Compound semiconductor device

Abstract: A compound semiconductor device includes an undoped semiconductor layer (2); a doped semiconductor layer (3) formed on the undoped semiconductor layer (2) and having smaller electron affinity than the undoped semiconductor layer (2), impurities being doped in the doped semiconductor layer (3); a gate electrode (4) formed on the doped semiconductor layer (3); and a source electrode (6) and a drain electrode (7) respectively formed at both sides of the gate electrode (4), wherein an impurity concentration of the doped semiconductor layer is selected such that a portion of the doped semiconductor layer (3) located immediately below the gate electrode (4) is not completely depleted in a state in which a gate voltage is not applied to the gate electrode (4), and is completely depleted in a state in which a negative voltage for minimizing a noise figure is applied to the gate electrode (4).

Description: BACKGROUND OF THE INVENTION The present invention relates to a compound semiconductor device such as a HEMT (High Electron Mobility Transistor). A HEMT structure is attracting attention as a field-effect transistor making use of a two-dimensional electron gas accumulated in a heterojunction interface. As shown in Fig. 1, this HEMT structure comprises an undoped semiconductor layer 2 disposed on a substrate 1; a doped semiconductor layer 3 whose electron affinity is smaller than that of the undoped semiconductor layer 2 and in which impurities are doped; a gate electrode 4 formed on the doped semiconductor layer 3; and a source electrode 6 and a drain electrode 7 which are respectively formed on a cap layer 5 at both sides of the gate electrode 4. Though the doped semiconductor layer 3 located immediately below the gate electrode is not completely depleted in a state in which the gate voltage is not applied, the two-dimensional electron gas accumulated in the heterojunction interface is directly controlled by the gate voltage which is under 0 volt. However, with the conventional HEMT structure in which the doped semiconductor layer located immediately below the gate electrode is completely depleted in the state in which the gate voltage is not applied, the source resistance is large, and the noise characteristic in a high frequency is deteriorated. To explain this point, first, the source resistance is expressed by the sum of the contact resistance between the source electrode 6 and the cap layer 5 and the sheet resistance of the epitaxial growth layer. As shown in Fig. 1, the sheet resistance of the epitaxial growth layer is further divided into portions (A) and (B), as shown in Fig. 1, and the resistance is particularly large in portion (B). The reason for this is that a plurality of current paths exist in portion (A), as shown at I1 and I2, and its resistance is therefore relatively small, whereas the number of paths is small in portion (B), as shown at I3. An energy band in a state in which the gate voltage is not applied and the doped semiconductor layer located immediately below the gate electrode is completely depleted, is slightly high as a whole for both portion (B) and the portion located immediately below the gate electrode, as shown by the solid line in Fig. 5. As such, practically no electrons remain in the doped semiconductor layer located immediately below the gate electrode and, likewise, practically no electrons remain in the doped semiconductor layer in portion (B). Then, when a negative voltage is applied to the gate electrode, the energy band for portion (B) is not as high as at the portion located immediately below the gate electrode, but rises as shown by the dotted line in Fig. 5. For this reason, the amount of electrons becomes small at portion (B) where the amount thereof is originally small. Accordingly, in a state of use, the electrons at portion (B) are not sufficient, and a resistance value becomes large, so that the noise is correspondingly large. To lower the resistance at portion (B), it suffices to increase the amount of electrons by increasing the impurity concentration of the doped semiconductor layer, but if the concentration is made excessively high, the portion of the doped semiconductor layer located immediately below the gate does not become depleted when the gate voltage is applied. Hence, if the device is operated in that state, it is impossible to obtain a satisfactory characteristic. IEEE Trans. on Microwave Theory and Techniques 36, 7; pp. 1124 to 1140 discloses the basic structure of a HEMT comprising an undoped GaAs-layer, an undoped AlGaAs spacer layer, an n + -AlGaAs layer, an n + -GaAs-layer, a gate electrode as well as source and drain electrodes. Among the plurality of doping levels of the n + -AlGaAs layer which are specified in this document, 5·10 18 at/cm 3 is mentioned. Moreover, Physica 117B and 118B, pp. 756 to 758, investigates the effects of free carrier screening on the electron mobility in a AlGaAs/GaAs heterostructure. As the authors found out, due to a loss of carrier screening the mobility of electrons is reduced when the carrier concentration is reduced. In the heterostructures employed for these investigations, the n + -doped AlGaAs layer is not fully depleted in a state where no gate voltage is applied. In addition, Applied Phys. Letters 22, 25; pp. 1729 to 1731, discloses a δ-doped GaAs field-effect transistor comprising a δ-doped electron channel, a series of 6-doped sheets and a series of six highly doped planes for nonalloyed ohmic contact formation. Furthermore, IEEE Trans. on Electron Devices 29-5, pp. 955 to 960 presents theoretical calculations for a two-dimensional electron gas FET (TEGFET) which is constituted by an AlGaAs/GaAs heterostructure. In Chapter IV the charge control law is established by assuming special boundary conditions for solving the Poisson equation. Moreover, IEEE Trans. on Electron Devices 28-7, pp. 790 to 795 as well presents theoretical calculations for an AlGaAs/GaAs heterostructure. In Chapter V these calculations are compared with real device data. In this chapter a problem of such AlGaAs/GaAs heterostructures, namely the parallel parasitic conductance, and possible solutions of this problem are addressed. SUMMARY OF THE INVENTION The present invention has been made in view of these points, and has an object to provide a compound semiconductor device of a HEMT structure in which the resistance value at the aforementioned portion (B) is reduced, and a noise figure is thereby improved. According to the present invention, the above objects are achieved by a compound semiconductor device comprising: an undoped semiconductor layer; a doped semiconductor layer formed on said undoped semiconductor layer and having a smaller electron affinity than said undoped semiconductor layer, impurities being doped in said doped semiconductor layer; a gate electrode formed on said doped semiconductor layer; and a source electrode and a drain electrode respectively formed at both sides of said gate electrode, wherein a planar doping region is provided in the vicinity of a center of said doped semiconductor layer, said doped semiconductor layer having an impurity concentration from 5 x 10 17 /cm 3 to 2.0 x 10 18 /cm 3 and said planar doping region having an impurity concentration from 5 x 10 11 /cm 2 to 6 x 10 12 /cm 2 such that impurities of said planar doping region located immediately below said gate electrode are not completely ionized in a state in which a gate voltage is not applied to said gate electrode, and are completely ionized in a state in which a negative gate voltage is applied to said gate electrode whereby a noise figure is minimized. In accordance with the present invention, since the impurities of said planar doping region located immediately below the gate electrode are not completely ionized in a state of non-application of a gate voltage, the amount of electrons in portion (B) is sufficiently large. Even if a negative voltage is applied to the gate, the amount of the electrons in portion (B) does not decrease substantially, so that the source-side resistance becomes small. Moreover, since the impurities of said planar doping region located immediately below the gate electrode are completely ionized in the state in which the negative voltage is applied to the gate voltage, the essential operating characteristic of the HEMT is not impaired. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram of the structure of a conventional compound semiconductor device of a HEMT structure; Figs. 2(a) and 2(b) are energy band diagrams of the device of a preliminary example which is not according to the present invention; Fig. 3 is a diagram of the structure of a compound semiconductor device according to the present invention; Fig. 4 is an energy band diagram of the device shown in Fig. 3; and Fig. 5 is an energy band diagram of a conventional device. DETAILED DESCRIPTION OF THE INVENTION In a preliminary example which is not according to the present invention, the structure of the compound semiconductor device itself is shown by a form similar to that of Fig. 1, but this example differs from the prior art in that the impurity concentration of the doped semiconductor layer is selected in such a manner as to satisfy the following conditions (1) and (2). That is, (1) The doped semiconductor layer located immediately below the gate electrode is not completely depleted in the state in which the gate voltage is not applied to the gate electrode. In other words, in the doped semiconductor layer located immediately below the gate electrode, the sum of a depletion layer extending from the gate electrode side and a depletion layer extending from the undoped semiconductor layer side is smaller than the thickness of the doped semiconductor layer. If this is shown in terms of an energy band, this means that, as shown by the solid line in Fig. 2(b), a portion in the vicinity of a Fermi level EF exists in the center of the doped semiconductor layer located immediately below the gate electrode. (2) In a state in which a negative voltage for minimizing the noise figure is applied to the gate electrode, the doped semiconductor layer located immediately below the gate electrode is completely depleted (as shown by dotted line in Fig. 2(b)). If the above-described measure is taken, the amount of electrons is large at portion (B) in the state of non-application of the gate voltage. If the negative voltage is applied to the gate electrode, the energy band immediately below the gate rises on the whole from the solid line (the non-applied state) to the dotted line (the applied state), as shown in Fig. 2(b). However, the effect of the negative voltage applied to the gate is small at portion (B) in the doped semiconductor layer, and the energy band rises only slightly from the solid line (the non-applied state) to the dotted line (the applied state), as shown in Fig. 2(a). Accordingly, since a state of sufficient existence of electrons is held in this portion (B), in the state of use (in the state in which the negative voltage is applied to the gate), the resistance at portion (B) is substantially low as compared with the prior art. A noise figure NF is given by NF = 1 + K(f/fT)√{gm(Rs + Rg)} In this formula, K is a fitting constant; f, a frequency; fT, a cutoff frequency; gm, the mutual conductance; Rs, the source resistance; and Rg, the gate resistance. As can be seen from this formula, the noise is reduced in the HEMT having a small source resistance Rs as in the above-described preliminary example which is not according to the present invention. In the above-described example, for instance, an undoped semiconductor layer 2 is formed of GaAs or InGaAs containing Ga at a ratio of 0.8. A doped semiconductor layer 3 is formed of n + AlGaAs (at a ratio of 0.22 of Al and 0.78 of Ga) doped with silicon having a concentration in the range of 2.5 x 10 18 /cm 3 to 6 x 10 18 /cm 3 as impurities. In addition, a cap layer 5 is constituted by n + GaAs. The undoped semiconductor layer 2, the doped semiconductor layer 3, and the cap layer 5 are formed consecutively on a substrate 1 through epitaxial growth. Next, according to the present invention, a planar doping region 9 is provided in the vicinity of the center of the doped semiconductor layer 3, as shown in Fig. 3. In this planar doping region 9, all the impurities thereof are not ionized in the state in which the voltage is not applied to a gate electrode 4, whereas all of them are ionized when a negative voltage for minimizing the noise is applied to the gate electrode. In the case of the present embodiment thus constructed, the source resistance becomes small for a reason similar to that for the above-described preliminary example, thereby reducing the noise. Fig. 4 shows an energy band for a portion located immediately below the gate electrode shown in Fig. 3. Here, the solid line is the case where the gate voltage is not applied, while the dotted line shows the case when the negative gate voltage is applied. In the embodiment shown in Fig. 3, the planar doping region 9 is a portion of the doped semiconductor layer 3 of AlGaAs (Al: 0.22, Ga: 0.8) doped with Si in the range of 5 x 10 17 /cm 3 to 2.0 x 10 18 /cm 3 , in which Si in the range of 5 x 10 11 /cm 2 to 6 x 10 12 /cm 2 is further doped. The other portions 2, 5 are formed of the same materials as those shown in Fig. 2. As described above, in accordance with the compound semiconductor device of the present invention, advantages are obtained in that the source resistance becomes small, and that the noise figure is improved.

Claims: A compound semiconductor device comprising: an undoped semiconductor layer (2); a doped semiconductor layer (3) formed on said undoped semiconductor layer (2) and having a smaller electron affinity than said undoped semiconductor layer (2), impurities being doped in said doped semiconductor layer (3); a gate electrode (4) formed on said doped semiconductor layer (3); and a source electrode (6) and a drain electrode (7) respectively formed at both sides of said gate electrode (4), characterized in that a planar doping region (9) is provided in the vicinity of a center of said doped semiconductor layer (3), said doped semiconductor layer (3) having an impurity concentration from 5 x 10 17 /cm 3 to 2.0 x 10 18 /cm 3 and said planar doping region (9) having an impurity concentration from 5 x 10 11 /cm 2 to 6 x 10 12 /cm 2 such that impurities of said planar doping region (9) located immediately below said gate electrode (4) are not completely ionized in a state in which a gate voltage is not applied to said gate electrode (4), and are completely ionized in a state in which a negative gate voltage is applied to said gate electrode (4) whereby a noise figure is minimized.

Inventors: NAKAGAWA YOSHIKAZU, NAKAGAWA, YOSHIKAZU, NAKAGAWA, YOSHIKAZU, C/O ROHM CO., LTD.
Applicants: ROHM CO LTD, ROHM CO., LTD.
Application year: 1992
Main classification: H01L29
IPC codes: H01L21, H01L29