Heterojunction Field Effect Transistor With Improve Carrier Density and Mobility

1. Field of the Invention

The present invention relates to a field effect transistor for use in high-speed logic integrated circuits, etc.

2. Description of the Prior Art

As a field effect transistor (FET) which is used as a device for high frequency amplifiers, high-speed computers, etc., a schottky qatc FET (MESFET) using, as its schottky gate, GaAs which has mobility in a low electric field higher than Si (GaAs MESFET), a high electron mobility transistor (HEMT), etc. have vigorously been developed recently.

In order to enhance the performance of a basic device for an integrated circuit for a high-speed computer, particularly the integration scale and the operation velocity, it is very important to realize high current driving-ability. For this, it is necessary to increase the velocity of a carrier and the carrier density, as it can easily be understood from a most simple model wherein the electric current flowing through a channel is in proportion to the product of the carrier density and the carrier velocity. It is also necessary to make the channel layer thinner, for suppressing short channel effects caused by shortening the gate length.

For these reasons, heretofore, for example in the case of said GaAs MESFET, a molecular-beam opitaxy (MBE) method, which is usable for depositing a thin film with the best controllability, has been used for growing the thin film in order to increase the carrier density, and make the channel thinner. The limit of doping with practical controllability by said MBE method, however, is about 2.times.10.sup.18 cm.sup.-3.

GaAs has a large electron mobility and so is advantageous for use as a high-speed device material, but owing to its low density state, its doping concentration is limited. On the other hand, Si has a lower electron mobility than GaAs, but has a higher density state sufficient for realizing a high density, not lower than 10.sup.20 cm.sup.-3. Thus the density state and the mobility of a carrier have a reverse correlation determined by an intrinsic band spectrum construction of the material. This means that in principle it is impossible to increase both of the density state and the carrier mobility.

An object of the present invention therefore is to realize a high performance FET which can improve integration scale, by effectively combining a merits of a material which has a low carrier density state but a high carrier mobility and a material having a low carrier mobility but a high carrier density state.

Thus the present invention provides a field effect transistor comprising first and second electrodes, semiconductor layers connected to these electrodes to form a carrier channel between them and a control electrode, said semiconductor layers consisting essentially of:

(a) a first semiconductor layer of a first semiconductor material having a low density state of electrons formed on a substrate,

(b) a second semiconductor layer of a second semiconductor material containing an n-type impurity element and having a high density state of electrons formed on the first semiconductor layer, and

(c) a third semiconductor layer of a third semiconductor material having a low density state of electrons formed on the second semiconductor layer.

The present invention also provides a field effect transistor comprising first and second electrodes, semiconductor layers connected to these electrodes to form a carrier channel between them and a control electrode, said semiconductor layers consisting essentially of:

(a) a first semiconductor layer of a first semiconductor material having a low density state of holes formed on a substrate,

(b) a second semiconductor layer of a second semiconductor material containing a p-type impurity element and having a high density state of holes formed on the first semiconductor layer, and

(c) a third semiconductor layer of a third semiconductor material having a low density state of holes formed on the second semiconductor layer.

According to the principle of the present invention, the intrinsic problem of a material that the mobility and the density state of a carrier are inversely proportional to each other is solved by using as a carrier supplying layer a semiconductor layer having a low mobility but a high density state and using as a carrier channel layer a semiconductor layer having a mobility as high as possible so that the carrier may flow therein by the merit of penetration of a wave function. For example, in the case of GaAs, the mobility in a low electric field is large, but the electron density is 4.7.times.10.sup.17 cm.sup.-3 which is two magnitudes lower than that of Si 2.8.times.10.sup.19 cm.sup.-3. In fact, owing to degeneration effect, doping can be made to a concentration as high as 5.times.10.sup.18 cm.sup.-3 which is higher than the density, even by said MBE method, but cannot be made to exceed 10.sup.20 cm.sup.-3. Accordingly, there is a limitation in making a channel having a density as high as possible and a thickness as thin as possible in order to obtain higher performance. According to the present invention, an extremely thin Ge layer having a large electron density of 1.times.10.sup.19 cm.sup.-3 is used as an electron supplying layer to enable n-type high concentration doping and to suppress decreases of electron average velocity less than a certain level by utilizing penetration effect of a wave function. Thus it becomes possible to obtain higher performance.

As for material of the electron supplying layer, that is, the second semiconductor material, it is possible and preferable to use Ge, Si or GeSi. As for material of the electron channel layer, that is, the first and the third semiconductor materials, it is possible and preferable to use GaAs or GaSb.

As for material of the hole supplying layer, it is possible and preferable to use Si or GaAs; and as for material of the hole channel layer, it is possible and preferable to use GeSi, Ge, InSb or GaSb.

These materials can be selected from element semiconductor materials or III-V compound semiconductor materials in accordance with the principle of the present invention to obtain the merit of the penetration of wave function as mentioned above. Apparently the first and the third semiconductor materials may be either the same or different materials. Further, even one of the first and the third semiconductor layers may be dispensed with, since the merit of the penetration of wave function can still be obtained.

The thickness of the electron or hole supplying layer should be one to effectively utilize the penetration wave function. In the case of electron supplying layer, it is preferably in the range of 20-100 .ANG., since if it is less than 20 .ANG., the effect obtainable by the existence of the electron supplying layer itself would be too small, and if it is much more than 100 .ANG., the effect obtainable by the penetration of wave function would be to small in view of the Dobye length considered as a reference of the penetration wave function. In the case of hole supplying layer, it is preferably at least 20 .ANG., since if it is less than 20 .ANG., the effect obtainable by the existence of the hole supplying layer itself would be too small. There is no critical limitation to the maximum thickness of the hole supplying layer, since holes overflow the hole supplying layer and substantially flow in the hole channel layer of high mobility as if they are confined in the channel layer of shape like a groove or well of their potential curve. The thickness of the hole channel layer therefore is preferably at least 100 .ANG., and there is no critical limitation to its maximum value.

FIG. 1 shows a cross section of an embodiment of the field effect transistor of the present invention.

FIG. 2 shows a schematic illustration of energy bands of the FET of FIG. 1 in a thermal equilibrium condition at a position immediately below its gate electrode. FIG. 3 shows a cross section of another embodiment of the field effect transistor of the present invention.

FIG. 4 shows a schematic illustration of energy bands of the FET of FIG. 3 in a depletion layer modulation mode operation condition when a positive voltage is applied to its gate, at a position immediately below its gate electrode.

The present invention will be explained below further in detail with respect to its some embodiments.

An FET as shown in FIG. 1 was prepared by the following steps. On a substrate 1 consisting of semi-insulating GaAs, a non-doped GaAs layer 2, a Ge layer 3 doped with 1.2.times.10.sup.19 cm.sup.-3 As and a non-doped GaAs layer 4 were grown sequentially by the MBE method, respectively in thicknesses of 600 nm, 10 nm and 20 nm, Then, as a control electrode, a gate electrode 7 consisting of for example WSi.sub.x was formed thereon by deposition by a sputtering method and by processing by a reactive dry etching method. Further, an n-type low resistance area 8 was formed by selectively implanting Si ions by using the gate electrode 7 and a photoresist as a mask under conditions of acceleration energy of 70 keV and dose of 1.times.10.sup.13 cm.sup.-2 and then carrying out an activation annealing at 800.degree. C. Finally, a source electrode 5 and a drain electrode 6, both of ohmic contact, were formed by evaporation of AuGe.multidot.Ni and an alloying heat treatment.

The obtained FET was found to have energy bands in a thermal equilibrium condition, at a position immediately below its gate electrode, as shown in FIG. 2, wherein its Fermi level of electrons 9, minimum point of conduction band 10 and maximum point of valence band 11 are shown. It can be seen in FIG. 2 that the wave function of electrons 12 penetrated into the non-doped GaAs layers 2 and 4 on both sides of the n-type Ge layer 3. By this, an average velocity of electrons in the non-doped GaAs layers 2 and 4 became larger than that in the Ge layer. Further, Ge having a state density of 1.times.10.sup.19 cm.sup.-3 was used as an electron supplying layer. It was thus made possible to effect higher doping higher than GaAs which is difficult to effect stable doping higher than 2.times.10.sup.18 cm.sup.-3 even if the electron degeneracy is taken into consideration.

As another embodiment, an FET as shown in FIG. 3 was prepared by the following steps. On an n-type Si substrate 21, sequentially an n-type Si layer 22 of 600 nm thickness, a non-doped Ge.sub.0.5 Si.sub.0.5 layer 23 of 30 nm thickness as a carrier channel layer, a p-type Si layer 24 of 5 nm thickness doped with 1.times.10.sup.19 cm.sup.-3 Ga as a carrier supplying layer, a non-doped Ge.sub.0.5 Si.sub.0.5 layer 25 of 10 nm thickness as a carrier channel layer, a low density n-type Si layer 26 of 20 nm thickness and an SiO.sub.2 layer 27 of 10 nm thickness formed by surface oxidation were grown by the MBE method. Then a gate electrode 7 consisting of for example WSi.sub.x was formed thereon by deposition by a sputtering method and by processing by a reactive dry etching method. Further, a p-type low resistance area 28 was formed by selectively implanting B ions by using the gate electrodes 7 and a photoresist as a mask under conditions of acceleration energy of 80 keV and dose of 1.4.times.10.sup.13 cm.sup.-2 and then carrying out an activation annealing at 900.degree. C. Finally, a source electrode 5 and a drain electrode 6 both of ohmic contact were formed by selective removal of the SiO.sub.2 layer, deposition of AlSi and alloying heat treatment.

The obtained FET was found to have energy bands in a depletion layer modulation mode operation condition when a positive voltage is applied to its gate, at a position immediately below its gate electrode, as shown in FIG. 4, wherein its Fermi level of electron 9, minimum point of conduction band 10 and maximum point of valence band 11 are shown. It can be seen in FIG. 4 that the wave function of holes 18 penetrated into the non-doped Ge.sub.0.5 Si.sub.0.5 layers 23 and 25 on both sides of the p-type Si layer 24. By this, an average velocity of holes in the non-doped Ge.sub.0.5 Si.sub.0.5 layers 23 and 25 became larger than that in the Si layer. Further, Si having a higher density of state was used as a hole supplying layer. It was thus made possible to achieve higher doping.

As explained above, higher concentration doping has been made possible by the present invention by applying the penetrating effect of a wave function and thereby suppressing decrease of carrier average velocity less than a certain level. Thus it has been made possible to realize a high performance FET suitable for larger scale integration.