field oxide (Fox ) regions, as is well known in the art. Also, while Fig. 1 shows the phototransistor 100 fabricated in an N-well 102 formed in a P-/P+ substrate 104. one of ordinary skill in the art will appreciate thai the starting semiconductor material could be an N-/N+ substrate. P-type region 106 is formed in N-well 102 between the birds beaks of the field oxide (Fox) regions to provide the base of the NPN phototransistor 100. Epitaxial N+ region 108 is formed where the polysilicon column sense line 1 10 contacts the base region 106 to form the phototransistor'. s emitter. A conductive row select line 1 12, comprising polysilicon, metal suicide or any other conductive layer available in the selected fabrication process, overlies a portion of the base region 106 and is separated therefrom by a layer of dielectπc material, typically silicon dioxide. As discussed in the '023 patent, for a read operation, the phototransistor 100 is biased as follows: ^ cυ i ~ C V ' V .._ C|| = 5V. At these conditions, and at high image intensity, the bipolar device 100 will not enter into saturation. Reference is now made to Fig. 2, which shows a phototransistor 200 that is similar to that disclosed in the '023 patent, but which includes ~2θA of tunnel oxide between the polysilicon column sense line 210 and the N+ epitaxial emitter region 208. Thus, the present invention provides a mechanism for decreasing the effective gain of the phototransistor 200 at high light levels, thereby making the imager useable over a greatly extended range of illumination conditions. In order to extend the dynamic range, the emitter sense line 210 is biased / 5 slightl) more negative than the common collector 202 of the arrav In the disclosed embodiment, the collector 202 is 0 5V more positive than the sense line 210 (e g , V t , - 4 5 V V„ „.„ = 5V) Furthermore the emitter junction 208 is arranged to have a non-idealuv factor less than that of the collector 202 especially at high currents This property can be arranged by for example as shown in Fig 2, inserting a thin approximately 2θA oxide between the emitter junction 208 and the sense line 210 The I V characteristic ot the oxide is a complex, near-exponential tunction of the voltage across the oxide whereas the emitter current is a steeper tunction of the base voltage multiplied by the exponential of the collector to emitter voltage, as shown in Fig 3 More specifically for purposes of this discussion, the I-V characteristics of the oxide are approximated as follows l ox L κT Whereas the junction current is given by In these expressions all voltages are expressed in terms of the thermal v oltage kT/q and the currents in terms ot the |unUion / The oxide is in scries with the emitter junction, but not with the collector junction P bHe, = K ej, + V o oxr and I ox = I j Therefore the total voltage is given b\ K* = kι/(l + 1^ - b/ 0 Because the collector voltage is offset from the emitter by .1 constant voltage 1 we have The current Gain is thus __£ = λ e ^ l bc At low base voltages the emitter current can flow through the oxide with voltage drop small compared with the base-emitter voltage At higher base voltages the oxide voltage increases faster than the base- emitter voltage, as shown in Fig 4 At very high base voltage, corresponding to very high light level, the voltage across the oxide approaches the collector emitter voltage, and the current into the sense line saturates This saturation happens very gracefully, as shown in Fig 5 The design criterion is that the oxide current at the full collector-emitter voltage multiplied by the sense enable pulse duration be just equal to the full-scale charge capability of the sense amplifier The minoπtv earner density as a function of position in the base region 206 of the bipolar transistor is shown in Figs 6a 6c for three base voltages, corresponding to three intensity levels near the level where it is desired that saturation occur In Fig 6a, the voltage drop across the oxide is much less than the collector-emitter voltage, and all current flows to the emitter In Fig 6b, the transistor is already beginning to saturate, and the effective current gain is already several times lower than its normal v alue. In Fig 6c, a condition reached at very high light levels, the collector current actually reverses, and the base charge accumulated from integrating the light is drained off into the collector Thus, a technique has been disclosed whereby an array of bipolar phototransistors can be utilized over an extremely wide range of light levels without saturating the sense amplifiers at the end of each sense line. This graceful saturation is accomplished without complication of the drive or sense electronics, and without sacrificing density in the array. The structure of the phototransistor 200 shown in Fig 2 may be fabricated according to essentially the same methods disclosed in the '023 patent, except that the 2θA tunnel oxide must be grown betore deposition of the emitter polysilicon This oxide may be grown by thermal oxidation at low pressure before CPCVD poly deposition in the same process chamber or furnace Alternatively, the 2()A tunnel oxide and associated read bias techniques may be incorporated into the structures disclosed in the following two co-pending and commonly-assigned applications, filed of even date herewith ( I ) Appln Serial No , filed , 1995, by Bergemont et al., titled Method of Manufacturing a Thin Poly Capacitor Coupled Contactless Imager with High Resolution and Wide Dvnamic Range (Docket No NSC 1-62200) and (2) Appln Serial No , filed , 1995, by Bergemont et al . titled Method of Manufacturing a Capacitor Coupled Contactless Imager With High Resolution and Wide Dynamic Range (Docket No NSC 1 -62300) Both of the aforementioned applications arc hereby incorporated by reference It should be understood that various alternatives to the embodiments of the invention descnbed herein may be employed in practicing the invention It is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby

Claims: What is claimed is 1 A method of manufactuπng a capacitor coupled contactless image structure in a semiconductor substrate, the method compnsmg forming a collector region of N-type conductivitv in the semiconductor substrate, forming a base region of P-type conductivity in the collector region, forming tunnel oxide about 2θA thick on a specified surtace ot the base region, forming a n-doped silicon emitter contact on the tunnel oxide such that an n+ epitaxial emitter region is defined in the base region beneath the emitter contact, forming silicon dioxide over the emitter contact and exposed surfaces of the base region; and forming a layer of second polysilicon on the silicon dioxide to define a base row select line 2 A capacitor coupled contactless imager structure formed in a semiconductor substrate, the image structure compπsing a collector region of N-type conductivity formed in the semiconductor substrate, a base region of P-type conductivity formed in the collector region, an n-doped polvsilicon emitter contact formed ov er a surface of the base region but separated trom said surface by tunnel oxide about 2θA thick such that an n+ epitaxial emitter region is tormeα in the base region beneath the emitter contact. silicon dioxide formed over the emitter contact and exposed surfaces of the base region, and a lav er ot second polvsilicon formed on the silicon dioxide to define a base row select line

Inventors: BERGEMONT ALBERT, CHI MIN-HWA, MEAD CARVER A, BERGEMONT, ALBERT, CHI, MIN-HWA, MEAD, CARVER, A.
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1996
Main classification: H01L27
IPC codes: H01L27, H01L31