Patent Publication Number: US-2004046176-A1

Title: Avalanche phototransistor

Description:
[0001] This application claims the priority of Korean Patent Application No. 2002-53450, filed Sep. 5, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to an avalanche photo detector, and more particularly, to an avalanche photo detector having low operation voltage, high operation speed and high sensitivity by applying a three-terminal structure to the photo detector and including a hot electron transition layer.  
       [0004] 2. Description of the Related Art  
       [0005] As demands for optical communication systems of super high speed and mass capacity and image processing systems have been recently increased, researches on photo detectors essentially used in these systems have been actively pursued. Most of such researches relate to methods for achieving high speed and high sensitivity for the photo detectors.  
       [0006] While most of the conventional photo detectors are of a PIN type having simple structure, a photo detector of various hetero-junction structures has been completed based on developments in semiconductor technologies such as molecular beam epitaxy and metal organic chemical vapor deposition. Thus, the PIN type photo detector of simple structure has been replaced with an avalanche photo detector (to be referred to hereinafter as an APD). Since the APD employs avalanche gain, the APD has an advantage in that sensitivity is higher than that of the PIN type photo detector.  
       [0007] So far, an avalanche photodiode has been used as APD. However, the avalanche photodiode has drawbacks in that a very high operation voltage is required for obtaining the avalanche gain and operation speed is low. Further, the avalanche photodiode has drawbacks in that an electric preamplifier is inevitably required because of low output current.  
       SUMMARY OF THE INVENTION  
       [0008] To solve the above and other problems, it is an aspect of the present invention to provide an improved avalanche photo detector which has features such as high gain, high sensitivity, high-saturation current, high output and high operation speed, even if a relative low operation voltage is applied.  
       [0009] Further, the present invention proposes an avalanche phototransistor as a new and high performance avalanche photo detector.  
       [0010] According to the above and other aspects of the invention, an avalanche phototransistor comprises a collector layer, a base layer and emitter layer which are sequentially laminated on a semiconductor substrate, an emitter photoabsorption layer which is formed between the emitter layer and the base layer, a thin avalanche-gain layered-structure which is formed between the photoabsorption layer and the base layer, and is comprised of a charge layer and a multiplication layer having a thickness of 5,000 Å or less, a hot electron transition layer which is formed between the base layer and the collector layer, and a collector electrode, a base electrode and an emitter electrode which respectively apply potential to the collector layer, the base layer and the emitter layer.  
       [0011] The photoabsorption layer is comprised of a bulk-type single material layer, a thin film layer having a thickness of 1000 Å or less, a self-assembled quantum dot layered-structure, a quantum well structure, a vertical type quantum dot array structure manufactured using a double-barrier quantum well structure or a multiple-barrier quantum well structure, or a quantum wire array structure. A spacer layer for distribution and control of impurities may be formed on the avalanche-gain layered-structure, if necessary.  
       [0012] The hot electron transition layer is composed of a semiconductor material having a bandgap wider than the base layer and the collector layer. Thus, the hot electron transition layer moves electrons at high speed, and may be a multilayer film comprised of a p-type semiconductor, an n-type semiconductor and an intrinsic semiconductor.  
       [0013] In the avalanche phototransistor according to the above and other aspects of the invention, electrons created in the photoabsorption layer by absorbing a light signal (infrared signal) are interband-transited or intersubband-transited. When an external voltage is applied, the created electrons are multiplicated by passing through the charge layer and the multiplication layer, and the multiplicated electrons move at high speed passing through the hot electron transition layer formed between the base layer and the collector layer. Thus, even if a relative low operation voltage is applied, high gain can be obtained. Further, high speed and low noise of the avalanche photo detector can be obtained by the thin multiplication layer.  
       [0014] Accordingly, since the avalanche phototransistor according to the present invention includes the avalanche-gain layered-structure, the hot electron transition layer and a three-terminal structure, high gain can be achieved. High sensitivity, low operation voltage, high output and high operation speed can be achieved due to the high gain. Stability can be ensured by suppressed breakdown of the photo detector. Further, since the low operation voltage is used, the avalanche phototransistor according to the present invention has many advantages. Since high gain is achieved, low photo-absorptivity can be compensated. Multiple operation functions can be obtained using the three-terminal structure. The infrared signal of various wavelengths can be detected, because the high degree of selection of the photoabsorption layer. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015] The above and other aspects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:  
     [0016]FIG. 1 is a cross sectional view of an avalanche phototransistor according to a first embodiment of the present invention;  
     [0017]FIG. 2 is a cross sectional view of an avalanche phototransistor according to a second embodiment of the present invention;  
     [0018]FIG. 3 is a cross sectional view of a waveguide type avalanche phototransistor according to a third embodiment of the present invention;  
     [0019]FIG. 4 is a cross sectional view of a waveguide-fed type avalanche phototransistor according to a fourth embodiment of the present invention;  
     [0020]FIGS. 5A to  10 B are diagrams illustrating various structures of a photoabsorption layer which can be applied to an avalanche phototransistor according to the present invention;  
     [0021]FIGS. 11A and 11B are respective schematic energy band diagrams under an equilibrium state not applying a voltage and a voltage applying state in an avalanche phototransistor according to the present invention; and  
     [0022]FIGS. 12A and 12B are respective schematic energy band diagrams under an equilibrium state not applying a voltage and a voltage applying state in an avalanche phototransistor according to the present invention, in a case of introducing a photoabsorption layer having a quantum structure. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0023] The present invention now will be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.  
     [0024] &lt;First Embodiment&gt; 
     [0025]FIG. 1 is a cross sectional view of an avalanche phototransistor according to a first embodiment of the present invention. The avalanche phototransistor shown in FIG. 1 has features such as high gain, high output, high operation speed and a three-terminal structure.  
     [0026] Referring to FIG. 1, the avalanche phototransistor is configured such that a collector layer  110 , a base layer  130  and an emitter layer  190  are sequentially laminated on a semiconductor substrate  100 . The avalanche phototransistor has a three-terminal structure in which a collector electrode  115 , a base electrode  135  and an emitter electrode  195  apply a potential to the collector layer  110 , the base layer  130  and the emitter layer  190 , respectively. The emitter electrode  195  is formed in the form of a ring on the emitter layer  190  so that the emitter electrode  195  defines a light-receiving part and is configured to receive an external predetermined voltage.  
     [0027] An emitter photoabsorption layer  170  is formed between the emitter layer  190  and the base layer  130 . The photoabsorption layer  170  absorbs a light signal so that electrons are created in the photoabsorption layer  170 . Further, a thin avalanche-gain layered-structure  160  is formed between the photoabsorption layer  170  and the base layer  130  so that the created electrons are multiplicated through the avalanche-gain layered-structure  160 . The avalanche-gain layered-structure  160  is comprised of a charge layer  150  and a thin multiplication layer  140  having a thickness of 5,000 Å or less. The multiplication layer  140  is composed of a bulk-type single material layer or a super lattice structure.  
     [0028] When a light signal having an energy higher than a bandgap energy is irradiated to the photoabsorption layer  170 , electrons are created in the photoabsorption layer  170  and transited to the conduction band leaving behind holes in the valance band. The photoabsorption layer  170  can be formed in various structures. For example, the photoabsorption layer  170  may be comprised of a bulk-type single material layer, a thin film layer having a thickness of 1000 Å or less, a self-assembled quantum dot layered-structure, a quantum well-structure, a vertical type quantum dot array structure manufactured by using a double-barrier quantum well structure or a multiple-barrier quantum well structure, or a quantum wire array structure. A spacer layer  180  functioning as a buffer layer may be optionally formed between the photoabsorption layer  170  and the emitter layer  190 .  
     [0029] A hot electron transition layer  125  is formed between the base layer  130  and the collector layer  110 . The hot electron transition layer  125  makes the electrons transited from the base layer  130  to move at a high-speed, and then the electrons are reached to the collector layer  110 . The hot electron transition layer  125  is made of a material having a bandgap wider than the base layer  130  and the collector layer  110 , and may be comprised of multilayer films  121 ,  122  and  123  as shown in FIG. 1.  
     [0030] In the structure of such avalanche phototransistor, the excited electrons in the photoabsorption layer  170  are multiplicated passing through the thin avalanche-gain layered-structure  160 , move at high-speed passing through the hot electron transition layer  125 , and reach the collector layer  110 . Thus, even if a lower voltage is applied to the avalanche transistor compared with the prior art, features such as high sensitivity, high gain, high output and high speed can be obtained.  
     [0031] In the phototransistor of the present invention, the collector layer  110 , the base layer  130  and the emitter layer  190  may be configured in either a pnp type or an npn type. Since factors such as material and doping concentrations for impurities of the collector layer, the base layer and the emitter layer and other elements for configuring the phototransistor considerably affect features such as the gain and speed of the APD, these factors must be carefully determined. For example, the emitter layer  190  may be composed of a p+-InAlAs layer, and the spacer layer  180  may be composed of an i-InAlAs layer. The photoabsorption layer  170  may be composed of an i-InGaAs single material having a thickness of 1,000 Å or less, and the avalanche-gain layered-structure  160  may be composed of a p-InAlAs charge layer  150  and a thin i-InAlAs multiplication layer  140  having a thickness of 5,000 Å or less. The base layer  130  may be composed of an n-InAlAs layer or a p-InAlAs layer having a thickness of 2,000-3,000 Å or less. The hot electron transition layer  125  may be a multilayer film composed of a p-InAlAs layer  123 , an n-InAlAs layer  122  having a thickness of 500 Å or less, and an i-InAlAs layer  121  having a thickness of about 2,000 Å. The collector layer  110  may be composed of an n-InAlAs layer, and n-InP may be used as the substrate  100 .  
     [0032] The present invention is not limited to the embodiments set forth herein and the present invention can be embodied while changing the kind of semiconductor material and the doping concentration for impurities, and so on; rather, the embodiment are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. Although the present invention has been described only the case where the electrons are majority carrier, it must be noted that the present invention can be applied to the case where the holes are majority carrier.  
     [0033] &lt;Second Embodiment&gt; 
     [0034] An avalanche phototransistor according to the present invention can be embodied as a resonant-cavity type avalanche phototransistor. FIG. 2 is a cross sectional view of such resonant-cavity type avalanche phototransistor. In FIG. 2, the same reference numerals as those in FIG. 1 represent the same element, and thus their description will be omitted.  
     [0035] Referring to FIG. 2, the resonant-cavity type avalanche phototransistor is characterized in that a lower mirror  101  comprised of a quarter-wave stack is interposed between the collector layer  110  and the substrate  100 , and an upper mirror  191  using dielectric multilayer is laminated on the emitter layer  190 . The lower mirror  101  has a lattice-matching structure on the substrate  100 , and may be comprised of a semiconductor DBR (Distributed Bragg Reflector) in which semiconductor layers having different refractive indexes are alternatively formed with several periods.  
     [0036] By applying the resonant-cavity type avalanche phototransistor using the mirror structure as described above, quantum efficiency can be increased, functions of elements can be improved, and high speed can be achieved. Accordingly, the phototransistor shown in FIG. 2 can be employed as a super high speed infrared signal detecting element.  
     [0037] &lt;Third Embodiment&gt; 
     [0038] An avalanche phototransistor according to the present invention can be embodied as a waveguide type avalanche phototransistor. FIG. 3 is a cross sectional view of such waveguide type avalanche phototransistor.  
     [0039] The waveguide type avalanche phototransistor shown in FIG. 3 is characterized in that first and second guiding layers  272  and  262  are respectively formed on an upper portion and a lower portion of a photoabsorption layer  270 . The waveguide type avalanche phototransistor includes an emitter layer  290  composed of a p+-InAlAs layer, the first guiding layer  272  composed of an i-InAlAs layer, the photoabsorption layer  270  composed of an i-InGaAs thin film layer, the second guiding layer  262  composed of an i-InAlAs layer, an avalanche-gain layered-structure  260  composed of a p-InAlAs charge layer  250  and a thin i-InAlAs multiplication layer  240  having a thickness of 2,000 Å or less, a thin base layer  230  having a thickness of 2,000 Å or less, a hot electron transition layer  225  composed of a p-InAlAs layer  223 , an n-InAlAs layer  222  having a thickness of 500 Å or less, and an i-InAlAs layer  221  having a thickness of about 2,000 Å, and a collector layer  210  composed of an n-InAlAs layer. The above layers are formed on a semiconductor substrate  200  such as an n-InP type substrate. The waveguide type avalanche phototransistor has a three-terminal structure in which a collector electrode  215 , a base electrode  235 , and an emitter electrode  295  apply potential to the collector layer  210 , the base layer  230  and the emitter layer  290 , respectively.  
     [0040] The emitter electrode  295  is formed on the emitter layer  290  in the form of sheet, and a light signal is incident on the photoabsorption layer  270  as indicated by the arrow of FIG. 3. Features which are not particularly described in the present embodiment are the same as in the first embodiment, and thus their description will be omitted.  
     [0041] &lt;Fourth Embodiment&gt; 
     [0042] An avalanche phototransistor according to the present invention can be embodied as a waveguide-fed type avalanche phototransistor. FIG. 4 is a cross sectional view of such waveguide-fed type avalanche phototransistor.  
     [0043] The waveguide-fed type avalanche phototransistor is characterized in that a waveguide layered-structure  304  is interposed between a collector layer  310  and a substrate  300 .  
     [0044] Specifically, the waveguide type avalanche phototransistor includes an emitter layer  390  composed of a p+-InAlAs layer, a spacer layer  380  composed of an i-InAlAs layer, a photoabsorption layer  370  composed of an i-InGaAs thin film layer, a graded spacer layer  361  composed of an i-InGaAlAs layer (i-InGa 0.47(1-x) Al 0.47x As, here x is in range from 0 to 1), an avalanche-gain layered-structure  360  composed of a p-InAlAs charge layer  350  and an i-InAlAs multiplication layer  340  having a thickness of 2,000 Å or less, a base layer  330  having a thickness of 2,000 Å or less, a hot electron transition layer  325  composed of a p-InAlAs layer  323 , an n-InAlAs layer  322  having a thickness of 500 Å or less, and an i-InAlAs layer  321  having a thickness of about 2,000 Å, a collector layer  310  composed of an n-InAlAs layer, and the waveguide layered-structure  304  composed of a guiding layer  303  composed of an InGaAlAs layer and an InAlAs layer  302 . The above layers are formed on a semiconductor substrate  300  such as an n-InP type substrate. The waveguide-fed type avalanche phototransistor has a three-terminal structure in which a collector electrode  315 , a base electrode  335  and an emitter electrode  395  apply a potential to the collector layer  310 , the base layer  330  and the emitter layer  390 , respectively. The emitter electrode  395  is formed on the emitter layer  390  in the form of sheet, and a light signal is incident on the waveguide layered-structure  304  as indicated by the arrow of FIG. 4. Features which are not particularly described in the present embodiment are the same as in the first embodiment, and thus their description will be omitted.  
     [0045] As described in the above first to fourth embodiments, since the avalanche phototransistor of the present invention as APD further includes the base layer and the hot electron transition layer compared with the conventional avalanche photodiode, the very thin avalanche-gain layered-structure can be applied so that high gain, high speed, high-saturated current and high output can be obtained compared with the conventional avalanche photodiode. Further, since the avalanche phototransistor of the present invention employs the three-terminal structure, multiple operation functions can be obtained.  
     [0046] &lt;Examples of Photoabsorption Layer&gt; 
     [0047] Next, various structures of a photoabsorption layer used in the avalanche phototransistor according to the present invention will be described. As described below, many various structures can be applied to the photoabsorption layer of the present invention. The infrared signal of various wavelengths can be detected, due to the high degree of selection assured by the photoabsorption layer via the various structures of the photoabsorption layer.  
     [0048]FIGS. 5A through 10B are structural horizontal cross sectional views and structural transverse cross sectional views of a photoabsorption layer capable of being used as the photoabsorption layers  170 ,  270  and  370  of FIGS. 1 through 4. FIGS. 5A, 6A,  7 A,  8 A,  9 A and  10 A are horizontal cross sectional views of the photoabsorption layer to the substrate, and FIGS. 5B, 6B,  7 B,  8 B,  9 B and  10 B are transverse cross sectional views of the photoabsorption layer to the substrate. Although only the photoabsorption layer  170  shown in FIGS. 1 and 2 is shown in FIGS. 5A through 10B for the sake of convenience, it is obvious to those skilled in the art that a layer as the photoabsorption layer  170  of FIGS. 1 and 2 can be applied to the photoabsorption layer  270  of FIG. 3 and the photoabsorption layer  370  of FIG. 4.  
     [0049]FIGS. 5A and 5B show the photoabsorption layer  170  composed of a bulk-type single material layer or a thin film layer having a thickness of 1,000 Å or less. The photoabsorption layer composed of an i-InGaAs thin film layer was introduced in the above first through fourth embodiments.  
     [0050]FIGS. 6A and 6C show the photoabsorption layer  170  comprised of a self-assembled quantum dot array layered-structure. As shown in FIG. 6C, the photoabsorption layer  170  can be comprised of the self-assembled quantum dot array layered-structure stacked several times. As well known, the self-assembled quantum dot is completed by laminating a material  163   b  having a large lattice constant, on a material  163   a  having a small lattice constant so that the material  163   b  is strained, agglomerating the material  163   b , and laminating the material  163   a  on the material  163   b . Generally, since a material of small lattice constant has a bandgap wider than a material of large lattice constant, the agglomerated material  163   b  surrounded by the materials  163   a  forms a narrow bandgap interposed between wide bandgaps, whereby the material  163   b  becomes quantum dots. Here, the reference numeral  163   a  may be, for example, a GaAs layer, and the reference numeral  163   b  may be, for example, an InAs quantum dot.  
     [0051]FIGS. 7A and 7B show the photoabsorption layer  170  comprised of a quantum dot array layered-structure through lateral confinement of a double barrier quantum well structure. A reference numeral  164   a  represents a quantum barrier layer composed of an i-InAlAs layer, a reference numeral  164   b  represents a quantum dot using an InGaAs quantum well layer having a thickness of 100 Å or less, and a reference numeral  164   c  represents an insulating layer such as SiN. As well known, the quantum barrier layer is referred to as a material layer having a wider bandgap than the quantum well layer.  
     [0052]FIGS. 8A and 8B show the photoabsorption layer  170  comprised of a quantum wire array layered-structure through lateral confinement in a double barrier quantum well type epitaxy structure. A reference numeral  165   a  represents a quantum barrier layer composed of an i-InAlAs layer, a reference numeral  165   b  represents a quantum wire using an InGaAs quantum well layer having a thickness of 100 Å or less, and a reference numeral  165   c  represents an insulating layer.  
     [0053]FIGS. 9A and 9B show the photoabsorption layer  170  comprised of a vertical quantum dot array layered-structure through lateral confinement in a triple barrier quantum well type epitaxy structure. A reference numeral  166   a  represents an i-AlAs layer, a reference numeral  166   b  represents a quantum dot using a GaAs quantum well layer having a thickness of 100 Å or less, and a reference numeral  166   c  represents an insulating layer. A method of forming a structure of the photoabsorption layer  170  of FIGS. 9A and 9B is similar to the method of FIGS. 7A and 7B.  
     [0054]FIGS. 10A and 10B show the photoabsorption layer  170  comprised of a vertical quantum wire array layered-structure using a triple wall quantum well structure. A method of forming the structure of the photoabsorption layer  170  of FIGS. 10A and 10B is similar to the method of FIGS. 8A and 8B. A reference numeral  167   a  represents a quantum barrier layer composed of an i-InAlAs layer, a reference numeral  167   b  represents a quantum wire using an InGaAs quantum well layer having a thickness of 100 Å or less, and a reference numeral  167   c  represents an insulating layer.  
     [0055] (Energy Band Diagram)  
     [0056]FIGS. 11A and 11B are schematic energy band diagrams illustrating an energy state of the avalanche phototransistor according to the present invention. Particularly, FIGS. 11A and 11B are schematic energy band diagrams in a case of not using the quantum structure as the photoabsorption layer of FIGS.  1  to  4 . In the drawings, reference elements (E), (B) and (C) represent an emitter layer, a base layer and a collector layer, respectively. Further, reference elements Ec and Ev represent the conduction band and the valance band, respectively.  
     [0057]FIG. 11A shows an energy band of the avalanche phototransistor under a thermal equilibrium state when being not applied a voltage from external. In FIG. 11A, a reference element V BI  represents a built-in potentional between the photoabsorption layer and the avalanche-gain layered-structure, and a reference element V′ BI  represents a built-in potentional between the base layer and the collector layer.  
     [0058]FIG. 11B shows an energy band of the avalanche phototransistor when a voltage is applied from external. Electrons in the photoabsorption layer absorb an infrared light, and then the electrons are interband-transited into the conduction band. The transited electrons are multiplicated by voltages V 1  and V 2  applied from the exterior and built-in potentionals V BI  and V′ BI  in the phototransistor while passing through the charge layer and the multiplication layer. Strength of electric field of the avalanche-gain layered-structure is controlled by a voltage, which is applied to both sides of the avalanche-gain layered-structure. V 1  is a voltage applied between the emitter layer and the base layer, and V 2  is a voltage applied between the base layer and the collector layer. The voltages V 1  and V 2  have reverse polarities to each other. In a case of a multiplication structure using electrons, the voltage V 1  is a negatively biased voltage, and the voltage V 2  is a positively biased voltage. The multiplicated electrons move at high speed while passing through the hot electron transition layer to reach to the collector layer, thereby producing a large electric signal (output).  
     [0059] The reason the electrons created in the photoabsorption layer are multiplicated by passing through the charge layer and the multiplication layer is that impact ionization occurs in the multiplication layer due to a very high electric field effect generated by applying the exterior reverse voltage. That is, since the energy level of the avalanche-gain layered-structure including the charge layer and the multiplication layer is lower than that of the emitter layer by the amount of the voltage V 1  so that a potential difference between the avalanche-gain layered-structure and the emitter layer is large and the strength of the electric field of the avalanche-gain layered-structure is high, the avalanche-gain by the impact ionization effect can be obtained. Further, the reason the moving speed of the multiplicated electrons is high by passing through the hot electron transition layer is that the energy level of the hot electron transition layer is lower than that of the avalanche-gain layered-structure by the amount of the V 2  so that the multiplicated electrons can be hot-electrons.  
     [0060] Accordingly, although the light signal of a very low intensity is applied to the avalanche phototransistor, since the potential difference between the layers occurs as described above, the avalanche phototransistor according to the present invention can sensitively detect the light signal.  
     [0061]FIGS. 12A and 12B are schematic energy band diagrams of the avalanche phototransistor according to the present invention in a case of applying a quantum structure to the photoabsorption layer of FIGS. 1 through 4. The quantum structure in FIGS. 12A and 12B is referred to as a quantum well structure, a quantum dot structure or a quantum wire array structure. Similar to FIGS. 11A and 11B, in FIGS. 12A and 12B, reference elements (E), (B) and (C) represent an emitter layer, a base layer and a collector layer, respectively. Further, a reference element Ec represents the conduction band.  
     [0062]FIG. 12A shows an energy band of the avalanche phototransistor under a thermal equilibrium state when an external voltage is not applied. In FIG. 12A, a reference element V BI  represents a built-in potential between the photoabsorption layer of the quantum structure and the avalanche-gain layered-structure, and a reference element V′ BI  represents a built-in potential between the base layer and the collector layer. Since the photoabsorption layer of the quantum structure is used in the avalanche phototransistor, the energy band of the photoabsorption layer is split into a number of sub-bands as shown in FIG. 12A.  
     [0063]FIG. 12B shows an energy band of the avalanche phototransistor when an external voltage is applied. Electrons in the photoabsorption layer absorb an infrared light, and the electrons are intersubband-transited into a band of sharp excitation level. The transited electrons, as described in FIG. 11B, are multiplicated by applying the external voltages V 1  and V 2  and the built-in potentials V BI  and V′ BI  while passing through the charge layer and the multiplication layer. The multiplicated electrons passes through the hot electron transition layer to reach to the collector layer. An infrared absorbing wavelength is determined by confinement energy level of quantum dot, quantum well or quantum wire  
     [0064] As described so far, since the avalanche phototransistor according to the present invention includes the avalanche-gain layered-structure, the hot electron transition layer, and a three-terminal structure, high gain can be achieved. Therefore, high sensitivity, low operation voltage, high output and high operation speed can be achieved. Stability can be ensured by suppressed breakdown of the photo detector. Further, since the low operation voltage is used, the avalanche phototransistor according to the present invention has many advantages. Since high gain is achieved, low photo-absorptivity can be compensated, and multiple operation functions can be obtained using the three-terminal structure. The infrared signal of various wavelengths can be selected and processed, because the high degree of selection of the photoabsorption layer.  
     [0065] The avalanche phototransistor of the present invention can be used for long-distance communication and in a case where a very high sensitivity is required, for example, for signal photon counting. Since the avalanche phototransistor of the present invention does not require an electric preamplifier, which is inevitably required in the avalanche photodiode, by accomplishing high gain, the avalanche phototransistor can be applied to a photo detector of high speed and high output, a high speed infrared signal detector, a high speed infrared signal amplifier or a light receiver. Further, the avalanche phototransistor can be applied to an ultra high speed switching device, a digital logic device or a high speed infrared digital logic device having multiple functions due to the increased degree of the freedom assured by the multi-terminal operation, for example, three or more terminals.  
     [0066] While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.