Thin film semiconductor device

Abstract: A thin film semiconductor device includes a gate electrode (401), a gate insulating layer (402), a thin film semiconductor layer (403), an ohmic layer (406) source and drain electrodes (404,405) and a protective layer (409). The protective layer contains an impurity for controlling conductivity.

Description: The present invention relates to thin film transistors such as those used in displays, image scanners, or the like. It concerns also arrays thereof and the operational use of such arrays. Related Background Art Along with recent office automation, input/output devices such as a display and an image scanner have received a great deal of attention as man-machine interfaces of OA equipment such as a wordprocessor, a personal computer, and a facsimile machine. Strong demand has arisen for lightweight,low-profile, low-cost input/output devices. Judging from these viewpoints, thin film transistors such as of hydrogenated amorphous silicon or polysilicon semiconductor materials are formed on an insulating substrate having a large area, and an active matrix liquid crystal display using thin film transistors or a photoelectric transducer apparatus using photosensors have been extensively developed. Fig. 1A is a sectional view illustrating a structure of a conventional thin film transistor (to be referred to as a TFT hereinafter). A gate insulating film 2 is deposited on a gate electrode 1, and a thin film semiconductor layer 3 consisting of, e.g., hydrogenated amorphous silicon (to be referred to as a-Si:H) and serving as a channel region is formed on the gate insulating film 2. N⁺-type layers 6 are formed between the thin film semiconductor layer 3 and metal electrodes as source and drain electrodes 4 and 5 to serve as ohmic junctions for electrons and blocking junctions for holes. The resultant transistor operates as an n-channel transistor. This transistor structure has a thin film semiconductor surface defining the upper surface of the channel region. Fig. 1B is a plan view illustrating the transistor structure of Fig. 1A. Fig. 1B particularly illustrates a TFT having a planar (the electrodes 4 and 5 in Fig. 1B have an interdigital structure) electrode structure proposed to prevent an increase in channel length and solve conventional fabrication problems. The TFT shown in Figs. 1A and 1B can be applied as a secondary photocurrent type photosensor (e.g., Japanese Kokai 60-101940). Figs. 2A and 2B are views for explaining the steps in manufacturing the conventional TFT shown in Figs. 1A and 1B (the method of manufacturing this TFT is shown in, e.g., Japanese Kokai 63-9157). A substrate comprises a glass substrate G, and Cr serving as a gate electrode 1 is formed on the glass substrate G. Cr is selectively etched by a photolithographic technique to form the gate electrode 1. A 300 nm (3000-Å) thick silicon nitride film 2 serving as a gate insulating film, a 500 nm (5000-Å) thick a-Si:H layer 3 serving as a semiconductor layer, and a 150 nm (1500-Å) thick n⁺-type layer 6 are continuously formed on the glass substrate G by, e.g. plasma CVD. Aluminium serving as a material for source and drain electrodes 4 and 5 is then deposited by sputtering. A photosensitive resin 8 is applied to the entire surface (Fig. 2A). The resin 8 is patterned into a desired shape upon exposure and development. The aluminium layer serving as the source and drain electrodes is patterned using the resist pattern (Fig. 2B). In this case, the photosensitive resin 8 as a resist pattern is present on the electrodes. The n⁺-type layer is etched to a depth of 180 nm (1800-Å) by etching such as RIE (Reactive Ion Etching) using the photosensitive resin as a mask. The photosensitive resin is then removed. Interelement isolation between TFTs is performed to prepare each TFT in Fig. 1A. After the above process, the surface of a semiconductor thin film of a conventional thin film transistor is susceptible to the influence of the ambient atmosphere. When oxygen gas or steam is directly brought into contact with, and adsorbed and diffused in the surface of the semiconductor thin film, electrical characteristics of the semiconductor thin film vary because the semiconductor thin film has a very small thickness. For this reason, it has been proposed to cover the element surface with an insulative protective film consisting of e.g. silicon nitride (Si₃N₄), aluminium oxide (Al₂O₃), or silicon oxide (SiO₂) (e.g. Japanese Kokai 59-61964). A method using a polyimide resin film polymerised in a heat treatment as a protective film is also proposed. In order to further improve stability of the element, a method of stacking a second protective film of the same material as that constituting the thin film semiconductor layer 4 on the polymerised polyimide resin film is also proposed (e.g. Japanese Kokai 1-137674). A plurality of thin film transistors and photosensors formed on a large substrate by the method described with reference to Figs. 2A and 2B are required to exhibit uniform characteristics within the large substrate. In the thin film transistor or photosensor formed in the process of Figs. 2A and 2B, particularly when RIE (Reactive Ion Etching) is used in the step of etching the n⁺-type layer in Fig. 2B, the surface of the thin film semiconductor layer is damaged by incident ions and the like by RIE, and the electrical characteristics are degraded. In addition, uniformity of the electrical characteristics tends to be lost by a distribution or the like of the RIE incident ions. For example, a threshold voltage which determines the operating characteristics of the thin film transistor varies within the range of several volts on the substrate, and a decisive drawback may be present in an actual application. For example, a display state is greatly changed in an active matrix display. In a sensor, the photocurrent and the dark current as the basic characteristic values greatly vary between elements, and the quality of the read image is greatly degraded, thus posing decisive problems in basic performance. When a protective film on TFT photosensors having nonuniform characteristics consists of an organic material such as polyimide, stability such as sufficiently high humidity resistance against envitonmental conditions cannot be expected. On the other hand, when the protective film consists of an inorganic material (e.g., a-SiN:H) which directly contacts the semiconductor, undesirable electrical characteristics or characteristic distributions are caused in accordance with the formation process of this protective film and the composition of the resultant protective film. For example, Hiranaka et. al. reported the relationship between the composition of the insulating layer and the thin film semiconductor layer 3 as a problem of a TFT gate interface. More specifically, according to Hiranaka et. al., as a problem of the gate interface between the gate insulating film 2 (SiN x :H) and the thin film semiconductor layer 3 (a-Si:H), the gate insulating film composition largely influences a band state of the thin film semiconductor layer 3 (J. Appl. Phys. 62(5), from P. 2129 (1987) and J. Appl. Phys. 60(12), from P. 4294 (1986)). The composition of the insulating layer as a protective film is also assumed to greatly influence humidity resistance. A fabrication process of a conventional thin film transistor will be described with reference to Figs. 3A to 3F. A gate electrode 302 is selectively formed on an insulating substrate 301. A gate insulating film 303, a thin film semiconductor layer 304, and an n⁺-type layer 305 are sequentially deposited on the insulating substrate 301 by the plasma CVD method (Fig. 3A). An electrode layer 310 serving as source and drain electrodes 306 and 307 is deposited, and a photoresist 308 for patterning the source and drain electrodes 306 and 307 is applied to the electrode layer 310 (Fig. 3B). The photoresist is patterned into a desired pattern, and the electrode layer 310 is etched by wet etching using the photoresist 308 as a mask, thereby forming the source and drain electrodes 306 and 307 (Fig. 3C). The n⁺-type layer 305 is etched using the photoresist 308 as a mask, and the photoresist 308 is removed (Fig. 3D). After the n⁺-type layer 305 is patterned to have a desired shape by using the photoresist pattern as a mask, the insulating layer 303, the thin film semiconductor layer 304, the n⁺-type layer 305, and the electrode layer 310 are etched to perform element isolation (Fig. 3E). A protective layer 309 is formed on the surface of the thin film semiconductor element by plasma CVD to finish the element (Fig. 3F). The conventional thin film transistor type photosensor is manufactured as described above. In the thin film transistor and the thin film transistor type photosensor formed by the method described with reference to Figs. 3A to 3F, when the n⁺-type layer is etched by reactive ion etching (to be referred to as RIE hereinafter), the surface of the thin film semiconductor layer is damaged by ions incident on the surface of the semiconductor surface. The important electrical characteristics of the thin film transistor type sensor may therefore be greatly degraded. When such an element is formed on a large substrate having a large size, the electrical characteristics of the thin film transistor and the thin film transistor type photosensor have a distribution having a wide range. In addition, when the surface protective layer of the thin film semiconductor consists of a silicon nitride film, electrical characteristic degradation such as an increase in dark current in the n-type thin film semiconductor surface may occur. As a result, the conventional thin film transistor and the conventional thin film transistor type photosensor cause degradation of display characteristics in a display and degradation of gradation by a low S/N ratio in a sensor in accordance with the degradation of the electrical characteristics and their wide distribution. In this manner, decisive problems associated with basic performance of the element often occur. The present invention is provided as a solution to the problems discussed above. In accordance with the invention (as defined in claim 1 of the claims appended) the protective layer of insulating material contains a p-type dopant impurity. This results in a depletion of electrons at and in the immediate vicinity of the semiconductor active layer. As a result of this the direct influence of the protective film on the electrical properties of the thin film transistor is much reduced. The freedom of choice of protective layer material accordingly is increased, and indeed this material may be chosen to optimise resistance to high humidity. The results reported below also demonstrate improvement in dark current, photocurrent, and temperature characteristics. It is found, however, for large area devices, e.g.photosensor arrays, that there is a spatial variance in the properties of the individual transistors, particularly in the temperature characteristic. This variance may nonetheless be accommodated by providing means of adjusting the respective operating point voltages applied to each transistor gate electrode and adjusting such voltages. While the above defined invention results in depletion of electrons at the surface of the active layer it is acknowledged that there is disclosed in an article entitled: "Step doping in hydrogenated amorphous silicon thin-film transistors for threshold voltage shifts", Matsumoto et al., Journal of Applied Physics 66, No. 10, pages 5058-5062 (November 1989) a structure having a step-doped semiconductor layer. This latter is provided as a means of controlling the threshold voltage in such devices. In particular, the effect is to deplete the undoped layer and shift the voltage threshold. In the accompanying drawings: Figs. 1A and 1B are sectional views of a conventional thin film transistor; Figs. 2A and 2B and Figs.3A to 3F are views for explaining the steps in manufacturing the conventional thin film transistor; Fig. 4 is a sectional view of a field effect transistor embodying the present invention; Figs. 5A to 5F are sectional views for explaining the steps of manufacturing the transistor of fig. 4; Fig. 6 is a view showing the positions of the transistors of fig. 4 arranged on a glass substrate of large area; Fig. 7 is a graph showing the distribution of dark currents Id and threshold voltages at operating points of the thin film transistors on the line A-A' of Fig. 6; Fig. 8 is a graph showing the distribution of room-temperature photocurrents and temperature characteristics at operating points of the thin film transistors on the line A-A' in fig. 6; Figs. 9A and 9B are energy band diagrams at points 0 and A₀ along the line A-A in Fig. 6; Fig. 10 is a graph showing the temperature characteristics of the photocurrent as a function of the gate voltage; Fig. 11 is a circuit diagram of a perfect contact sensor circuit; Fig. 12A is a plan view showing part of the perfect contact sensor formed on the basis of the circuit shown in Fig. 11; Figs. 12B and 12C are sectional views of the perfect contact sensor shown in Fig. 11; Fig. 13 is a graph showing comparison of the temperature characteristics of outputs under an illuminated condition of the perfect contact sensor shown in Figs. 12A, 12B, and 12C; Fig. 14 is a graph showing high-temperature, high-humidity test results of the perfect contact sensor; Figs. 15A and 15B are energy band diagrams estimated from the high-temperature, high-humidity test; Fig. 16 is a sectional view showing a structure of a lensless perfect contact sensor embodying the present invention; and Fig. 17 is a sectional view showing a perfect contact sensor with a lens also embodying the present invention. The solution to the aforementioned problems is to provide a protective insulating layer containing an impurity for controlling conductivity. The impurity for controlling conductivity is selected from elements belonging to Group III of the Periodic Table. More specifically, a p-type impurity is selected. The present invention will be described in detail with reference to a preferred embodiment in conjunction with the accompanying drawings. Fig. 4 is a sectional view of a thin film transistor type photosensor embodying the present invention. Figs. 5A to 5F are sectional views for explaining the steps in manufacturing the thin film transistor type photosensor shown in fig. 4. Cr for forming a gate electrode 302 is selectively formed on an insulating substrate 301, and a 300 nm (3000-Å) thick hydrogenated amorphous silicon nitride film (a-SiN x :H; to be referred to as a silicon nitride film) serving as a gate insulating film 303, a 500 nm (5000-Å) thick hydrogenated amorphous silicon layer (to be referred to as a-Si:H hereinafter) serving as a thin film semiconductor layer 304, and a 150 nm (1500 Å) thick n⁺-type layer 305 are sequentially formed on the insulating substrate 301 by a plasma CVD method, as shown in Fig. 5A). A 1-µm thick aluminum film serving as source and drain electrodes 306 and 307 is deposited by sputtering, and a photoresist layer 308 for patterning the source and drain electrodes is applied to the surface of the structure (Fig. 5B). The photoresist layer 308 is patterned to have a desired shape, and the source and drain electrodes 306 and 307 are formed by wet etching, using the patterned photoresist layer 308 as a mask (Fig. 5C). The n⁺-type layer 305 and a portion of the semiconductor layer 304 are etched to a depth of 170 nm (1700 Å) by RIE using the photoresist 308 as a mask, and then the photoresist layer 308 is removed (Fig. 5D). After patterning with a desired photoresist pattern is performed, unnecessary layers between the adjacent elements are removed (element isolation). Note that element isolation is performed by RIE (Fig. 5E). Subsequently, a protective layer 310 consisting of an impurity-doped silicon nitride film is formed on the surface of the thin film semiconductor formed in Fig. 5E by plasma CVD (Fig. 5F). A method of forming the protective film 310 consisting of the impurity-doped silicon nitride film will be described below. A plasma is generated by a gas mixture of SiH₄ and NHH₃, and silicon nitride is deposited on the substrate kept at a temperature T s = 330°C by plasma CVD. A volume ratio R (SiH₄/NH₃) of the gas mixture of SiH₄ and NH₃ between 10⁻¹ to 10 is changed to set an optical band gap energy E g to 1.8 to 5.0 eV. In this embodiment, B₂H₆ gas is mixed as a doping gas in the mixture of SiH₄ and NH₃. For example, by setting the volume ratio R of the gas mixture of SiH₄ and NH₃ to be R = about 3, and by adding 10⁴ ppm of B₂H₆ as a doping gas in the above gas mixture, the protective layer 310 consists of a silicon nitride film having a band gap energy E g = about 2.4 eV and an activation energy E a = about 1.3 eV. This protective layer substantially has a p conductivity type. The a-Si:H layer constituting the thin film semiconductor layer has a band gap energy E g = 1.7 to 1.8 eV and E a = 0.7 to 0.75 eV. When the protective layer comprising the impurity-doped silicon nitride film is formed on the surface of the a-Si:H layer, the surface region of the a-Si:H layer is almost fully depleted of electrons. As described above, a thin film transistor type photosensor is prepared. B₂H₆ is used as a doping gas in this embodiment. However, BH₃, B₂F₆ or BF₆ may be used in place of B₂H₆ as a doping gas. Subsequently, thin film transistor type photosensors (A) embodying the present invention, and thin film transistor type photosensors (B) formed by the conventional method (Figs. 3A and 3B) are each formed on a respective large glass substrate having a size of 300 mm□ at intervals of about 20 mm. A mark + in Fig. 6 represents a position of each thin film transistor type photosensor. A point 0 represents the approximate centre of the substrate. The electrical characteristics of the thin film transistor type photosensors are represented by dark currents Id, the threshold voltages V th of dark currents, photocurrents, and their temperature characteristics. Fig. 7 shows the distribution of the dark currents Id at an operating point of the thin film transistor type photosensors located on the line A-A' in Fig. 6, and the distribution of threshold voltages v th . The operating point is a gate voltage Vg = 0 V. As is apparent from Fig. 7, the dark current Id of the thin film transistor type photosensor (A) embodying the present invention is smaller than that of the conventional thin film transistor type photosensor (B), and the degree of non-uniformity (distribution) of the dark current Id of the thin film transistor type photosensor (A) is better than that of the conventional photosensor (B). The threshold voltages v th of the dark currents of the photosensors (A, B) are almost equal to each other, and a degree of non-uniformity (distribution) of the threshold voltages V th of the photosensor (A) is smaller than that of the photosensor (B). Fig. 8 shows the distribution of room-temperature photocurrents at an operating point of the thin film transistor photosensors located on the line A-A' of Fig. 6, and the temperature characteristic of the photocurrent (a photocurrent ratio at 60°C/25°C is here referred to as the temperature characteristic). If the gate voltage Vg is finely adjusted further in accordance with the distribution of the temperature characteristic of the photocurrent to correct the operating point, the distribution of the temperature characteristic of the photocurrent can also be eliminated in the conventional thin film transistor type photosensor (B). As is apparent from fig. 8, the photocurrent and the temperature characteristic of the thin film transistor type photosensor (A) embodying the present invention are larger and better than those of the conventional thin film transistor type photosensor (B). The temperature characteristic of the conventional thin film transistor type photosensor (B) is distributed in a range of 30% to 130% along the line A-A', and the distribution of photocurrents of the photosensors (B) ranges from about 10 to about 30% at room temperature. Energy band diagrams in a dark state are estimated from the above as follows. Fig. 9A shows energy band diagrams of the thin film transistor embodying the present invention near the threshold voltage V th at point 0 at the centre of the substrate and edge point A on the line A-A' in Fig. 6. Fig. 9B shows energy band diagrams of the conventional thin film transistor near the threshold voltage V th . As is apparent from these energy band diagrams, in the conventional thin film transistor type photosensor (B), the surface of the thin film semiconductor layer 304 includes an almost flat band state to a depletion state in a wide range from the centre point 0 to the edge point A. To the contrary, in the thin film transistor type photosensor (A) embodying the present invention, the surface of the thin film semiconductor layer 304 is slightly depleted by an influence of the surface protective layer 310, and the depletion distribution is relatively small. The temperature characteristic of the photocurrent is very important to determine an S/N ratio of a photosensor and is not necessarily satisfied by the temperature characteristics of the photocurrent shown in Fig. 8. A desired temperature characteristic of the photocurrent can be obtained by utilising the dependency of the temperature characteristic of the photocurrent on the gate voltage. Fig. 10 shows dependency of the temperature characteristics of the photocurrent on the gate voltage. As shown in Fig. 10, the above object can be achieved by the gate voltage, as shown. A drive circuit consisting of photosensors and thin film transistors manufactured by the process of the above embodiment may be used for a one-dimensional contact type sensor array. Fig. 11 shows an arrangement of a perfect contact type sensor array constituted by photosensors and thin film transistors embodying the present invention. A case of a sensor array having nine photosensors will be exemplified. Referring to Fig. 11, three out of nine photosensors E301 to E309 constitute each block, and three blocks constitute a photosensor array. This applies to capacitors C301 to C309 and switching transistors T301 to T309 in correspondence with the photosensors E301 to E309. Individual electrodes having identical numbers in the respective blocks constituted by the photosensors E301 to E309 are connected to one of common lines 3102 to 3104. More specifically, the first switching transistors T301, T304, and T307 of each block are connected to the common line 3102, the second switching transistors T302, T305, and T308 of each block are connected to the common line 3103, and the third switching transistors T303, T306, and T309 are connected to the common line 3104. The common lines 3102 to 3104 are connected to an amplifier 3105 through switching transistors T310 to T312. The gate electrodes of switching transistors ST301 to ST309 are commonly connected in units of blocks as in the gate electrodes of the switching transistors T301 to T309 and are connected to parallel output terminals of a shift register 3201 in units of blocks. The switching transistors ST301 to ST309 are sequentially turned on in units of blocks in response to shift timing pulses of the shift register 3201. Referring to Fig. 11, the common lines 3102 to 3104 are formed through capacitors C310 to C312, respectively and are grounded through switching transistors CT301 to CT303. The capacitance of each of the capacitors C310 to C312 is sufficiently larger than that of each of the capacitors C301 to C309. The gate electrodes of the switching transistors CT301 to CT303 are commonly connected, and the common node is connected to a terminal 3108. That is, when the terminal 3108 is set at high level, the switching transistors CT301 to CT303 are simultaneously turned on, and the common lines 3102 to 3104 are grounded. Each sensor comprises a thin film transistor type photosensor, and gate electrodes G301 to G309 correspond to the sensors E301 to E309, respectively. Fig. 12A is a plan view showing part of a perfect contact sensor array arranged on the basis of the circuit diagram of Fig. 11. Referring to Fig. 12A, a wiring unit 320 is formed in a matrix form. A photosensor unit 321 is formed by using a thin film transistor photosensor. A transfer switch 323a is arranged by using a thin film transistor embodying the present invention. A discharge switch 323b is arranged by utilizing the thin film transistor of the present invention to reset the charge of a charge storage unit 322. A lead line 325 connects a transfer switch signal output to a signal processing IC. A load capacitor 324 stores the charge transferred by the transfer switch 323a and reads out the transferred charge. In this embodiment, an a-Si:H film is used as a photoconductive semiconductor layer constituting the photosensor unit 321, the transfer switch 323a, and the discharge switch 323b. A plasma-CVD silicon nitride film is used as an insulating layer. Referring to Fig. 12A, only upper and lower wiring layers are illustrated for illustrative convenience, and the thin film semiconductor layer and the insulating layer are not illustrated. An n⁺-type layer is formed at the interface between the upper electrode wiring layer and the thin film semiconductor layer to achieve an ohmic contact. Fig. 12B is a longitudinal sectional view of the photosensor unit 321. The photosensor unit 321 comprises a lower electrode wiring layer 331 serving as a gate electrode, an insulating layer 332 serving as a gate insulating layer, a thin film semiconductor layer 333, an upper electrode wiring layer 335 serving as a source electrode, an upper electrode wiring layer 334 serving as a drain electrode, and a protective layer 340 converted into a p-type layer containing B (boron) as an impurity. Fig. 12C is a longitudinal sectional view of the transfer switch 323a and the discharge switch 323b. The transfer switch 323a comprises a lower electrode wiring layer 337 serving as a gate electrode, the insulating layer 332 serving as a gate insulating layer, the thin film semiconductor layer 333, and the upper electrode wiring layer 335 serving as a source electrode. The gate insulating layer and the thin film semiconductor layer of the discharge switch 323b are the same as the insulating film 332 and the thin film semiconductor layer 333. The source electrode consists of the upper electrode wiring layer 335, the gate electrode consists of the lower electrode wiring layer 337, and the drain electrode consists of an upper electrode wiring layer 336. The transfer switch 323a and the discharge switch 323b are covered with the protective layer 340 consisting of a p-type layer containing B (boron) as an impurity. As described above, an ohmic contact layer is formed between the upper electrode wiring layers 334, 335 and 336 of the photosensor unit, the transfer switch, and the discharge switch and the thin film semiconductor layer 333. Since the protective layer contains a p-type impurity, the temperature characteristic of the photocurrent can be set to fall within a range of 60 to 70%. At the same time, the dependency of the temperature characteristic of the photocurrent on the gate voltage as indicated in the above embodiment is utilised in the thin film transistor type photosensor to control the voltage of the gate electrode 331, thereby further improving the temperature characteristic of the photocurrent. Since the switching transistors T301 to T309, and ST301 to ST309 are covered with a protective layer containing p-type impurity, no practical problem is posed. Fig. 13 shows the distribution of the temperature characteristic of an output under an illuminated condition (an output obtained when a white original is read) in the one-dimensional perfect contact sensor formed on a large substrate. The one-dimensional perfect contact sensor is formed such that photosensors are aligned on the line A-A' in Fig. 6. The distribution of the temperature characteristic of outputs under illuminated condition in the one-dimensional contact sensor array (A) using photosensors and thin film transistors embodying the present invention and a one-dimensional perfect contact sensor array (B) using the conventional photosensors and thin film transistors are indicated by a broken curve (a) and an alternate long and short dashed curve (b) in Fig. 13, respectively. The distribution of outputs in the illuminated condition of the one-dimensional perfect contact sensor array (A) embodying the present invention is better than that of the conventional one-dimensional perfect contact sensor array (B), but has a poor temperature characteristic of outputs in the illuminated condition. Gate voltages are respectively applied to the gate electrodes G301 to G309 shown in Fig. 11 to improve the temperature characteristic of outputs in the illuminated condition, as indicated by a solid line (c) in fig. 13. The distribution of the temperature characteristic of outputs in the illuminated condition and the magnitude of the temperature characteristic are improved, as indicated by the solid curve (c) of Fig. 13. The one-dimensional contact sensor array embodying the present invention can be used as a lensless perfect contact sensor array for reading an original 16 upon radiation of light from a light source 13 to the lower surface of the sensor after an anti-wear layer 11 is formed on the upper surface of the photosensor, as shown in Fig. 16. The one-dimensional contact sensor array can also be used as a contact sensor array using a one-to-one size focusing lens (SELFOC lens available from Nippon Sheet Glass Co., Ltd.) 12 shown in Fig. 17. Results of a high-temperature, high-humidity test at 60°C and 90% of a contact type sensor array (A) of this embodiment and a contact type sensor array (B) formed by the conventional method are shown in Fig. 14. The perfect contact type sensor array of this embodiment and the conventional perfect contact type sensor array have better humidity resistance than a contact type sensor array having no protective layer due to the presence of the protective layer 309 (or 310). Moisture permeation is found even in the presence of the protective layer in accordance with high-temperature, high-humidity conditions and leaving time. Referring to Fig. 14, an operating point is set to be the gate voltage Vg = 0 V, and dark currents Id at points 0 and A₀ on the line A-A' of Fig. 6 are plotted as a function of leaving time. As is apparent from Fig. 14, the humidity resistance values are larger in an order of point (B)-0, point (A-0, point (A)-A₀, and point (B)-A₀. Figs. 15A and 15B show changes in energy band diagrams as a function of leaving time assumed in the test. Note that only the bands of the thin film semiconductor layer 304 are illustrated for illustrative convenience. The initial state is represented by the solid line, and the state after leaving is represented by the broken line. Fig. 15A shows changes in energy bands at points 0 and A₀ of the present invention (A). As is apparent from Fig. 15A, the surface layer is depleted after the test. To the contrary, Fig.15B shows changes in energy bands at point 0 according to the conventional method (B). In the energy band diagram, the surface layer is close to an almost flat band state or a storage state. In this thin film transistor, a change in surface state greatly influences a change in Id. The energy band diagram at point A₀ represents a depletion state. Large differences are present between changes at points 0 and A₀ in the humidity test, and an increase in distribution occurs. As described above, humidity resistance can be obtained by a protective layer. When the surface layer of the thin film semiconductor layer is set in a depletion state, an increase in dark current Id can be suppressed, and the distribution of charges is assumed to be limited accordingly. In the above embodiment, B (boron) is doped as an impurity in the protective layer. However, dark current, photocurrent, and their temperature characteristic can be controlled by the choice of dopant and the doping amount. In addition, it is also to improve the distribution. The main gist of the present invention is to improve sufficiently the humidity resistance and to correct the operating point so as to obtain desired characteristics. In the above embodiment, an impurity-doped protective layer is formed on a portion of the thin film semiconductor layer from which the n⁺-type layer is removed. However, an impurity-doped protective layer may be formed on a thin film semiconductor layer in advance, contact holes may be formed in the protective layer and an n⁺-type layer and metal layers for forming the source and drain electrodes may be formed. In the embodiments of the present invention, hydrogenated amorphous silicon is exemplified. However, another thin film semiconductor such as polysilicon or CdS may be used to obtain the same effect as described above. As has been described above, a p-type impurity is doped in the protective layer of a thin film transistor type photosensor, and the operating point of the thin film transistor type photosensor is corrected by the gate voltage. Therefore, a change in surface band of the thin film semiconductor generated during the fabrication process for the thin film transistor type photosensor, e.g., upon doping of a p-type impurity, can be prevented, and the distribution and magnitude of electrical characteristics (especially, dark currents, photocurrents, and their temperature characteristics) of the thin film transistor produced upon RIE or the like can be greatly improved. Problems posed by the fabrication process on a large substrate, and problems associated with reliability have been solved. In addition, the influences of protective film formation for improving humidity resistance and the composition of the protective film can be reduced. As a result, a degree of freedom in selection of the protective film material results.

Claims: An n-channel thin film field effect transistor comprising: a gate electrode (302); a gate insulation layer (303); a thin film semiconductor active layer (304); n⁺-type contacts (305) on said semiconductor layer (304); source and drain electrodes (306,307) contacting said n⁺-type contacts; and a protective layer (310) of insulative material disposed over and in contact with the surface of the semiconductor active layer (304) extending between said n⁺-type contacts (305); wherein said transistor is characterised in that: said protective layer (310) contains a p-type impurity which results in a depletion of electrons at and in the immediate vicinity of said surface of said semiconductor active layer (304). A transistor as claimed in claim 1 wherein said semiconductor layer (304) is of hydrogenated amorphous silicon. A transistor as claimed in claim 1 wherein said semiconductor layer (304) is of polysilicon. A transistor as claimed in claim 1 wherein said semiconductor layer (304) is of cadmium sulphide. A transistor as claimed in any preceding claim wherein said protective layer (310) is of silicon nitride. A transistor as claimed in claim 5 wherein said p-type impurity is boron. A transistor as claimed in any preceding claim including a light receptive area defined between said source and drain electrodes (306,307). An array of transistors each as claimed in any preceding claim together with gate voltage control means connected to respective gate electrodes (302) of said transistors (301 to 310), which array is characterised in that said gate voltage control means is operable to apply respective different operating point gate voltages to said respective gate electrodes (302) to compensate for a spatial variance in the temperature characteristic of said transistors (301 to 310). A method of operating the array of claim 8 wherein different operating point gate voltages are applied to said respective gate electrodes (302) to compensate for a spatial variance in the temperature characteristic of said transistors (301 to 310).

Inventors: ISHII TAKAYUKI, YAMANOBE MASATO, ISHII, TAKAYUKI, YAMANOBE, MASATO, ISHII, TAKAYUKI, C/O CANON KABUSHIKI KAISHA, YAMANOBE, MASATO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L29
IPC codes: H01L31, H01L27, H01L29