GB2248967A - A high mobility semiconductor device - Google Patents
A high mobility semiconductor device Download PDFInfo
- Publication number
- GB2248967A GB2248967A GB9022757A GB9022757A GB2248967A GB 2248967 A GB2248967 A GB 2248967A GB 9022757 A GB9022757 A GB 9022757A GB 9022757 A GB9022757 A GB 9022757A GB 2248967 A GB2248967 A GB 2248967A
- Authority
- GB
- United Kingdom
- Prior art keywords
- potential well
- electron energy
- energy level
- potential
- barrier layer
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
- 239000004065 semiconductor Substances 0.000 title claims abstract description 33
- 230000004888 barrier function Effects 0.000 claims abstract description 87
- 239000000463 material Substances 0.000 claims abstract description 32
- 230000003287 optical effect Effects 0.000 claims abstract description 25
- JBRZTFJDHDCESZ-UHFFFAOYSA-N AsGa Chemical compound [As]#[Ga] JBRZTFJDHDCESZ-UHFFFAOYSA-N 0.000 claims description 14
- 229910001218 Gallium arsenide Inorganic materials 0.000 claims description 13
- MDPILPRLPQYEEN-UHFFFAOYSA-N aluminium arsenide Chemical compound [As]#[Al] MDPILPRLPQYEEN-UHFFFAOYSA-N 0.000 claims description 11
- 230000005669 field effect Effects 0.000 claims description 3
- 239000002800 charge carrier Substances 0.000 abstract description 11
- 239000002772 conduction electron Substances 0.000 abstract description 10
- 239000002019 doping agent Substances 0.000 description 10
- 239000012535 impurity Substances 0.000 description 6
- 230000002829 reductive effect Effects 0.000 description 5
- 238000010586 diagram Methods 0.000 description 4
- 230000006798 recombination Effects 0.000 description 4
- 238000005215 recombination Methods 0.000 description 4
- 229910045601 alloy Inorganic materials 0.000 description 3
- 239000000956 alloy Substances 0.000 description 3
- 238000000034 method Methods 0.000 description 3
- 239000000758 substrate Substances 0.000 description 3
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 2
- 239000000969 carrier Substances 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 238000001451 molecular beam epitaxy Methods 0.000 description 2
- 229910001020 Au alloy Inorganic materials 0.000 description 1
- 229910000530 Gallium indium arsenide Inorganic materials 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
- 230000005496 eutectics Effects 0.000 description 1
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 description 1
- 239000010931 gold Substances 0.000 description 1
- 229910052737 gold Inorganic materials 0.000 description 1
- 230000002401 inhibitory effect Effects 0.000 description 1
- 230000000873 masking effect Effects 0.000 description 1
- 229910001092 metal group alloy Inorganic materials 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/015—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on semiconductor elements having potential barriers, e.g. having a PN or PIN junction
- G02F1/017—Structures with periodic or quasi periodic potential variation, e.g. superlattices, quantum wells
- G02F1/01716—Optically controlled superlattice or quantum well devices
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
- H01L31/0352—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
- H01L31/0352—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
- H01L31/035236—Superlattices; Multiple quantum well structures
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/0126—Opto-optical modulation, i.e. control of one light beam by another light beam, not otherwise provided for in this subclass
Landscapes
- Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- General Physics & Mathematics (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- Electromagnetism (AREA)
- Optics & Photonics (AREA)
- Computer Hardware Design (AREA)
- Nonlinear Science (AREA)
- Power Engineering (AREA)
- Chemical & Material Sciences (AREA)
- Nanotechnology (AREA)
- Life Sciences & Earth Sciences (AREA)
- Biophysics (AREA)
- Crystallography & Structural Chemistry (AREA)
- Junction Field-Effect Transistors (AREA)
Abstract
The device (1) has a conduction channel region (2) forming a heterostructure defining first and second potential wells (3 and 5) provided by layers of semiconductor material bounded by barrier layers (4, 7, 8). The second potential well (5) provides an electron energy level (e5) which is lower in electron energy than the lowest electron energy level (e3) of the first potential well (3) and the barrier layer (4) separating the first and second potential wells (3 and 5) provides an intermediate electron energy level (e4) which is lower in electron energy than the lowest electron energy level (e3) of the first potential well (3) but is higher in electron energy than the lowest electron energy level (e5) of the second potential well (5) for causing conduction through the conduction channel region (2) to occur along the plane of the second potential well (5) by means of electrons transferred to the second potential well (5) from the first potential well (3) via the intermediate electron energy level (e4) provided by the barrier layer (4). Thus, the source of the conduction charge carriers is well separated from the conduction channel (5) so reducing the possibility of scattering of the conduction electrons and improving their mobility along the conduction channel. The device may be a F.E.T. and the transfer of electrons between the potential wells may be effected by an optical beam source 50. <IMAGE>
Description
DESCRIPTION
A HIGH MOBILITY SEMICONDUCTOR DEVICE
This invention relates to a high mobility semiconductor device having a conduction channel region comprising a heterostructure defining first and second potential wells provided by layers of semiconductor material bounded by barriers. In particular, but not exclusively, this invention relates to a field effect transistor formed from III-V compound semiconductor material and having such a conduction channel region.
United States Patent (US-A-) 4163237 describes high mobility semiconductor devices having such conduction channel regions. In the devices described in US-A-4163237, the conduction channel region is formed of a heterostructure having a plurality of layers of narrow bandgap semiconductor material interleaved with doped layers of wider bandgap material so that the narrow bandgap semiconductor material layers form potential wells, in particular quantum wells, and each potential well provides a respective conduction channel so that conduction through the conduction channel region occurs along the plane of each quantum well with the conduction charge carriers being confined in the direction of thickness of the narrow bandgap layers.
Conduction occurs in the narrow bandgap material potential wells by means of charge carriers provided by virtue of the doping of the wide bandgap barrier layers bounding the narrow bandgap potential well layers. This phenomenon is known as 'modulation doping'. By thus separating the conduction charge carriers, for example electrons, from the dopant impurities which provide the conduction charge carriers, the possibility of scattering, and thus a reduction in the mobility, of conduction charge carriers is reduced. However, the presence of the dopant impurities in the barrier layers bounding the potential wells still results, particularly at low temperatures, in some scattering, and thus a reduction in mobility, of the charge carriers in the potential wells.
It is an aim of the present invention to provide a high mobility semiconductor device in which the possibility of scattering of the conduction charge carriers in the conduction channel region is even further reduced.
According to the present invention there is provided a high mobility semiconductor device having a conduction channel region comprising a heterostructure defining first and second potential wells provided by layers of semiconductor material bounded by barrier layers, characterised in that the second potential well provides an electron energy level which is lower in electron energy than the lowest electron energy level of the first potential well and in that the barrier layer separating the first and second potential wells provides an intermediate electron energy level which is lower in electron energy than the lowest electron energy level of the first potential well but is higher in electron energy than the lowest electron energy level of the second potential well for causing conduction through the conduction channel region to occur along the plane of the second potential well by means of electrons transferred to the second potential well from the first potential well via the intermediate electron energy level provided by the barrier layer separating the first and second potential wells.
Thus, in a high mobility semiconductor device in accordance with the invention, the second potential well provides the conduction channel and the conduction electrons are provided from the first potential well via the intermediate electron energy level provided by the barrier layer separating the first and second potential wells.
In practice, there may be many first and second potential wells so that a series of parallel conduction channels are provided by the second potential wells.
In a high mobility device in accordance with the invention, the source of the conduction charge carriers is well separated from the conduction channel so reducing the possibility of scattering of the conduction electrons and further improving their mobility along the conduction channel(s) provided by the second potential well(s).
In a preferred example of a high mobility device in accordance with the invention, the heterostructure is formed such that electrons are provided in the first potential well for transfer to the second potential well via the intermediate electron energy level provided by the intervening barrier layer by photogeneration of electron-hole pairs in the first potential well and the intervening barrier layer is sufficiently thick to inhibit tunnelling of holes from the first to the second potential well for confining holes of electron-hole pairs generated in the first potential well to the first potential well whilst facilitating transfer of the electrons of the electron-hole pairs from the first potential well to the second potential well via the intermediate electron energy level provided by the barrier layer.In such a case, an optical beam source may be provided in combination with the device to cause photogeneration of electron-hole pairs in the first potential well. Where the electrons are provided in the first potential well by photogeneration of electron-hole pairs, there is no necessity for any intentional doping of either the potential wells or the barrier layers and accordingly the possibility of scattering of the conduction electrons being caused by dopant impurities is significantly reduced so leading to significant improvements in electron mobility in the conduction channel provided by the each second potential well.
An optical beam of relatively low power density, for example 1 (one) Wcm-2 (watt per square centimetre) may be used to provide electron-hole pairs in the first potential well so that sufficient electrons transfer via the intermediate electron energy level provided by the barrier layer to the second potential well.
The differences in the energy levels of the first and second potential wells may be achieved by forming the first potential well as a relatively narrow potential well and the second potential well as a relatively wide potential well. In such circumstances, the first and second potential wells may be formed of the same material and the electron energy levels controlled by controlling the respective thicknesses of the first and second potential wells. In such a case, the first and second potential wells may comprise layers of gallium arsenide and the barrier layer may comprise a layer of aluminium arsenide. In this case, the first potential well is generally less than 3.5nm (namometers), typically 2.5nm, wide whilst the second potential well is generally greater than 3.5nm, typically 5nm, wide. The barrier layer generally has a thickness greater than 2.5nm, typically 8nm.Alternatively or additionally, the first and second potential wells may be formed by layers of different semiconductor material bounded by barrier layers so as to facilitate the provision within the second potential well of an electron energy level lower in electron energy than the lowest electron energy level of the first potential well.
This should enable greater flexibility in the difference in energy levels which may be achieved.
The heterostructure may comprise, in respect of the or each second potential well, a further potential well separated by a further barrier layer from the first potential well so that the first potential well provides an electron energy level which is lower in electron energy than the lowest electron energy level of the further potential well and the further barrier layer being formed so as to provide an intermediate electron energy level which is lower in electron energy than the lowest electron energy level of the further potential well but higher in electron energy than the lowest electron energy level of the first potential well for facilitating transfer of the electrons from the first potential well to the further potential well via the intermediate electron energy level provided by the further barrier layer which is sufficiently thick to inhibit tunnelling of holes from the further to the first potential well. This should enable the electrons and holes to be even further separated and may thus further reduce the possibility of scattering of the conduction electrons.
The conduction channel region may provide the conduction channel region of a field effect transistor.
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Figure 1 illustrates schematically and in part broken-away cross-section a first embodiment of a high mobility semiconductor device in accordance with the invention in combination with an optical beam source;
Figure 2 illustrates schematically an energy band diagram for part of the conduction channel region of the high mobility semiconductor device shown in Figure 1;
Figure 3 illustrates schematically and in part broken-away cross-section a second embodiment of a high mobility semiconductor device in accordance with the invention in combination with an optical beam source; and
Figure 4 illustrates illustrates schematically an energy band diagram for part of the conduction channel region of the high mobility semiconductor device shown in Figure 3;
It should be understood that the Figures are merely schematic and are not drawn to scale. In particular certain dimensions such as the thickness of layers or regions may have been exaggerated whilst other dimensions may have been reduced. It should also be understood that the same reference numerals are used throughout the
Figures to indicate the same or similar parts.
Referring now to the drawings, for example Figures 1 and 2, there is illustrated a high mobility semiconductor device 1 having a conduction channel region 2 comprising a heterostructure defining first and second potential wells 3 and 5 provided by layers of semiconductor material bounded by barrier layers 4,7,8.In accordance with the invention, the second potential well 5 provides an electron energy level e5 (Figure 2) which is lower in electron energy than the lowest electron energy level e3 of the first potential well 3 and the barrier layer 4 separating the first and second potential wells 3 and 5 provides an intermediate electron energy level e4 which is lower in electron energy than the lowest electron energy level e3 of the first potential well 3 but is higher in electron energy than the lowest electron energy level e5 of the second potential well 5 for causing conduction through the conduction channel region 2 to occur along the plane of the second potential well 5 by means of electrons transferred to the second potential well 5 from the first potential well 3 via the intermediate electron energy level e4 provided by the barrier layer 4 separating the first and second potential wells 3 and 5.
Thus, in a high mobility semiconductor device in accordance with the invention, the second potential well 5 provides the conduction channel and the conduction electrons are provided from the first potential well 3 via the intermediate electron energy level e5 provided by the barrier layer 4 separating the first and second potential wells 3 and 5.
The source of the conduction charge carriers is thus well separated from the conduction channel 5, so reducing the possibility of scattering of the conduction electrons and further improving their mobility along the conduction channel provided by the second potential well 5.
In a preferred example, and as in the embodiment shown in
Figure 1, the heterostructure 2 is formed such that electrons are provided in the first potential well 3 for transfer to the second potential well 5 via the intermediate electron energy level e4 provided by the intervening barrier layer 4 by photogeneration of electron-hole pairs in the first potential well 3 and the intervening barrier layer 4 is sufficiently thick to inhibit tunnelling of holes from the first 3 to the second 5 potential well for confining holes of electron-hole pairs generated in the first potential well 3 to the first potential well 3 whilst facilitating transfer of the electrons of the electron-hole pairs from the first potential well 3 to the second potential well 5 via the intermediate electron energy level e4 provided by the barrier layer 4.
It will of course be appreciated by those skilled in the art that the effective mass of a hole is much greater than that of an electron and that accordingly the probability of tunnelling of holes is much less than the probability of electrons tunnelling through the barrier layer 4.
In such a case an optical beam source 50 may be provided in combination with the device 1 to provide an optical beam 0 to cause photogeneration of electron-hole pairs in the first potential well 3. Where the electrons are provided in the first potential well 3 by photogeneration of electron-hole pairs, there is no necessity for any intentional doping of either the potential wells 3 and 5 or the barrier layers 4,7 and 8 and accordingly the possibility of scattering of the conduction electrons being caused by dopant impurities is significantly reduced, so leading to significant improvements in electron mobility in the conduction channel provided by the second potential well 5.
Referring now specifically to Figure 1, there is illustrated schematically, and in part broken-away cross-section, a first embodiment of a high electron mobility transistor (HEMT) 1 in accordance with the invention in which the conduction electrons are provided by optical means.
The HEMT 1 is provided on a substrate 10 which may comprise a semi-insulating monocrystalline gallium arsenide wafer 10a onto which a layer lOb of gallium arsenide is grown by a conventional epitaxial method, for example molecular beam epitaxy (MBE).
A buffer layer (not specifically shown) may be provided on the epitaxial layer lOb. The buffer layer may be in the form of an undoped superlattice of, for example, gallium arsenide and aluminium arsenide layers selected to have a composition equivalent to an A1,Gal-xAs alloy where x = 0.25. Where such a buffer layer is provided then the buffer layer and epitaxial layer 10b may each be about 0.5yam (micrometres) thick. If the superlattice buffer layer is omitted then the epitaxial layer lOb may have a thickness of about lym.
The conduction channel region or heterostructure 2 is then provided using, for example, conventional molecular beam epitaxy (MBE) growth techniques.
In the example shown in Figure 1, the conduction channel region 2 comprises a barrier layer 7 of an indirect gap material provided on the epitaxial layer lOb or buffer layer (not shown).
The barrier layer 7 may be formed of, for example, aluminium arsenide (AlAs) which is not intentionally doped. Typically, the barrier layer 7 may be about lOnm in thickness.
A relatively thick layer of a direct bandgap material, in this example not-intentionally doped gallium arsenide (GaAs), is then grown to form the second potential well 5. The second potential well 5 has a thickness which is typically greater than 3.5nm, generally 5 nm. The barrier layer 4 is then formed as a layer of an indirect bandgap material again, in this example, not-intentionally doped AlAs with a thickness which is greater than 2.5nm, generally 8nm, followed by the first potential well 3 which in this example is, like the second potential well 5, formed of not-intentionally doped gallium arsenide. In order to achieve higher electron energy levels, the first potential well 3 is thinner than the second potential well 5 and is typically less than 3.5nm wide, but greater than 1.9nm, generally 2.5nm wide. The final barrier layer 8, again in this example of not-intentionally doped AlAs, is then provided to complete the heterostructure 2.
The final barrier layer 8 may typically be about lOnm thick.
A capping layer 9 of gallium arsenide having a thickness of from about 10 to about 30nm and doped with an impurity of the one conductivity type, that is n conductivity in this example, to a dopant concentration of about 1.5 x 1018 atoms cm#3 is provided on the barrier layer 8.
Input and output or source and drain regions 20 and 21 are formed by local diffusion of dopants of the one conductivity type, in this example n conductivity type, into the ends of the conduction channel region 2 from the surface so as to extend just into the second potential well 5. The dopants may be introduced from a suitably doped metal alloy, for example an alloy of gold with an appropriate dopant, provided on the surface. So that subsequent source and drain electrodes 22 and 23 provide good ohmic contact, further gold may be deposited onto that used to dope the source and drain regions 20 and 21. A suitable alloy such as a eutectic AuGe alloy containing 5 per cent by weight of nickel may be used to form the source and drain electrodes 22 and 23.
A recess 24 formed by suitable masking and selective etching is provided between the source and drain electrodes 22 and 23 to expose an area 90 of the final barrier layer 8. An optical beam source 50 provides, as will be discussed below, an appropriate optical beam 0 directed at the exposed area 90.
Figure 2 illustrates schematically the energy band diagram for the conduction channel region or heterostructure 2 shown in Figure 1 with, as is conventional, Ec representing the conduction band and Ev the valence band of the heterostructure 2. The arrow labelled Be indicates the direction of increasing electron energy whilst the arrow labelled Eh indicates the direction of increasing hole energy. It will of course be appreciated that although only single hole energy levels h3, h5 have been shown, each of these levels represents a light hole and a heavy hole level with the heavy hole energy level being of lower hole energy in each case.
As can clearly be seen from Figure 2, the GaAs first potential well 3 forms a type II structure with the AlAs barrier layers 4 and 8, that is the lowest electron energy level e3 of the GaAs first potential well 3 is higher in electron energy than the X minima of the indirect gap AlAs barrier layer 4. The barrier layer 4 is sufficiently thick to inhibit tunnelling of holes from the first 3 to the second potential well 5.Thus, the first potential well 3 serves to confine holes whilst electrons within the first potential well may rapidly transfer to the intermediate energy level e4 of the barrier layer 4 and thence to the GaAs second potential well 5 which, being wider than the first potential well 3, has an electron energy level e5 which is lower in electron energy than the intermediate electron energy level e4 of the barrier layers 4 and 8 and a hole energy level hs. The wider second potential well serves to confine both electrons and holes.
Although Figures 1 and 2 show the heterostructure 2 as consisting of only one first potential well 3 and one second potential well 5, in practice there may be many first and second potential wells 3 and 5 separated by barrier layers similar to the barrier layer 4 so as to increase the current carrying capability of the conduction channel region 2 and, for example, a single second potential well 5 could be provided, separated by barrier layers, between two first potential wells 3 so that electrons can transfer to the second potential well from both first potential wells 3.
In the example shown in Figure 3 where the first potential well 3 has a second potential well 5 on only one side, the barrier layer 8 may be formed of a different material, for example, from the barrier layers 4 so that the barrier layer 8 does not provide an electron energy level lower in electron energy than the first potential well 3' thereby inhibiting transfer of electrons to the barrier layer 8, provided of course that the barrier layer 8 is optically transparent at the appropriate wavelengths.
Alternatively, the heterostructure 2 shown in Figure 1 may be modified to provide a further second potential well 5, separated by a barrier layer 4 from the first potential well 3, on the other side of the first potential well 3.
As mentioned above, a device 1 in accordance with the invention facilitates the transfer of electrons of photo-generated electron-hole pairs from the first potential well 3 to the second potential well 5 via the intermediate energy level e4 provided by the barrier layer 4 whilst the barrier layer 4 is sufficiently thick to inhibit tunnelling of holes and so acts to confine the holes of the photo-generated electron-hole pairs to the first potential well 3.
Thus, in operation of the device shown in Figure 1, when an appropriate voltage is applied between the source and drain regions 20 and 21 and the optical beam source 50 provides an optical beam
O with an appropriate wavelength (of the order of 750nm for the structure described above) equivalent to an energy hv equal to or greater than the electron-heavy hole (e-hh) exciton resonance of the first potential well 3 which is incident on the recess 90 (as shown perpendicularly of the layers of the conduction channel region 2), electron-hole pairs are generated in the first potential well 3. Of course, because the second potential well 5 is wider than the first potential well 3 and provides a lower electron energy level, the incident optical beam will also cause photogeneration of electron-hole pairs in the second potential well 5.
Normally, the photogenerated electrons and holes would recombine relatively rapidly and this will be the case for the photogenerated electrons and holes produced in the second potential well 5. However, as mentioned above, the barrier layer 4 provides an electron energy level e4 which is lower in electron energy than the lowest electron energy level e3 of the first potential well 3 and so facilitates transfer of photogenerated electrons from the first potential well 3 to the second potential well 5 via the intermediate electron energy level e4 provided by the barrier layer 4. The barrier layer 4 is sufficiently thick, 8nm in the example described above, to inhibit tunnelling of the photogenerated holes produced in the first potential well 3 into the second potential well 5 and accordingly the photogenerated holes remain confined in the first potential well 3.
The transferred electrons are thus separated from their corresponding photogenerated holes and provide a conduction channel in the second potential well 5 whilst an applied voltage is maintained between the source and drain regions 20 and 21. The time for the HEMT to return to the non-conducting state when the optical beam 0 is switched off will depend on the time taken for recombination of these photogenerated electrons and holes which is in turn governed by the time taken for the holes to tunnel from the first potential well 3 to the second potential well 5.
Because the electrons transferred from the first potential well 3 to the second potential well 5 cannot readily recombine with holes, a high population of electrons can be built up in the second potential well 5 relatively easily and only a low power density, typically of the order of 1 Cm~2, is required for the incident optical beam to generate sufficient electrons to provide an electron current along the conduction channel provided by the second potential well 5.
The current handling capability of the device 1 will depend on the number of second potential wells 3. However, because in the materials system described above, the absorption coefficient perpendicular to the layers of the structure is quite small, a number of first and second potential wells can be provided so that the conduction channel is provided by a number of parallel second potential wells 5 which should increase the current handling capability.
The time for recombination of the separated electrons and holes is governed by the time taken for the holes to tunnel from the first potential well 3 to the second potential well 5. This time is determined at least in part by the thickness of the barrier layer 4 and for the material system given above where the barrier layer is 8nm thickness then the recombination time may be, typically, of the order of 0.5 to a few microseconds. Accordingly, the time for the HEMT to return to a non-conducting state when the optical beam 0 is switched off will be determined at least in part by this recombination time. The switching off time may be shortened by decreasing the barriEr thickness towards the 2.5nm minimum although this may require a higher power density from the optical beam 0 to enable switching on and conduction of the HEMT.
The optically switched HEMT shown in Figure 1 has applications as an optical switch, for example for use in image processing or optical computing. The optical beam source 50 may be formed by a pulsed or continuous wave (cw) laser of suitable wavelength and output power density. The laser may be a dye lasers or even, because of the low power densities required, a semiconductor laser and could even be integrated on the same substrate.
The optical beam source 50 is selected to provide an optical beam 0 with at least a wavelength or range of wavelengths with an energy sufficient to provide the necessary electrons in the first potential well 3, that is with an energy equal to or greater than that of the e-hh exciton resonance of the first potential well 3.
Figure 3 shows schematically and in part broken away cross-section a second embodiment of an optically activated HEMT la in accordance with the invention whilst Figure 4 illustrates the energy band diagram for the conduction channel region in heterstructure 20 of the HEMT shown in Figure 3.
As can be seen most clearly from Figure 3, the heterostructure 20 of the HEMT la provides a further potential well 30 separated from the first potential well 3a by a further barrier layer 4a.
This heterostructure 20 is formed, using the conventional growth techniques mentioned above, so that the further barrier layer 4a provides a further intermediate electron energy level e4a lower in electron energy than the lowest electron energy level e30 of the further potential well 30 but higher than that e3a of the first potential well 3 whilst the further barrier layer 4a is sufficiently thick to inhibit tunnelling of holes from the further potential wells 30 to the first potential well 3a.
The outer barrier layers 7 and 8 may be formed of the same material as the barrier layers 4 and 4a, respectively and may again be 10nm or more thick. As mentioned above with respect to Figure 1, the barrier layer 8 may alternatively be formed of a different material providing a lowest electron energy level higher than the further potential well 30 so as to inhibit transfer of electrons to the barrier layer 8. Alternatively, the heterostructure 20 may be formed so as to be symmetrical about the centre of the third potential well 9.
The heterostructure 20 may be provided on a gallium arsenide substrate 10 is described above with reference to Figure 1 and may be formed so that the further, first and second potential wells 30, 3a and 5a are of the same material and are of increasing thickness so as to provide the necessary energy levels. As in the example described above, the potential wells 30,3a and 5a may be formed of gallium arsenide. In order to achieve the necessary electron energy levels the barrier layers 4a and 4 may be formed of aluminium arsenide and AlxGa1#xAs where x is greater than 0.45, for example Al0#8Ga0#2As, respectively. With such a material system, the barrier layers 4 and 4a may be about 8nm thick whilst the potential wells 30,3a and 5a may be 2.5nm, 3.5nm and 5.0nm respectively.
In other respects, the HEMT 1 shown in Figure 3 is similar to that shown in Figure 1.
In such a structure electrons photogenerated by an appropriate optical beam 0' in the further potential well 30 rapidly transfer via the intermediate electron energy level e4a of the further barrier layer 4a, the electron energy level e3a of the first potential well 3a and the intermediate electron energy level e4 of the barrier layer 4 to the second potential well 5a whilst the corresponding holes remain confined to the further potential well 30. In this case the separation of the electrons and holes is further increased and may further reduce the possibility of scattering which could otherwise detrimentally affect the mobility of the conduction electrons.
The modified HEMT shown in Figure 3 operates in a manner similar to that described above with reference to Figures 1 and 2.
Of course, the time required for the holes to tunnel to the second potential well 5a and thus the switching time of the HEMT will be increased. As in the case of Figure 1, a plurality of potential wells 30,3a and 5a may be provided with, for example, the structure being symmetrical about a third potential well 30.
Although in the examples described above, the free charge carriers are generated optically, it may be possible to provide electrons by doping the first potential well 3 with impurities of the one conductivity type or by providing a plane of dopants (so-called delta doping) at the interface between the barrier layer 8 and the first potential well 3. These may be provided in addition to or in place of the photogenerated carriers. It will of course be appreciated that sufficient energy will need to be imparted to thermalise the carriers provided by the dopants so as to allow the transfer of electrons out of the first potential well 3. This could be achieved electrically and/or optically.
Although the devices have been described above as being formed with AlAs barrier layers and GaAs potential wells, other III-V semiconductor materials could be used provided that the material used for the barrier layer(s) can provide the necessary electron energy level(s).
Although in the arrangements described above the differing electron energy levels of the first and second potential wells 3 and 5 (and the further potential well 30 if provided) are achieved by adjusting the thickness of the layers, it may be possible to achieve this effect by using a combination of different materials in place of or in addition to varying the layer thickness,.for example, by using InGaAs for the second potential well.
Other semiconductor materials may be used, for example II-VI semiconductor materials may be used or a combination of different types of semiconductor materials, provided that the material used for the first well is a direct gap material.
Also, the present invention may be applied to other semiconductor devices having controllable conduction channel regions.
From reading the present disclosure, other modifications and variations will be apparent to persons skilled in the art. Such modifications and variations may involve other features which are already known in the semiconductor art and which may be used instead of or in addition to features already described herein.
Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.
Claims (1)
- CLAIM(S)1. A high mobility semiconductor device having a conduction channel region comprising a heterostructure defining first and second potential wells provided by layers of semiconductor material bounded by barrier layers, characterised in that the second potential well provides an electron energy level which is lower in electron energy than the lowest electron energy level of the first potential well and in that the barrier layer separating the first and second potential wells provides an intermediate electron energy level which is lower in electron energy than the lowest electron energy level of the first potential well but is higher in electron energy than the lowest electron energy level of the second potential well for causing conduction through the conduction channel region to occur along the plane of the second potential well by means of electrons transferred to the second potential well from the first potential well via the intermediate electron energy level provided by the barrier layer separating the first and second potential wells.2. A device according to Claim 1, further characterised in that the heterostructure is formed such that electrons are provided in the first potential well for transfer to the second potential well via the intermediate electron energy level provided by the intervening barrier layer by photogeneration of electron-hole pairs in the first potential well and the intervening barrier layer is sufficiently thick to inhibit tunnelling of holes from the first to the second potential well for confining holes of electron-hole pairs generated in the first potential well to the first potential well whilst facilitating transfer of the electrons of the electron-hole pairs from the first potential well to the second potential well via the intermediate electron energy level provided by the barrier layer.3. A device according to Claim 2, in combination with an optical beam source for providing an optical beam for causing photogeneration of electron-hole pairs in the first potential well.4. A device according to any one of the preceding claims, further characterised in that the first potential well is relatively narrow whilst the second potential well is relatively wide and so provides an electron energy level which is lower in electron energy than the lowest electron energy level of the first potential well.5. A device according to Claim 4, further characterised in that the first and second potential wells are formed by layers of the same semiconductor material.6. A device according to Claim 5, further characterised in that the first and second potential wells comprise layers of gallium arsenide and the barrier layer comprises a layer of aluminium arsenide.7. A device according to Claim 4, 5 or 6, further characterised in that the first potential well is less than 3.5nm (nanometers) wide and the second potential well is greater than 3.5nm wide.8. A device according to Claim 7, further characterised in that the first potential well is 2.5nm wide and the second potential well is 5nm wide.9. A device according to any one of the preceding claims, further characterised in that the barrier layer separating the first and second potential wells is greater than 2.5nm wide.10. A device according to Claim 9, further characterised in that the barrier layer separating the first and second potential wells is 8nm wide.11. A device according to any one of Claims 1 to 4, further characterised in that the first and second potential wells are formed by layers of different semiconductor material bounded by barrier layers so as to facilitate the provision within the second potential well of an electron energy level lower in electron energy than the lowest electron energy level of the first potential well.12. A device according to any one of the preceding claims, further characterised in that the heterostructure defines a plurality of first and second potential wells.13. A device according to any one of the preceding claims, further characterised in that, in respect of the or each second potential well, the heterostructure comprises a further potential well separated by a further barrier layer from the first potential well, the first potential well providing an electron energy level which is lower in electron energy than the lowest electron energy level of the further potential well and the further barrier layer being formed so as to provide an intermediate electron energy level which is lower in electron energy than the lowest electron energy level of the further potential well but higher in electron energy than the lowest electron energy level of the first potential well for facilitating transfer of the electrons from the further potential well to the first potential well via the intermediate electron energy level provided by the further barrier layer.15. A device according to any one of the preceding claims, further characterised in that the conduction channel region provides the conduction channel region of field effect transistor.16. A high mobility semiconductor device, substantially as hereinbefore described with reference to the accompanying drawings.
Priority Applications (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9022757A GB2248967A (en) | 1990-10-19 | 1990-10-19 | A high mobility semiconductor device |
| DE69115205T DE69115205T2 (en) | 1990-09-24 | 1991-09-18 | Optically switchable device. |
| EP91202393A EP0478060B1 (en) | 1990-09-24 | 1991-09-18 | An optically switchable device |
| US07/764,670 US5119227A (en) | 1990-09-24 | 1991-09-23 | Optically switchable device |
| JP24365291A JP3260149B2 (en) | 1990-09-24 | 1991-09-24 | Optically switchable device |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9022757A GB2248967A (en) | 1990-10-19 | 1990-10-19 | A high mobility semiconductor device |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9022757D0 GB9022757D0 (en) | 1990-12-05 |
| GB2248967A true GB2248967A (en) | 1992-04-22 |
Family
ID=10684006
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9022757A Withdrawn GB2248967A (en) | 1990-09-24 | 1990-10-19 | A high mobility semiconductor device |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2248967A (en) |
Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4163237A (en) * | 1978-04-24 | 1979-07-31 | Bell Telephone Laboratories, Incorporated | High mobility multilayered heterojunction devices employing modulated doping |
| EP0155215A2 (en) * | 1984-03-08 | 1985-09-18 | Fujitsu Limited | High electron mobility semiconductor device employing selectively doped heterojunction |
| GB2189345A (en) * | 1986-04-16 | 1987-10-21 | Philips Electronic Associated | High mobility p channel semi conductor devices |
| US4806998A (en) * | 1986-06-30 | 1989-02-21 | Thomson-Csf | Heterojunction and dual channel semiconductor field effect transistor or negative transconductive device |
| GB2219130A (en) * | 1988-05-25 | 1989-11-29 | Philips Electronic Associated | A high mobility semiconductor device |
-
1990
- 1990-10-19 GB GB9022757A patent/GB2248967A/en not_active Withdrawn
Patent Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4163237A (en) * | 1978-04-24 | 1979-07-31 | Bell Telephone Laboratories, Incorporated | High mobility multilayered heterojunction devices employing modulated doping |
| EP0155215A2 (en) * | 1984-03-08 | 1985-09-18 | Fujitsu Limited | High electron mobility semiconductor device employing selectively doped heterojunction |
| GB2189345A (en) * | 1986-04-16 | 1987-10-21 | Philips Electronic Associated | High mobility p channel semi conductor devices |
| US4806998A (en) * | 1986-06-30 | 1989-02-21 | Thomson-Csf | Heterojunction and dual channel semiconductor field effect transistor or negative transconductive device |
| GB2219130A (en) * | 1988-05-25 | 1989-11-29 | Philips Electronic Associated | A high mobility semiconductor device |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9022757D0 (en) | 1990-12-05 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US5495115A (en) | Semiconductor crystalline laminate structure, forming method of the same, and semiconductor device employing the same | |
| US4984242A (en) | GaAs/AlGaAs heterostructure laser containing indium | |
| KR0153781B1 (en) | Elimination of heterojunction band discontinuities and its method | |
| US5047810A (en) | Optically controlled resonant tunneling electronic devices | |
| US5119227A (en) | Optically switchable device | |
| US4390889A (en) | Photodiode having an InGaAs layer with an adjacent InGaAsP p-n junction | |
| JPH06291359A (en) | Semiconductor light receiving device | |
| US5107514A (en) | Semiconductor optical element | |
| US4694318A (en) | Sawtooth photodetector | |
| US5107306A (en) | Semiconductor device having a waveguide structure | |
| US5266814A (en) | Optically activated resonant-tunneling transistor | |
| KR920006434B1 (en) | Resonant tunelling barrier structure device | |
| US4586067A (en) | Photodetector with isolated avalanche region | |
| CA2341110C (en) | Avalanche photodiode | |
| JP2604377B2 (en) | Resonant tunnel device | |
| US4561007A (en) | Double mesa avalanche photodetector | |
| US5286982A (en) | High contrast ratio optical modulator | |
| JPH06232442A (en) | Semiconductor photodetector | |
| EP0405832B1 (en) | Doping procedures for semiconductor devices | |
| US5757025A (en) | Infrared photodetector using delta-doped semiconductors | |
| US5377214A (en) | Tensile strained blue green II-VI quantum well Laser | |
| US4586066A (en) | Avalanche photodetector | |
| GB2248967A (en) | A high mobility semiconductor device | |
| US4755860A (en) | Avalanche photodetector | |
| JP3027116B2 (en) | Solar cell |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |