Abstract:
The invention relates to a semiconductor device of the hetero-junction transistor type comprising a stack of semiconductor layers which in combination constitute the source, drain and gate regions, while the current path between the source and drain regions is substantially at right angles to the various junctions. The gate region constitutes an electron accumulation region in the form of a two-dimensional quasi Fermi-Dirac gas which can be brought to the desired polarization potential of at least one gate electrode, while the electrons forming the source-drain current traverse this electron cloud without having a strong interaction with it, in ballistic or quasi-ballistic conditions.

Description:
This is a continuation of application Ser. No. 455,343, filed Jan. 3, 1983, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a semiconductor device of the hetero-junction transistor type comprising a stack of semiconductor layers which constitute a source region, a gate region and a drain region, while the current path established between the said source and drain regions is substantially at right angles to the various junctions. 
     Hetero-junction structures are well known from the prior art as designed and tested in the years 1950 ff. However, although they have very interesting potential properties superior to those of homo-junctions, they have been developed to only a small extent, except perhaps in opto-electronic devices, such as lasers. Thus, although numerous books and publications have appeared about semiconductor hetero-junction devices, the main applications developed for the market are of the homo-junction type. 
     One of the reasons for this slow development of hetero-junction devices resides in the poor crystalline and electrical quality of the layers at the interfaces due to the growing methods utilized thus far, which result in a considerable recombination of the charge carriers at the interfaces. 
     The novel growing methods developed since then, such as, for example, epitaxy from the vapor phase with the use of organometallic compounds, molecular beam epitaxy and other methods, have reduced to a large extent the aforementioned disadvantages and improved the steepness of the transitions and the freedom of choice of the configurations of the conduction bands. 
     Consequently, hetero-junction semiconductor devices have become more popular. 
     A recent example of a patent application for a hetero-junction semiconductor device is the European patent application EP No. 0 027 761 which discloses a horizontal transistor, the gate region of which comprises a P/N hetero junction (GaAsP/Ga 1-x  Al x  AsN), in which a charge inversion layer is formed for a positive polarization of the gate region. 
     SUMMARY OF THE INVENTION 
     A semiconductor device according to the present invention is constituted by a stack of semiconductor layers forming by combination the source, gate and drain regions, while the path of the current flowing from the source to the drain is substantially at right angles to the various junctions. 
     Such hetero-junction semiconductor devices are generally known from the prior Art; reference is made, for example, to U.S. Pat. No. 4,119,994, which discloses hetero-junction transistors of the conventional type, in which one zone of a first conductivity type (designated as the base zone) is arranged between two zones of a second conductivity type (designated as emitter zone and collector zone). 
     The semiconductor device according to the present invention does not belong to the transistors of this type; a novel structure is concerned, which belongs to the category of transistors in which all charge transport take place by electrons forming majority carriers. The device utilizes both ballistic transport or quasiballistic transport for the current flowing from source to drain and the conduction in a two-dimensional quasi Fermi-Dirac gas for the conduction in the gate. The current carriers flowing from source to drain traverse this Fermi-gas perpendicularly or substantially perpendicularly without having a strong interaction with the carriers forming the said gas. 
     Since at least 1970 numerous publications have appeared concerning ballistic transport and reference is made to the article in I.E.E.E. Transactions on Electron Devices, Vol. ED-26, no. 11, November 1979, entitled: &#34;Ballistic transport in semiconductor at low temperatures for low-power high-speed logic&#34; by M. S. SHUR and L. F. EASTMAN. 
     The two-dimensional electron gas in the proximity of a hetero-junction is described, for example, in the article of H. L. STORMER et al entitled: &#34;Two-dimensional electron gas at a semiconductor-semiconductor interface&#34; in &#34;Solid State Communications&#34;, Vol. 29, p. 705-709, 1979. 
     The semiconductor device according to the present invention is characterized in that the gate region is constituted by a layer of a first semiconductor material which forms a hetero-junction with the immediately adjacent layer of the drain region of a second semiconductor material, while the first and second semiconductor materials are chosen so that at the hetero-junction the lower level of the conduction band in the first semiconductor material is situated below the lower level of the conduction band in the second semiconductor material, and under the influence of the component of the electric field at right angles to the hetero-junction there is formed in the first semiconductor material in the proximity of the hetero-junction an accumulation of electrons in a potential well, the lower level of which is controlled by a gate electrode, in that the source region comprises at least one layer of a third semiconductor material chosen so that the lower level of the conduction band in the third semi-conductor material can be made higher than the lower level of the conduction band in the second semiconductor material by means of the polarization voltage applied thereto, and in that the said gate region is freed from impurities and is thinner than the free mean path of the charge carriers. 
     Thus, due to a difference in electron affinity between the first and the second semiconductor material, free electrons are transferred and are accumulatd in the potential well which has formed in the first semiconductor material in the proximity of the hetero-junction, and constitues a two-dimensional quasi Fermi-Dirac gas. The gate function is then obtained in an almost immaterial manner by this electron cloud which by an electrode can be brought to the desired polarization potential. The electrons forming the source-drain current thus traverse this electron cloud without having a strong interaction with it. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described more fully, by way of example, with reference to the accompanying drawing, in which: 
     FIG. 1 shows a device according to the prior art, the structure and the operation of which are related to the invention; 
     FIG. 2 shows an embodiment of the device in accordance with the invention; 
     FIGS. 3 and 4 show the variations of the doping and of the aluminum percentage of the various GaAlAs/GaAs layers of an embodiment of the device according to the invention; 
     FIG. 5 shows schematically the diagram of bands in the proximity of the main hetero-junction; and 
     FIGS. 6a, 6b and 6c show the diagram of bands of this novel hetero-structure, both quiescently and in operation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The semiconductor device according to the prior art shown in FIG. 1 is derived from the article &#34;Fabrication and Numerical Simulation of the Permeable Base Transistor&#34; by C. O. BOZLER and G. D. ALLEY in I.E.E.E. Transactions on Electron Devices, Vol. ED-27, no. 6, June 1980. An ohmic emitter contact 1 is made on a highly doped n +  -type region 2 of gallium arsenide; the flow of electrons emitted by this contact 1 is confined by insulating regions 3 obtained, for example, by a proton bombardment in a normally doped n-type region 4 and thus traverses a gate 5 constituted by a multitude of fingers embedded in the semiconductor substance in order to finally obtain the collector contact 6, which is likewise ohmic. 
     The gate 5 is made, for example, of tungsten, which establishes a Schottky contact with the gallium arsenide; it serves to control the flow of electrons which passes from the emitter to the collector in a manner somewhat analogous to a vacuum triode in spite of the important part played by the diffusion currents in this device. 
     With such a device the maximum transition frequency (the current amplification is equal to unity) is defined generally by the formula; ##EQU1## in which g m  represents the transconductance and C T  is calculated from the overall charge variation in the semiconductor ΔQ T  produced by a variation of the potential of the gate electrode ΔV GS  as follows: ##EQU2## 
     The devices thus realized have a maximum frequency f T  as defined above of the order of 40 GHz. The cut-off frequency or the maximum operating frequency is of the order of 10 GHz. 
     A device according to the present invention is shown in a sectional view in FIG. 2. On a semi-insulating substrate 7, for example, of gallium arsenide doped with chromium, there is grown epitaxially a strongly doped semiconductor layer 8 of the n ++  conductivity type and with a doping of the order of 10 18  atoms/cm 3  obtained, for example, by the introduction of silicon impurities and of a non-critical thickness, for example, 0.5 μm. This layer 8 serves as drain electrode and an ohmic contact 9, for example, of an alloy of gold-germanium, can be made on the periphery of the device, for example, in the form of a crown. 
     On this very highly doped layer 8 is formed a weakly doped layer 10 of the n-conductivity type with a doping of, for example, 10 16  atoms/cm 3 , which layer may even be undoped and may consist of a semiconductor material, such as gallium arsenide, and have a thickness of approximately 0.5 μm. This layer 10 mainly serves to separate the layer 8 from the upper layers and thus to reduce the capacity between the drain electrode constituted by the strongly doped layer 8 and the two-dimensional Fermi-Dirac gas serving as the gate electrode in the layer 12, while also increasing the drain gate breakdown voltage. 
     The layer 11 is a layer of a second semiconductor material, for example of gallium aluminum arsenide Ga 1-x  Al x  As having a thickness of the order of 100 nm. This layer 11 participates in the formation of the potential well as follows: 
     it serves as a barrier to prevent the electrons contained in the potential well from migrating towards the drain under the influence of the electric field; 
     it must not form a barrier for the electrons originating from the source and its height is therefore limited so that the Kinetic energy of these electrons exceeds it; 
     in a particular modification, it is possible to use this barrier for modulating the number of electrons originating from the source which reach the drain. 
     According to a first embodiment of the invention, this layer 11 is composed of weakly n-doped Ga 1-x  Al x  As with a constant and sufficient value of x. According to a second embodiment of the invention, this layer 11 is in fact constituted by three zones, that is to say in the proximity of the junction with the upper layer 12 a first undoped zone having a thickness of approximately 5 nm and a value of x varying continuously between about 0.2 and 0.3 then a second very highly doped zone of the n ++  conductivity type having a doping level of 10 18  atoms/cm 3 , a thickness of approximately 10 nm and a value for x which is constant at about 0.3, and finally a third undoped zone having a thickness of approximately 85 nm and a value of x which is constant at about 0.3 over the major part of the zone and then decreases towards the end of the zone, for example, in a linear manner. 
     Finally, according to a third embodiment, this layer 11 is undoped, is made of gallium aluminum arsenide Ga 1-x  Al x  As and has a value of x varying stepwise, as in the preceding embodiment. 
     The layer 12 is a layer made of another semiconductor compound chosen so that its lower level of the conduction band is situated below the lower level of the band of conduction of the layer 11 and that under the influence of the component of the electric field at right angles to the hetero-junction between the two aforementioned layers 11/12 a potential well is formed therein; the curvature of the conduction band has to be sufficient so that the lower level exceeds the Fermi level in the proximity of the hetero-junction, thus authorizing an accumulation of electrons in this proximity, as is shown in FIG. 5, which will be described in greater detail hereinafter. 
     According to an embodiment of the invention, the layer 11 being made of gallium aluminum arsenide Ga 1-x  Al x  As, the layer 12 is made of undoped gallium arsenide GaAs. 
     A possible example of the choice of the materials constituting the layers 11 and 12 is the pair GaAlAs/GaAs, but other materials may also be used. 
     However, taking into account the knowledge and experiments regarding this subject matter, the Applicant has preferred the use of the pair GaAlAs/GaAs, which has the advantage of an equivalent structure and crystal lattice, without a noticeable disturbance of the crystalline order. 
     In this layer 12 of undoped GaAs, due to its larger electronic affinity with respect to the layer 11 of Ga 1-x  Al x  As, free electrons are transferred and are thus accumulated at the lowest energy levels which are completely unoccupied and are situated between the lower level of the conduction band and the Fermi level. This electronic enhancement zone occupies a very small thickness of less than 5 nm in a layer of GaAs having a thickness of approximately 50 nm which constitutes the so-called gate region of this novel structure. The gate contact capable of controlling the Fermi level (or quasi level) of the aforementioned gate region is obtained by a layer 13 in ohmic contact with the said layer 12 of GaAs deposited, for example, in the form of a crown on the periphery of this intermediate plate, for example, a gold layer disposed on a germanium layer. 
     The source region of the novel device is constituted by the assembly of the layers 14 and 15 on which a source electrode 16 is deposited. The layer 14 is an intermediate layer between the layer 15 serving as electron source and the lower layers of the gate region. The main function of this layer 14 is to give the electrons leaving this layer 14 and entering the lower layer 12 of the gate region, in a direction at right angles to the interface between the two aforementioned layers 14/12, a mean kinetic energy about equal to the difference in electronic affinity between the materials forming these layers designated as third and first semiconductor materials for the layers of the source and gate regions, respectively. 
     According to an embodiment of the invention, the layer 14 is made of undoped gallium aluminum arsenide having a small thickness of, for example, 25 nm. 
     The layer 15 constituting the electron source is also made of gallium aluminum arsenide, which, however, is very highly n-doped and has a doping level of more than 10 18  atoms/cm 3  and a non-critical thickness of, for example, 200 nm. The source electrode is obtained in the form of an ohmic contact 16. According to an embodiment, the latter can be realized in the form of a layer of gallium arsenide of n-conductivity type doped, for example, with arsenic at 10 20  atoms/cm 3  and on which finally a metallic contact is made, for example, by a deposition of gold or aluminum. 
     FIG. 3 shows the variations of the doping in number of atoms per cm 3 . The thickness of the layers (one unit is 50 nm) is plotted substantially on the abscissa while the doping, which varies substantially from 10 14  (intrinsic semiconductor) to 10 19  atoms/cm 3  (semiconductor of the n ++  -type), is plotted on the ordinate. The doping of n-conductivity type of the semiconductor compounds GaAs and Ga 1-x  Al x  As is effected, for example, by atoms of impurities, as silicium (Si) or selenium (Se). The substrate is semi-insulating and is obtained, for example, by doping gallium arsenide (GaAs) with chromium (Cr). The various source, gate and drain zones are also represented in this Figure by the symbols S, G and D, respectively. Moreover, dotted lines indicate the various aforementioned embodiments. 
     FIG. 4 shows the variations of the value x of the compound Ga 1-x  Al x  As in the case of the described embodiment with the pair GaAs/GaAlAs. From a source region in the form of a plate with a value x of, for example, 0.45, an inflection is found at about 0.3 (layer 14) until a value of about 0.2 is attained, which inflection corresponds to a change in the slope of the curve which represents the aforementioned variations. Symmetrically, a second plate-shaped region with a value x of, for example, 0.3 (layer 11) is attained after an equivalent and symmetrical inflection at about x=0.2. 
     The inflection with a change of the slope of the curves of the variations of the concentration x of aluminum of the compound Ga 1-x  Al x  As mainly has for its object to smooth the curve of the band levels at the interface in order to reduce the quantous inflection phenomenon which occurs during the passage of an electron in the proximity of a discontinuity of the potential. 
     FIG. 5 shows the band diagram on both sides of the main hetero-junction. The lefthand part of the band diagram represents the material GaAs and the righthand part the material Ga 0 .7 Al 0 .3 As (by way of example). In such a band diagram, the symbols E c  and E v  designate the lower and upper levels of the conduction band or valency band. The symbol E f  designates the Fermi level, that is to say the energy level for which the occupation probability is 0.5. In the band diagram, the band curves are such that an electron accumulation zone 17 is formed. Free electrons originating from the region 18 of Ga 1-x  Al x  As (Si-doped) leave the latter due to the difference in electronic affinity between the two materials and are accumulated at the lowest levels of free energy in this thin region 17 near the interface. A thin undoped layer 19 in the material Ga 1-x  Al x  As is intended to grant a good separation between the donor impurities (for example, Si) and this layer of localized electrons. This layer of localized electrons 17 forms a kind of two-dimensional quasi Fermi-Dirac gas, while the polarization means 13 permit bringing it to the desired potential, on the order of a few tenths of a volt, for example, 0.2 V, with respect to the source, and of thus controlling the lower level of the potential well. 
     Finally, FIG. 6 shows the band diagrams of the device assembly at the same potential (FIG. 6-a) or in the polarized state (FIG. 6-b and 6-c). When the two materials are at the same potential V 1  =V 2 , the band diagram (FIG. 6-a) causes a potential well to appear such that the source-drain current is zero. When the potentials are not equal, if V 1  &lt;V 2 , electrons emitted by the source can traverse this potential well. 
     Moreover, the gate region being sufficiently thin and freed from impurities, the electron transfer is effected through this region by a ballistic effect. The term &#34;ballistic effect&#34; is to be understood to mean herein that the charge carrier, in this case the electron, is accelerated without a noticeable collision either with the impurities present in the crystal lattice or with the phonons (lattice vibrations) over a distance less than a critical (so-called free) path length. 
     At lower operating temperatures, for example, of the order of 77° K., at which only the phenomenon of collision with the impurities is predominant (&#34;impurity scattering&#34;), the mean free pathlength before the collision is larger, for example, of the order of a micron. 
     FIG. 6-b shows the band diagram when the source potential (V 1 ) is lower than the drain potential (V 2 ) and the potential (V 3 ) has an intermediate value: 
     
         V.sub.1 &lt;V.sub.3 &lt;V.sub.2. 
    
     The influence on the band diagram of this potential distribution is as follows: On the one hand, a smoothing (arrow a) of the first potential peak in the source region, on the other hand a tilting (arrow b) of the band level about the quasi Fermi level in the potential well of the gate region, causing the formation of an electron accumulation zone 17, finally a lowering (arrow c) of the curve below the level of the first potential peak in the drain region. 
     The operation of such a device is then clearly apparent: free electrons originating from the source region leave the latter due to the polarization, traverse the gate region by a ballistic effect and return to the state of equilibrium in the drain region mainly by interaction with an impurity (impurity scattering), this interaction being represented by an asterisk in this diagram. 
     A fraction α of the electrons emitted by the source owing to the potential difference between source and gate pass the gate region with a sufficient energy and pulse storage so that these electrons traverse the discontinuity in the conduction band between the gate and drain regions. The value of the flow of electrons emitted by the source is determined by the potential difference between source and gate. The potential of the gate is fixed by the two-dimensional quasi Fermi level of the electron gas. 
     Two modes of operation of the device can be distinguished: 
     (1) Application as a FET: α≈1. The current in the gate is small with respect to the current of the drain; 
     (2) α&lt;1. Between gate and source there is a negative resistance. FIG. 6-c shows schematically the band diagram in this application, in which only a part of the electrons can traverse the potential well. 
     Its operation in the first case can be similar to that of a vacuum tube in which also a movement of charge carriers of a single sign (electrons) under the influence of an electric field produced by the potential difference between emission electrode (cathode) and a reception electrode (wafer) takes place, the current being modulated by the potential of an intermediate electrode (gate). 
     However, the device according to the invention has properties more interesting than any of the devices known thus far, especially in that the maximum transition frequency f T , which can be approached by the following formula: ##EQU3## is higher than 100 GHz. 
     It is clear to those skilled in the art that numerous modifications of the invention can be realized without departing from the scope of the invention.