High electron mobility transistor

A high electron mobility transistor including an n-type semiconductor layer having a mixed crystal of aluminum gallium arsenide with an aluminum mixed ratio set to fall in the range of 0.2.about.0.3, and an undoped semiconductor layer forming a superlattice structure of an electron supplying layer, the undoped semiconductor layer having a mixed crystal of aluminum gallium arsenide with an aluminum mixed ratio set to fall in the proximity of a critical mixed crystal ratio between direct transition and indirect transition.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention generally relates to semiconductor devices, and in
 particular to a high electron mobility transistor that uses a mixed
 crystal of a compound semiconductor material.
 2. Description of the Related Art
 Recently as seen in a portable phone, satellite broadcasting and satellite
 communication, numerous communication systems using high-frequency waves
 such as microwaves or millimeter waves have been developed. In these
 systems, a high-power amplifier is indispensable for the final stage
 amplifier of the signal-transmitter unit thereof. Therefore, in view of
 the use of ultra high-frequency band in such systems, a high electron
 mobility transistor, also called HEMT, is widely used as the high-power
 output device. It should be noted that a HEMT is a transistor having
 superior high-frequency characteristics.
 Hereinafter the structure and operation of a conventional HEMT used for
 high frequency and high power application will be described with reference
 to FIGS. 1-6.
 FIG. 1 is a diagram showing the structure of a conventional HEMT. As shown
 in FIG. 1, a conventional HEMT includes an undoped GaAs layer 2 acting as
 a channel layer on a semi-insulating substrate 1 of GaAs. On the undoped
 GaAs layer 2, an undoped Al.sub.x Ga.sub.1-x As layer 4 is grown as a
 spacer layer, and an n.sup.+ -type Al.sub.x Ga.sub.1-x As layer 5 is grown
 on the undoped Al.sub.x Ga.sub.1-x As layer 4 as an electron supplying
 layer. On the n.sup.+ -type Al.sub.x Ga.sub.1-x As layer 5, there are
 provided an undoped Al.sub.x Ga.sub.1-x As layer 6, an undoped GaAs layer
 7, and an n.sup.+ -type GaAs layer 8. Further, a two-dimensional electron
 gas 3 is formed in the undoped GaAs layer 2 along an interface to the
 undoped Al.sub.x Ga.sub.1-x As layer 4. The conventional HEMT also
 includes a gate electrode 9, a source electrode 10, and a drain electrode
 11.
 As seen above, the HEMT for high-power applications has the undoped layer 6
 (which may also be an n.sup.- -type layer) of Al.sub.x Ga.sub.1-x As for
 increasing the breakdown voltage of the HEMT. Further, a pair of undoped
 GaAs layers 7 are provided so as to laterally sandwich the gate electrode
 9. In the structure of FIG. 1, the Al-content X of the layer 4 or 5,
 represented by the composition Al.sub.x Ga.sub.1-x As, is desired to have
 a large value for improving the sheet density of the two-dimensional
 electron gas 3. however, when the parameter X is too large, the electron
 density of the two-dimensional electron gas 3 is easily saturated due to
 the fact that the donor impurity level becomes too deep. Further, the
 operation of the HEMT tends to become unstable as the HEMT begins to show
 an optical response. Therefore, the compositional parameter X of the
 Al.sub.x Ga.sub.1-x As in the layer 4 or layer 5 has been generally set to
 fall in the range of 0.2.about.0.3.
 FIG. 2 is a diagram showing the band structure of the HEMT of FIG. 1 under
 a thermal equilibrium state. In FIG. 2, the relationship between the
 valence band Ev, the conduction band Ec, the Fermi level E.sub.F, the
 ground state energy level Eo of electrons, and the first-excited energy
 level E.sub.1, of the electrons, is represented.
 FIG. 3 is a diagram showing the electron density distribution of the
 conventional HEMT of FIG. 1 under a thermal equilibrium state, wherein
 FIG. 3 shows the case in which the HEMT constitutes a normally-on device.
 FIG. 4 is a diagram showing the band structure of the HEMT under a biased
 state in which a gate bias voltage is applied in the three-terminal
 circuit model for causing the HEMT to turn on. Further, FIG. 5 is a
 diagram showing the electron density distribution of the HEMT in the
 aforementioned biased state.
 Referring to FIG. 2 and FIG. 3, the electron supplying layer 5 is entirely
 depleted under the condition of thermal equilibrium. When a bias voltage
 is applied to the electrode 9, on the other hand, an electrically neutral
 region appears in the layer 5 and grows with an increase of the biased
 voltage. Thus, as shown in FIG. 5, the electron density of the n.sup.+
 -type Al.sub.x Ga.sub.1-x As layer 5 increases with the gate voltage. It
 should be noted that the drift velocity of the electrons in the electron
 supplying layer 5 of n.sup.+ -type Al.sub.x Ga.sub.1-x As is lower than
 that in the channel layer 2 of undoped GaAs. Further, in view of the fact
 that the electrons in the layer 5 flow to the gate electrode 9 under such
 a state, the HEMT of FIG. 1 suffers from the problem of drastic decrease
 of the transconductance g.sub.m, which tends to occur when the gate bias
 voltage is increased.
 FIG. 6 is a diagram showing the relationship between the gate voltage
 V.sub.g and the transconductance g.sub.m. In FIG. 6, the broken line 60
 shows the characteristic of the conventional HEMT, while the solid line 61
 shows the characteristic of the HEMT of the present invention to be
 described later.
 Further, there is a HEMT having the electron supplying layer 5 formed of a
 superlattice structure of n.sup.+ -type GaAs and i-type AlAs. In this
 prior art HEMT, the aluminum atoms and the silicon atoms, the silicon
 atoms being doped as donors, are separated spatially from each other so as
 to minimize the interaction between the aluminum atoms and the silicon
 atoms. It should be noted that it is this interaction between Al and Si
 that makes the donor impurity level deep. Thereby the HEMT successfully
 avoids the problem of saturation of the electron density in the
 two-dimensional electron gas 3 and the problem of unstability of the HEMT
 operation caused by the optical response.
 FIG. 7 is a diagram showing the band structure of the electron supplying
 layer having the superlattice structure consisting of n.sup.+ -type GaAs
 13 and i-type AlAs 12, wherein the bend of the energy band is omitted. As
 shown in FIG. 7, the effective energy band gap Eg is defined as the
 difference between the energy level E.sub.Qe for the ground state of the
 electrons and the energy level E.sub.Qh for the ground state of the holes.
 It should be noted that the energy level of the electrons and holes is
 quantized as a result of formation of the superlattice structure. By
 choosing a proper thickness for the n.sup.+ -type GaAs layer 13, the
 energy gap Eg can be set equal to or greater than the gap energy for the
 case in which the compositional parameter X of the n.sup.+ -type Al.sub.X
 Ga.sub.1-X As layer 5 is set to about 0.3.
 Therefore the electrons in the electron supplying layer having such a
 superlattice structure are not confined in the quantum well (n.sup.+ -type
 GaAs layer 13) of the superlattice under the thermal equilibrium state. In
 FIG. 7, it should be noted that the conduction band Ec of the i-type AlAs
 layer 12 is for the one at the .GAMMA.-valley.
 However, even though the HEMT has such a structure, the electrons in the
 two-dimensional electron gas 3 are accelerated and flow easily into the
 electron supplying layer when a large drain current flows. As a result,
 the drastic decrease of the transconductance g.sub.m is still caused.
 FIG. 8 is a diagram of the three-terminal characteristic of the high-power
 operation of the HEMT of FIG. 7 together with a load line 80. As shown in
 FIG. 8, the output power decreases due to the decrease of the
 transconductance g.sub.m in the high-current region 81. Further, it can be
 seen that the electric power gain, which depends on the mean value of the
 transconductance g.sub.m, decreases also along the entire load line 80.
 Further, there is induced a decrease in the drain efficiency and power
 added efficiency as a result of the decrease of the transconductance
 g.sub.m at the proximity of a knee voltage. These drawbacks of
 conventional HEMT cause serious problems particularly when the HEMT is
 used for high-power applications.
 SUMMARY OF THE INVENTION
 It is a general object of the present invention to provide a novel and
 useful high electron mobility transistor wherein the foregoing problems
 are eliminated.
 A more specific object of the present invention is to provide a high
 electron mobility transistor realizing an operation with high power, high
 gain and high efficiency.
 The above object of the present invention is achieved by a high electron
 mobility transistor including an n-type semiconductor layer having a mixed
 crystal of aluminum gallium arsenide represented as, with the aluminum
 content x being set to fall in the range of 0.2.about.0.3, and an undoped
 semiconductor layer forming a superlattice structure of an electron
 supplying layer, the undoped semiconductor layer having a mixed crystal of
 aluminum gallium arsenide represented as Al.sub.y Ga.sub.1-y As, with the
 aluminum content y being set to fall in the proximity of a critical
 composition in which an AlGaAs mixed crystal experiences a transition from
 direct transition type to an indirect transition type.
 The above object of the present invention is also achieved by a high
 electron mobility transistor including a channel layer of a mixed crystal
 of indium gallium arsenide represented by In.sub.x Ga.sub.1-x As with the
 indium content x being set to fall in the range of 0.1.about.0.3, and an
 electron supplying layer provided above said channel layer, electron
 supplying layer having a superlattice structure and including an n-type
 semiconductor layer of a mixed crystal of aluminum gallium arsenide
 represented as Al.sub.y Ga.sub.1-y As with the aluminum content y being
 set equal to or smaller than 0.3, and an undoped semiconductor layer of a
 mixed crystal of aluminum gallium arsenide represented as Al.sub.x
 Ga.sub.1-z As, with the aluminum content z being set to fall in the range
 of 0.4.about.0.5.
 According to the present invention, the high electron mobility transistor
 can maintain the transconductance at a high value even when the high
 electron mobility transistor is operated under a condition in which a
 large drain current flows.
 Other objects and further features of the present invention will become
 apparent from the following detailed description when read in conjunction
 with the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Hereinafter, a preferred embodiment of the present invention will be
 described with reference to the drawings.
 FIG. 9 is a diagram showing a structure of a high-power HEMT according to a
 preferred embodiment of the present invention. The structure of the HEMT
 shown in FIG. 9 is generally identical to the structure of the
 conventional HEMT shown in FIG. 1. Thus, those parts corresponding to the
 parts described previously are designated by the same reference numerals
 and the description thereof will be omitted. Referring to FIG. 9, the
 difference exists in the materials forming the layers of the HEMT. In
 particular, the channel layer is formed by In.sub.0.2 Ga.sub.0.8 As which
 provides a higher mobility and a higher drift velocity for the electrons.
 More specifically, the HEMT of FIG. 9 includes a buffer layer 1a of i-type
 GaAs formed on a semi-insulating substrate 1 of GaAs with a thickness of
 5000 .ANG.. On the buffer layer 1a, there is provided a channel layer 2a
 of undoped In.sub.0.2 Ga.sub.0.8 As with a thickness of 130 .ANG., and an
 undoped spacer layer 4a of Al.sub.0.5 Ga.sub.0.5 As is provided on the
 channel layer 2a with a thickness of 40 .ANG.. Further, an electron
 supplying layer 5a of n.sup.+ -type Al.sub.0.2 Ga.sub.0.8 As, doped to a
 carrier concentration level of 2.times.10.sup.18 cm.sup.-3, is provided on
 the spacer layer 4a with a thickness of 40 .ANG.. In such a
 heteroepitaxial structure, a two-dimensional electron gas 3 is formed in
 the channel layer 2a along the interface to the spacer layer 4a. The
 layers 4a and 5a are repeated alternately to form a superlattice
 structure. On the electron supplying layer 5a, there is formed a barrier
 layer 6a of Al.sub.0.2 Ga.sub.0.8 As, doped to a carrier concentration
 level of 2.times.10.sup.16 cm.sup.-3, with a thickness of 200 .ANG., and a
 cap layer 7a of GaAs is formed on the barrier layer 6a with a thickness of
 300 .ANG.. The cap layer 7a is doped to a carrier concentration level of
 2.times.10.sup.16 cm.sup.-3.
 Further, an etching-stopper layer 7b of n.sup.+ -type Al.sub.0.2 Ga.sub.0.8
 As, doped to a carrier concentration level of 2.times.10.sup.18 cm.sup.-3,
 is formed on the cap layer 7a with a thickness of 20 .ANG., and an ohmic
 contact layer 8a of GaAs, doped to a carrier concentration level of
 2.times.10.sup.18 cm.sup.-3, is formed on the etching-stopper layer 7b
 with a thickness of 800 .ANG.. On the cap layer 7a and the contact layer
 8a, there is provided a passivation film 17 of SiN as shown in FIG. 9.
 Moreover the gate electrode 9 is provided in Schottky contact with the
 barrier layer 6a via an opening formed in the cap layer 7a so as to expose
 the barrier layer 6a, and the source electrode 10 and the drain electrode
 11 are provided in ohmic contact with the contact layer 8a.
 The gate electrode 9 has a T-type structure including therein a stacking of
 a WSi layer and a Au layer, and may have a gate length of 0.25 .mu.m. The
 source electrode 10 and the drain electrode 11 are formed by AuGe/Ni/Au,
 and are separated from each other by a distance of about 4 .mu.m.
 According to the evaluations of the power characteristic conducted on the
 above HEMT of FIG. 9, it was confirmed that the gain is improved by about
 1 dB over a conventional HEMT at the frequency of 20 GHz. Further under
 the same condition, it was indicated that the output power is improved by
 about 0.5 dB, and the power added efficiency is improved also by about 5%.
 In the present invention, the channel layer 2a may be formed of i-type GaAs
 in place of InGaAs, in the same way as in the device of FIG. 1.
 Hereinafter the characteristic of the HEMT of FIG. 9 will be explained.
 FIG. 10 is a diagram showing the band structure of the HEMT of FIG. 9 under
 a thermal equilibrium state. On the other hand, FIG. 11 is a diagram
 showing the band structure of the same HEMT under a biased state for
 causing to flow a large drain current. In FIG. 11, the ground energy level
 E.sub.Qe of the electrons in the electron supplying layer 5a is
 illustrated. It should be noted that the energy level of the electrons is
 quantized in the electron supplying layer 5a as a result of the
 superlattice formation.
 In this embodiment, as a result of the quantization, the energy level
 E.sub.Qe can be higher than the energy level of the electrons in the
 two-dimensional electron gas 3 even when the HEMT is biased to flow a
 large drain current.
 FIG. 12 is a diagram showing the electron density distribution of the above
 HEMT under the foregoing in biased condition. As shown in FIG. 12, the
 probability of the electrons existing in the electron supplying layer 5a
 is reduced substantially from the conventional diagram of FIG. 5. Thereby
 as illustrated by the solid line 61 in FIG. 6, the transconductance is
 maintained at a high level value even though the HEMT is biased to flow a
 large drain current, wherein FIG. 6 reflects the situation in which the
 electron density of the two-dimensional electron gas is increased
 substantially as compared with the conventional HEMT.
 FIG. 13 is a diagram showing the conduction band energy of a Al.sub.x
 Ga.sub.1-x As mixed crystal as a function of the Al-content x.
 Generally, in the band structure of a semiconductor crystal, there are
 several different minima in the conduction band called .gamma.-valley,
 X-valley and L-valley with respective, different crystal momenta.
 As shown in FIG. 13, the energy level Ec(.GAMMA.) at the .gamma.-valley of
 the conduction band becomes higher as the value of Al-content X gets
 larger. On the contrary, the energy level at the X-valley or L
 -valley(Ec(X,L))of the conduction band becomes lower. Therefore the energy
 levels at the X-valley and the .gamma.-valley are lower than the energy
 level at the .gamma.-valley in the case the Al-content X is equal to or
 larger than a critical composition Xc, where Xc takes a value of about
 0.5. It should be noted that the AlGaAs layer is a direct transition type
 in the case that the Al-content X is smaller than the critical value Xc
 and becomes an indirect transition type when the Al-content X exceeds the
 critical value Xc.
 In the electron supplying layer forming the superlattice structure, it is
 desired that the layer 4a has as high an energy level for the conduction
 band Ec as possible for increased barrier height. For this, it is desired
 that the layer 4a has the Al-content x as large as possible. However in
 the case that the Al-content X of the Al.sub.x Ga.sub.1-x As layer 4a
 exceeds the critical ratio Xc, the conduction band energy level Ec(X, L)
 for any of the X-valley and the L-valley becomes lower than the conduction
 band energy level Ec(.GAMMA.) for the .GAMMA.-valley, and the effective
 barrier height of the AlGaAs layer 4a becomes low. Therefore, about 0.5 is
 the most preferable value for the Al-content X of the Al.sub.x Ga.sub.1-x
 As layer 4a.
 On the other hand, the n.sup.+ -type Al.sub.x Ga.sub.1-x As, which is to be
 a material with a narrower band gap, is also desired to have as high an
 energy level of the conduction band Ec as possible for confirming
 electrons in the two-dimensional electron gas 3 effectively. However, too
 large of the Al-content X causes a problem that the impurity level formed
 therein becomes deep. It should be noted that Si is doped in the layer 5a
 as a donor. Accordingly, about 0.2 is the most preferable value for the
 Al-content X of the n.sup.+ -type Al.sub.x Ga.sub.1-x As layer 5a.
 In addition, the above semiconductor material is not limited to the series
 of GaAs and AlGaAs or the superlattice structure consisting of n.sup.+
 -type Al.sub.0.2 Ga.sub.0.8 As and i-type Al.sub.0.5 Ga.sub.0.5 As as
 disclosed heretofore.
 The present invention is not limited to the specifically disclosed
 embodiment, and variations and modifications may be made without departing
 from the scope of the present invention.
 The present application is based on Japanese priority application
 No.11-76736 filed on Mar. 19, 1999, the entire contents of which are
 hereby incorporated by reference.