Semiconductor composite apparatus, print head, and image forming apparatus

A semiconductor composite apparatus, includes a first substrate, a semiconductor thin film layer, active devices, first driving circuits, and second driving circuits. The semiconductor thin film layer is formed on the first substrate and is formed of a first semiconductor material. The active devices are formed in the semiconductor thin film layer. The first driving circuits is formed of a second semiconductor material and performing a first function in which the active devices are driven. The second driving circuits are formed of a third semiconductor material and performing a second function in which the active devices are driven, the second function being different from the first function.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor composite apparatus such as a light emitting diode (LED), a print head that uses such a semiconductor composite apparatus, and an image forming apparatus.

2. Description of the Related Art

For example, Japanese Patent (Kokai) Publication No. 10-63807 discloses a conventional semiconductor composite apparatus in which light emitting elements and their driver circuits and semiconductor integrated circuits are fabricated. This semiconductor composite apparatus is a card type information-controlling apparatus that includes light-emitting/light-receiving elements and associated electronics on a substrate. The light-emitting/light-receiving elements are formed of semiconductor thin films, and convert electrical signals into light signals and light signals into electrical signals. The electronics includes semiconductor integrated circuit elements, capacitors, and inductors, and communicates display information with the light-emitting/light-receiving elements.

Among recent optical printheads used as a light source for an electrophotographic printer is a semiconductor composite apparatus (“Design of Light Printer” pp 114-116 (under editorship of Mukita Yoshihiro, published by TRICEPS). Light emitting diode (LED) array chips and their drive circuit chips are mounted on a printed circuit board.

The above reference “Design of a Light Printer” discloses a conventional LED unit that employs light emitting diodes (LEDs) The LED unit includes a plurality of LED chips in which a plurality of LEDs are arranged, driving integrated circuits (driving ICs) that drive the LED chips, and a unit substrate on which the LED chips and driving ICs are mounted. The LED chips are electrically connected to the driving ICs by wire bonding. The LED chips and driving ICs have a thickness of approximately 300 μm, and are die-bonded to the unit substrate by a die-bond paste.

The aforementioned conventional semiconductor composite apparatuses suffer from the following drawbacks.

For example, because the LED chips and driving ICs are connected by wire-bonding, the LED chips and driving ICs are required to have large pads. The bonding pads are, for example, 100 μm×100 μm, much larger than the light emitting regions. In other words, the pads that do not directly contribute to emission of light occupy a larger area than regions that directly contribute to emission of light. This is uneconomical in terms of efficient utilization of material.

Wire bonding is done by applying a predetermined force on the wire and the pad. Therefore, the LED chips and driving ICs need to be as thick as several hundred microns, e.g., 300 μm. Wire bonding process also requires bonding pads to be thick. Thus, the thickness of the chips cannot be reduced any further as long as the chips are mounted on the unit substrate by the processes of dicing, picking-up, and bonding. Thus, reduction of the manufacturing cost cannot be achieved by decreasing the thickness of the chip.

SUMMARY OF THE INVENTION

The present invention was made in view of the aforementioned drawbacks of the prior art apparatuses.

An object of the invention is to provide a semiconductor composite apparatus that can be manufactured at low cost while maintaining sufficient functions and performance.

An object of the invention is to provide a printhead that uses the semiconductor composite apparatus, and an image forming apparatus that uses the printhead.

A semiconductor composite apparatus includes a first substrate, a semiconductor thin film layer, active devices, first driving circuits, and second driving circuits. The semiconductor thin film layer is formed on the first substrate and is formed of a first semiconductor material. The active devices are formed in the semiconductor thin film layer. The first driving circuits are formed of a second semiconductor material and perform a first function in which the active devices are driven. The second driving circuits are formed of a third semiconductor material, and perform a second function in which the active devices are driven. The second function is different from the first function.

With the semiconductor composite apparatus according to the present invention, the functions required for driving active devices are apportioned into first driving circuits and second driving circuits.

In this specification, the term “High speed” covers an operating frequency not lower than 5 MHz. The second driving circuits operate as a high-speed circuit that is connected to the active devices and directly drives the active devices. The first driving circuits operate as a low-speed circuit that is connected to the second driving circuits and controls the second driving circuits. The first driving circuits operate at a lower speed than the second driving circuits.

With the semiconductor composite apparatus, the high-speed circuits are formed of a third semiconductor material, which is expensive but is capable of fabricating high speed circuits that directly drive the active devices. The low-speed circuits are formed of a second semiconductor material, which is inexpensive but is only capable of fabricating low speed circuits that control the high speed circuits. In this manner, the functions required for driving active devices are apportioned into the high-speed circuits and the low-speed circuits.

In this specification, the term “Large-current” covers a driving current not lower than 5×10−5A. The term “Small-current” covers a driving current not more than 5×10−5A. The second driving circuits directly drive the active devices, and therefore are capable of driving a large current. The first driving circuits control the second driving circuits, and therefore are capable of driving only a small current. The second driving circuits are formed of the third semiconductor material capable of driving a large current. The first driving circuits are formed of the second semiconductor material capable of driving only a small current. In this manner, the functions required for driving active devices are apportioned into the large-current circuits and the low-speed circuits.

DETAILED DESCRIPTION OF THE INVENTION

First Embodiment

A semiconductor composite apparatus is a semiconductor thin film. High-speed circuits are formed of a single crystal semiconductor, which is expensive but is capable of fabricating high speed circuits that directly drive the active devices. Low-speed circuits are formed of a poly crystal semiconductor, which is inexpensive but is only capable of fabricating low speed circuits that control the high speed circuits. In other words, the functions of the semiconductor composite apparatus are apportioned into the high speed circuits and low speed circuits, thereby preventing the deterioration of functions as well as maintaining low manufacturing cost.

Large-current driving circuits are formed of a material capable of fabricating large-current circuits. Small-current driving circuits are formed of a material capable of only fabricating small-current circuits that drive the large-current driving circuits. In other words, the functions of the driving circuits are apportioned into the large-current circuits and small-current circuits, thereby preventing deterioration of functions of the semiconductor composite apparatus and increase in manufacturing cost. In this specification, the term “High speed” covers an operating frequency not lower than 5 MHz. “Low speed” covers an operating frequency lower than 5 MHz. The term “Large-current” covers a driving current not lower than 5×10−5A. The term “Small-current” covers a driving current not more than 5×10−5A.

For the sake of convenience, the invention will be described, by way of example, with respect to a semiconductor composite apparatus that can be applied to an LED printhead for use in, for example, an electrophotographic printer.

FIG. 1is a top view of a pertinent portion of the semiconductor composite apparatus.FIG. 2is a cross sectional view taken along a line A-A ofFIG. 1.FIG. 3is a cross sectional view taken along a line B-B ofFIG. 1.

Referring toFIG. 1, the semiconductor composite apparatus includes a semiconductor thin film layer104fabricated on a first substrate101formed of an inorganic material such as a glass substrate or an organic material such as a plastics substrate. The semiconductor thin film layer104is formed of a first semiconductor material, which is, for example, a single layer of a compound semiconductor such as AlxGa1-xAs, AlxGayIn1-x-yP, or GaxIn1-xAsyP1-y(1≧x≧0, 1≧y≧0) or a stacked structure of more than one of these layers with different values of the parameters x and y. The semiconductor thin film layer104includes light emitting regions104aas a group of active devices having a pn junction.FIG. 1illustrates a plurality of light emitting regions104aproduced by dividing the respective pn junctions by mesa etching, the light emitting regions being aligned in line at predetermined intervals. However, the plurality of light emitting regions104amay include a plurality of groups of light emitting regions104a, each of group having a plurality of light emitting elements104astaggered with respect to each other in the direction perpendicular to the direction in which they are aligned.

Referring toFIGS. 2 and 3, formed on the first substrate101is a reflection layer105that reflects light emitted downwardly from the light emitting region104, the light being reflected back toward the light emitting region104. The reflection layer105is formed of a metal material in the form of a single layer formed of at least one selected from the group consisting of Ti, Pt, Au, Ge, Ni, Al, Cu, Pd, and W; a stacked structure of layers each of which is formed of at least one selected from the group consisting of Ti, Pt, Au, Ge, Ni, Al, Cu, Pd, and W; or an alloy film containing at least one selected from the group consisting of Ti, Pt, Au, Ge, Ni, Al, Cu, Pd, and W. The reflection layer105is electrically connected to a common electrode contact108. The common electrode108is connected through a connection pad123to a ground terminal, not shown, of second driving circuits103, which will be described later, to the ground. In other words, the reflection layer105also serves as a common wire to plurality of light emitting elements104a. Also formed on the first substrate101are wiring layers, i.e., individual wires106and a common wire107. The individual wire106is provided for a corresponding light emitting region104a. The individual wire106is in the form of a stacked structure of a layer of Au and a layer of one selected from the group of Ti/Pt, Ge, and Ni, or an alloy of Au and at least one selected from the group of Ti, Pt, Ge, and Ni. One example of such a stacked structure is Ti/Pt/Au. The individual wire106may also take the form of an alloy film that contains Al, e.g., AlSiCu and Ni/Al. The common wire107is in the form of a stacked structure of a layer of Au and a layer of either Ge or Ni. The common wire107may also be an alloy of Au and at least one of Ge and Ni. For example, if the upper surface of the light emitting region104ais a p-type semiconductor layer, the common wire107is a contact formed on an n-type semiconductor layer. The common wire107is electrically connected to the reflection layer105. These wiring layers may be formed by using a semiconductor photolithographic process such as a lift-off process.

Formed on the first substrate101are first driving circuits102that control second driving circuits103(which will be described later) to drive the respective LEDs. The first driving circuits102are formed of a second semiconductor material whose base material is, for example, a poly crystal Si. The first driving circuits102are formed on a glass substrate simultaneously, and include circuits that control all of the LEDs. A power supply, not shown, supplies electric power to the first driving circuits102through connection pads109. The first driving circuits102perform logical processes of input signals received from external circuits through the corresponding connection pads109, thereby generating driving signals. For example, the first driving circuits102are digital circuits that process the input signals and hold the processed input signals, and may be implemented by using low-speed circuits, in which case the signals are processed at a low speed throughout the first driving circuits102. The outputs of the first driving circuits102are fed to the second driving circuits103through the wires131and connection pads121, and control the second driving circuits103.

The second driving circuits103are conventional semiconductor chips formed of a third semiconductor material whose base material is, for example, a single crystal Si. The second driving circuits103are electrically connected to the connection regions in wiring regions formed on the first substrate101, being bonded by flip-chip bonding or wire bonding. Specifically, the outputs of second driving circuits103are fed to the individual wires106for the respective LEDs through connection pads122. In other words, the second driving circuits103are electrically connected to the three connection pads121,122, and123. The second driving circuits103control current that energizes LEDs to light up. For example, the second driving circuits103convert the drive signals, logically processed by the first driving circuits102, into a drive power for driving the LEDs. The second driving circuits103are controlled by the first driving circuits102in such a way that the second driving circuits103selectively drive the LEDs to emit light in accordance with the drive signals.

An interlayer dielectric film is formed in an appropriate region of a semiconductor composite apparatus to prevent short circuit. For this purpose, interlayer dielectric films (i.e., passivation film)210and212are formed on the first substrates101as shown inFIG. 2. The interlayer dielectric films210and212are formed preferably by plasma-activated chemical vapor deposition (PCVD) in which good coverage is obtained not to cause short circuit or cutting-off of wires at stepped portions. Alternatively, the stepped portions may be planarized by using a polyimide film, a spin-on glass (SOG) film, or any suitable technique suitable depending on the shapes of the stepped portions. Specifically, the interlayer dielectric film210is formed, preferably, of a polyimide film, a coating of organic film, or a spin-on glass film. The interlayer dielectric film212is preferably, for example, a SiN film.

The semiconductor thin film layer104includes an upper layer region and a lower layer region formed by mesa etching. The upper layer includes a pn junction. The lower layer region forms a common electrode. The second driving circuits103include connection pads (connection posts)121a,121c,122a, and122c, which are electrically connected to one another via paste layers121band122b. The reflection layer105also serves as a conductive layer that connects between the common electrode and the second driving circuits103(FIG. 3).

FIG. 4is a cross sectional view of a pertinent portion including the semiconductor thin film layer104.

Specifically, a bonding layer104afacilitates bonding of the semiconductor thin film layer104onto the upper surface of the interlayer dielectric film210. Reference numeral104bdenotes a conductive layer. A contact layer104cis of a second conductivity type. An etching stopping layer104dstops mesa etching at the surface of the etching stopping layer but can be selectively etched by another etching method to expose the contact layer104c. The contact layer104ccannot be provided without the etching stopping layer104despecially when the contact layer104cis very thin. A buffer layer104eprovides high quality epitaxial layers grown on the etching stopping layer104d. The reference numerals104f,104g,104h, and104idenote a lower cladding layer, an active layer, an upper cladding layer, and a first conductivity type contact layer, respectively. The bonding layer104a, conductive layer104b, contact layer104c, etching stopping layer104d, buffer layer104e, lower cladding layer104f, active layer104g, upper cladding layer and114h, and first conductivity type contact layer and104iare formed in this order from the bottom.

The bonding layer104ais a semiconductor layer having a smaller energy band gap than the active layer104g, and is, for example, an n-type GaAs layer. The conductive layer104bis, for example, an n-type AltGa1-tAs (1>t>0). The second conductivity type contact layer104cis an n-type GaAs layer. The etching stopping layer104dmay be selectively etched together with the contact layer104cand an upper layer above the etching stopping layer104d. When the contact layer104cis an n-type GaAs layer, the etching stopping layer104dis preferably an n-type InsGa1-sP layer (1≧s≧0). The buffer layer104eis, for example, an n-type GaAs layer having a smaller energy band gap than the active layer104g, and plays an important role in improving the quality of the epitaxial grown layer formed above the etching stopping layer104d. The lower cladding layer104fis, for example, an n-type AlzGa1-zAs layer (1>z>0). The active layer104gis an n-type AlxGa1-xAs layer (1>y>0). The upper cladding layer104his a p-type AlxGa1-xAs layer (1>x>0). The contact layer104iof a first conductivity type is a p-type GaAs layer. The composition ratios of the respective layers are preferably such that t>y, z>y, and x>y. Of course, the semiconductor thin film layer104may have an appropriate composition ratio such that the relations among the x, y, z, and t are satisfied.

A portion of the flux of light emitted from the active layer104gtransmits downward, and passes through the buffer layer104e, the contact layer104c, and the bonding layer104ato reach the reflection layer105. Then, the light is reflected back by the reflection layer105, and passes through the gluing layer104a, contact layer104c, buffer layer104e, and contact layer104ito be emitted outwardly of the side of the semiconductor thin film layer104. Thus, the thinner the respective layers, the smaller the absorption of light. However, if the respective layers are too thin, then defects in the layers and/or sagging in the boundaries of stacked layers are caused, so that required functions cannot be obtained. From this point of view, the buffer layer104e, contact layer104c, and bonding layer104ahave a thickness preferably in the range of 5 to 100 nm. The etching stopping layer104dhas a thickness preferably in the range of 5 to 100 nm. The thickness of the semiconductor thin film layer104is preferably in the range of 0.1 to 20 μm for the following reasons.

When the semiconductor thin film layer104is very thin, the stress exerted by the interlayer dielectric film and wires causes cracks in the semiconductor thin film layer104. Experiment by the inventors reveals that the cracks are hard to avoid when the thickness is less than 0.1 μm. A semiconductor thin film layer104having a relatively small thickness makes it difficult to handle when the semiconductor thin film layer104is bonded to the first substrate101. A semiconductor thin film layer104having a relatively large thickness results in an increase in defects in the wires between LEDs and driving circuits. This is due to the fact that a large thickness causes a large step between the semiconductor thin film layer104and the first substrate101, and therefore causes defective coverage of the interlayer dielectric layer and wires on the side surface of the semiconductor thin film layer104. A semiconductor thin film layer104having a thickness more than 20 μm causes a large variation in the focal depth of an exposing apparatus during a photolithographic process for fabricating wire patterns, resulting in defective wiring patterns. Further, a small amount of resist tends to remain on the side surface of the semiconductor thin film layer104. Residual resist is also a source of defective wiring pattern. One way of reducing the step between the semiconductor thin film layer104and the first substrate101is to form a thicker interlayer dielectric film. However, void of the semiconductor thin film layer104and deterioration of LEDs tends to occur due to the stress exerted by the interlayer dielectric film. From these points of view, the thickness of the semiconductor thin film is preferably in the range of 0.1 to 20 μm.

When signals are input from an external circuit, not shown, into the first driving circuits102, the first driving circuits102performs various types of digital processing on the signals: resetting of the driving data and settings of the addresses of LEDs to be energized, the duration of energizing the LEDs, the timing of energizing the LEDs, and the current flowing through the LEDs. The, the first driving circuits102outputs the thus produced driving signals to the second driving circuits103. In accordance with the driving signals, the second driving circuits103control the current that flows through the LEDs. Thus, the LEDs are properly energized.

The base material of the first driving circuits102is a poly crystal Si. A poly crystal Si allows forming of the first driving circuits102in a large area simultaneously. However, if the crystal grains are not so large, there are too many grain boundaries, the life of carriers is shorter, the resistance of the material is higher, the mobility of carriers is lower. Thus, such a crystal does not lend itself to high speed applications. The poly crystal Si may be re-crystallized into larger crystals but is not as good as the single crystal in a large-current driving operation and a high-speed operation. Thus, the semiconductor composite apparatus of the invention does not need poly crystal Si circuit elements for controlling driving current of the LEDs.

The control of the current fed into the LEDs is performed by the second driving circuits103whose base material is a single crystal. The functions of driving the LEDs are apportioned into the first driving circuits102and second driving circuits103. Digital processing (logic processing) is performed only by the first driving circuits102that are formed of a poly crystal Si. In this manner, all of the required functions are accomplished at low manufacturing costs.

As described above, because the LEDs in the semiconductor thin film layer104are connected to their driving circuits through thin film wiring patterns, the semiconductor thin film layer104having LEDs formed therein can be greatly reduced in thickness. Further, the driving circuits for controlling the current flowing through the LEDs are formed on a material whose base material is a single crystal Si. The semiconductor circuits for processing the drive signals are formed of a material whose base material is a poly crystal Si. In other words, the functions are apportioned into circuit devices according to the semiconductor materials of which the circuit devices are formed, thereby reliably controlling the current required for energizing the LEDs. Further, the circuit elements formed on a poly crystal Si can be formed simultaneously in a large area. This is greatly effective in reducing the manufacturing cost as opposed to forming all the circuit elements on a single crystal Si.

The semiconductor thin film layer104may be of another configuration.

FIG. 5illustrates a modification to the semiconductor composite apparatus according to the first embodiment. Referring toFIG. 5, the semiconductor thin film layer104may be replaced by a semiconductor thin film layer304having a pn junction of a type different from the semiconductor thin film layer104. An impurity of a first conductivity type (p-type impurity) is selectively diffused into a semiconductor layer of a second conductivity type (n-type semiconductor layer) to form a p-type diffusion region330(contact layer of the first conductivity type) The p-type diffusion region330is as deep as the diffusion front reaches the active layer104g. A diffusion region330athat has diffused into the active layer104gis, for example, a p-type AlyGa1-yAs layer. A diffusion region330bformed in the upper cladding layer104his a p-type AlxGa1-xAs layer. An uppermost contact layer330cof a first conductivity type is separated from a contact layer of a second conductivity type (n-type contact layer) The common wires107are formed on the second conductive contact layer (n-type contact layer) of a second conductivity type.

FIG. 6illustrates another modification to the semiconductor composite apparatus according to the first embodiment;

Still alternatively, the semiconductor thin film layer may be a semiconductor thin film layer404with the common wires107(common contact layer) formed on the uppermost layer104ias shown inFIG. 6.

Another form of the pn junction in the semiconductor thin film layer may be such that the light emitting elements are divided deep enough to the lowermost layer by etching. Still another way of driving the LEDs may be a time-division driving method instead of a simultaneous driving method.

The first driving circuits102may also be implemented as a low speed circuit, and the second driving circuits103may also be implemented as a high speed circuit. The first driving circuits102may be in digital form and the second driving circuits103may be in analog form. The first driving circuits102may be implemented as a large scale logic circuit, and the second driving circuits103may be implemented as a small scale circuit. Such a configuration allows fabrication of the first driving circuits102on the poly crystal Si that can occupy a relatively large area on, for example, a glass substrate. This eliminates limitations in the scale of circuit and reduces the manufacturing cost. The second driving circuits103are required to operate at a high speed, and are preferably formed in a single crystal semiconductor (e.g., single crystal silicon or single crystal compound semiconductor such as a single crystal GaAs) where the mobility of electrons is high.

Second Embodiment

In the first embodiment, the second driving circuits103are formed of the third semiconductor materials on a substrate. A second embodiment differs from the first embodiment in that the second driving circuits103are formed of the third semiconductor materials in a semiconductor thin film. Configurations similar to those in the first embodiment have been given the same reference numerals and their description is omitted.

FIG. 7is a top view illustrating a pertinent portion of the semiconductor composite apparatus.FIG. 8is a cross sectional view taken along a line A-A ofFIG. 7.FIG. 9is a cross sectional view taken along a line B-B ofFIG. 7. For better understanding of the structure, the interlayer dielectric film is omitted fromFIG. 7.

In other words, the semiconductor composite apparatus includes first driving circuits102and the aforementioned semiconductor thin film layer104in the first embodiment, formed on the first substrate such as a glass substrate or a plastics substrate. The first driving circuits102are formed of second semiconductor materials such as a poly crystal Si and an organic semiconductor material. The poly crystal Si and organic semiconductor material may be formed on the first substrate101simultaneously. The semiconductor thin film layer104may be replaced by the semiconductor thin film layers304and404previously mentioned in the first embodiment. When the first driving circuits102control the light emitting elements to light up, the first driving circuits102operate from the electric power supplied via the connection pads109from a power supply, not shown. The first driving circuits102perform the logic processing of the signals received via the connection pads109from external circuits not shown, and produce driving signals.

The semiconductor composite apparatus is provided with second driving circuits503in place of the second driving circuits103in the first embodiment, the second driving circuits503controlling the current that flows through the LEDs. The second driving circuits503are semiconductor thin films formed of a third semiconductor material whose base material is a single crystal Si. Individual wires106, common wire107, and common electrode108are electrically connected to the second driving circuits503. The first driving circuits102and second driving circuits103are interconnected through wires131. A reflection layer105, the individual wires106, and the common wire107may be formed of the same materials as those in the first embodiment.

The electrical connections between the second driving circuits503and LEDs and between the second driving circuits503and the first driving circuits102are preferably made using thin film wires formed in a semiconductor photolithographic process. For this reason, the semiconductor thin film of the second driving circuits503is preferably not more than 20 μm thick.

As described above, because the second driving circuits503are semiconductor thin films formed on a substrate whose base material is single crystal Si and are bonded on the first substrate101, all of the devices can be interconnected using thin film wires that make it easy to form and connect wire patterns. Thus, the electrical connections are highly reliable.

Just as in the first embodiment, the semiconductor material, metal material, and/or substrate materials for the semiconductor composite apparatus may be selected appropriately according to specific requirements.

Third Embodiment

A third embodiment differs from the first and second embodiments in the configuration of first driving circuits and second driving circuits. Elements similar to those in the first and second embodiments have been given the same reference numerals and their description is omitted.

FIG. 10is a top view of a pertinent portion of a semiconductor composite apparatus according to the third embodiment.FIG. 11is a cross sectional view taken along a line A-A ofFIG. 10.FIG. 12is a cross sectional view taken along a line B-B ofFIG. 10. For better understanding of the embodiment, an interlayer dielectric film is omitted fromFIG. 10. While the third embodiment will be described in terms of the semiconductor thin film layer104in the first embodiment, the semiconductor thin film layers304and404in the first embodiment may of course be used.

The second driving circuits103in the first embodiment and the second driving circuits503in the second embodiment are formed of a material whose base material is a single crystal Si, on a substrate or within a semiconductor thin film. In contrast, second driving circuits603in the third embodiment are formed within a semiconductor thin film whose base material is a poly crystal Si.

The first driving circuits102in the first and second embodiments are formed of a semiconductor thin film whose base material is a poly crystal Si. In contrast, first driving circuits602in the third embodiment are formed of a second semiconductor material whose base material is an organic semiconductor material.

The poly crystal Si for the second driving circuits603is formed by the re-crystallization, using a laser, of a poly Si formed on the first substrate101. The second driving circuits603has a mobility of electrons not less than 300 Vcm/s so that the second driving circuits603can operate at a high speed and controllably drive a current on the order of 1 mA.

The organic semiconductor material for the first driving circuits602also has a mobility of electrons not less than 1 Vcm/s.

The second driving circuits603formed of a material whose base material is poly crystal Si, and the first driving circuits602formed of a material whose base material is an organic semiconductor material. The first driving circuits602and second driving circuits603are provided on the first substrate101simultaneously. The electrical connection between the second driving circuits603and LEDs and between the second driving circuits603and the first driving circuits602are made via thin film wires formed in a semiconductor photolithographic process.

As described above, the first driving circuits602are formed of a material whose base material is an organic semiconductor material, and the second driving circuits603are formed of a material whose base material is a poly crystal Si. Thus, the first driving circuits602and second drive circuits603can be formed simultaneously in a large area. Thus, the semiconductor composite apparatus according to the third embodiment is effective in greatly reducing the manufacturing costs while also maintaining the necessary overall functions.

Fourth Embodiment

A semiconductor composite apparatus according to a fourth embodiment differs from the first to third embodiments in that an optical lens is provided under a first substrate and focuses light emitted from LEDs. Elements similar to those in the first to third embodiments have been given the same reference numerals and their description is omitted.

FIG. 13is a top view illustrating a pertinent portion of the semiconductor composite apparatus.FIG. 14is a cross sectional view taken along a line A-A ofFIG. 13.FIG. 15is a cross sectional view taken along a line B-B ofFIG. 13. For better understanding of the embodiment, an interlayer dielectric film is omitted fromFIG. 13. While the fourth embodiment will be described in terms of the semiconductor thin film layer104, first driving circuits602, and second driving circuits603in the third embodiment, the semiconductor thin film layers304and404in the first embodiment may of course be used in place of the semiconductor thin film layer104. Further, the first driving circuits102(first embodiment) may be used in place of the first driving circuits602. Still further, the second driving circuits103(first embodiment) and503(second embodiment) may be used in place of the second driving circuits603.

The semiconductor composite apparatus according to the first to third embodiments has a reflection layer105formed under the semiconductor thin film layer104that contains LEDs, the reflection layer105reflecting the light emitted toward the lower surface of the first substrate101back toward the upper surface.

The semiconductor thin film layer104according to the fourth embodiment has a reflection layer on the surface above the LEDs and an optical lens on the side of the substrate remote from the surface in which the LEDs are formed. The optical lens gathers the light and focuses the light into small spots on an image forming surface of a photoconductive drum of an image forming apparatus.

Specifically, the upper surface of the semiconductor thin film104is covered with an individual electrode744that also serves as a reflection layer. Thus, the light emitted upward is reflected downward by the individual electrode744. The individual electrode744may be in the form of a stacked structure of a layer of Au and a layer of one selected from the group of Ti/Pt, Ge, and Ni, or an alloy of Au and at least one selected from the group of Ti, Pt, Ge, and Ni. One example of such a stacked structure is Ti/Pt/Au. The individual wire106may also take the form of an alloy film that contains Al, e.g., AlSiCu and Ni/Al. A transparent conductive film740is formed of ITO or ZnO through which light emitted from the LEDs is transmitted. A common electrode contact108connects the common electrode to the second driving circuits603through the transparent conductive film740. Therefore, a common wire may be routed on the other side of the common electrode contact108. While the transparent conductive film740is used to route the common wire, a wire may also be formed above the semiconductor layer104in which case the transparent conductive film740need not be conductive and only needs to be a thin film transparent to light.

An optical lens750is formed on the underside of the first substrate101, gathering the light reflected downward by the individual electrode744and focuses the reflected light into small spots on the image forming surface of the photoconductive drum. The optical lens750is a rod lens array in which rod lenses are aligned across the full-length of a line of LEDs. The first substrate101has a thickness preferably substantially equal to the focal length of the optical lens750such that the light is focused by the optical lens750into small spots on the image forming surface of the photoconductive drum. Thus, the distance between the optical lens750and the LEDs need not be adjusted, so that the assembly process of the optical lens750may be simplified.

The semiconductor composite apparatus requires to be configured so that light emitted from the LEDs is incident on the optical lens750via the first substrate101. Thus, the first substrate101is preferably transparent to the wavelength of the light emitted from the LEDs as possible. For this reason, the first substrate101may be formed of optically transparent glass substrate or a plastics substrate.

As described above, the optical lens750is provided on the underside of the first substrate101having a thickness substantially equal to the focal length of the optical lens750, simplifying the adjustment of the focal distance of the optical lens750, the adjustment of the position of the optical lens750relative to the LEDs, and the assembly process of the optical lens750.

Fifth Embodiment

A semiconductor composite apparatus according to a fifth embodiment includes first driving circuits, second driving circuits, and LEDs, which are formed on a first substrate. The semiconductor composite apparatus further includes respective devices, connectors through which signals and electric power are fed from external circuits and electric power, wires that electrically connect between connectors, and third driving circuits that communicate signals between the first driving circuits and between the second driving circuits. Elements similar to those in the first to fourth embodiments have been given the same reference numerals and their description is omitted.

FIG. 16is a top view of the semiconductor composite apparatus according to the fifth embodiment.FIG. 17is a cross sectional view taken along a line A-A ofFIG. 16. The semiconductor composite apparatus will be described in terms of the semiconductor thin film layer104, first driving circuits102and second driving circuits103described in the first embodiment. However, the semiconductor thin film layers304and404in the first embodiment may also be used in place of the semiconductor thin film layer104. Further, the first driving circuits102may be replaced by the first driving circuits602(third embodiment), and the second driving circuits103may be replaced by the second driving circuits503(second embodiment) and603(third embodiment).

Specifically, the semiconductor composite apparatus includes wiring regions860and862formed on a first substrate101as shown inFIG. 16. Signals and electric power are received from external circuits via a connector864. The signals are directed to the first driving circuits102via the wiring regions860and862. The electric power and the ground potential are supplied to the first driving circuits102and second driving circuits103via the wiring regions860and862. The first driving circuits102send signals to the external circuits via the wiring regions860and862. The wiring regions860and862are also used as a region on which electronic components and a memory are mounted for processing signals, stabilizing the electric power, dividing the supply voltage, and/or compensating for temperature changes.

The third driving circuits850communicate signals between the first driving circuits102and between the second driving circuits103. The third driving circuits850are formed of a material whose base materials are, for example, both a single crystal Si and a poly crystal Si. Alternatively, the third driving circuits850may be formed of materials whose base material is a single crystal Si, poly crystal Si, or an organic semiconductor material. The third driving circuits850perform logical processing of the signals received from the external circuits before driving the LEDs. More specifically, the third driving circuits850are mounted on a wiring layer871by flip chip bonding. A plurality of electrodes851are formed on the third driving circuits850. The electrodes851are electrically connected to connection posts872by means of pastes873. The wiring layer871is formed on a dielectric layer870provided on the first substrate101. The third driving circuits850may take a variety of forms, and can be fabricated in a variety of methods. For example, the third driving circuits850may be wire-bonded.

As described above, the semiconductor composite apparatus includes the respective devices, first driving circuits102, second driving circuits103, semiconductor thin film104(LEDs), the connector864, wiring regions860and862, and third driving circuits850, thereby simplifying the configuration of the external circuits.

Sixth Embodiment

A semiconductor composite apparatus according to a sixth embodiment differs from the fifth embodiment in that third driving circuits are formed in a semiconductor thin film instead of on a substrate. Elements similar to those in the fifth embodiment have been given the same reference numerals and the description is omitted.

FIG. 18is a top view of the semiconductor composite apparatus.FIG. 19is a cross sectional view taken along a line A-A ofFIG. 18. While the semiconductor composite apparatus will be described in terms of the semiconductor thin film layer104, first driving circuits102, and second driving circuits103described in the first embodiment, the semiconductor thin film layer104may be replaced by the semiconductor thin film layers304and404previously mentioned in the first embodiment. Further, the first driving circuits102may be replaced by the first driving circuits602(third embodiment) and the second driving circuits103may be replaced by the second driving circuits503(second embodiment) and603(third embodiment).

Specifically, the semiconductor composite apparatus includes third driving circuits950capable of performing logical processing of the signals received from the external circuits for driving the LEDs. The third driving circuits950are formed in the form of a semiconductor thin film as shown inFIG. 19.

Because the third driving circuits950are in the form of a semiconductor thin film, thin film wires can be used to make the electrical connection between the third driving circuits950and other circuits and the electrical connection between the third driving circuits950and wiring regions. Therefore, the wiring structure is simplified, and is highly reliable. The third driving circuits can be formed simultaneously in a large area.

The third driving circuits950formed of a semiconductor thin film allows the following modifications to the semiconductor composite apparatus.

FIG. 20illustrates a modification to the semiconductor composite apparatus according to the sixth embodiment;

FIG. 21illustrates another modification to the semiconductor composite apparatus according to the sixth embodiment;

FIG. 22illustrates a still another modification to the semiconductor composite apparatus according to the sixth embodiment;

As shown inFIG. 20, a third substrate951may be provided on the upper surface of the third driving circuits950. The third substrate951may be a sapphire substrate. The third substrate951having the third driving circuits950thereon is electrically connected to the first substrate101by flip chip bonding or a wire bonding.

The third substrate951strongly holds the third driving circuits950and provides good isolation between the third driving circuits950and the first substrate101, thereby minimizing leakage current as well as improving the performance.

As shown inFIG. 21, the third driving circuits950in the form of a thin film may be bonded on the first driving circuits102. Further, as shown inFIG. 22, the third driving circuits950in the form of a thin film may be provided on the first driving circuits102with a planarizing film952(dielectric film) formed between the third driving circuits950and first driving circuits102, thereby saving the material cost of the first substrate101by a space required for third driving circuits950.

Seventh Embodiment

A semiconductor composite apparatus according to a seventh embodiment differs from the fifth and sixth embodiments in that the third driving circuits are not used but a different configuration is employed. Elements similar to those in the fifth and sixth embodiments have been given the same reference numerals and their description is omitted.

FIG. 23is a top view of the semiconductor composite apparatus.FIG. 24is a top view illustrating a pertinent portion of the semiconductor composite apparatus.FIG. 25is a cross sectional view taken along a line A-A ofFIG. 24. While the semiconductor composite apparatus will be described in terms of the semiconductor thin film layer104, first driving circuits102, and second driving circuits103described in the first embodiment, the semiconductor thin film layer104may be replaced by the semiconductor thin film layers304and404previously mentioned in the first embodiment. Further, the first driving circuits602(third embodiment) may be replaced by the first driving circuits102and the second driving circuits603(third embodiment) may be replaced by the second driving circuits103(first embodiment) and503(second embodiment).

As shown inFIG. 23andFIG. 24, the semiconductor composite apparatus according to the seventh embodiment does not employ the third driving circuits850and950described in the fifth and sixth embodiments but employs only wiring regions860and862formed on the first substrate110. The semiconductor composite apparatus may be configured such that the first driving circuits102and the wiring region share an area. In other words, the wiring region860may be formed under the first driving circuits102as shown inFIG. 25. Thus, signals are communicated with external circuits through a connector864, and electric power is received from the external circuits through the connector864. The received signals are supplied to the first driving circuits102. The electric power and the ground potential are supplied to the first driving circuits102and second driving circuits103through the wiring regions860and862. Further, the signals are outputted to the external circuits from the first driving circuits102through the wiring regions860and862.

Because the third drive circuits850and950are not provided on the semiconductor composite apparatus but on a separate substrate, the configuration of the semiconductor composite apparatus may be simplified as compared with a case in which the third driving circuits850and950are provided, thereby implementing a low cost semiconductor composite apparatus.

FIG. 26illustrates a modification to the semiconductor composite apparatus according to the seventh embodiment.

Referring toFIG. 26, instead of employing the first driving circuits102whose base material is a poly crystal Si, first driving circuits602whose base material is an organic semiconductor material described in the third embodiment may be employed though operating speed decreases. For such a semiconductor composite apparatus, a simple technique such as printing may be used in forming the first driving circuits602simultaneously, achieving a great cost reduction.

FIG. 27illustrates another modification to the semiconductor composite apparatus according to the seventh embodiment.

As shown inFIG. 27, instead of employing the second driving circuits103whose base material is a single crystal Si, the second driving circuits603whose base materials is a poly crystal Si described in the third embodiment may be used. For such a semiconductor composite apparatus, the second driving circuits603whose base material is a poly crystal Si can drive LEDs having a high light-emitting efficiency, saving the manufacturing cost greatly.

FIG. 28illustrates a still another modification to the semiconductor composite apparatus according to the seventh embodiment. The modification inFIG. 27may further be modified as shown inFIG. 28. In other words, as shown inFIG. 28, the individual electrode744described in the fourth embodiment covers the semiconductor thin film104. A transparent conductive film740is formed in place of the reflection layer105. An optical lens750may also be provided on the side of the first substrate101remote from the LEDs, the optical lens750gathering the light emitted from the LEDs and focusing the light into small spots on the surface of an image bearing body of an image forming apparatus. The seventh embodiment allows configuring of a printhead in unitary construction with the optical lens, simplifying the assembly process.

Eighth Embodiment

A semiconductor composite apparatus according to an eighth embodiment differs from the fifth to seventh embodiments in that the connector and wiring region according to the fifth to seventh embodiments are fabricated on a second substrate. Elements similar to those in the fifth to seventh embodiments have been given the same reference numeral sand their description is omitted.

FIG. 29is a top view of the semiconductor composite apparatus.FIG. 30is a cross sectional view taken along a line A-A. While the semiconductor composite apparatus will be described in terms of the semiconductor thin film layer104, first driving circuits602, and second driving circuits603described in the third embodiment, the semiconductor thin film layer104may be replaced by the semiconductor thin film layers304and404previously mentioned in the first embodiment. Further, the first driving circuits602may be replaced by the first driving circuits102(first embodiment) and the second driving circuits603may be replaced by the second driving circuits103and503(second embodiment).

As shown inFIG. 29, the semiconductor composite apparatus according to the eighth embodiment employs a second substrate1001that includes the connector864and wiring regions860and862described in the fifth to seventh embodiments. Specifically, a second substrate1001has a wiring/electronic components region1002similar to the wiring regions860and862, and a connector1003similar to the connector864. The second substrate1001is a substrate formed of an inorganic material such as a glass substrate, a glass epoxy substrate, a ceramic substrate, or a plastics substrate.

A first substrate101is bonded on the second substrate1001by an adhesive as shown inFIG. 30, thus mounting a semiconductor composite apparatus1010(same configuration as that inFIG. 11) on the second substrate1001. The connection pad109provided on the semiconductor composite apparatus1010is wire-bonded to the connection pad1005in the wiring/electronic components region1002on the second substrate1001, thus establishing electrical connection between the semiconductor composite apparatus and the second substrate1001. In other words, the signals and electric power can be relayed through the second substrate1001.

The signals received through the connector1003from external circuits are supplied to the first driving circuits602via the wiring/electronic components region1002. Also, electric power and the ground potential are supplied to the first driving circuits602and second driving circuits603via the wiring/electronic components region1002. Further, the first driving circuits602can output signals to the external circuits via the wiring/electronic components region1002.

As described above, the second substrate1001provided with a connector and a wiring structure fabricated thereon allows smooth communication of signals between the semiconductor composite apparatus and the external circuits, and permits supplying of electric power from the external circuits to the semiconductor composite apparatus. The eighth embodiment is effective in implementing the first substrate101having a narrower width, ensuring reduction of the manufacturing cost. The eighth embodiment offers more degrees of freedom, for example, multi-layer wires, in the basic design of the semiconductor composite apparatus.

Ninth Embodiment

A ninth embodiment differs from the eighth embodiment in that a semiconductor composite apparatus is mounted on the second substrate in a different way. Elements similar to those in the eighth embodiment have been given the same reference numerals and their description is omitted.

FIG. 31is a top view of a semiconductor composite apparatus according to the ninth embodiment.FIG. 32is a cross sectional view taken along a line A-A ofFIG. 31.FIG. 33is a cross sectional view taken along a line B-B ofFIG. 31. While the semiconductor composite apparatus will be described in terms of the semiconductor thin film layer104, first driving circuits602, and second driving circuits603described in the third embodiment, the semiconductor thin film layer104may be replaced by the semiconductor thin film layers304and404previously mentioned in the first embodiment. Further, the first driving circuits602may be replaced by the first driving circuits102(first embodiment) and the second driving circuits603(third embodiment) may be replaced by the second driving circuits103(first embodiment) and503(second embodiment).

In the eighth embodiment, a first substrate101is bonded on a second substrate1001by an adhesive, thus mounting a semiconductor composite apparatus1010on the second substrate1001. In the ninth embodiment, the first substrate101is electrically connected to the second substrate1001having a connector and wires formed thereon.

Specifically, as shown inFIG. 32andFIG. 33, a connection pad109on the semiconductor composite apparatus1010is positioned relative to a connection pad1005on a wiring/electronic component region1002formed on the second substrate1001. Then, the connection pad1005is electrically connected to the connection pad109either directly or by means of a bonding paste layer1101, thus bonding the second substrate1001on the semiconductor composite apparatus1010throughout a connection region1110as shown inFIG. 31. The second substrate1001may be fixed by using, for example, a UV curing resin after the second substrate1001is connected to the semiconductor apparatus1010.

The ninth embodiment is more effective in implementing a narrow-width semiconductor composite apparatus than the eighth embodiment. Further, the width of the second substrate1001in the ninth embodiment can be narrower than the width of the first substrate101.

The semiconductor composite apparatus may be modified as follows: Referring toFIG. 34, the individual electrodes744described in the fourth embodiment is used to cover the upper surface of the semiconductor thin film104. The transparent conductive film740described in the fourth embodiment is formed. An optical lens750is provided on the side of the first substrate101remote from the LEDs, gathering light emitted from the LEDs and focusing the light into small spots on an image forming surface of a photoconductive drum of an image forming apparatus. The first substrate101with the optical lens750provided thereon may be electrically connected onto the second substrate1001.

Tenth Embodiment

A tenth embodiment is directed to an LED printhead to which the semiconductor composite apparatuses according to the first to third embodiments and the fifth to eighth embodiments are applied.

The LED printhead according to the tenth embodiment is used as an exposing unit for use in, for example, an electrophotographic printer or an electrophotographic copying machine. Specifically, the LED printhead includes an LED unit1502carries LEDs thereon and is supported on a base member1501. The LED unit1502includes semiconductor composite apparatus according to the first to third embodiments and the fifth to eighth embodiments is mounted on a substrate. A rod lens array1503is disposed above the LEDs of a light emitting unit1502a. The rod lens array1503includes a plurality of cylindrical optical lenses aligned side by side along a straight line of LEDs. The cylindrical optical lenses are supported on a lens holder1504. The LEDs of the light emitting unit1502acan be any one of the semiconductor thin film layers104,304, and404.

The lens holder1504covers the base member1501and LED unit1502. The base member1501, LED unit1502, and lens holder1504are held together in a sandwiched relation by means of clampers1505, which extend through openings1501aformed in the base member1501and through openings1504aformed in the lens holder1504. In other words, the base member1501and the lens holder1504serve as a supporting frame that supports the LED unit1502and the rod lens array1503.

The rod lens array1503gathers light emitted from the LED unit1502and focuses the light into small spots on an external member (e.g., photoconductive drum).

Because the LED unit1502employs the semiconductor composite apparatus according to the first to third embodiments and the fifth to eighth embodiments, the LED unit1502can be miniaturized and provides high quality images. Because the driving circuits for the LEDs may be divided in terms of required functions, the manufacturing cost may be partially reduced according to the specifications of the respective driving circuits, implementing excellent performance of the LED unit1502at low cost.

Eleventh Embodiment

An LED printhead according to an eleventh embodiment is directed to an LED print head to which the semiconductor composite apparatus according to the ninth embodiment is applied.

The LED printhead according to the eleventh embodiment is used as an exposing unit for use in, for example, an electrophotographic printer or an electrophotographic copying machine. Specifically, as shown inFIG. 36, the LED printhead includes an LED unit1602carries LEDs thereon and is supported on a base member1601. The first substrate101is fixed on the base member1601, thus mounting the semiconductor composite apparatus according to the ninth embodiment. A rod lens array1603is disposed above the LEDs formed of any one of the semiconductor thin films104,304, and404. The rod lens array1603gathers light emitted from the LEDs and focuses the light into small spots. The rod lens array1603includes a plurality of cylindrical optical lenses aligned side by side along a straight line of LEDs. The cylindrical optical lenses are supported on a lens holder1604.

The lens holder1604covers the base member1601and LED unit1602. The base member1601, LED unit1602, and lens holder1604are held together in a sandwiched relation by means of clampers1605, which extend through openings1601aformed in the base member1601and through openings1604aformed in the lens holder1604. In other words, the base member1601and the lens holder1604serve as a supporting frame that supports the LED unit1602and the rod lens array1603.

Because the LED unit1602employs the semiconductor composite apparatus according to the ninth embodiment, the reliability of the wiring region can be greatly improved.

Twelfth Embodiment

A twelfth embodiment is directed to an LED printhead having an optical lens which is a modification to the fourth and seventh embodiments or an LED printhead having an optical lens which is a modification to the ninth embodiment.

The LED printhead according to the twelfth embodiment is used as an exposing unit for use in, for example, an electrophotographic printer or an electrophotographic copying machine. Specifically, as shown inFIG. 37, the LED printhead includes an LED unit1703that carries LEDs thereon, and is supported on supporting frames1701and1702. The first substrate101is supported by the supporting frames1701and1702. Fixing pins1704hold the supporting frames1701and1702together, so that the semiconductor composite apparatus having the optical lens750is mounted on the LED unit.

Because the LED unit1703uses the semiconductor composite apparatus having the optical lens750, a lens holder for supporting a rod lens array is not necessary as opposed to the tenth embodiment of the eleventh embodiment. This makes it easy to assemble the LED unit, and offers a miniaturized and simplified structure of the LED unit. Heat dissipating fins, not shown, may be provided to the supporting frames1701and1702as required.

Thirteenth Embodiment

A thirteenth embodiment is directed to an image forming apparatus that employs an LED printhead to which any one of the tenth to twelfth embodiments is applied.

Referring toFIG. 38, the image forming apparatus includes a paper cassette2001that holds a stack of non-printed recording medium P. A hopping roller2002contacts the top page of the stack of the recording medium P. When the hopping roller2002rotates, the hopping roller2002feeds the top page of the stack of recording medium P into a transport path. A pinch roller2003, registration roller2004, pinch2005, and registration roller2006cooperate with one another to feed the page of the recording medium P to the process unit2007Y in timed relation with image formation performed in the process unit2007Y. Then, the page of the recording medium P is advanced through the process units2007M,2007C, and2007B.

The process units2007Y,2007M,2007C, and2007B are aligned from upstream to downstream of the direction of travel of the recording medium P. Each of the process units2007Y,2007M,2007C, and2007B may be substantially identical; for simplicity only the operation of the process units2007Y for forming yellow images will be described, it being understood that the other process units may work in a similar fashion.

The process unit2007Y includes a photoconductive drum2007athat is driven in rotation by a drive source and gears, not shown, in such a direction as to transport the recording medium P to the adjacent process unit. Disposed around the photoconductive drum2007aare a charging unit2007b, an exposing unit2007c, a developing unit2007d, and a cleaning unit2007e. The charging unit2007bcharges the surface of the photoconductive drum2007a. The exposing unit2007cselectively energizes LEDs, not shown, in accordance with print data to illuminate the charged surface of the photoconductive drum2007ato form an electrostatic latent image on the photoconductive drum2007a. The developing unit2007dsupplies yellow toner to the electrostatic latent image on the photoconductive drum2007ato develop the electrostatic latent image into a yellow toner image. The cleaning unit2007eremoves residual toner from the photoconductive drum2007a.

A transfer roller2008is formed of, for example, a semi-conductive rubber and opposes the photoconductive drum2007a. A voltage is applied to the transfer roller2008to create a potential difference between the photoconductive drum2007aand the transfer roller2008, thereby transferring the toner image onto the recording medium P.

A fixing unit2009is located downstream of the process unit2007B, and includes a heat roller and a pressure roller that form a fixing point between the heat roller and pressure roller. When the recording medium P passes through the fixing point, the toner images of the respective colors are fused under pressure and heat into a permanent full color image.

The recording medium P is advanced at a predetermined speed through the process units in sequence, so that toner images of the respective colors are transferred onto the recording medium P one over the other in registration.

A pinch roller2010, discharge roller2011, pinch roller2012, and discharge roller2013are driven in rotation by a drive source, not shown, to discharge the recording medium P through the discharge path onto a stacker2014.

In this manner, a full color image is formed on the recording medium P.

As described above, because the image forming apparatus employs an LED print head according to the tenth to twelfth embodiments, the image forming apparatus is good in space utilization efficiency, high quality image formation, and reduction of the manufacturing cost.

The present invention is not limited to the aforementioned embodiments. For example, the embodiments have been described in terms of a semiconductor composite apparatus that can be applied to an LED printhead. However, the present invention may be applicable to anything provided that arbitrary active devices are controllably driven. The present invention may also be applied to arbitrary light emitting elements, for example, a semiconductor laser.

The conductivity type of the semiconductor thin film layer in the present invention may be changed from an n-type a p-type and vice versa. Also, the conductivity type of the active layer may be changed in a variety of forms such as n-type, p-type, and non-doped.

While the first semiconductor materials for forming the semiconductor thin film in the embodiments have been described with respect to GaAs, AlGaAs, and AlGaAsP, the first semiconductor materials may contain one nitride semiconductor material such as GaAs1-xNx, GaP1-xNx, InAs1-xNx, InP1-xNx, InGa1-xAs1-yNy, InP1-x-yAsxNy, GaP1-x-yAsxNy, InxGa1-xN, AlxGa1-xN, InxAl1-xN, or GaN (1≧x≧0, 1≧y≧0), or more than one of these materials.

The LEDs have been described with respect to a double hetero junction, the present invention may also be of other structure such as a single hetero junction or a homo junction.

While the semiconductor thin films have been described with respect to an LED epitaxial film, the present invention may be applied to arbitrary light emitting elements. For example, the light emitting elements may be replaced by light receiving elements.

The LED printhead and image forming apparatus are not limited toFIGS. 35-38.