A PNPN THYRISTOR

Abstract: A pnpn thyristor comprises an anode and cathode region, and a base region which is positioned between the anode and cathode regions. The base region is composed of a p-base layer facing the cathode region, and a first to third n-base layers. Among the first to third n-base layers, the first n-base layer faces the p-base layer, and the third n-base layer faces the anode region. The anode and cathode regions are wider in their forbidden bandwidth than the first and third n-base layers, and the second n-base layer is narrower in its forbidden bandwidth than the first and third n-­ base layers so that an optical coupling property is improved, and a hight output of light is obtained.

Description: FIELD OF THE INVENTION The invention relates to an optical pnpn thyristor, and more particularly to an optical pnpn thyristor which is applicable for instance to an optical semiconductor memory in an image processing system or an optical computer. BACKGROUND OF THE INVENTION One of bistable semiconductor lasers is described in detail on a paper of "The Institute of Electronics, Information and Communication Engineers, 1985". In the bistable semiconductor laser, light is supplied from a light emitting diode through a lens to one facet thereof to be focussed into an active layer of an unexcited state so that a laser oscillation is obtained with the trigger light. Such a bistable semiconductor laser in which a laser oscillation is performed with a minute trigger light, and the laser oscillation thus induced is continued thereafter even in the absence of the trigger light is applicable to an optical semiconductor memory which is an important device for instance in a prospective optical switching system or parallel optical information processing system. Another conventional optical semiconductor memory is described on pages 596 to 600 of "Journal of Applied Physics, 59(2), 15 January 1986". The optical semiconductor memory is of an optical pnpn thyristor to be provided with an n-base layer of n-GaAs having a narrower forbidden bandwidth between an anode region of p-AlGaAs and cathode region of n-AlGaAs. In the optical thyristor, a bias voltage is applied thereto to provide a turning-on thereof wherein a transition is performed from a high impedance state to a low impedance state so that carriers are injected into the n-base region to be confined therein. As a result, a spontaneous emission light is radiated from the optical thyristor. According to the former of the bistable semiconductor laser, however, there is a disadvantage that an optical coupling of the trigger light is difficult at the facet thereof, thereby increasing the coupling loss of the trigger light, although an induced emission light which is desirable in regard to light output power is obtained therein. According to the latter of the optical pnpn thyristor, further, such a high output of light is not obtained as in the bistable semiconductor laser because an output light is a spontaneous emission light. SUMMARY OF THE INVENTION Accordingly, it is an object of the invention to provide an optical pnpn thyristor in which an optical coupling of trigger light is easy to be improved. It is a further object of the invention to provide an optical pnpn thyristor in which a high output of light is obtained. According to the invention, an optical pnpn thyristor comprises, anode and cathode regions, and a base region which is positioned between said anode and cathode regions, said base region including a p-base layer facing said cathode region, and first to third n-base layers among which said first n-base layer is facing said p-base layer, and said third n-base layer is facing said anode region, wherein respective forbidden bandgaps of said anode and cathode regions are wider than those of said first and third n-base layers, and the forbidden bandgap of said second n-base layer is narrower than those of said first and third n-base layers, and wherein the second n-base layer is provided with resonance planes at its extremities for the emission of laser light in response to trigger light. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in more detail in conjunction with following drawings wherein, Fig. 1 is a cross sectional view showing a conventional optical pnpn thyristor, Fig. 2 is a perspective view showing an optical pnpn thyristor in a first embodiment according to the invention, Figs. 3A to 3B are energy band diagrams showing an operation of the optical pnpn thyristor in the first embodiment, Fig. 4 is an explanatory diagram showing a relation between a bias voltage and current flowing in the optical pnpn thyristor in the first embodiment, Fig. 5 is a timing chart showing the operation of the optical pnpn thyristor in the first embodiment, Fig. 6 is a spectrum diagram of output light in the optical pnpn thyristor in the first embodiment, Fig. 7 is a design chart for the optical pnpn thyristor in the first embodiment, Figs. 8A to 8C are energy band diagrams showing an operation of an optical pnpn thyristor in a second embodiment according to the invention, Fig. 9 is a cross sectional view showing an optical pnpn thyristor in a third embodiment according to the invention, Figs. 10A to 10C are energy band diagrams showing an operation of the optical pnpn thyristor in the third embodiment, and Fig. 11 is a cross sectional view showing a device to which an optical pnpn thyristor in the first to third embodiments according to the invention is applied. DESCRIPTION OF PREFERRED EMBODIMENTS Before explaining an optical pnpn thyristor in an embodiment according to the invention, an optical semiconductor memory of an optical pnpn thyristor mentioned before will be again explained briefly. The optical semiconductor memory comprises a semiconductor substrate 11 of GaAs, a buffer layer 12 of GaAs, an anode region 13 of p-AlGaAs, an n-base layer 14 of n-GaAs having a narrower forbidden bandgap, a p-base layer 15 having a thickness of 3nm (30Å), a cathode region 16 of n-AlGaAs, a cap layer 17 of GaAs, and a cathode and anode electrodes 18 and 19 provided respectively on the upper surface of the cap layer 17 and back surface of the semiconductor substrate 11. In operation, a predetermined level of a bias voltage is applied across the cathode and anode electrodes 18 and 19, and trigger light is supplied as shown by an arrow so that a transition is peformed from a high impedance state to a low impedance state whereby carriers are injected into the n-base layer 14 to be confined therein. As a result, spontaneous emission light is radiated therefrom as shown by an arrow. However, the output light is not so high in its intensity as expected in the bistable semiconductor laser. In Fig. 2, there is shown an optical pnpn thyristor in the first embodiment according to the invention. The optical thyristor is applicable to an optical semiconductor memory for 0.8µm wavelength band which is composed of AlGaAs/GaAs system semiconductor. The optical pnpn thyristor comprises a semiconductor substrate 21 of n-GaAs, a cathode region 22 of n-Al 0.4 Ga 0.6 As having a thickness of 2µm and density of 5x10¹⁷cm⁻³, a p-base layer 23 of p-Al 0.25 Ga 0.75 As having a thickness of 5nm (50Å) and density of 1x10¹⁹cm⁻³, a first n-base layer 24 of undoped n-Al 0.25 Ga 0.75 As having a thickness of 0.3µm and density of 1x10¹⁵cm⁻³, a second n-base layer 25 of undoped n-GaAs having a thickness of of 0.1µm and density of 1x10¹⁵cm⁻³, a third n-base layer 26 of n-Al 0.25 Ga 0.75 As having a thickness of 0.5µm and density of 1x10¹⁵cm⁻³, an anode region 27 of p-Al 0.4 Ga 0.6 As having a thickness of 1µm and density of 5x10¹⁸cm⁻³, a cap layer 28 of p-GaAs having a thickness of 0.2µm and density of 2x10¹⁹cm⁻³, and an anode and cathode electrodes 29 and 30 provided respectively on the upper surface of the cap layer 28 and back surface of the semiconductor substrate 21. The optical pnpn thyristor is shaped to have a stripe configuration from the upper portion of the first n-base layer 24 to the cap layer 28 to be formed in an etching process and is provided with polyimide layers 31 on the both sides of the strip configuration whereby the difference of refractive indexes between an active layer and both sides thereof becomes smaller so that a transverse mode oscillation can be controlled. The polyimide layers 31 function also as passivation. Resonance planes are formed at cleaved facets to provide a resonator having a length of 100µm. In Fig. 3A to 3C, there are shown energy band diagrams of the optical pnpn thyristor in which a zero bias state, high impedance state and on state thereof are shown respectively. For simplifying the illustrations thereof, such matters as a discontinuity at a heterostructure junction which are not directly related to the feature of the invention is approximately drawn. As shown at the top of Fig. 3A, forbidden band energies of the n-cathode region 22 and p-anode region 27 are E₁ and E₄ (E₁=E₄ in the embodiment) respectively, while a forbidden band energy Eg of the n-base layer is composed of forbidden band energies E₃ of the first and third base layers 24 and 26, and a forbidden band energy Ea of the second n-base layer 25. Among the first to third n-base layers 24 to 26, the second n-base layer 25 of the forbidden band energy Ea acts as an active layer for a laser oscillation. Further, a forbidden band energy of the p-base layer 23 is E₂. In regard to the values of the forbidden band energies mentioned above, E₁ and E₄ are larger than E₃, and Ea is smaller than E₃, that is, E₁, E₄>E₃>Ea. On the other hand, carrier densities of the n-cathode region 22, and first to third n-base layers 24 to 26 are n₁, n₂, n a , and n₂ respectively, while those of the p-base layers 23 and p-anode region 27 are p₁ and p₂ respectively. The carrier densities n₂ and n a are set to be low to some extent. In operation, zero bias state of the optical pnpn thyristor is shown in Fig. 3A. A predetermined level of a bias voltage is then applied across the anode and cathode electrodes 29 and 30. At the starting stage, substantially no current is flowing through the optical pnpn thyristor due to a high impedance state shown in Fig. 3B. A depletion layer is formed to be extended into the n-base region because the n-base layers 24 to 26 are of low carrier densities so that almost all of the voltage is applied across a p-n junction of the p and n-base layers 23 and 24. At the present stage, holes in the p-anode region 27 can not extend a potential barrier formed at a p-n junction between the p-anode region 27 and third n-base layer 26 so that the high impedance state under which substantially no current is flowing results. When the applied voltage is further increased up to a predetermined level to be larger than a break over voltage V BO shown in Fig. 4, current is suddenly flowing to result in an on state shown in Fig. 3C, as known in operation of a normal optical thyristor. The bias voltage V₁ can be decreased down to V₂ as shown in Figs. 3B and 3C. Under the on state, the voltage applied across the optical thyristor is substantially equal to a voltage to be applied to a semiconductor device having a single p-n junction. The narrow forbidden bandgap Ea(Eg=Ea) of the second n-base layer 25 is provided in the invention so that a portion of holes and electrons are dropped into a hollow portion of potential. In this occasion, a laser oscillation is induced in the optical thyristor when a gain exceeds a loss, if a reflecting mirror is provided at the outside thereof. Electrons flowing into the anode region 27 and holes flowing into the cathode region 22 can be decreased in regard to the number thereof so that the relaxation of carriers becomes easier in the active layer of the n-base layer 25 because the forbidden bandgaps E₁ and E₄ of the n-cathode and p-anode regions 22 and 27 are wider than those E₃ of the n-base layers 24 and 26. As a result, the emission efficiency is increased in the optical thyristor. In a case where a voltage applied across the optical thyristor is set to be V X shown in Fig. 4, and an appropriate amount of light is supplied thereto, the light is absorbed in the n-base layers 24, 25 and 26 so that holes are injected into the p-base layer 23. The n-base layer 25 is thin and the n-base layers 24 and 26, mainly, function for absorbing layers. The holes thus injected thereinto increase the number of electrons passing through the p-base layer 23 because the transistor effect is expected therein. The electrons moderate the inclination of potential in the n-base layers 24 to 26, thereby increasing the number of holes injected from the anode region 27 into the n-base layer 26. In such a manner, a positive feedback effect is performed to shift the optical semiconductor memory from a high impedance state to an on state. As a result, a laser oscillation is induced in the optical thyristor to which trigger light is supplied as shown in Fig. 5. The laser oscillation is continued even after the turning-off of the trigger light, but stopped therein when an applied voltage V X is reset as shown by a reference symbol P R in Fig. 5. Reset voltage value is set to be lower than the holding voltage value V H . As clearly understood from the above, the sensitivity of trigger light is increased therein as an applied voltage V X thereacross approaches the break over voltage V BO . In the first embodiment, several tens mW of output light are obtained in the optical thyristor when several tens µW of trigger light are supplied thereto. At the time, external differential quantum efficiency is 23 percents at one facet, while the wavelength of the laser oscillation is approximately 875.2 nm wherein an induced emission light is obtained corresponding to the bandgap of GaAs. Further, a light emission is not obtained at a wavelength corresponding to the bandgap of the n-base layers 24 and 26, but an effective confinement of injected carriers is performed in the n-base layer 25 under the on state as shown in Fig. 6 wherein current is flowing in an amount of 190mA which is 1.2 times a current threshold of 160mA under the applied bias voltage of 1.7V at the room temperature. If it is assumed that a single n-base layer having a thickness of approximately 0.1µm equal to that of the active layer in the first embodiment and having a small forbidden bandgap is provided in place of the first to third n-base layers in the first embodiment, the thickness is too thin to absorb trigger light so that the sensitivity of the trigger light becomes lowered. In Fig. 7, there is shown a design chart by which parameters as carrier density and layer thickness in a base region of GaAs for a normal optical pnpn thyristor used as an electric switching means are determined. Accordingly, the design chart can be used to design an optical pnpn thyristor of the invention. In an optical thyristor, the transition from a high impedance state to an on state is controlled dependent on a punch through voltage V PT of an n-base region and avalanche breakdown voltage V B1 . In a case where an n-base region of the optical thyristor is of a single semiconductor layer, a turning-on voltage is determined by use of a step junction approximation, as defined in the following equation (1) according to a punch through limitation, and in the following equation (2) and (3) according to an avalanche breakdown limitation. where q is 1.6x10⁻¹⁹ coulomb, N is a carrier density (cm⁻³ ) of a base layer, d is a thickness of the base layer, εs is a permittivity thereof, α₁ and α₂ are current gains of npn and pnp transistors respectively, n is a constant, and Eg is a forbidden band energy (eV). As clearly understood from the above, a turning-on voltage is determined in regard to a carrier density in the design chart. In a case where the design chart is applied to an optical pnpn thyristor according to the invention, parameters are determined to be shifted in the direction of a higher voltage to some extent. Next, there will be explained an optical pnpn thyristor in the second embodiment according to the invention. The optical pnpn thyristor is different from one in the first embodiment only in that a thickness of a p-base layer 23 in the second embodiment is smaller than that in the first embodiment such that a depletion layer is formed, for instance, even in a case where an externally applied voltage is zero. In operation, an optical current gain in a transistor consisting of an n-cathode region 22, p-base layer 23, and n-base layer 24 becomes higher so that the sensitivity of trigger light is further increased. Energy band diagrams in the second embodiment are shown in Figs. 8A to 8C. In "Journal of Applied Physics, 59(2), 15 January 1986" mentioned before, an optical thyristor having a thinned p-base layer is described. According to the paper, it is reported that current of more than 100mA can flow in the optical thyristor which is turned on by applying trigger light of several µW thereto. In a case where such an amount of current is flowing into an active layer effectively, a laser oscillation can be completely performed. The optical pnpn thyristor in the second embodiment is operated based on the principle as described above. Figs. 8A to 8C are self-explanatory in view of the descriptions in Figs. 3A to 3C except that the bias voltages V₁ and V₂ are replaced by bias voltages V'₁ and V'₂ so that repeated explanations are omitted here. In Fig. 9, there is shown an optical pnpn thyristor in the third embodiment according to the invention. The optical pnpn thyristor comprises a semiconductor substrate 91 of p-GaAs, a buffer layer 92 of p-GaAs having a thickness of 0.5µm, an anode region 93 of p-Al 0.4 Ga 0.6 As having a thickness of 0.5µm and impurity density of 1x10¹⁸cm⁻³, a first and third n-base layers 96 and 94 each of n-Al Z Ga 1-Z As (0.25≦Z≦0.3) having a thickness of 0.5µm and impurity density of 5x10¹⁶cm⁻³, a second n-base layer 95 of n-GaAs having a thickness of 0.1µm and impurity density of 5x10¹⁶cm⁻³, a p-base layer 97 of p-Al 0.3 Ga 0.7 As having a thickness of 0.05µm and impurity density of 1x10¹⁹cm⁻³, a cathode region 98 of n-Al 0.4 Ga 0.6 As having a thickness of 0.3µm and impurity density of 1x10¹⁸cm⁻³, a cap layer 99 of n-GaAs having a thickness of 0.1µm and impurity density of 5x10¹⁸cm⁻³, a cathode electrode 100 of AuGe-Ni, and an anode electrode 101 of AuZn. In the optical pnpn thyristor, respective forbidden bandgaps of the first to third n-base layers 96, 95 and 94 are narrower than those of the anode and cathode regions 93 and 98, while forbidden bandgaps of the first and third n-base layers 96 and 94 are wider than that of the second n-base layer 95 to be increased in proportion to a distance of the formers from the respective junction planes on the upper and back surfaces of the latter. For this purpose, a value "Z" for defining an impurity density of the first and third n-base layers 96 and 94 is increased linearly from 0.25 up to 0.3 in proportion to the distance as mentioned above. In Figs. 10A to 10C, there are shown energy band diagrams in which a zero bias state, high impedance state, and on state are shown for the optical pnpn thyristor in the third embodiment. As understood from the explanations in conjunction with Figs. 3A to 3C, each pn junction in the optical thyristor is placed under a forward bias state as shown in Fig. 10C after the optical thyristor is turned on. In a case where a base layer is of a single semiconductor layer as in a conventional optical thyristor, carriers injected into the base layer are diffused thereinto. In the optical thyristor in the third embodiment, however, carriers are drifting along inclinations of potential because a band structure is formed as shown in Fig. 10C so that the carriers are dropped without any delay into the thin n-base layer 95 effectively. The first to third n-base layers 96, 95 and 94 are deemed in operation to be a single n-base layer having a thickness equal approximately to that of the second n-base layer 95. In a case where a wavelength of trigger light is selected to be smaller than the bandgap wavelength which is determined by a maximum forbidden bandgap in the n-base layer, an absorption coefficient is not decreased in the structure described above. Further, an external differential quantum efficiency is increased by five percents in the thyristor in which the forbidden band is gradually increased as compared to an optical thyristor in which n-base layers each of n-Al 0.25 Ga 0.75 As are provided. In Fig. 11, there is shown a device to which an optical pnpn thyristor in the first to third embodiments according to the invention is applied. The device comprises an optical thyristor 111 in the embodiments, a forty-five degree reflecting mirror 112, and a plate member 113 having a window (not shown) through which trigger light is passed. The optical thyristor 111 and forty-five degree reflecting mirror 112 are provided on the plate member 113 wherein the forty-five degree reflecting mirror 112 is formed in an etching process to be positioned near a resonance plane of the optical thyristor 111. In operation, a predetermined level of a bias voltage is applied across the optical thyristor 111, and the trigger light is supplied through the window of the plate member 113 to the optical thyristor 111 so that output light is radiated in the horizontal direction. The output light thus radiated is reflected in the vertical direction by the forty-five degree reflecting mirror 112. The device is especially useful in an parallel optical information processing means. A reflecting angle of the mirror 112 may be changed to control the direction of output light.

Claims: An optical pnpn thyristor comprising (a) an anode region (27; 93) and a cathode region (22; 98) and (b) a base region (23-26; 94-97) positioned between the anode and cathode regions, (c) said base region including four adjacent layers starting in the following order from said cathode region: a p-base layer (23; 97), a first n-base layer (24; 96), a second n-base layer (25; 95) and a third n-base layer (26; 94), (d) said second n-base layer (25; 95) being provided with resonance planes (31) at its extremities for the emission of laser light in response to trigger light, (e) wherein the band gaps (E₄, E₁) of the anode and cathode regions, respectively, are wider than the band gap (E₃) of the first (24; 96) and the third (26; 24) n-base layers and the band gap (E a ) of the second n-base layer (25; 95) is narrower than the band gap (E₃) of the first (24; 96) and the third (26; 24) n-base layers. An optical pnpn thyristor according to claim 1, wherein said p-base layer (23; 97) is of such a thickness that a depletion layer is formed in the entire p-base layer even if no external voltage is applied. An optical pnpn thyristor according to claim 1 or 2, wherein the band gaps of the first and third n-base layers increase with increasing distance from the second n-base layer (25; 95). Use of the optical pnpn thyristor as claimed in claims 1, 2 or 3 in an optical device comprising (a) a mirror (112) having a predetermined reflecting angle, which mirror is positioned near a resonance plane (31) of the optical pnpn thyristor (111), and (b) a plate member (113) having a window for passing trigger light and carrying that optical pnpn thyristor (111) and said mirror (112).

Inventors: KASAHARA, KENICHI
Applicants: NEC CORPORATION
Application year: 1987
Main classification: H01L31
IPC codes: G11C11, H01L31, G02F3