SEMICONDUCTOR DEVICE, LIGHT-EMITTING DIODE AND TRANSISTOR STRUCTURE MAKING USE OF RESONANT TUNNELING

Abstract: The present invention provides a semiconductor device comprising: a first layer of semiconductor material of the one conductivity type; a second layer of semiconductor material of the other conductivity type; and a structure for resonant tunneling between the first and second layer of semiconductor material. The resonant tunneling structure (RTS) makes use of the extremely rapid tunneling effect wherein charge carriers tunnel through an energy barrier. The tunneling time is decisive here and not the radiative recombination. Extremely rapid optical oscillations can be obtained herewith with a frequency for instance in the region of 750 GHz. Electrical oscillations of such frequencies have already been realized.

Description: SEMICONDUCTOR DEVICE, LIGHT-EMITTING DIODE AND TRANSISTOR STRUCTURE MAKING USE OF RESONANT TUNNELING Existing semiconductor devices of III-V material, for instance GaAs, are known for emitting light (LEDs) . Radiation occurs herein with the recombination of charge carriers. Switching on and off of the radiation in these known semicon- ductor devices is comparatively slow. The present invention provides a semiconductor device comprising: - a first layer of semiconductor material of the one conductivity type; - a second layer of semiconductor material of the other conductivity type; and - a structure for resonant tunneling between the first and second layer of semiconductor material. The resonant tunneling structure (RTS) makes use of the extremely rapid tunneling effect wherein charge carriers tunnel through an energy barrier. The tunneling time is decisive here and not the radiative recombination. Extremely rapid optical oscillations can be obtained herewith with a frequency for instance in the region of 750 GHz. Electrical oscillations of such frequencies have already been realized. The semiconductor device according to the present invention can be operated at room temperature (± 300 K) , although the effects are more pronouncedly present at tempe¬ ratures in a region of approximately 80 K. At this low tempe- rature the light intensity can, within a voltage range of several mV (millivolts) or several tens of mV, be switched between two levels with a mutual intensity ratio of at least 10 (or even 100) . The present invention further provides a semiconductor device according to claim 8, that is, provided with a third layer of the first conductivity type. When such a semiconductor device is used in a transistor, and more particularly in an electro-optical heterodyne converter an optical signal is generated at the output thereof by the internal oscillation of the transistor, the amplitude of which signal is modulated with the electrical signal applied to the base. If such a transistor structure is placed in a cavity, for instance using the so- called icrostripline technology, the emitter collector current can oscillate at extremely high frequency, for instance 420 GHz for GaAs tunnel diodes. The oscillation in the emitter- collector current occurs locally, that is, only in the connection between the transistor and the capacitance which serves as supply voltage stabilization and in the transistor itself. In such a small circuit all parasitical effects such as capacitative coupling and outward radiation have a limited effect, which makes possible the said extremely high frequencies in emitter-collector current. A signal of for instance several MHz could be applied to the base, which signal is then modulated in the 100 GHz band. Further advantages, features and details of the present invention will become apparent in the light of a description of a preferred embodiment thereof with reference to the annexed drawing, in which: fig. l shows a diagram of a preferred embodiment of the semiconductor device according to the present invention; fig. 2 shows graphs of current-voltage characteristics of the preferred embodiment of the semiconductor device shown in fig. 1 at two different temperatures; fig. 3 shows a graph of the spectral distribution of the radiation emitted by the semiconductor device depicted schematically in fig. 1 under respectively different condi¬ tions; fig. 4 shows a graph of the light intensity charted against the applied voltage at T=77k; fig. 5 shows a graph of the light intensity charted against the applied voltage in a semiconductor device as according to fig. 1 at T»300k; fig. 6 shows a diagram of a second preferred embodiment of a semiconductor device according to the present invention; fig. 7 shows a graph of the emitter current I E and the base current I B as a function of the emitter voltage V E at different base voltages, and fig. 8 shows a graph of the spectral distribution of radiation emitted by the semiconductor device of fig. 6. A semiconductor device 1 (fig. 1) comprises a 500 nm thick GaAs p + (2xl0 18 cm '3 ) buffer layer 1 of GaAs, a non-doped spacer layer 2 (200 nm thick) of GaAs, a resonant tunneling structure (RTS) 3, a 200 nm thick, non-doped GaAs spacer layer 4, and a GaAs n + (3xl0 18 cm *3 ) layer of GaAs 5. The layers are applied using epitaxial growth making use of molecular bundles. The p + layer 1 is provided with an electrical terminal 6 connected to the schematically depicted conducting substrate 7. The n + layer 5 is provided with an electrode 8 and a schematically designated terminal 9. Arranged in the electrode 8 in a manner not shown is a window for passage of radiation or light generated in the semiconductor device. The resonant tunneling structure (RTS) according to the embodiment shown comprises a 4 nm thick AlAs barrier layer 11, a 6 nm thick GaAs well layer 12 and a 4 nm thick barrier layer of AlAs 13. Shown in the graphs of fig. 2, in which the forward voltage ϋ is charted in volts (V) against the current in milliamperes (mA) through the above described preferred embodiment of the semiconductor device according to the present invention, is a voltage region for both a temperature T of 300 K and for a temperature of 80 K (in a liquid nitrogen environment) within which a differential resistance has a negative value such as indicated by the curved line segments lτ and 1 2 respectively. The ratio of the current at U = 2.55 V and U = 2.50 V amounts to 2.1 at 80 K. This phenomenon is attributable to resonant tunneling of electrons in the conduction band. The hole current can consist of "light" holes because of the greater chance of tunneling of such "light" holes. Tunneling of holes is equally conceivable, although somewhat slower. Recent magnetic tunneling experiments perhaps point however in the direction of mixing of "light" and "heavy" holes, wherein the most significant contribution to the hole current is provided by "heavy" holes. Hole current generated by thermal emission and/or by tunneling results in a reducing of the above stated ratio between maximum and minimum current. In fig. 3 the wavelength X is charted in nanometers (nm) against a relative electroluminescence intensity ELRH.. The peak at 726 nm is caused by recombination of electrons and holes in the well layer. The charge carriers, both holes and electrons, tunnel through the AlAs barrier layer into the quantum well of the RTS. In fig. 4, in which the forward voltage U is charted in volts (V) against the relative electroluminescence intensity ELREL, it can be seen that around the region of negative differential resistance a relative ratio of the light intensity ELREL exists of 10. By changing the applied voltage U within a range of only several tens of mV an ON/OFF switching of the light is therefore obtained. Fig. 5 shows a graph corresponding to that of fig. 4, but at room temperature (± 300 K) . A considerable change in the light intensity ELRE L as a function of the applied voltage is also observable here. Fig. 6 shows a further preferred embodiment of a semiconductor device 20 in the form of a transistor structure provided with an emitter terminal 21, a base terminal 22 and a collector terminal 23. The emitter terminal 21 is coupled via a conducting contact 24 to a first layer 25 of a semiconductor material of the one conductivity type, for instance p-type. The base terminal 22 is coupled via a conducting contact 26 to a second layer 27 of the other conductivity type, for instance n- type. The collector terminal 23 is coupled via a conducting contact 28 to a layer of semiconductor material 29. Situated between the layer 25 and layer 27 is an RTS structure or tunnel structure 30. The layer 22 is preferably embodied with extremely small thickness so that it exhibits properties of a two-dimensional electron gas (2DEG) . This 2DEG is not capable of completely shielding the collector from the emitter. In this manner tunnel characteristics are obtained between collector and emitter and the emitter-collector current can begin to oscillate in the region of negative differential resistance. The base contact 26 is used to inject minority charge carriers via the 2DEG. An optical signal is herein obtained at an exciton wavelength, wherein at the output an oscillating optical signal of high frequency is obtained which is modulated with the signal of lower frequency applied to the base. As the charge carriers in the quantum well can disappear due to the tunneling as well as due to recombination, the light yield will perhaps be less than in the case of a comparable LED, as described above. The velocity of this semiconductor device is not however limited by a recombination time but by the tunneling time of the majority charge carriers, which is extremely short. Under the influence of a voltage between emitter and collector, holes tunnel from the emitter to the collector. As these holes pass through the 2DEG in the base with a high energy (in so-called unbound states) there is only a small chance of the holes falling back to the energy level of the base. With a correct choice of the voltage between emitter and collector the current (and therefore the hole population) oscillates in the potential well normally between twice the nominal value and the zero point. Electrons can be injected into the quantum well via the 2DEG of the base. When the extent of injection is not too great these electrons will recombine in the quantum well and the outcoming light will have mainly the so-called exciton wavelength. This exciton luminescence can be interpreted as a product of the oscillating hole population of the quantum well and the electron injection via the base. The transistor operates as heterodyne converter with an optical output signal. When the base layer consists of n-type semiconductor material, electrically conducting contacts can easily be arranged in accordance with known PdGe technology on the extremely thin base layer. The 2DEG of n-type has a favourable effect on the resistance of the base. If in contrast to the above described embodiment use is made of electron majority charge carriers and the base is therefore of the p-type, higher frequencies can be achieved than with the aid of the tunneling of holes. In the realization of the structure shown in fig. 6, that is, with n-type base, in an MBE process, the base is a 10 nm thick InGaAs layer. The 2DEG of the base lies 10 nm below the tunnel structure RTS. In measurements (fig. 7) on a realized semiconductor device as described above, curves I 3 -I 7 indicate the measured base current in milliampere (mA) at a temperature of 77 K and at five different base voltages of -0.2 V, -0.1 V, 0 V, 0.1 V and 0.2 V respectively as a function of the applied emitter voltage VE in volts (V) . The curves 1 8 , 1 9 , 1 10 , In and 1 T2 designate the measured emitter current I E in milliamperes (mA) as a function of the applied emitter voltage V E at the above mentioned respective base voltages. The region of negative differential resistance is clearly visible in fig. 7. The transistor can herein no longer be used as amplifier for electrical signals, while it is still possible to inject electrons from the base through the tunnel structure to the emitter. The base current determines the hole injection with a negligible influence on the current amplification. In fig. 8 the spectrum E (energy in electrovolts eV) is charted at a base emitter voltage in the region of negative differential resistance, that is, at approximately 4 V (and a temperature of about 80 K) . The voltage applied to the base amounted to 1 V. Charted along the vertical axis is a relative scale of the light intensity (EL R H.) . Adjacent to the exciton light at 1.636 eV can be seen a quantity of luminescence radiation caused by the p-GaAs and originating from tunneling electrodes and reabsorption. The above described relates to first results on a first embodiment according to the present invention. Improvements to these results can be expected by enlarging the dosing of the p + layer in order to further occupy the hole band. It is anticipated that reducing the thickness of the spacer layer will likewise improve the operation. Further of importance is reduction of the dispersion in the barrier layers since this can have adverse effects on the resonance. Improvements are likewise conceivable in respect of optical filtering of the radiation or the light within the peak intensity shown in fig. 3. Through use of a number of heterodyne converters as described above which are each set on a distinguishable wave¬ length in the 100 GHz band, different corresponding electrical signals can be transmitted via an optical cable.

Claims: CLAIMS 1. Semiconductor device comprising: - a first layer of semiconductor material of the one conductivity type; - a second layer of semiconductor material of the other conductivity type; and - a structure for resonant tunneling between the first and second layer of semiconductor material. 2. Semiconductor device as claimed in claim 1, provided with a spacer layer between the structure for resonant tunneling and at least one of both layers of semiconductor material. 3. Semiconductor device as claimed in claim 1 or 2, wherein the structure for resonant tunneling comprises: - a non-doped AlAs barrier layer; - a non-doped GaAs well layer; and - a non-doped AlAs barrier layer, wherein the barrier layers and the well layer have a considerably smaller thickness than the first mentioned first and second layers of semiconductor material. 4. Semiconductor device as claimed in any of the claims 1-3 provided with conducting contacts on the semiconductor layers of the one and the other conductivity type. 5. Semiconductor device as claimed in any of the claims 1-4, wherein the first layer of semiconductor material is a GaAs p + layer with a thickness of approximately 300 nm, the structure for resonant tunneling comprises an AlAs barrier layer with a thickness of approximately 4 nm, a well layer of GaAs with a thickness of approximately 6 nm and an AlAs barrier layer with a thickness of approximately 4 nm, and the second layer of semiconductor material comprises a GaAs n + layer of approximately 300 nm. 6. Semiconductor device as claimed in any of the claims 1-5 provided with a window for passage of radiation or light generated in the semiconductor device. 7. Light-emitting diode comprising a semiconductor device as claimed in any of the claims 1-6. 8. Semiconductor device as claimed in claim 1, com¬ prising: - a third layer of the first conductivity type. 9. Semiconductor device as claimed in claim 8, wherein the RTS structure has a small thickness such that in operation it assumes the form of a two-dimensional electron gas. 10. Semiconductor device as claimed in claim 8 or 9, wherein the second layer of semiconductor material is provided with a base or gate contact for injecting minority charge carriers into the RTS structure. 11. Transistor structure comprising a semiconductor device as claimed in any of the claims 8-10. 12. Electro-optical heterodyne converter comprising a semiconductor device as claimed in claim 10, wherein an oscillating electrical signal is applied to the base and wherein an optical signal is obtained at the output which is modulated with the electrical signal.

Inventors: BORGHS GUSTAAF, GENOE JAN, VAN HOOF CHRIS, BORGHS, GUSTAAF, GENOE, JAN, VAN HOOF, CHRIS
Applicants: IMEC INTER UNI MICRO ELECTR, INTERUNIVERSITAIR MICRO-ELEKTRONICA CENTRUM VZW
Application year: 1992
Main classification: H01L33
IPC codes: H01L29, H01L33