Semiconductor device having level shift diode

Abstract: A semiconductor device comprises a substrate (31-33), at least one field effect transistor (Q3, Q4, Q11, Q12, Q13, Q22) provided on the substrate, and at least one level shift diode (D1-D4) provided on the substrate, where the level shift diode comprises a first layer (12, 34B, 54b, 62a, 62c, 74b, 82a, 82c) made of a first compound semiconductor and a second layer (13, 34C, 61a, 61c, 63a, 63c, 81a, 81c, 83a, 83c) made of a second compound semiconductor having an electron affinity smaller than that of the first compound semiconductor. The first and second layers form a heterojunction therebetween. The first and second compound semiconductors are both made of either one of n-type and i-type (or either one of p-type and i-type) semiconductors, and the first layer is used in common with the field effect transistor as a layer of the field effect transistor.

Description: The present invention generally relates to semiconductor devices having level shift diodes with an arbitrary level shift quantity by utilizing a discontinuity in the energy band at a heterojunction between two semiconductors. Conventionally, in a semiconductor device based on a silicon layer, a level shift diode is constructed by utilizing the fact that a forward voltage drop is approximately constant at a pn junction. The level shift diode is used to shift a voltage level by a predetermined value in a logic circuit and the like employing transistors. When a voltage drop introduced by one pn junction is insufficient, a plurality of level shift diodes are connected in series according to the needs. In a semiconductor device based on a gallium arsenide (GaAs) layer, a level shift quantity of a level shift diode utilizing a Schottky junction is a constant value which is approximately in a range of 0.6 V to 0.8 V per Schottky junction, because the Schottky barrier height is in the range of 0.6 V to 0.8 V regardless of the kind of metal formed on the GaAs layer. Hence, even when a plurality of Schottky junctions are utilized, it is possible to obtain only a level shift quantity which is an integral multiple of the constant value. When making a Schottky diode, the Schottky junction is made by forming a metal layer on the GaAs substrate. But the surface of the GaAs substrate is exposed to the air when forming the metal layer thereon. The surface of the GaAs substrate includes irregularities when compared to that of the silicon substrate, and for this reason, it is difficult to accurately control the level shift quantity of the Schottky diode because the Schottky diode is easily affected by a leakage current. In addition, since it is necessary to utilize a plurality of Schottky junctions in order to obtain a relatively large level shift quantity, a large area of the semiconductor device is occupied by the Schottky diodes for providing the desired level shift. Therefore, there is a demand for a semiconductor device which has level shift diodes with arbitrary level shift quantities, and in which the level shift diodes only occupy a small area on the semiconductor device. Needless to say, it is essential that the level shift quantity of each level shift diode on the semiconductor device is accurately controlled to a desired value. Accordingly it is desirable to provide a novel and useful semiconductor device having level shift diodes, in which the problems described before are eliminated and the demands are satisfied. "MONTE CARLO SIMULATION OF GaAs SUBMICRON n⁺-n-n⁺ DIODE WITH GaAlAs HETEROJUNCTION CATHODE" by K. Tomizawa, Y.Awano and N. Hashizume, ELECTRONIC LETTERS, 9th December 1982, Vol. 18, No. 25/26, shows a current density versus anode voltage characteristics of a GaAs diode having a Al x Ga 1-x As heterojunction cathode for different Al content x. IEEE Electron Device Letters, Vol. EDL-7, No. 3, March 1986, pp 145-149, discloses a semiconductor device according to the preamble of accompanying claims 1 and 4. In this device, the diode is a Schottky diode in series with the gate of the FET. US-A-4 353 081 discloses a semiconductor diode having a GaAs/AlGaAs heterojunction, employing a graded composition to provide control over the shape of the potential barrier. A series arrangement of such diodes is also disclosed, in accordance with the preamble of accompanying claim 1. According to the present invention, there is provided a semiconductor device comprising a substrate and at least two level shift diodes provided on said substrate, said diodes having a stacked structure comprising:- a first layer of a first compound semiconductor having a first electron affinity; a second layer of a second compound semiconductor formed on said first layer and having a second electron affinity less than said first electron affinity, forming a first heterojunction between said first and second layers, said first and second compound semiconductors being any arbitrary combination of n-type and i-type semiconductors; a third layer comprising said first compound semiconductor with the first electron affinity and formed on said second layer; at least one fourth layer of similar composition to said second layer, formed on said third layer; a first electrode formed on said first layer; and a second electrode formed on said third layer; characterised in that:- at least one field effect transistor is provided on said substrate whereby at least a portion of said first layer (74a) is used in common with said field effect transistor as a layer of said field effect transistor and as a layer of said diodes, and said diodes are level shift diodes in which said third layer forms a second heterojunction with said second layer; and said second to fourth layers are formed selectively at a plurality of mutually-isolated locations on said substrate, whereby at a first location a first of said level shift diodes is formed which comprises said second layer and said third layer and at a second location a second of said level shift diodes is formed which comprises said second layer, said third layer and said fourth layer, said level shift diodes having mutually opposite bias directions. It is possible to arbitrarily set the level shift quantity by stacking a plurality of heteroiunctions and by appropriately changing the composition of the compound semiconductor heterojunction forming layer constituting the heterojunction which generates a discontinuity in the energy band. In addition, the area of the level shift diodes on the semiconductor device is small compared to that of a Schottky diode. Moreover, it is possible to accurately control the level shift quantities of the level shift diodes. Reference is made, by way of example, to the accompanying drawings, in which:- Figs. 1A and 1C are cross-sectional views each showing a level shift diode which is useful for explaining the semiconductor device according to the present invention; Figs. 1B and 1D are energy band diagrams of the level shift diode shown in Figs. 1A and 1C, respectively; Figs. 2A through 2C are energy band diagrams for explaining the operation of the level shift diode shown in Fig. 1A; Fig. 2D is an energy band diagram for explaining the operation of the level shift diode shown in Fig. 1C; Fig. 3 shows an anode voltage versus anode current characteristic of the level shift diode shown in Fig. 1A; Fig. 4A is a cross-sectional view showing another level shift diode; Figs. 4B and 4C are energy band diagrams for explaining the operation of the level shift diode shown in Fig. 4A; Fig. 4D is a cross-sectional view showing another level shift diode; Fig. 4E is an energy band diagram of the level shift diode shown in Fig. 4D; Fig. 4F is a graph of aluminum composition of the level shift diode shown in Fig. 4D; Fig. 5 shows an anode voltage versus anode current characteristic of the level shift diode shown in Fig. 4A; Fig. 6 is a circuit diagram showing a semiconductor device useful for explaining the present invention; Fig. 7 is a cross-sectional view showing an essential part of the device of Fig. 6; Fig. 8 is a cross-sectional view on an enlarged scale showing an essential part of a level shift diode used in the device of Fig. 6; Fig. 9 is a cross-sectional view showing an essential part of a modification of the Fig. 6 device; Fig. 10 is a circuit diagram showing another semiconductor device useful for explaining the present invention; Fig. 11 is a cross-sectional view showing an essential part of the device of Fig. 10; Fig. 12 is a cross-sectional view on an enlarged scale showing a level shift diode region in the device of Fig. 10; Fig. 13 is a circuit diagram showing an embodiment of the semiconductor device according to the present invention; Fig. 14 is a cross-sectional view showing an essential part of the embodiment; and Fig. 15 is a cross-sectional view on an enlarged scale showing level shift diode regions of the embodiment. Fig. 1A shows a level shift diode which is useful for explaining the semiconductor device according to the present invention. A level shift diode 10 comprises an n⁺-type gallium arsenide (GaAs) anode contact layer 11, an n-type GaAs heterojunction forming layer 12, an n-type (or i-type) aluminum gallium arsenide (AlGaAs) heterojunction forming layer 13, an n⁺-type AlGaAs cathode contact layer 14, an anode electrode 15, and a cathode electrode 16. FIG.1B shows an energy band diagram of the level shift diode 10. In FIG.1B, E c denotes the bottom of the conduction band, and ΔE a denotes the barrier height at the energy band gap produced by the difference in the electron affinities of the n-type GaAs heterojunction forming layer 12 and the n-type AlGaAs heterojunction forming layer 13. According to the level shift diode 10, a barrier height E B becomes small and a current starts to flow when a positive voltage is applied to the anode electrode 15, a negative voltage is applied to the cathode electrode 16 and the two voltages are increased. FIGS.2A through 2C show the energy band diagram of the level shift diode 10 for different anode voltages V a , and FIG.3 shows the anode voltage V a versus anode current I a characteristic of the level shift diode 10. In FIGS.2A through 2C and 3, those parts which are the same as those corresponding parts in FIGS.1A and 1B are designated by the same reference numerals and characters. FIG.2A shows the energy band diagram for the case where V a = 0. In this case, the barrier height ΔE a is sufficiently high, and no anode current I a flows. FIG.2B shows the energy band diagram for the case where V a ≦ ΔE a /e, where e denotes the electronic charge. In this case, the barrier height ΔE a decreases compared to the case shown in FIG.2A, and a slight anode current I a starts to flow. FIG.2C shows the energy band diagram for the case where V a ≅ ΔE a /e. In this case, the barrier height ΔE a decreases even further from the case shown in FIG.2B, and a large anode current I a flows thereby reaching saturation. Accordingly, in the level shift diode 10, the level (voltage) shift quantity changes depending on the barrier height ΔE a at the heterojunction interface. But the barrier height ΔE a can be varied by changing the composition of the n-type AlGaAs heterojunction forming layer 13 which is one of the semiconductor layers for forming the heterojunction interface, that is, by varying Al content x of Al x Ga 1-x As which constitutes the n-type AlGaAs heterojunction forming layer 13. As a result, unlike the Schottky diode utilizing a Schottky junction where the level shift quantity can only be changed in steps, the level shift quantity of the level shift diode 10 can be changed continuously by appropriately varying the Al content x of Al x Ga 1-x As. Furthermore, it is possible to more accurately control the level shift quantity compared to the Schottky diode. FIG.1C shows a level shift diode with p-type semiconductors in which the carriers are holes. FIG.1D is an energy band diagram of the level shift diode in FIG.1C. E v denotes the bottom of the valence band, and E barrier denotes the barrier height at the energy band gap produced by the difference in the energy level of the valence band for the holes. In this case, the energy level of E v in the first layer 12 is higher than that of the second layer 13 at the heterojunction. FIG.2D shows the energy band diagram for the case where V a ≅ ΔE barrier /e. In this case, holes are injected from the anode to the cathode. FIG.4A shows another level shift diode which is useful for explaining the present invention. In FIG.4A, those parts which are the same as those corresponding parts in FIG.1A are designated by the same reference numerals, and a description thereof will be omitted. In FIG.4A, a level shift diode 20 essentially comprises two level shift diodes 10 which are connected in series. FIG.4B shows the energy band diagram of the level shift diode 20 for the case where V a = 0, and FIG.4C shows the energy band diagram of the level shift diode 20 for the case where V a = 2 x ΔE a /e. In addition, FIG.5 shows an anode voltage V a versus anode current I a characteristic of the level shift diode 20. In FIGS.4A through 4C and 5, those parts which are the same as those corresponding parts in FIGS.1A and 1B are designated by the same reference numerals and characters. Because two level shift diodes 10 are essentially connected in series as shown in FIG.4A, a diode coupled in an opposite direction is introduced in the middle between the two level shift diodes 10. However, the heterojunction in the middle essentially does not act as a barrier as may be seen from FIG.4C. As a result, the level (voltage) shift quantity is 2 x ΔE a /e as shown in FIG.5. It is assumed heretofore that the two level shift diodes 10 constituting the level shift diode 20 are identical both in construction and characteristic. However, the two level shift diodes 10 may have the same construction but different barrier heights ΔE a . In other words, the Al content x of the Al x Ga 1-x As which constitutes the n-type AlGaAs heterojunction forming layer 13 of one of the two level shift diodes 10 may be different from that of the other of the two level shift diodes 10. In this case, it is easier to control the level (voltage) shift quantity of the level shift diode 20 to a desired value than simply controlling to the same value the Al content x of the Al x Ga 1-x As which constitutes the n-type AlGaAs heterojunction forming layer 13 of the two level shift diodes 10 having identical construction and characteristic. FIG.4D shows the level shift diode which has a Al x Ga 1-x As layer 17 with graded composition x in substitute of the layers 11 through 14 in FIG.4A. FIG.4E shows the energy band diagram of the level shift diode of FIG.4D in which the energy level of the conduction band in the layer 17 changes gradually. FIG.4F shows Al composition of FIGS.4D and 4E. Next, a description will be given of a semiconductor device useful for explaining the present invention. FIG.6 is a circuit diagram of the device. In FIG.6, the semiconductor device is a buffered field effect transistor logic (BFL) circuit comprising transistors Q1 through Q4 and a level shift diode D1. IN and OUT respectively denote input and output terminals, V DD denotes a positive power source voltage, and V SS denotes a negative power source voltage. FIG.7 shows an essential part of the device indicated by a phantom line in FIG.6. In FIG.7, those parts which are the same as those corresponding parts in FIG.6 are designated by the same reference numerals and characters. In FIG.7, the semiconductor device comprises a semiinsulating GaAs substrate 31, an undoped (intrinsic) GaAs active layer 32, an n⁺-type AlGaAs electron supplying layer 33, a level shift diode region 34, an element isolation region 35, alloyed regions 36, a two dimensional electron gas (2DEG) layer 37, source electrodes SQ3 and SQ4, drain electrodes DQ3 and DQ4, gate electrodes GQ3 and GQ4, and a cathode electrode CD of the level shift diode D1. High electron mobility transistors (HEMTs) are used for the transistors Q3 and Q4. FIG.8 shows on an enlarged scale the level shift diode region 34 shown in FIG.7 together with the n⁺-type AlGaAs electron supplying layer 33. The level shift diode region 34 comprises an n⁺-type GaAs layer 34A, an n-type GaAs layer 34B, an n-type AlGaAs layer 34C, an n⁺-type AlGaAs layer 34D, an n-type AlGaAs layer 34C, an n-type GaAs layer 34B, an n⁺-type GaAs layer 34A, an n-type GaAs layer 34B, an n-type AlGaAs layer 34C, and an n -type AlGaAs layer 34D. The n⁺-type AlGaAs electron supplying layer 33 and the level shift diode region 34 constitute the level shift diode D1 which is essentially the same as the level shift diode 20 described before in conjunction with FIGS.4A through 4C and 5. Hence, the drain electrode DQ4 functions as the anode electrode of the level shift diode D1. Since the level shift diode region 34 comprises highly doped layers, the level shift diode region 34 can also function satisfactorily as an electrode contact layer. As may be seen from FIGS.7 and 8, the level shift diode D1 is connected between the source electrode SQ3 of the transistor Q3 and the drain electrode DQ4 of the transistor Q4, and the semiconductor device shown in FIG.7 is identical to the essential part of the BFL circuit indicated by the phantom lines in FIG.6. FIG.9 shows an essential part of a modification of the device in Fig. 6. In FIG.9, those parts which are the same as those corresponding parts in FIGS.7 and 8 are designated by the same reference numerals, and a description thereof will be omitted. One n-type GaAs layer 34B and one n-type AlGaAs layer 34C respectively in the middle are omitted from the level shift diode region 34 shown in FIG.8. In addition, an alloyed region 39 is formed in the top n⁺-type AlGaAs layer 34D. As described before in conjunction with FIG.4C, these omitted layers 34B and 34C have little effect and are omitted in this modification for this reason. The illustration of the connection between the cathode electrode CD and the source electrode SQ3 is omitted in FIG.9. In FIGS.7 through 9 and FIGS.11, 12, 14 and 15 which will be described later, the symbols of the diode and/or transistor are shown in the cross sectional view to facilitate the understanding thereof. Next, a description will be given of a second semiconductor device useful for explaining the present invention. FIG.10 is a circuit diagram of the second device. In FIG.10, the semiconductor device is a Schottky diode field effect transistor logic (SDFL) circuit comprising transistors Q11 through Q13 and a level shift diode D2. IN and OUT respectively denote input and output terminals, V DD denotes a positive power source voltage, and -V SS denotes a negative power source voltage. FIG.11 shows an essential part of the second device. In FIG.11, those parts which are the same as those corresponding parts in FIG.10 are designated by the same reference numerals and characters. In FIG.11, the semiconductor device comprises a semiinsulating GaAs substrate 51, an undoped (intrinsic) GaAs active layer 52, an n⁺-type AlGaAs electron supplying layer 53, a GaAs layer 54, an element isolation region 55, alloyed regions 56, a two dimensional electron gas (2DEG) layer 57, a level shift diode region 58, source electrodes SQ11, SQ12 and SQ13, drain electrodes DQ11, DQ12 and DQ13, gate electrodes GQ11, GQ12 and GQ13, and a cathode electrode CD of the level shift diode D2. As shown in FIG.12, the GaAs layer 54 is made up of an n⁺-type GaAs layer 54a and an n-type GaAs layer 54b. The level shift diode region 58 comprises an AlGaAs layer 61, an GaAs layer 62 and an AlGaAs layer 63. HEMTs are used for the transistors Q11 through Q13. The AlGaAs layer 61 comprises an n-type AlGaAs layer 61a, an n⁺-type AlGaAs layer 61b and an n-type AlGaAs layer 61c. The GaAs layer 62 comprises an n-type GaAs layer 62a, an n⁺-type GaAs layer 62b and an n-type GaAs layer 62c. The AlGaAs layer 63 comprises an n-type-AlGaAs layer 63a and an n⁺-type AlGaAs layer 63b. As may be seen from FIGS.10 and 11, the level shift diode D2 is connected between the cathode electrode CD and the drain electrode DQ11 of the transistor Q11, and the semiconductor device shown in FIG.11 is identical to the SDFL circuit shown in FIG. 10. Next, a description will be given of an embodiment of the semiconductor device according to the present invention. FIG.13 is a circuit diagram of the embodiment. In FIG.13, the semiconductor device is a field effect transistor logic with Schottky diode and coupling capacitor (FLSC) circuit comprising transistors Q21 and Q22, level shift diodes D3 and D4 and a resistor R. IN and OUT respectively denote input and output terminals, V DD denotes a positive power source voltage, and -V SS denotes a negative power source voltage. FIG.14 shows an essential part of the embodiment. In FIG.14, those parts which are the same as those corresponding parts in FIG.13 are designated by the same reference numerals and characters. In FIG.14, the semiconductor device comprises a semiinsulating GaAs substrate 71, an undoped GaAs active layer 72, an n⁺-type AlGaAs electron supplying layer 73, a GaAs layer 74, an element isolation region 75, alloyed regions 76, a two dimensional electron gas (2DEG) layer 77, level shift diode regions 78 and 79, a source electrode SQ22, a drain electrode DQ22, a gate electrode GQ22, an anode electrode AD of the level shift diode D4, and a cathode electrode CD of the level shift diode D3. As shown in FIG.15, the GaAs layer 74 is made up of an n⁺-type GaAs layer 74a and n-type GaAs layer 74b. The level shift diode region 78 comprises an AlGaAs layer 81, a GaAs layer 82 and an AlGaAs layer 83. The level shift diode region 79 comprises the AlGaAs layer 81 and the GaAs layer 82. HEMTs are used for the transistors Q21 and Q22. The AlGaAs layer 81 comprises an n-type AlGaAs layer 81a, an n⁺-type AlGaAs layer 81b and an n-type AlGaAs layer 81c. The GaAs layer 82 comprises an n-type GaAs layer 82a, an n -type GaAs layer 82b and an n-type GaAs layer 82c. The AlGaAs layer 83 comprises an n-type AlGaAs layer 83a and an n⁺-type AlGaAs layer 83b. As may be seen from FIGS.13 and 14, the level shift diodes D3 and D4 are connected in parallel, and the semiconductor device shown in FIG.13 is identical to the essential part of the FLSC circuit shown in FIG. 14. According to the invention, it may be seen that the level shift quantity of the level shift diode in the semiconductor device is accurately controlled to a desired value because the level shift diode utilizes the heterojunction. In addition, the level shift diode only occupies a small area on the semiconductor device compared to the conventional semiconductor using a Schottky diode. Moreover, after a plurality of the n⁺-type GaAs layer, the n-type GaAs layer, the n-type AlGaAs layer and the n⁺-type AlGaAs layer are stacked, it is possible to obtain a desired level shift quantity by eliminating a predetermined numbers of layers of the stacked level shift diodes. It may be seen that after a plurality of the n⁺-type GaAs layer, the n-type GaAs layer, the n-type AlGaAs layer and the n⁺-type AlGaAs layer are stacked, it is possible to form a plurality of level shift diodes on the same substrate by isolating level shift diode regions and removing a predetermined number of layers from arbitrary ones of the level shift diode regions. In this manner, it is possible to form level shift diodes coupled in opposite directions on the same substrate, by simply making the number of layers of the level shift diode regions different from each other. In the devices shown in FIG.7 and after, it is assumed for convenience' sake that the level shift diode at least comprises a first layer made of a first compound semiconductor and a second layer made of a second compound semiconductor having an electron affinity smaller than that of the first compound semiconductor, where the first and second compound semiconductors are both made of n-type semiconductors. However, the first and second compound semiconductors may both be made of either one of n-type and i-type semiconductors. Moreover, the first and second compound semiconductors may both be made of either one of p-type and i-type semiconductors. It will be readily apparent to those skilled in the art that the described embodiment may be modified to suit the case where the first and second compound semiconductors are both made of either one of p-type and i-type semiconductors, for example.

Claims: A semiconductor device comprising a substrate (71, 72, 73) and at least two level shift diodes (D3, D4) provided on said substrate, said diodes having a stacked structure comprising:- a first layer (74a, 74b) of a first compound semiconductor having a first electron affinity; a second layer (81) of a second compound semiconductor formed on said first layer and having a second electron affinity less than said first electron affinity, forming a first heterojunction between said first and second layers, said first and second compound semiconductors being any arbitrary combination of n-type and i-type semiconductors; a third layer (82) comprising said first compound semiconductor with the first electron affinity and formed on said second layer (81); at least one fourth layer (83) of similar composition to said second layer, formed on said third layer; a first electrode formed on said first layer (74a, 74b); and a second electrode (AD) formed on said third layer (82); characterised in that:- at least one field effect transistor (Q22) is provided on said substrate whereby at least a portion of said first layer (74a) is used in common with said field effect transistor as a layer of said field effect transistor and as a layer of said diodes, and said diodes (D3, D4) are level shift diodes in which said third layer (82) forms a second heterojunction with said second layer (81); and said second to fourth layers are formed selectively at a plurality of mutually-isolated locations on said substrate, whereby at a first location a first (D4) of said level shift diodes (D3, D4) is formed which comprises said second layer (81) and said third layer (82) and at a second location a second (D3) of said level shift diodes (D3, D4) is formed which comprises said second layer (81), said third layer (82) and said fourth layer (83), said level shift diodes (D3, D4) having mutually opposite bias directions. A semiconductor device as claimed in claim 1, wherein said first compound semiconductor is gallium arsenide (GaAs) and said second compound semiconductor is aluminum gallium arsenide (AlGaAs). A semiconductor device as claimed in claim 1, wherein said first compound semiconductor is gallium arsenide (GaAs) and said second compound semiconductor is aluminum gallium arsenide Al x Ga 1-x As, said level shift diodes (D3, D4) having level shift quantities determined by the Al content x. A semiconductor device as claimed in any preceding claim, wherein a composition of said second compound semiconductor is selected so that said level shift diodes (D3, D4) have predetermined level shift quantities. A semiconductor device as claimed in any preceding claim, wherein said first to fourth layers are formed at said second location on said substrate in such a way as to form two said second level shift diodes (D3) connected in series with mutually similar bias directions. A semiconductor device as claimed in any preceding claim, wherein the sequence in which said layers are formed is the same at all of said locations on said substrate. A semiconductor device as claimed in any preceding claim, wherein said field effect transistor is a high electron mobility transistor.

Inventors: AWANO YUJI, AWANO, YUJI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1987
Main classification: H01L27
IPC codes: H01L27, H01L29, H01L21