Semiconductor device having an organic semiconductor material

A semiconductor device is provided with an organic material which is formed by a solid-state mixture of organic donor and organic acceptor molecules. A semiconducting solid-state mixture is known with molar ratios between donor and acceptor molecules of 1.3:2 and 1.66:2. The known solid-state mixture has the disadvantage that its electrical conductivity is comparatively high, so that it is not possible to manufacture switchable devices from the mixture. Here the material includes an n- or p-type semiconductor material, the n-type semiconductor material having a molar ratio between the donor and acceptor molecules below 0.05, and the p-type semiconductor material having this ratio above 20. These solid-state mixtures may be used for manufacturing switchable semiconductor devices. The n- and p-type organic solid-state mixtures can be used for manufacturing transistors, diodes, and field effect transistors in a same manner as, for example, doped silicon or germanium.

BACKGROUND OF THE INVENTION 
The invention relates to a device provided with an organic material which 
is formed by a solid-state mixture of organic donor and organic acceptor 
molecules. "Donor molecule" is here understood to mean a molecule which 
can give off an electron comparatively easily, and "acceptor molecule" a 
molecule which can take up an electron comparatively easily. 
A solid-state mixture of the kind mentioned in the opening paragraph is 
known from European Patent Application no. 423956. The solid-state mixture 
is semiconducting at molar ratios between donor and acceptor molecules of 
1.3:2 and 1.66:2. The known solid-state mixture described has the 
disadvantage that the electrical conductivity of the known solid-state 
mixture is comparatively high, so that it is not possible to influence the 
conductivity of the solid-state mixture to the extent that switchable 
devices can be manufactured. 
SUMMARY OF THE INVENTION 
The invention has for its object inter alia to counteract this 
disadvantage. 
According to the invention, the device is for this purpose characterized in 
that the material comprises an n- or p-type semiconductor material, 
wherein the n-type semiconductor material has a molar ratio between the 
donor and acceptor molecules below 0.05, and wherein the p-type 
semiconductor material has this ratio greater than 20. It is accordingly 
possible with the donor/acceptor combination to manufacture an n-type as 
well as a p-type organic semiconductor. 
The solid-state mixtures according to the invention may be used for 
manufacturing switchable semiconductor devices. The n- and p-materials may 
be used for manufacturing transistors, diodes, and field effect 
transistors in the same manner as, for example, doped silicon or 
germanium. 
The known solid-state mixtures are used for manufacturing an organic 
conductor. Given a molar ratio between donor and acceptor molecules of 
1:1, the solid-state mixture becomes semi-metallic. At donor/acceptor 
ratios of 1.3:2 and 1.66:2, i.e. comparatively close to 1:1, the 
conductivity of the solid-state mixture is lower than in the case of 
purely metallic conductivity, but the conductivity is still so high that 
no switching elements can be made with the solid-state mixture. In 
addition, the known solid-state mixtures do not exhibit n- or p-type 
behavior, i.e. the conductivity is not determined by comparatively loosely 
bound electrons or holes. The solid-state mixture according to the 
invention, on the other hand, behaves as an n- or p-type semiconductor. 
This means that effects such as depletion, enhancement, injection of 
charge carriers, field effect, etc., known from other semiconductor 
materials, can be used for making switching elements. The solid-state 
mixture behaves as an n-type semiconductor in the case of a molar 
solid-state mixture ratio between donors and acceptors below 0.05, i.e. 
with a relatively very large number of acceptor molecules. The solid-state 
mixture behaves as a p-type semiconductor in the case of a molar ratio 
above 20, i.e. with a relatively very large number of donor molecules. In 
contrast to the mixture according to the invention, the presence of 
acceptor atoms leads to a p-type behavior in semiconductor materials such 
as e.g. silicon, whereas the presence of donor atoms leads to an n-type 
behavior. 
It is suspected that the behavior of the solid-state mixture with 
comparatively low and high molar donor/acceptor ratios is caused by a 
so-called hopping mechanism. Thus n-type behavior of the solid-state 
mixture with a low molar donor/acceptor ratio could be caused by the fact 
that the conductivity is determined by electrons of donor molecules, of 
which there are relatively few, which move ("hop") to lattice locations 
(holes) on acceptor molecules, of which there are comparatively many 
available. The reverse could be the case for p-type material. 
Preferably, the device comprises both an n-type region manufactured from 
the n-type material and a p-type region manufactured from the p-type 
material. The solid-state mixture used in a semiconductor device according 
to the invention may be manufactured comparatively easily through 
codeposition from two vapor sources, one for donor molecules and one for 
acceptor molecules, at a reduced pressure, for example lower than 
1.3.times.10.sup.-3 N/m.sup.2 (10.sup.-5 torr). The molar donor/acceptor 
ratio may be changed comparatively easily in that the yields of the 
sources are adapted, for example by adapting a source temperature. Thus 
both n-type and p-type regions of the organic semiconductor can be made in 
one vapor deposition process. The semiconductor device according to the 
invention, accordingly, is much easier to manufacture than, for example, 
silicon semiconductor devices where high-temperature diffusion plays a 
part in manufacturing n- and p-type regions. Preferably, the device 
comprises a pn junction between the p- and the n-type regions. Such a pn 
junction behaves as a diode and is a basic form of a switching element. 
The pn junction can also be manufactured in a simple manner, as can the 
separate p- and n-type regions, through variation of the yields of vapor 
deposition sources of donor and acceptor molecules. 
An additional advantage is obtained when the device comprises a further 
region manufactured from a solid-state mixture of the organic donor and 
organic acceptor molecules, where the molar ratio between the donor and 
acceptor molecules is substantially equal to one. Such a region has 
semi-metallic properties and may thus be used, for example, as a 
connection region, buried conductor, or interconnection between 
semiconducting regions. Semi-metallic regions can be manufactured in one 
process step, along with n- and p-type regions, in that the molar 
donor-acceptor ratio is changed to approximately 1:1 during manufacture. 
An additional metallization step is necessary in the manufacture of such a 
conductive region in the case of semiconductor materials such as silicon. 
Preferably, the device comprises a region adjoining a surface and made from 
the n-type material provided with a passivated surface layer. It is found 
in practice that n-type material forms a passivating surface layer when it 
is exposed to air (a mixture of N.sub.2 and O.sub.2) after its manufacture 
under reduced pressure. If the semiconductor device is so manufactured 
that regions of n-type material adjoin a surface, then the device is 
passivated after vapor deposition under vacuum conditions when it is 
exposed to an atmosphere containing oxygen. This effect may be compared to 
passivation of a silicon surface through formation of silicon dioxide. 
Preferably, the device comprises a field effect transistor, with a source 
and a drain region, and with an interposed n-type channel region 
manufactured from the n-type material, which channel region is provided 
with a gate electrode which is separated from the channel region by an 
insulating layer, while a side of the channel region facing away from the 
gate electrode is provided with a passivated surface layer which adjoins a 
surface. The side of the channel region facing away from the gate 
electrode is passivated upon exposure to air. A comparatively narrow 
channel region results from this. Such a narrow channel region has a 
favorable influence on a so-called on/off ratio of the field effect 
transistor, i.e. the difference in conductivity in the channel region when 
the channel is blocked or rendered conducting in known manner via the gate 
electrode. 
Known donor molecules are, for example, TTF: tetrathiafulvalene, TMTTF: 
tetramethyltetrathiafulvalene, TSF: tetraselenafulvalene, TMTSF: 
tetramethyltetrasilenafulvalene. Known acceptor molecules are, for 
example, TCNQ: tetracyanoquinodimethane, TNAP: 
tetracyanonaphtoquinodimethane, and TCNDQ: tetracyanodiquinodimethane. All 
these molecules can be used as donor and acceptor molecules in a 
solid-state mixture according to the invention. For further examples of 
organic donor and acceptor molecules, the reader is referred to the book: 
"Organic Charge-Transfer Complexes" by R. Foster, Academic Press 1969, 
Table 1.1, pp. 5-11. Preferably, the organic donor molecule comprises TTF: 
tetrathiafulvaiene and the organic acceptor molecule comprises TCNQ: 
tetracyanoquinodimethane. These materials are comparatively easily 
available and can be readily applied at a temperature below 200.degree. C. 
An additional advantage is obtained when a surface of the device is 
provided with a surface layer which seals the device off against oxygen. 
The stability of the solid-state mixture is increased thereby. Preferably, 
the surface layer comprises silicon monoxide. Silicon monoxide can be 
applied at a comparatively low temperature of approximately 200.degree. 
C., so that the organic donor and acceptor molecules are not attacked.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The Figures are purely diagrammatic and not drawn to scale. Corresponding 
parts have been generally given the same reference numerals in the 
Figures. 
FIGS. 2, 4 and 6 show semiconductor devices provided with an organic 
semiconductor material formed by a solid-state mixture of organic donor 
and organic acceptor molecules. A donor molecule is here understood to be 
a molecule which can give off an electron comparatively easily, and an 
acceptor molecule is understood to be a molecule which can take up an 
electron comparatively easily. 
Known semiconducting solid-state mixtures have molar ratios between donor 
and acceptor molecules of 1.3:2 and 1.66:2. Such solid-state mixtures have 
the disadvantage that the electrical conductivity is comparatively high, 
so that it is not possible to manufacture switchable devices with the 
mixture. 
According to the invention, the semiconductor material comprises an n- or 
p-type semiconductor material such that the n-type material has a molar 
ratio between the donor and acceptor molecules below 0.05, and the p-type 
material has this ratio above 20. The solid-state mixture behaves as an 
n-type semiconductor when it has a molar ratio between donors and 
acceptors below 0.05, so with a relative very great number of acceptor 
molecules. The solid-state mixture behaves as a p-type conductor at a 
molar ratio above 20, so with a relative very large number of donor 
molecules. 
FIG. 1 shows how the conductivity S varies as a function of the molar 
donor/acceptor ratio D/A in an n-type semiconductor material manufactured 
from TCNQ as the acceptor molecule and TTF as the donor molecule. The 
electrical conductivity can accordingly be varied over many orders of 
magnitude in that the molar ratio between donor and acceptor molecules is 
controlled. FIG. 1 shows that the mixtures according to the invention have 
an electrical conductivity which differs very strongly from semi-metallic 
conductivity such as is found for known solid-state mixtures with a molar 
donor/acceptor ratio D/A of approximately 1. 
FIG. 2 shows a field effect transistor with a source region 1, a drain 
region 2 and an interposed n-type channel region 3 made from the n-type 
material, the channel region 3 being provided with a gate electrode 4 
which is separated from the channel region 3 by an insulating layer 5, 
while a side 3' of the channel region 3 facing away from the gate 
electrode 4 is provided with a passivated surface layer 7 which adjoins a 
surface 6. Such a device is manufactured as follows. A strongly doped 
p-type silicon slice (approximately 0.02 .OMEGA.cm) is used as the gate 
electrode 4. On this slice, a 50 nm thick silicon dioxide layer is grown 
thermally in known manner as the insulating layer 5. On this insulating 
layer 5 a source region 1 and drain region 2 are formed from a 
vapor-deposited gold layer of approximately 0.1.mu. thickness, which is 
patterned in known manner by photolithography and an etching process. The 
distance between the source and drain regions, the channel length L, is 5 
.mu.m, while the channel width Z, i.e. the width of the channel region 3 
transverse to the plane of drawing, is 10 min. The electrical resistance 
between source 1/drain 2 and the gate electrode 4 is more than 10.sup.12 
.OMEGA. here. Then a solid-state mixture is provided on the insulating 
layer 5 and on the source and drain regions 1, 2. The silicon slice is for 
this purpose placed in a vaporizing bell jar where a solid-state mixture 
of donor and acceptor molecules, TTF and TCNQ, respectively, in a molar 
ratio of approximately 1:200 is provided to a thickness of 0.17 .mu.m at a 
pressure of 1.3.times.10.sup.-4 N/m.sup.2 (1.times.10.sup.-6 torr). The 
TCNQ and TTF are provided from different vapor sources which are kept at a 
temperature of approximately 150.degree. C. The electrical conductivity of 
the solid-state mixture applied is 5.times.10.sup.-6 Scm.sup.-1. The side 
3' of the channel region 3 facing away from the gate electrode 4 is 
passivated upon exposure of the channel region 3 to air during the removal 
from the bell jar. The device according to the invention then comprises a 
channel region 3 made of n-type semiconductor material of the solid-state 
mixture provided with a passivated surface layer 7. It is found in 
practice that n-type material 3 forms a passivating surface layer 7 of 
approximately 0.15 .mu.m when it is exposed to an atmosphere containing 
oxygen after its manufacture in vacuum. The oxygen renders the n-type 
solid-state mixture more insulating. This effect is strongest at a 
boundary surface between n-type mixture and the atmosphere and becomes 
progressively less so towards the bulk of the solid-state mixture. A 
comparatively shallow channel region 3 is created by this passivation. 
Such a shallow channel region 3 favorably affects a so-called on/off ratio 
of the field effect transistor, i.e. there is a great difference in 
conductivity in the channel region 3 when the channel is blocked in known 
manner via the gate electrode 4 and when the channel is made conducting. 
The conductivity of the channel region in the conducting state does 
decrease owing to the passivation because the channel region 3 becomes 
shallower (less thick). FIG. 3 is a graph in which a current I.sub.sd 
between source 1 and drain 2 is plotted horizontally and a voltage V.sub.g 
applied to the gate electrode 4 is plotted horizontally. The voltage 
between source 1 and drain 2 was set for a fixed value of 20V here. The 
curve of FIG. 3 was registered 47 days after manufacture, so with a fully 
passivated surface. It is evident from FIG. 3 that the on/off ratio 
(measured for V.sub.g of +20 and -20V) is great: 3.times.10.sup.-7 
/10.sup.-9, i.e. approximately 300. 
FIG. 4 shows a so-called MIS (Metal Insulator Semiconductor) diode 
according to the invention. This MIS diode is manufactured in a manner 
analogous to that of the field effect transistor of the preceding example, 
but no source and drain regions are provided now, whereas a second gold 
electrode 8 is provided on the surface 6. The MIS diode has a surface area 
of 0.31 mm.sup.2. The MIS diode behaves as a capacitance which can store 
charge Q at its electrodes 8 and 4. FIG. 5 shows a differential 
capacitance dQ/dV.sub.g as a function of the voltage V.sub.g applied to 
the gate electrode 4. FIG. 5 was registered at a frequency of 1 kHz and an 
amplitude of 0.5V, the voltage V.sub.g across the electrodes 4 and 8 being 
varied at a rate of 20V/minute. The broken line 10 gives the differential 
capacitance of the device without the semiconductor layer 3, the full line 
11 the differential capacitance of the device with the semiconductor layer 
3. The differential capacitance value of curve 11 approaches the value of 
curve 10 for V.sub.g greater than approximately 20V. This indicates that 
the semiconductor material 3 is enhanced with electrons, so that the 
semiconductor region 3 is regarded as a conductor for the differential 
capacitance dQ/dV and the differential capacitance is determined by the 
insulating layer 5, as is the case for curve 10, where the electrode 8 
lies directly on the insulating layer. The differential capacitance value 
of curve 11 approaches that value of dQ/dV which belongs to an insulating 
layer comprising both the insulating layer 5 and the semiconductor region 
3 when V.sub.g becomes lower than approximately -20V. This is an 
indication that the semiconductor region 3 is fully depleted of charge at 
these voltages. FIG. 5 shows clearly, therefore, that the solid-state 
mixture behaves as an n-type semiconductor. It is noted that the MIS diode 
exhibits a hysteresis in its differential capacitance curve. The cause of 
this hysteresis is not clear, but it could be due to various mechanisms 
such as charge being held at a boundary surface, oxide charge, or 
migration of donor or acceptor molecules. 
FIG. 6 shows a device which comprises both an n-type region 23 manufactured 
from the n-type material and a p-type region 22 manufactured from the 
p-type material, a pn junction 35 being formed between the p- and n-type 
regions. The device also comprises a further region 24 made from a 
solid-state mixture of the organic donor and organic acceptor molecules in 
which the molar ratio between the donor and acceptor molecules is 
substantially equal to one. The device of FIG. 6 is a diode. Such a device 
is manufactured as follows. Solid-state mixtures of the donor and acceptor 
molecules are provided on an insulating substrate 20 made of glass. The 
silicon slice is for this purpose placed in a vaporizing bell jar where a 
solid-state mixture of TTF and TCNQ as the donor and acceptor materials, 
respectively, is provided from different vapor sources at a pressure of 
1.3.times.10.sup.4 N/m.sup.2 (1.times.10.sup.-6 torr). The temperatures of 
the vapor sources are set in dependence on the desired ratio between donor 
and acceptor molecules. Different layers 21 to 24 are provided in one 
process sequence, i.e. without the substrate 20 being taken from the bell 
jar. First a semi-metallically conducting layer 21 comprising a 
solid-state mixture with a molar ratio TTF/TCNQ of approximately 1:1 is 
provided to a thickness of 0.2 .mu.m. This layer 21 acts as a first 
electrode of the semiconductor device. The electrical conductivity of the 
solid-state mixture provided is approximately 1 Scm.sup.-1. Then a p-type 
semiconductor layer 22 of 0.2 .mu.m thickness is provided, comprising a 
solid-state mixture with a molar ratio TTF/TCNQ of approximately 200: 1, 
without the substrate 20 being removed from the bell jar. The electrical 
conductivity of the solid-state mixture provided is 5.times.10.sup.-6 
Scm.sup.-1. On the p-type layer 22, an n-type semiconductor layer 23 is 
provided, comprising a solid-state mixture with a molar ratio TTF/TCNQ of 
approximately 1:200. The electrical conductivity of the solid-state 
mixture provided is 5.times.10.sup.-6 Scm.sup.-1. On the n-type layer 23, 
a semi-metallic layer 24 is provided, comprising a solid-state mixture of 
the organic donor and the organic acceptor molecules, the molar ratio 
between the donor and acceptor molecules being substantially equal to one 
here. A gold layer 25 of 0.2 .mu.m thickness is provided on this 
semi-metallic layer 24 as a second electrode. The gold layer is shaped in 
known manner by vapor deposition, a photolithographical process, and 
etching. The organic layers 21 to 24 are then patterned by plasma etching. 
Subsequently, according to the invention, the surface 30 and the lateral 
sides of the device created by plasma etching are provided with a surface 
layer 26 which seals the device off against oxygen. 
Such a layer is manufactured at a comparatively low temperature 
(200.degree. C. or lower) in a low-temperature CVD (Chemical Vapor 
Deposition) process. The layer seals the solid-state mixture off against 
oxygen, whereby the stability of the solid-state mixture increases. 
Preferably, the surface layer 26 comprises silicon monoxide. Silicon 
monoxide can be provided in a known manner at a comparatively low 
temperature of approximately 200.degree. C. The silicon monoxide layer 
ensures that the organic donor and acceptor molecules are not attacked. 
Known donor molecules are, for example, TTF: tetrathiafulvalene, TMTFF: 
tetramethyltetrathiafulvalene, TSF: tetraselenafulvalene, TMTSF: 
tetramethyltetrase. lenafulvalene. Known acceptor molecules are, for 
example, TCNQ: tetracyanoquinodimethane, TNAP: 
tetracyanonaphtoquinodimethane, and TCNDQ: tetracyanodiquinodimethane. All 
these molecules can be used as donor and acceptor molecules in a 
solid-state mixture according to the invention. The mention of the above 
donor and acceptor material is not to be regarded as limitative. Further 
examples of organic donor and acceptor molecules can be found in the book: 
"Organic Charge-Transfer Complexes" by R. Foster, Academic Press 1969, 
Table 1.1, pp. 5-11. A device according to the invention is also possible 
with donor and acceptor molecules other than those mentioned, for example, 
with the said donor or acceptor molecules which have in addition been 
provided with groups such as long carbon chains or benzene rings 
(macromolecules). Preferably, the organic donor molecule comprises TTF: 
tetrathiafulvalene, and the organic acceptor molecule TCNQ: 
tetraeyanoquinodimethane. These materials are comparatively easily 
available and can be readily applied at a temperature below 200.degree. C. 
The invention is not limited to the embodiments described above. The 
semiconductor device may comprise, instead of one switching element, many 
switching elements on a common substrate. The semiconductor device may 
also comprise other switching elements such as, for example, bipolar 
transistors, diodes, field effect transistors, or thyristors. These 
devices are designed on the analogy of semiconductor devices known from 
silicon technology. The semiconductor devices may be made in that the 
solid-state mixture is patterned by known techniques such as 
photolithography and etching, for example, plasma etching. Conductive, 
p-type and n-type regions can be manufactured and shaped then by means of 
the solid-state mixture according to the invention.