Driver circuit for a pnpn semiconductor element

Abstract: The invention provides an optical functioning device which emits and receives light and a driver circuit for controlling the device with light. In the device, elements in which semiconductor multilayer-film reflecting mirrors are provided at both the upper and lower ends of a pnpn structure of direct transition semiconductors and which have light-emitting and light-receiving functions to act as optical resonators are integrated two-dimensionally with electrodes which are provided for the layers of the pnpn structure. An active layer of gradually varying composition is inserted at an interface of a quantum well structure. The driver circuit is constituted from a transistor obtained from a pnp or npn portion of the pnpn structure, and the transistor is caused to act as a phototransistor into which light is introduced.

Description: Background of the Invention 1. Field of the Invention This invention relates to a driver circuit for a pnpn semiconductor element. 2. Description of the Related Art Utilization of light in information processing is expected to allow high-speed processing and large capacity. Optical information processing is employed to effect processing of an optical signal using optical functioning elements having functions of threshold value processing, calculation, storage and so forth. It is considered an advantage of optical information processing over electric information processing that it allows parallel processing of a high density through the use of the incoherence of light. Therefore, investigations are directed to optical functioning elements which can be two-dimensionally integrated at a high density. As examples of optical functioning elements, a switch and an optical thyristor of pnpn structure in which a phototransistor and a pn junction light-emitting element are integrated have been reported. The switch is disclosed in detail in "Optical Devices of InGaAsP/InP Heterojunction Phototransistors and Inner Stripe Light-Emitting Diodes," IEEE Transaction on Electron Devices, Vol. ED-32, 1985, pp. 2656-2660, and the optical thyristor is disclosed in detail in "Double heterostructure optoelectronic switch as a dynamic memory with low-power consumption," Applied Physics Letter, Vol. 52, 1988, pp. 679-681. As the optical thyristor, an element of a surface-emitting laser type into which a vertical resonator structure is introduced in order to enhance efficiency and speed of emission of light is disclosed in "Surface-emitting laser operation in vertical-to-surface transmission electrophotonic devices with a vertical cavity," Applied Physics Letter, Vol. 58, 1991, pp. 1250-1251. Further, for the switch, a resetting operation has been reported wherein, using a phototransistor inside the structure of an optical switch, light is introduced into the phototransistor by a driver circuit connected in parallel to the optical switch in order to return an optical switch from the ON state to the OFF state. A pnpn semiconductor structure according the preamble of claim 1 is disclosed in "Functional Integration of Light-Triggered Static Induction Thyristor and the Static Induction Phototransistor", J. Nishizawa et al; IEEE-Electron Device Letters Vol.EDL-7, No 4, 265 (1986). However, the functioning elements mentioned above require operations to control the bias voltage when light is to be received or emitted and either lowering the voltage to a level close to zero for resetting in order to restore the initial state after switching or drawing out carriers by means of a gate electrode, and where they are two-dimensionally integrated at a high density, it is difficult to lay electric wiring lines for electrically driving individual devices from external electric circuits. Further, since the reported resetting operation using a phototransistor involves lowering the voltage applied to an optical switch by means of a phototransistor connected in parallel to the optical switch, high power and a considerable amount of time are required to carry out the resetting operation. Summary of the Invention It is an object of the present invention to provide a driver circuit for a pnpn semiconductor element wherein the drawbacks described above are eliminated and wherein bias control and a resetting operation can be performed with light. According to an aspect of the present invention, a driver circuit for a pnpn semiconductor element having the constructions according the preamble of claim 1 is constructed in such a manner that the circuit is constituted from a bipolar transistor element obtained by using a portion of the pnpn layer structure of each of the pnpn elements disposed two-dimensionally on a substrate, the circuit including first to third semiconductor layers or second to fourth semiconductor layers and being connected to at least one of a first gate electrode and a second gate electrode of the pnpn element so that supply and drawing out of current of the transistor circuit occurs through the gate electrodes, and that light is introduced into the bipolar transistor so that the bipolar transistor acts as a phototransistor. Further advantageous features of the driver circuit for a pnpn- semiconductor element according the invention are given in the subclaims 2 and 3. The above and other objects, features, and advantages of the present invention will become apparent from the following description in conjunction with the accompanying drawings which illustrate an example of a preferred embodiment of the present invention. Brief Description of the Drawings Fig. 1 is a schematic view showing the construction of an embodiment of a pnpn semiconductor element of the present invention; Fig. 2 is a diagrammatic view showing the construction of an active layer of the pnpn semiconductor element; Fig. 3 is a circuit diagram showing the construction of a driver circuit for the pnpn semiconductor element; and Fig. 4 is a similar view showing another example of the construction of a driver circuit for the pnpn semiconductor element. Detailed Description of the Preferred Embodiments In the present invention, a driver circuit for a pnpn-semiconductor element having a pnpn structure made from direct transition semiconductor material is epitaxially grown on a substrate, and an electrode is formed for each pnpn layer of each of the elements separated from one another by etching. An element of the pnpn structure exhibits the characteristics of a thyristor when a forward voltage is applied to the p and n electrodes on the opposite ends of the semiconductor element. In particular, an element of pnpn structure remains in a high-resistance condition in which no current flows through it when the applied forward voltage is lower than a threshold voltage, but when the applied forward voltage exceeds the threshold voltage, the element is switches into a low-resistance condition in which current flows through it. This switching also takes place when light is introduced into the element while the applied forward voltage is lower than the threshold voltage. In a low-resistance condition, the element emits light when current is supplied to it, and an optical resonator, with which laser oscillation is obtained, is provided by forming, on each of the upper and lower ends of the pnpn structure of the element, a multilayer-film reflecting mirror having thickness equal to one fourth the wavelength of light to be emitted and composed of alternating layers, these layers being of two semiconductor types and two differing refractive indices. Here, by inserting into the active layer a structure composed of layers of gradually varying composition at the interfaces of a quantum well, in the ON state, the supplied current can be effectively trapped in the active layer and laser oscillation can be obtained at a low current supply level. When an electric field is applied, upon switching, to a non-doped portion of the quantum well including the active layer, the heterobarrier at the interface of the quantum well is reduced by the inserted layers of gradually varying composition, consequently preventing the current generated by light from being trapped by the quantum well. A bipolar transistor constituting a circuit for controlling the pnpn semiconductor element with light can be obtained by using three pnp or npn electrodes of the electrodes of the pnpn element. Since the threshold voltage of the pnpn element drops by supplying current to the element from the gate electrode, if the bipolar transistor is caused to act as a phototransistor into which light is introduced and the phototransistor is connected to the gate electrode to supply current to the phototransistor, the threshold voltage of the pnpn element can be controlled by means of the intensity of light introduced into the phototransistor. Further, resetting can be performed at high speed by drawing out current from the gate electrode of the pnpn element in the ON state. If the phototransistor is connected so that current is drawn out from the gate electrode, resetting of the pnpn element can be performed at high speed by introducing light into the phototransistor. The preferred embodiments of the present invention are described below with reference to the drawings. Fig. 1 shows an embodiment of a pnpn semiconductor element of the present invention. The pnpn semiconductor element is constructed such that a pair of multilayer-film reflecting mirrors 12 and 13 each formed from alternating layers of GaAs and AlAs having thickness equal to λ/4n are disposed at the upper and lower ends of an AlGaAs/GaAs layer structure 11 which includes p-type and n-type direct transition semiconductor layers in a pnpn structure, the above structure being formed on a substrate 25 of semi-insulating GaAs. In the expression λ/4n, which defines the thickness of the multilayer-film reflecting mirrors 12 and 13, λ is the wavelength of laser oscillation and n is the refractive index of GaAs or AlAs. The upper multilayer-film reflecting mirror 12 is of the p-type, and the lower multilayer-film reflecting mirror 13 is of the n-type. The reflective index of the semiconductor multilayer-film reflecting mirrors, which is approximately 99.3%, is obtained with 15 GaAs/AlAs layers. An active layer has a quantum well structure of InGaAs and AlGaAs and is inserted between the center p layer and n layer. The element is formed in a mesa shape in two etching steps. In the first step, the multilayer film and the p-type semiconductor layer are etched into the n-type AlGaAs layer. In the second step, several layers are etched into the lower semiconductor multilayer film, following which etching to the semi-insulating substrate is carried out. An anode electrode 21 is formed on the p-type multilayer film reflecting mirror, an n gate electrode 22 is formed on n-type AlGaAs, a p gate electrode 23 is formed on the diffused region 26 which is formed by diffusion of a p-type impurity into the n-type AlGaAs and extends to the p-type GaAs layer, and a cathode electrode 24 is formed on the second n-type semiconductor multilayer film. Fig. 2 is a diagrammatic view showing the structure of the active layer portion of the pnpn semiconductor element. At the interface of the quantum well structure wherein InGaAs is held between AlGaAs, a structure is inserted in which the period of the superstructure of InGaAs and AlGaAs is artificially varied so that the average composition varies gradually. Fig. 3 shows an embodiment wherein a driver circuit for the pnpn semiconductor element is formed with a configuration wherein a pair of npn-type phototransistors are connected to an n gate electrode. The npn-type transistors can be realized by driving the pnpn structure of Fig. 1 so that the cathode electrode 24 may act as the emitter, the p gate electrode 23 may act as the base and the n gate electrode 22 may act as the collector. In the present embodiment, since the npn-type transistors are used as phototransistors, the bases are open. The emitter of phototransistor 40 is connected to the n gate electrode, the collector of phototransistor 40 is connected to the positive power source 50, while the collector of phototransistor 41 is connected to the n gate electrode, and the emitter of phototransistor 41 is connected to negative power source 51. The anode of the pnpn element is connected to the positive power source through a load resistor, and the cathode of the pnpn element is connected to the ground. The positive power source is set to a value lower than the threshold voltage of the pnpn element so that a fixed current may be supplied to the pnpn element when the pnpn element is in the ON state. The intensity of light 60 to be introduced into the pnpn element is set to a value at which the element cannot be switched at the voltage of the positive power source, and control light 61 is introduced into phototransistor 41 while current is supplied from the n gate electrode. Consequently, the threshold voltage of the pnpn element drops so that it is switched by incident light 60. When reset light 62 is introduced into phototransistor 40 with the element in the ON state, carriers accumulated in the pnpn structure are drawn out from the n gate electrode by the phototransistor so that the element is reset to the OFF state. Fig. 4 shows another embodiment wherein a driver circuit for the pnpn semiconductor element is realized with a configuration wherein a pair of npn-type phototransistors are connected to an n gate electrode and a p gate electrode. The emitter of phototransistor 40 is connected to the n gate electrode, and the collector of phototransistor 40 is connected to the positive power source 50. The emitter of phototransistor 41 is connected to the p gate electrode, and the collector of phototransistor 41 is connected to the positive power source 50. The operation of the present embodiment is similar to that of the embodiment of Fig. 3, and control light 61 is introduced into phototransistor 41 whereas reset light 62 is introduced into phototransistor 40. In contrast to the embodiment of Fig. 3, the present construction requires only one power source. When the phototransistors in the embodiments described above are replaced by phototransistors of the pnp-type, similar operation can be achieved by reversing the connections of the emitters and the collectors. As described in detail above, according to the present invention, a pnpn semiconductor element having light-receiving and vertical emission laser outputting functions and a driver circuit which can effect bias control and resetting of the pnpn semiconductor element with light can be realized using transistors obtained from the pnpn structure. Although a specific embodiment of the present invention has been described above, it is to be understood that the variations of the embodiment are possible and that the scope of the present invention is defined in the appended claims.

Claims: A driver circuit for a pnpn semiconductor element having a pnpn layer structure (11) including at least a first semiconductor layer of the direct transition type of either p or n conduction type, a second semiconductor layer of conduction type opposite to that of the first semiconductor layer, a third semiconductor layer of the same conduction type as that of the first semiconductor layer, and a fourth semiconductor layer of the same conduction type as that of the second semiconductor layer, electrodes (21, 24), provided for each of said first and fourth semiconductor layers, a first gate electrode (23) provided for said second semiconductor layer, and a second gate electrode (22) provided for said third semiconductor layer, characterized in that the circuit is constituted from a bipolar transistor element which is obtained by using a portion of the pnpn layer structure of each of said pnpn elements (11) disposed two-dimensionally on a substrate (25), the circuit including first to third semiconductor layers or second to fourth semiconductor layers and being connected to at least one of a first gate electrode and a second gate electrode of the pnpn element such that the supply and drawing out of current of the transistor circuit is performed through said gate electrodes, and that light is introduced into said bipolar transistor so that said bipolar transistor acts as a phototransistor. A driver circuit for a pnpn semiconductor element as claimed in claim 1, wherein a pair of reflecting mirrors (12, 13) each formed from a semiconductor multilayer film are formed at the upper and lower ends of said pnpn layer structure (11), the energy gaps of said first and fourth semiconductor layers are greater than the energy gaps of said second and third semiconductor layers, and a non-doped layer structure which includes an active layer is formed between said second semiconductor layer and said third semiconductor layer. A driver circuit for a pnpn semiconductor element as claimed in claim 2, wherein said active layer has a quantum well structure wherein a semiconductor having a small energy gap and having a thickness of several tens of nm is held between a pair of layers of another semiconductor having an energy gap greater than the former semiconductor, and a semiconductor layer wherein the energy gap varies gradually within the range of several tens of nm is inserted at an interface of said quantum well structure.

Inventors: OGURA ICHIRO, OGURA, ICHIRO, OGURA, ICHIRO, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1993
Main classification: H01L31
IPC codes: H01L31, G02F3, H01L33, G02F1, H01L29, H03K17