Semiconductor device and method for producing same

Abstract: A semiconductor device composed of an E-mode transistor and a D-mode transistor and utilizing a two-dimensional electron gas, comprising: a semi-­ insulating semiconductor substrate (41); a channel layer (42); a carrier-supply layer (43); a threshold voltage adjusting layer (44); a first etching-stoppable layer (45); an ohmic-contactable layer; a second etching-­ stoppable layer (49); and a contact cap layer (50), these layers being epitaxially and successively formed on the substrate (41); source and drain electrodes (60 - 63) formed on the contact cap layer (50); a first gate electrode (66) of the E-mode transistor formed in a first recess (57E); and a second gate electrode (67) of the D-mode transistor formed in a second recess (57D); characterized in that the device further comprises a third etching-stoppable layer (47) which is formed in the ohmic-contactable layer to divide it into an upper portion layer (48) and a lower portion layer (46) thinner than the upper layer (48). In the formation of the first recess (57E), the upper portion layer (48) and the lower portion layer (46) are selectively and isotropically etched, and in the formation of the second recess (57D), the contact cap layer (50) and the upper portion layer (48) are selectively and isotropically etched.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device comprising an enhancement-mode (E-mode) field-effect transistor (FET) and a depletion-mode (D-mode) FET which are operated at a high speed by using a two dimensional electron gas (2 DEG), and to a method of producing the semiconductor device. 2. Description of the Related Art Each of the above-mentioned FETs has a hetero-junction and is well-known as a high electron mobility transistor (HEMT). In general, the above FET's comprise a semi-insulating gallium-arsenide (GaAs) substrate, an undoped GaAs channel layer, an n-type aluminum-gallium-arsenide (AlGaAs) electron-supply layer, and an n-type GaAs cap layer, which layers are successively formed on the substrate. A threshold voltage Vth of the FET depends on the thickness of a semiconductor layer including the n-type AlGaAs electron-supply layer between the undoped GaAs channel layer and a contacting bottom of a gate electrode. This thickness is controlled to a depth of a gate electrode recess formed in the semiconductor layers by an etching process. Furthermore, where an integrated circuit device is composed of HEMTs, DCFL (direct coupled FET logic) circuits are widely used as basic inverter circuits. The inverter circuit has an enhancement/depletion (E/D) constitution consisting of an E-mode HEMT at a drive side and a D-mode HEMT at a load side and in an E/D constitution semiconductor device for the inverter circuit, it is necessary to form the E-mode HEMT having one threshold voltage and the D-mode HEMT having another threshold voltage in the same semiconductor substrate. Current demands for an increase in the speed of computer systems have led to the development of compound semiconductor integrated circuit devices having a very high-speed operation. In order to meet the above demand, it is necessary not only to improve the high-speed operation but also to increase the reliability of these devices, and therefore in the HEMTs constituting the integrated circuit device, the gate length of the HEMT is shortened to enable improve a higher speed operation. Further, it is necessary to provide the following properties: (a) low source resistance Rs; (b) small parasitic capacity Cp; and (c) excellent gate properties including a high gate breakdown voltage. Furthermore, it is necessary to easily and simultaneously form the E-mode and D-mode HEMTs having the above properties on the same semiconductor substrate. A method of producing the E/D constitution semiconductor device in which the E-mode and D-mode HEMTs are simultaneously formed on the same substrate has been proposed, for example, in U.S. Patent No. 4,615,102. According to this U.S. Patent, the semiconductor device is produced in the following manner (refer to Figs. 1 to 8). As illustrated in Fig. 1, on a semi-insulating GaAs substrate 1, an undoped GaAs channel layer (a first semiconductor layer) 2, an n-type AlGaAs electron-supply layer (a second semiconductor layer having an electron affinity smaller than that of the GaAs layer 2) 3, an n-type GaAs threshold voltage adjusting layer for the D-mode HEMT (a third semiconductor layer) 4, an n-type AlGaAs first stoppable layer (a fourth semiconductor layer) 5, an n-type GaAs ohmic-contactable layer (a fifth semiconductor layer) 6, an n-type AlGaAs second etching-stoppable layer (a sixth semiconductor layer) 7, and an n-type GaAs ohmic-contactable layer (a seventh semiconductor layer) 8 are formed (i.e., epitaxially grown) in sequence by a molecular beam epitaxy (MBE) method or a metal organic chemical vapor deposition (MOCVD) method. As illustrated in Fig. 2, to isolate the E-mode HEMT and the D-mode HEMT from each other, the semiconductor layers 8 to 2 and the semiconductor substrate 1 are selectively etched by a wet etching method using a suitable etchant, e.g., hydrofluoric acid (HF), to form a groove 9, an E-mode transistor region "E" island, and a D-mode transistor region "D" island. This groove 9 extends into the substrate 1. In place of the etching treatment an ion-implantation treatment can be adopted, and in this case, oxygen ions or protons are doped into the portion corresponding to the groove 9 to form an insulating region. As illustrated in Fig. 3, portions of the n-type GaAs layer 8 and the n-type AlGaAs layer 7 corresponding to a gate region of the E-mode transistor are selectively etched by a suitable etching method (e.g., a wet etching method using HF solution to form a recess 10 in the E region. A portion of the n-type GaAs layer 6 is exposed in the recess 10 and may be simultaneously etched. As illustrated in Fig. 4, an insulating layer 11 being of, e.g., silicon dioxide (SiO₂), is formed over the whole of the exposed surface by a chemical vapor deposition (CVD) method. The insulating (SiO₂), layer 11 is coated with a resist layer (not shown) having openings and is then selectively etched by e.g., a wet etching method using an HF solution, to form contact openings therein. Leaving the resist layer, a metal film of AuGe/Au, AuGe/Ni/Au, AuGe/Ni, or the like is formed on the resist layer and the exposed portions of the seventh n-type GaAs layer 8 in the openings by a vapor deposition method. The resist layer is then removed by a suitable solvent, whereby a portion of the metal film on the resist layer is also removed. As the result of such a lift-off process, electrode metal portions 12, 13, 14, and 15 remain on the seventh n-type GaAs layer 8, and a heat treatment for alloying is carried out to form ohmic-contactable electrodes 12 to 15 of the source and drain electrodes of the E-mode and D-mode HEMTs. As illustrated in Fig. 5, a resist layer (a masking layer) 16 is coated, exposed, and developed to form openings 17E and 17D for the formation of grooves for gate electrodes of the E-mode and D-mode transistors, respectively. By using the resist layer 16 as a mask, the SiO₂ layer 11 is etched through the openings 17E and 17D by, e.g., a wet etching method using an HF solution, so that openings 18E and 18D are formed in the SiO₂ layer 11. Then a selective dry etching treatment, in this case a reactive ion etching (RIE) treatment using an etchant gas comprising CCl₂F₂ , is performed to anisotropically etch the fifth n-type GaAs layer 6 in the E region through the opening 17E and the seventh n-type GaAs layer 8 in the D region, to form openings 19E and 19D, respectively. The fourth n-type AlGaAs layer 5 in the E region and the sixth n-type AlGaAs layer 7 in the D region serve as etching-stoppable layers. The above-mentioned RIE method using CCl₂F₂ gas can etch GaAs about 200 times faster than AlGaAs. The etching is automatically stopped at the surfaces of the n-type AlGaAs layer 5 and 7. As illustrated in Fig. 6, the exposed portions of the AlGaAs layers 5 and 7 in the openings 17E to 19E and 17D to 19D are etched by, e.g., a wet etching method using an HF solution, to expose the third n-type GaAs layer 4 in the E region and the fifth n-type GaAs layer 6 in the D region. Since this etching treatment is performed for the purpose of removing a AlGaAs layer, and since the thickness of the AlGaAs layers 5 and 7 is thin, it is easy to control the etching of the AlGaAs layers 5 and 7 without the complete removal of the third and fifth GaAs layers 4 and 6. As the result of the etching, openings 20E and 20D are formed in the AlGaAs layers 5 and 7 and portions of the GaAs layer 4 and 6. It is possible to perform the etching treatment by a dry etching method. As illustrated in Fig. 7, a selective dry etching (in this case, RIE) treatment using CCl₂F₂ gas is performed so as to etch the third GaAs layer 4 in the E region through the opening 17E and the fifth GaAs layer 6 in the D region through the opening 17D so that openings 21E and 21D are formed, respectively. The second n-type AlGaAs layer 3 in the E region and the fourth n-type AlGaAs layer 5 in the D region serve as etching-stoppable layers. As the result of the above-mentioned etching treatments, grooves 22E and 22D consisting of the openings 18E to 21E and 18D to 21D, respectively, are completed. As illustrated in Fig. 8, a metal layer for a Schottky barrier of, e.g., aluminum (Al), is formed by, e.g., a vapor deposition method, on the remaining resist layer 16 and in the grooves 22E and 22D. The resist layer 16 is then removed by a suitable solvent, whereby a portion of the metal film on the resist layer 16 is also removed. As the result, metal portions, i.e., gate electrodes 23 and 24 of the E-mode and D-mode HEMTs, respectively are formed. Therefore, a semiconductor device comprising the E-mode and D-mode HEMTs is obtained. According to the above-mentioned production method of the prior art, the etching treatments and the gate electrode formation through the openings 18E and 18D of the insulating layer 11 are performed in a self-alignment system, and it is possible to easily produce a semiconductor device comprising the E-mode and D-mode HEMTs and having accurately controlled threshold voltages Vth. To increase the operation speed of a DCFL circuit composed of the E-mode and D-mode HEMTs shown in Fig. 8, it is necessary to enable the drive-side transistor, i.e., the E-mode HEMT, to operate at a higher speed. As an expedient to this end, the gate length of the E-mode HEMT is shortened to 1 µm or less, i.e., on the order of submicrons. But, in this case, a lowering of the source resistance Rs and the parasitic capacitor Cp become serious problems. In the semiconductor device shown in Fig. 8, the regulation of the thickness of the n-type GaAs ohmic-contact layer 6 may be used to solve these problems. Since the n-type GaAs layer 4 serves to adjust the threshold voltage Vth, the thickness thereof is determined in accordance with the circuit performance conditions of the integrated circuit semiconductor device, and can not be easily changed. Furthermore, the n-type AlGaAs etching stoppable layers 5 and 7 are substantially formed in a thin state, and thus these layers can not be included in the parameters for solving the problems. To lower the source resistance Rs of the E-mode transistor, the thickness of the n-type GaAs layer 6 can be increased, but as apparent from Fig. 8, the increased layer thickness involves an increase of the contact area between the layer 6 and the gate electrodes 23 and 24, with the result that the parasitic capacitance Cp is increased and a leakage current between the electrodes 23 and 24 and the layer 6 is also increased, whereby the gate breakdown voltage of the E-mode and D-mode transistors is lowered. On the other hand, to increase the gate breakdown voltage and to lower the parasitic capacitance Cp of the transistors, preferably the n-type GaAs layer 6 is made thinner. The thinned layer 6, however, brings an increase of the source resistance Rs and a lowering of the transconsconductance gm of the transistors. An improved gate structure for the E/D constitution semiconductor device has been proposed, by which the above-mentioned antinomy is eliminated. This improved gate structure is formed in the following manner, referring to Fig. 9 and 10 in which the reference numerals and symbols used in Fig. 1 to 8 represent the same portions or corresponding portions of the device of Figs. 1 to 8. Figure 9 shows a production stage corresponding to that shown in Fig. 5 of the prior art. In this case, the simultaneous etching step of the n-GaAs layer 6 in the E-mode portion and the n-GaAs layer 8 in the D-mode portion is carried out in an isotropic manner, instead of the anisotropic manner used in Fig. 5. Accordingly, the formed openings 19E and 19D extend in a transverse direction. The isotropic etching can be performed by controlling a gas pressure and a vias voltage in an RIE method using CCl₂F₂ etching gas with maintaining the high selectivity of GaAs to AlGaAs. Figure 10 shows a production stage corresponding to that shown in Fig. 8. The production steps up to this stage are the same as those carried out in Figs. 6 to 8. In the produced semiconductor device having the improved gate structure, the gate electrodes 23 and 24 do not come into contact with the n-type GaAs layers 6 and 8, respectively, as shown in Fig. 10. Therefore, the obtained semiconductor device exhibits a lower parasitic capacitance Cp and a smaller amount of leakage current to thereby improve the gate breakdown voltage, compared with the semiconductor device explained in connection with Fig. 1 to 8. Furthermore, since the gate electrode 23 of the E-mode region does not come into contact with the n-type GaAs layer 6, an increase of the thickness of the layer 6 for reducing the source resistance Rs does not cause an increase of the parasitic capacitance of the E-mode transistor. Nevertheless, the increase of the thickness of the n-type GaAs layer 6 still causes an increase of the contact area between the gate electrode 24 and the layer 6 in the D-mode region, which increases the gate leakage current and reduces the gate breakdown voltage of the D-mode transistor, as mentioned above. Therefore, the deterioration of the gate properties of the D-mode transistor prevents the formation of Schottky diodes between the gate electrode 24 and the source and drain electrodes 14 and 15, hinders the designing of an integrated circuit, and lowers the reliability of the semiconductor device. To solve the above mentioned problem in the D-mode transistor, a simultaneous etching of the n-type GaAs layer 4 in the E-mode region and the n-type GaAs layer 6 in the D-mode region has been considered for forming the openings 21E and 21D shown in Figure 7 in the isotropic manner instead of the anisotropic manner, so that the openings 21E and 21D extend in a transverse direction, as shown in Figure 11. In this case, the gate electrode 24 is formed to not come into contact with the n-type GaAs layer, but a surface depletion layer of the E-mode transistor reaches the undoped GaAs channel layer 2 to hinder the operation of the E-mode transistor. Therefore, the last etching step for a formation of recesses (grooves) for the gate electrodes should be carried out anisotropically to bring a portion of the electrode 23 into contact with the layer 4. In the above mentioned explanation the E-mode and D-mode transistors are HEMTs utilising a two dimensional electron gas, but it is possible to use transistors utilising a two dimensional hole gas instead of the electron gas. A "two dimensional carrier gas" includes the two dimensional electron gas and two dimensional hole gas. DE-A-3,706,274 discloses another method for overcoming the problems described above. According to this disclosure, the gate electrode of a FET is formed in a recess, and is separated from the side walls of the recess by an insulating layer. In this way the device's parasitic capacitance is lowered, and its breakdown voltage increased. An object of the present invention is to provide an E/D constitution semiconductor device utilising a two dimensional carrier gas, an enhancement mode transistor which has a small source resistance Rs, a small parasitic capacitance Cp, and a high breakdown voltage, and a depletion mode transistor which also has a high breakdown voltage. Another object of the present invention is to provide a method of producing the above mentioned improved E/D constitution semiconductor device. Another object of the present invention is to provide the E/D constitution semiconductor device composed of E-mode and D-mode transistors having gate lengths of 1 µm or less. According to a first aspect of the present invention, a semiconductor device comprises an enhancement mode transistor and a depletion mode transistor formed on the same substrate and isolated from each other, the device utilising a two dimensional carrier gas, the semiconductor device including: a semi-insulating semiconductor substrate; a first semiconductor layer which is a channel layer, in which the two dimensional carrier gas is generated; a second semiconductor layer which forms a heterojunction with the first semiconductor layer; a third semiconductor layer which is a threshold voltage adjusting layer for the depletion mode transistor and which has an etch rate larger than that of the second semiconductor layer for a predetermined etchant; a fourth semiconductor layer which has a first etching stoppable layer; a fifth semiconductor layer which is an ohmic-contactable layer and has an etch rate larger than that of the fourth semiconductor layer for the pre-determined etchant; a sixth semiconductor layer which is a second etching stoppable layer; a seventh semiconductor layer which is a contact cap layer and has an etch rate larger than that of the sixth semiconductor layer for the predetermined etchant, the first to seventh semiconductor layers being successively formed on the substrate; source and drain electrodes partially lying on the seventh semiconductor layer; a first gate electrode for the enhancement mode transistor formed on a first recess extending to the second semiconductor layer; a second gate electrode for the depletion mode transistor formed in a second recess extending to the fourth semiconductor layer; characterised in that the semiconductor device further comprises an eighth semiconductor layer which is a third etching stoppable layer having an etch rate smaller than that of the fifth semiconductor layer for the predetermined etchant, and is formed in the fifth semiconductor layer to divide the layer into an upper portion layer and a lower portion layer thinner than the upper portion layer, wherein the first gate electrode is in contact with the third semiconductor layer, and does not come into contact with any of the fourth to seventh semiconductor layers, and the second gate electrode is in contact with the lower portion layer and does not come into contact with the eighth semiconductor layer, the upper portion layer, or the sixth or seventh semiconductor layers. According to a second aspect of the present invention, a method of producing a semiconductor device comprising an enhancement mode transistor and a depletion mode transistor formed on the same substrate and isolated from each other, the device utilising a two dimensional carrier gas, the method including the steps of: epitaxially and successively forming, on a semi-insulating semiconductor substrate; (a) a first semi-conductor layer which is a channel layer, in which the two dimensional carrier gas is generated; (b) a second semi-conductor layer which forms a heterojunction with the first semiconductor layer, and which serves as a carrier supply layer; (c) a third semiconductor layer which is a threshold voltage adjusting layer for the depletion mode transistor and has an etch rate larger than that of the second semiconductor layer for a pre-determined etchant; (d) a fourth semiconductor layer which is a first etching stoppable layer; (e) a lower portion layer of a fifth semi-conductor layer having an etch rate which is larger than that of the fourth semiconductor layer for the predetermined etchant; (f) an eighth semiconductor layer which is a third etching stoppable layer; (g) an upper portion layer of said fifth semiconductor layer which is thicker than the lower portion layer having an etch rate larger than that of said eighth semiconductor layer for the predetermined etchant, said fifth semiconductor layer being an ohmic-contactable layer; (h) a sixth semiconductor layer which is a second etching stoppable layer; and, (i) a seventh semiconductor layer which is a contact cap layer and has an etch rate larger than that of the sixth semiconductor layer for the predetermined etchant; selectively etching the seventh and sixth semiconductor layers at a first area corresponding to a gate electrode of the enhancement mode transistor; coating the surface with a masking layer and forming openings in said masking layers at said first area and at a second area which corresponds to a gate electrode of the enhancement-mode and depletion-mode transistors respectively; isotropically etching said upper portion layer of the fifth semiconductor layer to form a first opening at said first area and the seventh semiconductor layer to form a second opening at said second area, said eighth semiconductor layer at said first area and said sixth semiconductor layer at said second area serving as etching stoppers; etching said eighth semiconductor layer at said first area and said sixth semiconductor layer at said second area, thereby extending said first and second openings; isotropically etching said lower portion layer of said fifth semiconductor layer to extend the first opening at said first area and said upper portion layer of said fifth semiconductor layer to extend the second opening at said second area, the fourth semiconductor layer at said first area and the eighth semiconductor layer at said second area serving as etching stoppers; etching said fourth semiconductor layer at said first area and said eighth semiconductor layer at said second area; anisotropically etching said third semiconductor layer to form a first recess at the first area and said lower portion layer of said fifth semiconductor layer to form a second recess at the second area, the second semiconductor layer at said first area and the fourth semiconductor layer at the second area serving as etching stoppers; and simultaneously forming the first gate electrode within the first recess on said second semiconductor layer connected to the third semiconductor layer but not coming into contact with the fifth semiconductor layer and the second gate electrode within the second recess on said fourth semiconductor layer, in contact with said lower portion layer of said fifth semiconductor layer but not coming into contact with said upper portion layer of said fifth semiconductor layer. In the present invention, a gap is formed between the gate electrode of the E-mode transistor and the lower portion layer and the upper portion layer of the fifth semiconductor layer, whereby a parasitic capacitance Cp is reduced and the gate breakdown voltage is increased. Furthermore, the upper portion layer is formed to a thickness large enough to reduce the source resistance Rs of the E-mode transistor. Since another gap is formed between the gate electrode of the D-mode transistor and the upper portion layer of the fifth semiconductor layer, the formation of the lower portion layer having a required small thickness increases the gate breakdown voltage of the D-mode transistor. Namely, the lowering of the source resistance Rs of the E-mode transistor and the increase of the gate breakdown voltage are simultaneously obtained by dividing the fifth semiconductor layer into the thin lower portion layer and the thick upper portion layer. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more apparent from the description of the preferred embodiments set forth below, with reference to the accompanying drawings, in which: Figs. 1 through 8 are schematic sectional views of an E/D constitution semiconductor device in various stages of production in accordance with a prior art technique; Figs. 9 and 10 are schematic sectional views of an E/D constitution semiconductor device produced in accordance with a proposed technique; Fig. 11 is a schematic sectional view of an E/D constitution semiconductor device having another gate structure; and Figs. 12 through 22 are schematic sectional views of an E/D constitution semiconductor device in various stages of production in accordance with a method of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figs 12 through 22, a method of producing an E/D constitution semiconductor device in accordance with a preferred embodiment of the present invention is now explained. As illustrated in Fig. 12, on a semi-insulating substrate 41, an undoped GaAs channel layer (a first semiconductor layer) 42, an n-type AlGaAs electron-supply layer (a second semiconductor layer) 43, an n-type GaAs threshold voltage adjusting layer (a third semiconductor) 44 for a D-mode transistor, an n-type AlGaAs first etching-stoppable layer (a fourth semiconductor layer) 45, an n-type GaAs lower portion layer 46, an n-type AlGaAs third etching-stoppable layer (an eighth semiconductor layer) 47, an n-type GaAs upper portion layer 48, an n-type AlGaAs second etching-stoppable layer (a sixth semiconductor layer) 49 and an n-type GaAs contact cap layer (a seventh semiconductor layer) 50 are formed (i.e., epitaxially grown) in sequence by an appropriate method, such as an MBE method and an MOCVD method. The lower portion layer 46 and upper portion layer 48 and eighth semiconductor layer 47 correspond to a fifth semiconductor layer of an n-type GaAs ohmic-contactable layer. Preferably, the thickness of the lower portion layer 46 is thinner than a half, particularly one fourth, of that of the upper portion layer 48. The thinner the lower portion layer, the smaller a gate leakage current of the D-mode transistor, with the result that the gate breakdown voltage is improved (increased). For example, each of the grown semiconductor layers has the following thickness and donor concentration: (a) Undoped GaAs layer 42: Thickness: 500 nm (b) n-type AlGaAs layer 43: Thickness: 30 nm Donor concentration: 2 x 10¹⁸ cm⁻³ (c) n-type GaAs layer 44: Thickness: 10 nm Donor concentration: 2 x 10¹⁸ cm⁻³ (d) n-type AlGaAs layer 45: Thickness: 5 nm Donor concentration: 2 x 10¹⁸ cm⁻³ (e) n-type GaAs layer 46: Thickness: 15 nm Donor concentration: 2 x 10¹⁸ cm⁻³ (f) n-type AlGaAs layer 47: Thickness: 5 nm Donor concentration: 2 x 10¹⁸ cm⁻³ (g) n-type GaAs layer 48: Thickness: 60nm Donor concentration: 2 x 10¹⁸ cm⁻³ (h) n-type AlGaAs layer 49: Thickness: 5 nm Donor concentration: 2 x 10¹⁸ cm⁻³ (i) n-type GaAs layer 50: Thickness 40 nm Donor concentration: 2 x 10¹⁸ cm⁻³ As illustrated in Fig. 13, to isolate the E-mode FET and the D-mode FET from each other, the semiconductor layers 50 to 42 and the semiconductor substrate 41 are selectively etched by a wet etching method using a suitable etchant, e.g., hydrofluoric acid (HF), to form a groove 51, an E-mode FET region "E" island, and a D-mode FET region "D" island. This groove 51 extends into the substrate 41. In place of the etching treatment an ion-implantation treatment can be adopted. In this case, oxygen ions or protons are doped into the portion corresponding to the groove 51 to form an isolating region. As illustrated in Fig. 14, portions of the n-type GaAs layer 50 and the n-type AlGaAs layer 49 corresponding to a gate region of the E-mode FET are etched by a suitable etching method (e.g., a wet etching method using HF solution) to form a recess 52 in the E region. A portion of the n-type GaAs upper portion layer 48 in the recess 52 is exposed and may be simultaneously etched. Instead of the above-mentioned etching step, the recess 52 may be formed by a dry etching method, e.g., an RIE method using an etching gas comprising CCl₂F₂ , to selectively etch the n-type GaAs layer 50, and then a wet etching method using an HF solution to etch the n-type AlGaAs (second etching-stoppable) layer 49. As illustrated in Fig. 15, an insulating layer 53 being of, e.g., silicon dioxide (SiO₂), is formed over the whole of the exposed surface by a chemical vapor deposition (CVD) method. The SiO₂ layer 53 has a thickness of, e.g., about 300 nm. The insulating (SiO₂) layer 53 is coated with a resist layer (not shown) having openings and is then selectively etched by, e.g., a wet etching method using an HF solution, to form contact openings therein. Leaving the resist layer, a metal film of AuGe/Au, AuGe/Ni/Au, AuGe/Ni, or the like is formed on the resist layer and the exposed portions of the seventh n-type GaAs layer 50 in the openings by a vapor deposition method. For example, the AuGe/Au film consists of an AuGe layer having a thickness of about 30 nm and an Au layer having a thickness of 270 nm. The resist layer is then removed by a suitable solvent, whereby a portion of the metal film on the resist layer is also removed. As the result of such a lift-off process, electrode metal portions 60, 61, 62, and 63 remain on the seventh n-type GaAs layer 50. A heat treatment for alloying (e.g., at about 450°C for 1 minute), is carried out to form ohmic-contact electrodes 60 to 63 of the source and drain electrodes of the E-mode and D-mode FET's. As illustrated in Fig. 16, a resist layer (a masking layer) 64 is coated, exposed, and developed to form openings 54E and 54D for the formation of grooves for gate electrodes of the E-mode and D-mode FET's, respectively. As illustrated in Fig. 17, by using the resist layer 64 as a mask, the SiO₂ layer 53 is etched through the openings 54E and 54D by, e.g., a wet etching method using an HF solution, so that openings 55E and 55D are formed in the SiO₂ layer 53. The etching of SiO₂ may be performed by a RIE method using an etching gas, such as CF₄ , CHF₃ , C₂F₆ and C₃F₈. Then a selective dry etching treatment, in this case a reactive ion etching (RIE) treatment using an etchant gas comprising CCl₂F₂ , is performed at a pressure of 20 Pa and a bias voltage of 30V, so as to isotropically etch the n-type GaAs upper portion layer 48 in the E region through the opening 54E and the seventh n-type GaAs layer 50 in the D region through the opening 54D, to form openings 56E and 56D, respectively. The eighth n-type AlGaAs layer 47 in the E region and the sixth n-type AlGaAs layer 49 in the D region serve as etching-stoppable layers. If the RIE method using CCl₂F₂ gas is performed under the conditions of a pressure of 5 Pa and a bias voltage of 100 V, the GaAs can be anisotropically etched. The formation of the openings 56E and 56D can be performed by the anisotropical etching process. As illustrated in Fig. 18, the exposed portions of the AlGaAs layers 47 and 49 in the openings 54E to 56E and 54D to 56D are etched by, e.g., a wet etching method using an HF solution, to expose the n-type GaAs lower portion layer 46 in the E region and the n-type GaAs upper portion layer 48 in the D region. Since this etching treatment is performed for the purpose of removing a AlGaAs layer, and since the thickness of the AlGaAs layers 47 and 49 is very thin (5 mm), it is easy to control the etching of the AlGaAs layers 47 and 49 without the complete removal of the lower and upper GaAs layers 46 and 48. As a result of the etching, openings 56E and 56D extend through the AlGaAs layers 47 and 49 and into portions of the GaAs layer 46 and 48. It is possible to perform the etching treatment by a dry etching method using an etchant gas, e.g., Ar, He, H₂, or BCl₃ instead of the wet etching methods. Also, it is possible to use a wet etching of a mixture of I₂ and KI. This mixture can etch AlGaAs about 30 - 40 times faster than it etches GaAs. As illustrated in Fig. 19, by using the resist layer 64 as a mask, the exposed portions of the n-type GaAs lower portion layer 46 in the E region and the n-type GaAs upper portion layer 48 are isotropically etched by an RIE method using a CCl₂F₂ etching gas at a pressure of 20 Pa and at a bias voltage of 30 V, and thus the openings 56E and 56D are further extended, the fourth AlGaAs layer 45 in the E region and the eighth AlGaAs layer 47 in the D region serving as etching-stoppable layers. As illustrated in Fig. 20, the AlGaAs (first and third etching-stoppable) layers 45 and 47 are selectively etched by a wet etching method using an HF solution to expose the third n-type GaAs layer 44 in the E region and the n-type GaAs lower portion layer 46 in the D region, and thus the openings 56E and 56D are further extended. A suitable dry etching method may be adopted instead of the wet etching method. As illustrated in Fig. 21, a selective dry etching (in this case, RIE) treatment using CCl₂F₂ gas is performed at a pressure of 5 Pa and a bias voltage of 100 V, to anisotropically etch the third GaAs layer 44 in the E region through the opening 56E and n-type GaAs lower portion layer 46 in the D region through the opening 56D, respectively. The second n-type AlGaAs layer 43 in the E region and the fourth n-type AlGaAs layer 45 in the D region serve as etching-stoppable layers. The openings 56E and 56D are extended to complete recesses (grooves) 57E and 57D defining the distances (semiconductor layer thickness) between the undoped GaAs channel layer surface and the bottom of the recesses, which distances obtain the predetermined threshold voltages of the E-mode and D-mode transistors, respectively. If it is necessary to minutely control the threshold voltage Vth, the exposed second and fourth AlGaAs layers 43 and 45 are slightly etched by a suitable etching method. Even if the fourth AlGaAs layer (first etching-stoppable layer) 45 in the D region is removed to expose the third GaAs (threshold voltage adjusting layer) 44, this causes no problem in the operation of the D-mode transistor. As illustrated in Fig. 22, a metal layer for a Schottky barrier of, e.g., aluminum (Al), is formed by, e.g., a vapor deposition method, on the remaining resist layer 64 and in the recesses 57E and 57D. The thickness of the metal layer is, e.g., about 400 nm. The resist layer 64 is then removed by a suitable solvent, whereby a portion of the metal film on the resist layer 64 is also removed. As a result, metal portions, i.e., gate electrodes 66 and 67 of the E-mode and D-mode transistors, respectively, are formed, and thus a semiconductor device comprising the E-mode and D-mode HEMTs is obtained. Each of the E-mode and D-mode transistors of the obtained E/D constitution semiconductor device has an accurately controlled threshold voltage Vth, high gate breakdown voltage, and low source resistance Rs. The gate electrode 66 of the E-mode transistor is formed such that the side of the electrode 66 does not come into contact with the GaAs layers 46 and 48 corresponding to the fifth ohmic-contactable layer of a conventional E-mode transistor but comes into contact with the threshold voltage adjusting GaAs layer 44, and simultaneously, the gate electrode 67 of the D-mode transistor is formed such that the side of the electrode 67 does not come into contact with the upper portion layer (GaAs layer) 48 but comes into contact with the lower portion layer (GaAs layer) 46 thinner than the layer 48. Therefore, when the gate length of each of the E-mode and D-mode transistors is shortened to 1 µm or less, to increase the operating speed, the source resistance Rs of the E-mode transistor is reduced by increasing the total thickness of the GaAs layers 46 and 48 without increase of the parasitic capacitor and without an extension of a surface depletion layer to the channel layer 42, and simultaneously, the gate leakage current of the D-mode transistor is reduced to increase the gate breakdown voltage. Namely, the gate breakdown voltage of the D-mode transistor can be improved without a deterioration of the properties of the E-mode transistor. Furthermore, the E-mode and D-mode transistors are easily, simultaneously and self-aligningly formed on the same substrate in a manner similar to that used in the conventional case. The conductivity type of the grown semiconductor layers 44 to 50 can be changed from n-type to p-type. In this case, a two dimensional hole gas is generated in the channel layer 42. If the electron affinity of the second (electron-supply) layer 43 is smaller than that of the first (channel) layer 42 and the lattice constant of the electron-supply layer 43 is equal or similar to that of the channel layer 42, germanium (Ge), indium-antimonide (InSb), or indium-arsenide (InAs) may be used for the channel layer 42 and AlGaAs, GaAs, cadmium-telluride (CdTe), or gallium-antimonide (GaSb) may be used for the electron supply layer 43. It is possible to use combinations of Ge (first layer) and AlGaAs (second layer), Ge and GaAs, InSb and CdTe, and InAs and GaSb in addition to the combination of GaAs and AlGaAs. The layers 44, 46, 48 and 50 may be made of Ge, InSb, or InAs and the etching stoppable layers 45, 47 and 49 may be made of AlGaAs, CdTe, or GaSb. It is possible to adopt a suitable etchant in accordance with compound semiconductor materials used for a semiconductor device according to the present invention.

Claims: A semiconductor device comprising an enhancement mode transistor and a depletion mode transistor formed on the same substrate and isolated from each other, the device utilising a two dimensional carrier gas, the semiconductor device including: a semi-insulating semiconductor substrate (41); a first semiconductor layer (42) which is a channel layer, in which the two dimensional carrier gas is generated; a second semiconductor layer (43) which forms a heterojunction with the first semiconductor layer (42); a third semiconductor layer (44) which is a threshold voltage adjusting layer for the depletion mode transistor and which has an etch rate larger than that of the second semiconductor layer (43) for a predetermined etchant; a fourth semiconductor layer (45) which has a first etching stoppable layer; a fifth semiconductor layer (46, 48) which is an ohmic-contactable layer and has an etch rate larger than that of the fourth semiconductor layer (45) for the predetermined etchant; a sixth semiconductor layer (49) which is a second etching stoppable layer; a seventh semiconductor layer (50) which is a contact cap layer and has an etch rate larger than that of the sixth semiconductor layer (49) for the predetermined etchant, the first to seventh semiconductor layers being successively formed on the substrate (41); source and drain electrodes (60 to 63) partially lying on the seventh semiconductor layer (50); a first gate electrode (66) for the enhancement mode transistor formed on a first recess (57E) extending to the second semiconductor layer (43); a second gate electrode (67) for the depletion mode transistor formed in a second recess (57D) extending to the fourth semiconductor layer (45); characterised in that the semiconductor device further comprises an eighth semiconductor layer (47) which is a third etching stoppable layer having an etch rate smaller than that of the fifth semiconductor layer for the predetermined etchant, and is formed in the fifth semiconductor layer to divide the layer into an upper portion layer (48) and a lower portion layer (46) thinner than the upper portion layer (48), wherein the first gate electrode (66) is in contact with the third semiconductor layer, and does not come into contact with any of the fourth to seventh semiconductor layers (45 to 50), and the second gate electrode (67) is in contact with the lower portion layer (46) and does not come into contact with the eighth semiconductor layer (47), the upper portion layer (48), or the sixth or seventh semiconductor layers (49, 50). A semiconductor device according to claim 1, wherein each of the first and second gate electrodes (66, 67) has a gate length of 1 µm or less. A semiconductor device according to claim 1 or 2, wherein the thickness of the lower portion layer (46) is smaller than a half of a thickness of the upper portion layer (48). A semiconductor device according to claim 3, wherein the thickness of the lower portion layer (46) is smaller than one fourth of a thickness of the upper portion layer. A semiconductor device according to any preceding claim, wherein the two dimensional carrier gas is a two dimensional electron gas. A semiconductor device according to claim 5, wherein the transistors are high electron mobility transistors. A semiconductor device according to any preceding claim, wherein the first, third, fifth and seventh semiconductor layers are of GaAs and the second, fourth, sixth and eighth semiconductor layers are of AlGaAs. A method of producing a semiconductor device comprising an enhancement mode transistor and a depletion mode transistor formed on the same substrate and isolated from each other, the device utilising a two dimensional carrier gas, the method including the steps of: epitaxially and successively forming, on a semi-insulating semiconductor substrate (41); (a) a first semi-conductor layer (42) which is a channel layer, in which the two dimensional carrier gas is generated; (b) a second semi-conductor layer (43) which forms a heterojunction with the first semiconductor layer (42), and which serves as a carrier supply layer; (c) a third semiconductor layer (44) which is a threshold voltage adjusting layer for the depletion mode transistor and has an etch rate larger than that of the second semiconductor layer (43) for a pre-determined etchant; (d) a fourth semiconductor layer (45) which is a first etching stoppable layer; (e) a lower portion layer (46) of a fifth semiconductor layer (46,48) having an etch rate which is larger than that of the fourth semiconductor layer (45) for the predetermined etchant; (f) an eighth semiconductor layer (47) which is a third etching stoppable layer; (g) an upper portion layer (48) of said fifth semiconductor layer which is thicker than the lower portion layer (46) having an etch rate larger than that of said eighth semiconductor layer (47) for the predetermined etchant, said fifth semiconductor layer (46,48) being an ohmic-contactable layer; (h) a sixth semiconductor layer (49) which is a second etching stoppable layer; and, (i) a seventh semiconductor layer (50) which is a contact cap layer and has an etch rate larger than that of the sixth semiconductor layer (49) for the predetermined etchant; selectively etching the seventh and sixth semiconductor layers (50,49) at a first area corresponding to a gate electrode of the enhancement mode transistor; coating the surface with a masking layer (64) and forming openings (54E,54D) in said masking layers (64) at said first area and at a second area which corresponds to a gate electrode of the enhancement-mode and depletion-mode transistors respectively; isotropically etching said upper portion layer (48) of the fifth semiconductor layer (46,48) to form a first opening (56E) at said first area and the seventh semiconductor layer (50) to form a second opening (56D) at said second area, said eighth semiconductor layer (47) at said first area and said sixth semiconductor layer (49) at said second area serving as etching stoppers; etching said eighth semiconductor layer (47) at said first area and said sixth semiconductor layer (49) at said second area, thereby extending said first and second openings (56E,56D); isotropically etching said lower portion layer (46) of said fifth semiconductor layer (46,48) to extend the first opening (56E) at said first area and said upper portion layer (48) of said fifth semiconductor layer (46,48) to extend the second opening (56D) at said second area, the fourth semiconductor layer (45) at said first area and the eighth semiconductor layer (47) at said second area serving as etching stoppers; etching said fourth semiconductor layer at said first area and said eighth semiconductor layer at said second area; anisotropically etching said third semiconductor layer (44) to form a first recess (57E) at the first area and said lower portion layer (46) of said fifth semiconductor layer (46,48) to form a second recess (57D) at the second area, the second semiconductor layer (43) at said first area and the fourth semiconductor layer (45) at the second area serving as etching stoppers; and simultaneously forming the first gate electrode (66) within the first recess (57E) on said second semiconductor layer (43) connected to the third semiconductor layer (44) but not coming into contact with the fifth semiconductor layer (46,48) and the second gate electrode (67) within the second recess (57D) on said fourth semiconductor layer (45), in contact with said lower portion layer (46) of said fifth semiconductor layer (46,48) but not coming into contact with said upper portion layer (48) of said fifth semiconductor layer (46,48). A method according to claim 8, wherein each of the first and second gate electrodes (66, 67) is formed by depositing metal to make a gate length of 1 µm or less. A method according to claim 8 or 9, wherein the lower portion layer (46) has a thickness smaller than a half of a thickness of the upper portion layer (48). A method according to claim 10, wherein the lower portion layer (46) has a thickness smaller than one fourth of a thickness of the upper portion layer (48). A method according to claim 8, 9, 10 or 11, wherein the third semiconductor layer (44) is provided with an n-type conductivity, so that the two dimensional carrier gas is a two dimensional electron gas. A method according to claim 12, wherein the transistors are high electron mobility transistors. A method according to any of claims 8 to 13, wherein the first, third and seventh semiconductor layers and the lower and upper portion layers are made of GaAs and the second, fourth, sixth and eighth semiconductor layers are made of AlGaAs. A method according to any of claims 8 to 14, wherein the second and fourth semiconductor layers are slightly etched to adjust the threshold voltages of the transistors.

Inventors: SUEHIRO HARUYOSHI, SUEHIRO, HARUYOSHI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: H01L27
IPC codes: H01L29, H01L27, H01L21