Radiation source and method for the operation thereof

Abstract

The invention relates to a radiation source, comprising at least one semiconductor substrate, on which at least two field-effect transistors are formed, which each contain a gate electrode, a source contact, and a drain contact, which bound a channel, wherein the at least two field-effect transistors are arranged adjacent to each other on the substrate, wherein each field-effect transistor has exactly one gate electrode and at least one source contact and/or at least one drain contact is arranged between two adjacent gate electrodes, wherein a ballistic electron transport can be formed in the channel during operation of the radiation source. The invention further relates to a method for producing electromagnetic radiation having a vacuum wavelength between approximately 10 μm and approximately 1 mm.

Claims

1. A radiation source, comprising at least one semiconductor substrate on which at least two field effect transistors are formed, each of which has a gate electrode having a gate length L.sub.G, a source contact and a drain contact, which bound a channel, wherein the at least two field effect transistors are arranged adjacent to one another on the substrate, wherein each field effect transistor has exactly one gate electrode, and at least one source contact and/or at least one drain contact is arranged between two adjacent gate electrodes, wherein adjacent source contacts and/or drain contacts all have a distance Λ with respect to one another, which complies with the following condition: $\Lambda =\frac{4{\mathrm{nL}}_G}{2k-1},$ wherein L.sub.G is the gate length of the field effect transistors and n and k are natural numbers, wherein k can adopt values from 1 to 2n, wherein the plurality of source contacts and drain contacts create a one dimensional diffraction grating for plasmonic waves resulting in the channels.

2. The radiation source according to claim 1, wherein a two-dimensional electron gas is formed in the channel of the field effect transistors, at least when the radiation source is operated.

3. The radiation source according to claim 1, wherein the plurality of source contacts and drain contacts is between approximately 1000 and approximately 4000.

4. A radiation source according to claim 1, wherein a ballistic electron transport is formed in the channel when the radiation source is operated.

5. The radiation source according to claim 1, wherein the source contacts and the drain contacts and the gate electrodes all have equal widths L.sub.G.

6. The radiation source according to claim 1, wherein the channel length is less than 300 nm.

7. The radiation source according to claim 1, wherein the mean free path length of the electrons in the channel is longer than twice the width L.sub.G.

8. The radiation source according to claim 1, wherein the mobility μ.sub.e of the electrons in the channel is greater than approximately 8000 cm.sup.2.Math.(Vs).sup.−1.

9. The radiation source according to claim 1, wherein the semiconductor substrate contains or consists of Si or a III-V compound semiconductor or that the semiconductor substrate contains or consists of InP and/or GaAs and/or InAlGaAs and/or InGaAs and/or InAlAs.

10. The radiation source according to claim 1, wherein the gate electrodes, the source contacts and the drain contacts are arranged parallel to one another.

11. The radiation source according to claim 1, wherein the gate length L.sub.G of at least one transistor of the radiation source is selected depending on the material parameters, i.e. charge carrier mobility μ and effective electron mass m.sub.eff, in such a way that $\frac{{\mu }^2.Math.m_{\mathrm{eff}}}{e.Math.L_G}\ge a,$ wherein e is the elementary charge and a is a constant.

12. The radiation source according to claim 11, wherein the constant a is selected from the interval of 4.Math.10.sup.−7 cm.Math.V.sup.−1 to 1.Math.10.sup.−5 cm.Math.V.sup.−1.

13. The radiation source according to claim 1, further containing a constant power source.

14. A probe or catheter having a radiation source according to claim 1.

15. A method for producing electromagnetic radiation having a vacuum wavelength between approximately 10 μm and approximately 1 mm, said method using an arrangement having at least two field effect transistors which are arranged on a semiconductor substrate and each of which contains a gate electrode having a gate length L.sub.G, a source contact and a drain contact, which bound a channel, wherein the at least two field effect transistors are arranged adjacent to one another on the substrate, wherein each field effect transistor has exactly one gate electrode, and at least one source contact and/or at least one drain contact is arranged between two adjacent gate electrodes, wherein adjacent source contacts and/or drain contacts all have a distance Λ with respect to one another, which complies with the following condition: $\Lambda =\frac{4{\mathrm{nL}}_G}{2k-1},$ wherein L.sub.G is the gate length of the field effect transistors and n and k are natural numbers, wherein k can adopt values from 1 to 2n, wherein the plurality of source contacts and drain contacts create a one dimensional diffraction grating for plasmonic waves resulting in the channels, wherein a first electric voltage (V.sub.gate) is applied to the gate electrodes and a second electric voltage (V.sub.source) is applied to the source contacts and a third electric voltage (V.sub.drain) is applied to the drain contacts so as to create between the source contacts and the drain contacts in each case a ballistic flow of electrons which excites plasmon oscillations in at least one channel of the field effect transistors.

16. The method according to claim 15, wherein the second electric voltage (V.sub.source) is zero and/or a mass potential and/or the third electric voltage (V.sub.drain) is selected in such a way that the current supplied to or drained from the source contact reaches saturation.

17. The method according to claim 15, wherein in each case a two-dimensional electron gas is formed between the source and drain contacts.

18. The method according to claim 15, characterized in that wherein the third electric voltage (V.sub.drain) is selected depending on the charge carrier mobility μ, the effective electron mass m.sub.eff and the gate length L.sub.G in such a way that $\frac{{\mu }^2.Math.m_{\mathrm{eff}}.Math.V_{\mathrm{Drain}}}{e.Math.L_G^2}>1,$ wherein e designates the elementary charge.

19. The method according to claim 15, wherein a constant electric current is supplied to the arrangement.

20. The method according to claim 15, wherein the electromagnetic waves excited in the respective field effect transistors interfere with one another in a constructive way.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention shall be explained in more detail below without limiting the general inventive concept, wherein

(2) FIG. 1 shows a top view of a radiation source according to the invention.

(3) FIG. 2 shows a cross-section through a radiation source according to a first embodiment of the invention.

(4) FIG. 3 shows a cross-section through a radiation source according to a second embodiment of the invention.

DETAILED DESCRIPTION

(5) FIG. 1 shows a top view of a radiation source according to the present invention. The radiation source 1 contains a semiconductor substrate on which a plurality of source contacts 11, 12 and 13, a plurality of drain contacts 41, 42, and 43 and gate electrodes 21, 22, 23, 24 are arranged. A first gate electrode 21 is disposed between the first source contact 11 and the first drain contact 41. A second gate electrode 22 is disposed between the first drain contact 41 and the second source contact 12. Together with the layers which are arranged in the substrate 10 and which are explained below by means of FIGS. 2 and 3, the first source contact 11 forms a first field effect transistor 31 together with the first drain contact 41 and the first gate electrode 21. Likewise, the first drain contact 41, the second source contact 12 and the second gate electrode 22 form a second field effect transistor 32. The structure explained by way of example by means of FIG. 1 can be continued periodically so as to arrange several hundred or several thousand source and drain contacts and gate electrodes on the surface of the semiconductor substrate. Consequently, a plurality of field effect transistors 31, 32, 33, 34 . . . is arranged on the substrate.

(6) In the exemplary embodiment shown, adjacent transistors of the periodic arrangement share, in each case, a source or drain contact. In other embodiments of the invention, dedicated source and drain contacts can be available for each field effect transistor.

(7) In the exemplary embodiment shown, all source and drain contacts and gate electrodes have the same width L. In some embodiments of the invention, the width L can be between approximately 3 nm and approximately 300 nm in order that the natural frequencies of the plasmons in the channel correspond to the respective frequency of the FIR radiation. In some embodiments of the invention, the distance L.sub.SP between the gate electrodes and the respectively adjacent source and drain contacts can be three times the width of the contacts, i.e. L.sub.SP=3L. The periodic arrangement of the field effect transistors on the surface of the semiconductor substrate can have a width of approximately 2.5 μm to approximately 250 μm, and therefore the width is approximately a quarter of the wavelength of the electromagnetic radiation produced by the radiation source.

(8) The first contacts 11, the second contacts 12, the third contacts 13 and the assigned gate electrodes 21, 22, 23 and 24 can have a longitudinal extension of approximately 100 nm to approximately 2000 μm or of approximately 500 nm to approximately 2500 μm or of approximately 100 nm to approximately 250 μm.

(9) Furthermore, FIG. 1 shows optional fourth contacts 50 on the margin. Said contacts can be disposed on the same electric potential as the adjacent source or drain contacts 11. The fourth contacts 50 can serve for preventing the drop in the plasmonic waves on the margins of the arrangement. In some embodiments of the invention, the optional fourth contacts 50 can ensure, for the field effect transistors arranged on the margin, an antenna effect which is equal or similar to that of the transistors inside the arrangement.

(10) The periodic arrangement of the source and drain contacts does not only form the field effect transistors, as described above, but simultaneously a diffraction grating for the plasmonic waves resulting in the channels. For this purpose, the source contacts (11, 12, 13) and the drain contacts are arranged in such a way that adjacent source contacts (11, 12, 13) and drain contacts (41, 42, 43) all have a distance Λ relative to one another that complies with the following condition:

(11) $\Lambda =\frac{4{\mathrm{nL}}_G}{2k-1},$
wherein L.sub.G is the gate length of the field effect transistors (31, 32, 33, 34) and n and k are natural numbers, wherein k can adopt values from 1 to 2n. As a result, at least a partial surface has a periodic arrangement of source and/or drain contacts.

(12) FIG. 2 shows a cross-section through a radiation source according to a first embodiment of the invention. Equal components are provided with equal reference signs, and therefore the description is limited to the essential differences. FIG. 2 shows a semiconductor substrate 10 which may contain a III/V compound semiconductor, for example. A heterostructure 30 can be formed in substrate 10 and can produce a 2-DEG in channel 311, 321, 331 and 341 of the field effect transistors 31, 32, 33 and 34.

(13) Furthermore, it is evident from FIG. 2 how to introduce source and drain contacts 11, 12, 13, 41, 42 and 43 into substrate 10 to produce appropriate field effect transistors 31, 32, 33 and 34. On the one hand, contacts 41, 42, 43, 11, 12 and 13 serve for electrically contacting the semiconductor heterostructure 30 to enable a ballistic flow of electrons between adjacent source and drain contacts. At the same time, at least some of the plasmons created in channels 311, 321, 331 and 341 are reflected by contacts 41, 42, 43, 11, 12 and 13 in such a way that contacts 41, 42, 43, 11, 12 and 13 also produce a one-dimensional diffraction grating 15 for the plasmons. This diffraction grating likewise acts as an antenna for the produced FIR radiation. The grating constant Λ is given by the distance of adjacent contacts and their width, as illustrated in FIG. 1 by way of example.

(14) The gate contacts 21, 22, 23 and 24 are arranged on insulation layers 20a, 20b, 20c and 20d. The insulation layers 20 can contain or consist of an oxide, a nitride or an oxynitride, for example. During the operation of the radiation source, the gate voltage is chosen in such a way that sufficient charge carriers are formed in the channel of the field effect transistor. Furthermore, the third electric voltage (V.sub.drain) can be selected in such a way that the current transported in the channel reaches saturation.

(15) In some embodiments of the invention, the third electric voltage (V.sub.drain) can be chosen depending on the charge carrier mobility μ, the effective electron mass m.sub.eff and the gate length L.sub.G in such a way that

(16) $\frac{{\mu }^2.Math.m_{\mathrm{eff}}.Math.V_{\mathrm{Drain}}}{e.Math.L_G^2}>1,$
wherein e designates the elementary charge. In this case, the condition for a ballistic transport of the charge carriers in the channel is complied with, and therefore plasmon oscillations with high efficiency are excited. The third electric voltage (V.sub.drain) is here chosen depending on the gate length L.sub.G below the material-dependent breakdown field strength. For example, the field strength

(17) $V_{\mathrm{Drain}}/L_G$
can be less than 1.Math.10.sup.5 V.Math.cm.sup.−1 when the radiation source contains or consists of germanium. The field strength

(18) $V_{\mathrm{Drain}}/L_G$
can be less than 3.Math.10.sup.5 V.Math.cm.sup.−1 when the radiation source contains or consists of silicon. The field strength

(19) $V_{\mathrm{Drain}}/L_G$
can be less than 4.Math.10.sup.5 V.Math.cm.sup.−1 when the radiation source contains or consists of GaAs. The field strength

(20) $V_{\mathrm{Drain}}/L_G$
can be less than 2.2.Math.10.sup.6 V.Math.cm.sup.−1 when the radiation source contains or consists of SiC. The field strength

(21) 0$V_{\mathrm{Drain}}/L_G$
can be less than 5.0.Math.10.sup.5 V.Math.cm.sup.−1 when the radiation source contains or consists of InP. The field strength

(22) $V_{\mathrm{Drain}}/L_G$
can be less than 5.0.Math.10.sup.6 V.Math.cm.sup.−1 when the radiation source contains or consists of GaN.

(23) In some embodiments of the invention, the gate length L.sub.G of at least one transistor of the radiation source can be chosen depending on the material parameters, i.e. charge carrier mobility μ and effective electron mass m.sub.eff, in such a way that

(24) $\frac{{\mu }^2.Math.m_{\mathrm{eff}}}{e.Math.L_G}\ge a,$
wherein e designates the elementary charge and a is a constant. In some embodiments of the invention, the constant a can be chosen from an interval of 4.Math.10.sup.−7 cm.Math.V.sup.−1 to 1.Math.10.sup.−5 cm.Math.V.sup.−1. The constant a can be greater than 1.Math.10.sup.−5 cm.Math.V.sup.−1 when the radiation source contains or consists of germanium. The constant a can be greater than 3.3.Math.10.sup.−6 cm.Math.V.sup.−1 when the radiation source contains or consists of silicon. The constant a can be greater than 2.5.Math.10.sup.−6 cm.Math.V.sup.−1 when the radiation source contains or consists of GaAs. The constant a can be greater than 4.5.Math.10.sup.−7 cm.Math.V.sup.−1 when the radiation source contains or consists of SiC. The constant a can be greater than 2.0.Math.10.sup.−7 cm.Math.V.sup.−1 when the radiation source contains or consists of GaN. The constant a can be greater than 2.0.Math.10.sup.−6 cm.Math.V.sup.−1 when the radiation source contains or consists of InP.

(25) It should be noted that in other embodiments of the invention the semiconductor heterostructure 30 can also be omitted so as to obtain the two-dimensional electron gas on the basis of the band bending induced by the gate voltage. In some embodiments, at least two contacts 11, 41 and at least one gate electrode 21 can form a field effect transistor having high electron mobility (HEMT). The mobility β.sub.e of the electrons in the channel can then be higher than approximately 8000 cm.sup.2.Math.(Vs).sup.−1 or higher than approximately 9500 cm.sup.2.Math.(Vs).sup.−1.

(26) The plasmonic waves forming in the semiconductor heterostructure 30 and/or underneath a boundary of the gate electrodes, all produce electromagnetic FIR radiation. The respective partial intensities emitted from the field effect transistors can sum up, thus yielding a radiation source of high intensity and/or high brilliance.

(27) FIG. 3 shows a section of a second embodiment of the present invention. Equal constituents of the invention are provided with equal reference signs, and therefore the following description is limited to the essential differences.

(28) The second embodiment also has a semiconductor substrate 10. An optional semiconductor heterostructure can be deposited on the semiconductor substrate 10 and can provide a 2-DEG having a high charge carrier mobility. In contrast to the first embodiment, the source contacts 11 and 12, the drain contacts 41 and 42 and the gate electrodes 21, 22 and 23 are not arranged in trenches inside the substrate 10. The contacts 41, 42, 11 and 12 are rather formed as ohmic contacts on the surface of the semiconductor substrate. This can considerably facilitate the production of the radiation source.

(29) The gate electrodes 21, 22 and 23 are also formed as a metal layer on the surface of the semiconductor substrate 10. The gate electrodes 21, 22 and 23 are made as Schottky contacts. In some embodiments of the invention, this can be effected by using a metal or an alloy having more or less work function. The production of the radiation source 1 then merely calls for the production of the corresponding contacts and gate electrodes by depositing and structuring two different metal layers on the semiconductor substrate 10 with the heterostructure 30.

(30) In some embodiments of the invention, the surface of the semiconductor substrate 10 can be removed at least in some segments by etching to expose the heterostructure 30. As a result, the production of the ohmic contacts and/or the Schottky contacts can be facilitated or the quality thereof can be improved. In some embodiments of the invention, at least some areas of the semiconductor substrate 10 can be doped so as to be conductive to provide ohmic contacts having better quality. In some embodiments of the invention, at least some areas of the semiconductor substrate 10 can be sparsely doped and/or insulated and/or poorly conductive to provide Schottky contacts having better quality.

(31) However, since contacts 41, 42, 11 and 12 are ohmic contacts and the gate electrodes are Schottky contacts, the effect thereof on the electrodes in the heterostructure 30 is different. Nevertheless, all metallizations have a similar influence on the plasmons forming, and therefore the grating constant Λ of the resulting diffraction grating 15 and/or of the antenna formed by the diffraction grating 15 is smaller than in the first exemplary embodiment. The smaller grating constant Λ can have an advantageous effect on the formation and amplification of the plasmons and/or the FIR radiation emission when the radiation source is operated. As a result, the second embodiment shown in FIG. 3 can have a higher intensity electromagnetic radiation and/or provide a radiation having higher frequency. The plasmons forming in the channels of the field effect transistors couple electromagnetically with the grating produced by the metallizations 41, 42, 11, 12, 21, 22 and 23. This grating acts like an antenna, thus converting the plasmons into free electromagnetic radiation.

(32) In some embodiments of the invention, the semiconductor substrate 10 and/or the ohmic contacts 41, 42, 11 and 12 and/or the Schottky contacts 21, 22 and 23 can consist of or obtain a material which absorbs FIR radiation to a minor degree.

1st Exemplary Embodiment

(33) A radiation source according to the invention contains field effect transistors which are produced from GaAs on a semiconductor substrate. The mobility μ.sub.e of the electrons is μ.sub.e=9500 cm.sup.2.Math.(Vs).sup.−1. The effective mass m.sub.eff of the electrons in the semiconductor is 0.067 m.sub.o, wherein m.sub.o designates the mass of the free electron. The individual field effect transistors of the radiation source all have a gate length of 300 nm. The field effect transistors are operated by a third voltage V.sub.drain of 1 V. Therefore, the following applies:

(34) $\frac{{\mu }^2.Math.m_{\mathrm{eff}}.Math.V_{\mathrm{Drain}}}{e.Math.L_G^2}=4.06>1,$
i.e. a ballistic transport of the electrons occurs in the channel of the field effect transistors. As a result, such a radiation source can emit FIR radiation.

2nd Exemplary Embodiment

(35) A radiation source according to the invention contains field effect transistors which are arranged on a semiconductor substrate made of silicon. The mobility μ.sub.e of the electrons is μ.sub.e=300 cm.sup.2.Math.(Vs).sup.−1. The effective mass m.sub.eff of the electrons in the semiconductor is 0.36 m.sub.o, wherein m.sub.o is the mass of the free electron. Each of the individual field effect transistors of the radiation source has a gate length of 15 nm. The field effect transistors are operated with a third voltage V.sub.drain of 1 V. Therefore, the following applies:

(36) $\frac{{\mu }^2.Math.m_{\mathrm{eff}}.Math.V_{\mathrm{Drain}}}{e.Math.L_G^2}=8.19>1,$
i.e. a ballistic transport of the electrons occurs in the channel of the field effect transistors. Therefore, such a radiation source can emit FIR radiation.

3rd Exemplary Embodiment

(37) A radiation source according to the invention contains field effect transistors which are arranged on a semiconductor substrate made of silicon. The mobility μ.sub.e of the electrons is μ.sub.e=300 cm.sup.2.Math.(Vs).sup.−1. The effective mass m.sub.eff of the electrons in the semiconductor is 0.36 m.sub.o, wherein m.sub.o is the mass of the free electron. Each of the individual field effect transistors of the radiation source has a gate length of 30 nm. The field effect transistors are operated with a third voltage V.sub.drain of 1 V. Therefore, the following applies:

(38) $\frac{{\mu }^2.Math.m_{\mathrm{eff}}.Math.V_{\mathrm{Drain}}}{e.Math.L_G^2}=2.36>1,$
i.e. a ballistic transport of the electrons only just occurs in the channel of the field effect transistors. Thus, such a radiation source can also emit FIR radiation.

Comparative Example

(39) A field effect transistor is arranged on a semiconductor substrate made of silicon. The mobility μ.sub.e of the electrons is μ.sub.e=300 cm.sup.2.Math.(Vs).sup.−1. The effective mass m.sub.eff of the electrons in the semiconductor is 0.36 m.sub.o, wherein m.sub.o designates the mass of the free electron. Each of the individual field effect transistors of the radiation source has a gate length of 100 nm. The field effect transistors are operated with a third voltage V.sub.drain of 1 V. Therefore, the following applies:

(40) $\frac{{\mu }^2.Math.m_{\mathrm{eff}}.Math.V_{\mathrm{Drain}}}{e.Math.L_G^2}=0.18<1,$
i.e. there is no ballistic transport of the electrons in the channel of the field effect transistor. Thus, no radiation source according to the invention can be realized on the basis of such transistors.

(41) Of course, the invention is not limited to the embodiments shown in the figures. Therefore, the above description should not be considered to be limiting but explanatory. The below claims should be understood to the effect that an indicated feature is available in at least one embodiment of the invention. This does not exclude the presence of further features. If the claims and the above description define “first” and “second” features, this designation serves for distinguishing between two similar features without determining an order.