RESONANT TUNNELING SEMICONDUCTOR DEVICE

Abstract: A resonant tunneling semiconductor device having a large peak-to-valley current density J p /J v ratio comprises an InP substrate (11), a first contact compound semiconductor (12) lattice-matched to InP, a first barrier layer (13) of (In 0.52 A1 0.48 As) z (In 0.53 Ga 0.47 As) 1-z , (0 < z ≦αµρ¨), a well layer (14) of In 1-y Ga y As, (0.48 ≧ y ≧ 0.46), a second barrier layer (15) of (In 0.52 A1 0.48 As) z (In 0.53 Ga 0.47 As) 1-z , (0 < z < 1), and a second contact layer compound semiconductor (16) lattice-matched to InP: The first and second barrier layers and the well layer forming a quantum-well structure. Instead of the quantum-well structure, it is possible to adopt the quantum-well structure of strained-layers comprising the first and second barrier layers which are of In 1-x Al x As (0.48 < z ≦αµρ¨ 1) and have a thickness of 0.5 to 10.0 nm.

Description: The present invention relates to a compound semiconductor device, and more particularly, to a resonant tunneling semiconductor device having a negative differential resistance characteristic caused by resonant tunneling phenomenon. The resonant tunneling semiconductor device uses a resonant tunneling barrier structure, ie. a quantum-well structure, comprising two barrier layers and a well layer sandwiched therebetween. The resonant tunneling of a carrier (ie. electron) occurring in the structure provides the negative resistance characteristic. Some examples of known resonant tunneling semiconductor devices will now be described with reference to Figures 1 to 5 of the accompanying drawings, in which:- Figure 1 is a schematic sectional view of a conventional RTB diode; Figure 2 is a conduction energy-band diagram of the RTB diode under a bias condition; Figure 3 is a current-voltage characteristics diagram of the RTB diode; Figure 4 is a current-voltage characteristics diagram of the RTB diode (50 µm in diameter) at 77 K; and, Figure 5 is a graph showing a relationship between J p /J v ratios and the J p of AlGaAs/GaAs RTB diodes. Resonant tunneling semiconductor devices such as a resonant tunneling barrier (RTB) diode, a resonant tunneling bipolar transistor (RBT) and a resonant tunneling hot electron transistor (RHET), have been proposed. For example, EP-A-0 177 374 (corresponding to US Serial No.754,416) discloses a type of RHET. Figure 1 shows a known RTB diode. This RTB diode comprises a substrate 1, a first contact layer 2, a first barrier layer 3, a well layer 4, a second barrier layer 5, a second contact layer 6, a protective insulating layer 7, a first ohmic-contact electrode 8, and a second ohmic-contact electrode layer 9. The data of the above-mentioned parts of the RTB diode is as follows, by way of example. (a) Substrate 1 Material: semi-insulating GaAs (b) First contact layer 2 Material: n-type GaAs Impurity concentration: 1 × 10¹⁸cm⁻³ Thickness: 0.5µm (c) First barrier layer 3 Material: i-type Al 0.33 Ga 0.67 As Thickness: 50 Å (5.0 nm) (d) Well layer 4 Material: i-type GaAs Thickness: 56 Å (5.6 nm) (e) Second barrier layer 5 Material: i-type Al 0.33 Ga 0.67 As Thickness: 50 Å (5.0 nm) (f) Second contact layer 6 Material: n-type GaAs Impurity concentration: 1 × 10¹⁸ cm⁻³ Thickness: 0.3 µm (g) Protective insulating layer 7 Material: Silicon dioxide (SiO₂) Thickness: 0.3 µm In the RTB diode, the first and second barrier layers 3 and 5 and the well layer 4 having a thickness of less than the de Broglie wavelength of the carrier (i.e., electron) form a quantum-well structure. Figure 2 shows a conduction energy-band diagram of the RTB diode, for explaining the operation thereof when a bias voltage is applied to the RTB diode. In the drawing, "E c " and "E₀" indicate the bottom of the conduction band and the first resonant level for the electron in the well layer 4, respectively. Where the first contact layer 2 has a positive potential with respect to the second contact layer 6, to bring the bottom E c of the conduction band of the second contact layer 6 into correspondence with the first resonant level E₀ in the well layer 4, as shown in Fig. 2, namely, where the voltage V between the first and second contact layers 2 and 6 becomes: V ≈ 2E₀/q wherein q is the electric charge of the electron, electrons tunnel from the second contact layer 6 to the first contact layer 2 through the barriers due to the resonant tunneling effect, and thus a current flows in the RTB diode. On the other hand, where the voltage V becomes V > 2E₀/q or V < 2E₀/q, so that the voltage conditions are different to the resonant tunneling condition, the amount of electrons tunneling into the first contact layer 2 is remarkably decreased, and thus very little current flows. Figure 3 shows a current-voltage characteristic of the RTB diode, in which the abscissa axis and the ordinate axis represent the voltage and the current density, respectively. In the drawing, "V p " indicates a voltage at which a maximum current (peak current) flows due to the occurrence of resonant tunneling, "V v " indicates a voltage at which a minimum current (valley current) flows when resonant tunneling does not occur, J p indicates a maximum current density, and J v indicates a minimum current density. As can be seen from Fig. 3, the current first increases as voltage increases. When the voltage becomes V p , the current reaches the maximum current density J p , and then the current sharply decreases. When the voltage becomes V v , the current reaches the minimum current density J v , and thereafter, the current increases again. Namely, where the voltage varies from V p to V v , the current density decreases from J p to J v for the voltage increase, and thus a negative resistance characteristic appears. Figure 4 shows a current-voltage characteristic of a conventional RTB diode having a diameter of 50 µm, which was obtained by measurement under a temperature of 77 K. In the drawing, the abscissa axis represents applied voltage and each graduation on the scale thereof represents 200 mV. The ordinate axis represents current and each graduation on the scale thereof represents 50 mA. In this case, as can be seen from Fig. 4, the applied voltage corresponding to the minimum current I v is shifted to a lower voltage side with respect to the other voltage corresponding to the maximum current I p , which occurs because of variations of the amount of voltage drop due to a series resistance component. The above-mentioned negative differential resistance characteristics in the RTB diode can be also obtained in an RHET and can be used in the constitution of logic circuits, frequency multipliers, oscillation circuits, etc., to reduce the number of elements required for those functions. In such cases, in order to operate the circuits at a high-speed and with a large noise margin, the ratio of the maximum current density to the minimum current density (peak-to valley current density ratio, J p /J v ) must be large and the maximum (peak) current density J p must be large. The negative differential resistance characteristics including the J p /J v ratio and the J p value can vary in accordance with barrier height of the quantum-well structure, a width (thickness) of the barrier layers, width (thickness) of a well layer, impurity concentration of the contact layers lying outside the barrier layers, and the like. Conventional AlGaAs/GaAs RTB diodes have maximum (peak) current density J p values and peak-to-valley current density (J p /J v ) ratios, as shown in Fig. 5. As can be seen from the drawing, as the maximum current density J p increases, the J p /J v ratio decreases. For example, when the J p value is approximately equal to 2 × 10⁴ A/cm², the J p /J v ratio is approximately equal to 3. Thus, in the conventional negative resistance element of an AlGaAs/GaAs system, where the J p value is made larger, the B>p/J v ratio decreases. Another example of a known resonant tunneling device is described in IEDM 85, Technical Digest, pages 764-765. The device described comprises a first contact compound semiconductor layer formed on the substrate and lattice-matched to InP, a first barrier layer of In o.53 Ga 0.47 As formed on the first contact layer and lattice-matched to InP, a well layer of In o.53 Ga o.47 As formed on the first barrier layer and substantially lattice-matched to InP, a second barrier layer of In o.52 Al o.48 As formed on the well layer and lattice-matched to InP, and a second contact compound semiconductor layer formed on a barrier layer and lattice-matched to InP. This device suffers from the same problems as other prior art devices described above. According to the present invention, there is provided a resonant tunneling semiconductor device comprising an InP substrate ; a first contact compound semiconductor layer formed on the InP substrate and lattice-matched to InP; a first barrier layer of In 1-x Al x As (0.48 < x ≦ 1) formed on the first contact layer and not lattice matched to InP; a well layer of In 1-y Ga y As (0.46≦ y ≦ 0.48) formed on the first barrier layer and substantially lattice-matched to InP; a second barrier layer of In 1-x A1 x As (0.48 < x ≦ 1) formed on the well layer and not lattice-matched to InP; and a second contact compound semiconductor layer formed on the second barrier layer and lattice-matched to InP wherein the first and second barrier layers have a thickness up to 10 nm and wherein the first barrier layer , the second barrier layer and the well-layer form a quantum-well structure of strained-layers and the bottom of the conduction band (E c ) of the first and second contact layers is lower than the first resonant level (E₀) in the quantum-well at the non-biased condition. In this case, In 1-x Al x As (0.48 < x ≦ l) is adopted for the barrier layer instead of InAl(Ga)As of the IEDM article mentioned above, to increase the barrier height of the quantum-well structure. Therefore, as the molar ratio "x" approaches the maximum value "l", the barrier height increases. The large barrier height provides a greater separation between the first resonant level E₀ and the second resonant level E₁. The large barrier height and the greater separation cause the minimum (valley) current density J v to decrease, which contributes to the increase of the J p /J v ratio. On the other hand, since the In 1-x A1 x As (0.48 < x ≦ 1) layer is not lattice-matched to InP, the layer thickness is restricted, to prevent misfit dislocations from occurring and to form a strained-layer structure. Accordingly, the InA1As barrier layer preferably has a thickness of from 0.5 to 10.0 nm, more preferably, from 1.0 to 5.0 nm. The present invention provides a resonant tunneling semiconductor device having negative differential resistance characteristics in which a maximum (peak) current density J p and a maximum/minimum (peak-to-valley) current density (J p /J v ) ratio are high. The present invention also improves the operating speed and the noise margin of electric circuits including the resonant tunneling semiconductor devices. Some examples of semiconductor devices according to the present invention will now be described with reference to the accompanying drawings, in which:- Figure 6 is a schematic sectional view of an RTB diode according to the present invention; Figure 7 is a diagram showing the relationship between J p /J v ratios and the J p values of examples of RTB diodes according to the invention at 77K; Figure 8 is a diagram showing the relationship between J p /J v ratios and the J p values of examples of RTB diodes according to the invention at 300K; Figure 9 is a diagram showing the relationship between J p /J v ratios and the J p values of examples of RTB diodes according to the invention having In 1-x A1 x As barrier layers; Figures 10 and 11 are current density-voltage characteristics diagrams of an RTB diode having the AlAs barrier layers according to the present invention, at 77K and 300K, respectively; Figure 12 is a schematic sectional view of an example of a RHET according to the present invention; and, Figure 13 is a schematic sectional view of an example of resonant tunneling bipolar transistor according to the invention. The RTB diode shown in Figure 6 comprises an InP substrate 11, a first contact layer 12 of a ternary or quaternary compound semiconductor, a first barrier layer 13 of In 1-x Al x As (0.48 < x ≦ 1),a well layer 14 of In 1-y Ga y As (0.46 ≦ y ≦ 0.48), a second barrier layer 15 of In 1-x A1 x As (0.48 < x ≦ 1), a second contact layer 16 of a ternary or quaternary compound semiconductor, a protective insulating layer 17, a first ohmic-contact electrode 18, and a second ohmic-contact electrode 19. The above-mentioned RTB diode is produced as follows. The first contact layer 12, the first barrier layer 13, the well layer 14, the second barrier layer 15, and the second contact layer 16 are consecutively grown on the InP substrate 11 by a molecular beam epitaxy (MBE) method. The formed multilayer body is selectively etched from the top surface to the first contact layer 12 by a conventional photolithography method, as shown in Fig. 6, so that a mesa portion of the multilayer body having a diameter of approximately 50 µm is formed. Then a protective insulating layer 17 is formed on the exposed surface by a chemical vapor deposition (CVD) method. The protective insulating layer 17 is selectively etched by a conventional photolithograph method, to open windows for the first and second contact layers 18 and 19. Then the contact layers 18 and 19 are formed by a vapor deposition method and a lift-off method. In 1-x Al x As (0.48 < x ≦ l) having a forbidden band width (energy bandgap) larger than that of the quaternary compound semiconductor of (In 0.52 Al 0.48 As) z (In 0.53 Ga 0.47 As) 1-z , (0 < z ≦ l), [e.g., In 0.52 Al 0.48 As] is adopted for the barrier layer material. However, the In 1-x Al x As (0.48 < x ≦ l) layer is not lattice-matched with InP. In this case, it is possible to prevent misfit dislocation from occurring by adopting a strained-layer structure. By way of example, the data of the parts of the RTB diode is as follows. (a) InP substrate 11 Material: semi-insulating single crystalline InP (b) First contact layer 12 Material: n-type In 0.53 Ga 0.47 As Impurity concentration: 1 x 10¹⁸ cm⁻³ Thickness: 0.5 µm (c) First barrier layer 13 Material: i-type AlAs Thickness: 24 Å (2.4 nm) (d) well layer 14 Material: i-type In 0.53 Ga 0.47 As Thickness: 44 Å (4.4 nm) (e) Second barrier layer 15 Material: i-type AlAs Thickness: 24 Å (2.4 nm) (f) Second contact layer 16 Material: n-type In 0.53 Ga 0.47 As Impurity concentration: 1 x 10¹⁸ cm⁻³ Thickness: 0.3 µm (g) Protective insulating layer 17 Material: Silicon dioxide (SiO₂) Thickness: 0.3 µm (h) First and second electrode layers 18 and 19 Material: AuGe/Au or Cr/Au Thickness: 20 nm/200 nm or 10 nm/200 nm Alloying treatment: No It is also possible to introduce undoped In 0.53 Ga 0.47 As spacer layers between the first contact layer 12 and the first barrier layer 13 and between the second barrier layer 15 and the second contact layer 16. In this case, the barrier layer 13 and 15 are obtained when the mole fraction "x" in In 1-x Al x As is equal to one. The energy bandgap of AlAs is 3.03 eV. Therefore, where the molar ratio "x" in the In 1-x Al x As barrier layer varies from more than 0.48 up to 1, the energy bandgap thereof varies from more than 1.47 eV up to 3.03 eV, which is larger than that of the (In 0.52 Al 0.48 As) z (In 0.53 Ga 0.47 As) 1-z barrier layer according to the first embodiment of the present invention. Consequently, the barrier height of the In 1-x Al x As barrier layers becomes more than 0.53 eV. As the molar ratio "x" increases, the separation between the first and second resonant layers in the well layer increases. The larger barrier height and the greater separation contribute to a decrease of the minimum (valley) current density J v and increase of the J p /J v ratio. In order to make the quantum-well structure including the In 1-x Al x As barrier of strained-layers, it is necessary to restrict the thickness of the In 1-x Al x AS barrier layers up to a critical layer thickness of 100 Å (10.0 nm). Therefore, the In 1-x Al x As barrier layer his a thickness of from 5 to 100 Å (0.5 nm to 10.0 nm), preferably, 10 Å to 50 Å (1.0 nm to 5.0 nm). The RTB diodes comprising the above-mentioned parts have maximum (peak) current density J p values and peak-to-valley current density J p /J v ratios, at 77 K, indicated with open circles (o), as shown in Fig. 7. As can be seen from the drawing, the J p /J v ratios of the RTB (AlAs/InGaAs) diodes according to the embodiment of the present invention are much larger than those of conventional RTB (InAlAs/InGaAs) and RTB (AlGaAs/GaAs) diodes. Figure 8 is similar to Fig.7 and shows data of the J p values and J p /J v ratios for the RTB diodes according to the embodiment of the invention and the conventional RTB diodes, of which data were obtained by testing at 300 K. The J p /J v ratios (at 300 K) of the RTB (AlAs/InGaAs) diodes according to the embodiment of the invention are remarkably than those (at 77K) shown in Fig. 7 but are much larger than those of the conventional RTB (InAlAs/InGaAs) diodes and the conventional RTB (AlGaAs/GaAs) diodes. Figure 9 shows data of the RTB diodes including the In 1-x Al x As (0.48 ≦ x ≦ 1) barrier layers of the present invention, in which the mole fraction "x" is used as a parameter. The RTB diodes comprise the above-mentioned parts (shown in Fig. 6) except for the variation of the composition of In(Al)As barrier layers 13 and 15 and the insertion of spacer layers of undoped InGaAs between the first contact layer 12 and the first barrier layer 13 and between the second barrier layer 15 and the second contact layer 16. The thickness (L N ) of the spacer layers is approximately 15 Å (1.5 nm). Therefore, the quantum-well structure is formed of the In 1-x Al x As barrier layers 13 and 15 having a thickness (L B ) of 24 Å (2.4 nm) and the InGaAs well layer 14 having a thickness (L W ) of 44 Å (4.4 nm). As can be seen from Fig. 9 the J p /J v ratios (at 77 K and at 300 K) of the RTB diodes having the AlAs barrier layer at the mole fraction "x" of 1 are superior to those of the RTB diodes having the In 0.52 Al 0.48 As barrier layers of the prior art. Taking Figs. 7 and 8 into consideration, it is obvious that the J p /J v ratios shown in Fig. 9 are superior to those of the conventional RTB (AlGaAs/GaAs) diodes. The RTB diode comprising the AlAs barrier layers according to the present invention was tested at temperatures of 77 K and 300 K, to obtain the current density-voltage characteristics shown in Figs. 10 and 11 respectively. As can be seen from the drawings, although the J p values are almost constant, the J v value at 77 K is lower than the J v value at 300 K and approaches zero. Accordingly the J p /J v ratio at 77 K becomes very much larger than that at 300 K. Referring to Fig. 12 an RHET (resonant tunneling hot election transistor) according to the present invention is explained hereinbelow. The RHET comprises an InP substrate 21, a collector layer 22 of n⁺-type In 0.53 Ga 0.47 As, a collector barrier layer 23 of i-type InGaAlAs, a base layer 24 of n-type In 0.53 Ga 0.47 As, a first barrier layer 25, a well layer 26 of i-type In 0.53 Ga 0.47 As, a second barrier layer 27, an emitter layer 28 of n⁺-type In 0.53 Ga 0.47 As, a protective insulating layer 29 of SiO₂ , and ohmic-contact electrodes 30, 31 and 32. The barrier layers 25 and 27 and a well layer 26 form a quantum-well structure in accordance with the first or second embodiment of the present invention. For example, the barrier layers 25 and 27 are of i-type In 0.52 Al 0.48 As and have a thickness of 41 Å (4.1 nm), or are of i-type AlAs and have a thickness of 24 Å (2.4 nm). Referring to Fig. 13 an RBT (resonant tunneling bipolar transistor) according to the present invention is explained hereinbelow. The RBT comprises an InP substrate 33, a collector contact layer 34 of n⁺-type In 0.53 Ga 0.47 As, a collector layer 35 of n-type In 0.53 Ga 0.47 As, a base layer 36 of P-type In 0.53 Ga 0.47 As, a first barrier layer 37, a well layer 38 of i-type In 0.53 Ga 0.47 As. a second barrier layer 39, an emitter layer 40 of n⁺-type In 0.53 Ga 0.47 As, a protective insulating layer 41 of SiO₂ , and ohmic-contact electrodes 42, 43 and 44. The barrier layers 37 and 39 and a well layer 38 form a quantum-well structure in accordance with the present invention. For example, the barrier layers 37 and 39 are of i-type In 0.52 Al 0.48 As and have a thickness of 41 Å (4.1 nm), or are of i-type AlAs and have a thickness of 24 Å (2.4 nm). Although the above-mentioned quantum-well structure has a symmetrical structure in which the composition (i.e., barrier height) and the thickness (i.e., barrier width) of the barrier layers are equal to each other, it is possible to adopt an asymmetrical structure in which the composition and/or the thickness of one of the barrier layers is different from the other barrier layer.

Claims: A resonant tunneling semiconductor device comprising an InP substrate (11); a first contact compound semiconductor layer (12) formed on the InP substrate (11) and lattice-matched to InP; a first barrier layer (13) of In 1-x Al x As (0.48 < x ≦ 1) formed on the first contact layer (12) and not lattice matched to InP; a well layer (14) of In 1-y Ga y s (0.46≦ y ≦ 0.48) formed on the first barrier layer (13) and substantially lattice-matched to InP; a second barrier layer (15) of In 1-x Al x As (0.48 < x ≦ 1) formed on the well layer (14) and not lattice-matched to InP; and a second contact compound semiconductor layer (16) formed on the second barrier layer (15) and lattice-matched to InP wherein the first and second barrier layers (13,15) have a thickness up to 10 nm and wherein the first barrier layer (13), the second barrier layer (13) and the well-layer (14) form a quantum-well structure of strained-layers and the bottom of the conduction band (E c ) of the first and second contact layers (12,16) is lower than the first resonant level (E₀) in the quantum-well at the non-biased condition. A resonant tunneling semiconductor device according to claim 1, wherein the mole fraction "x" of the first barrier layer (13) is equal to 1, the first barrier layer comprising AlAs. A resonant tunneling semiconductor device according to claim 1 or claim 2, wherein the mole fraction "x" of the second barrier layer (15) is equal to 1, the second barrier layer comprising AlAs. A resonant tunneling semiconductor device according to any of claims 1 to 3, wherein the first and second barrier layers (13,15) have a thickness of from 0.5 to 10.0 nm. A resonant tunneling semiconductor device according to claim 4, wherein the thickness of the first and second barrier layers (13,15) is from 1.0 to 5.0 nm. A resonant tunneling semiconductor device according to any of the preceding claims, wherein the first contact layer (12) is of a material selected from the group consisting of a ternary compound semiconductor and a quaternary compound semiconductor. A resonant tunneling semiconductor device according to claim 6, wherein the ternary semiconductor is In 0.53 Ga 0.47 As. A resonant tunneling semiconductor device according to any of the preceding claims, wherein the mole fraction "y" is 0.47 and the well layer (14) comprises In 0.53 Ga 0.47 As. A resonant tunneling semiconductor device according to any of the preceding claims, wherein the second contact layer (16) is of a material selected from the group consisting of a ternary compound semiconductor and a quaternary compound semiconductor. A resonant tunneling semiconductor device according to claim 9, wherein said ternary semiconductor is In 0.53 Ga 0.47 As. A resonant tunneling semiconductor device according to any of the preceding claims, further comprising a first electrode (18) contacting the first contact compound layer; and a second electrode (19) contacting the second contact compound layer. A resonant tunneling semiconductor device according to any of the preceding claims, further comprising a compound semiconductor collector layer (22), formed on the InP substrate (21), the collector layer having a first conductivity type; and a compound semiconductor collector barrier layer (23) formed on the collector layer (22), the collector barrier layer (23) forming a barrier at the junction to the collector layer (22); wherein the first contact compound semiconductor layer comprises a base layer (24) of the first conductivity type, and the second contact compound semiconductor layer comprises an emitter layer (28) of the first conductivity type. A resonant tunneling semiconductor device according to any of claims 1 to 11, further comprising a compound semiconductor collector layer (34) formed on the InP substrate (33), the collector layer having a first conductivity type, wherein the first contact compound semiconductor layer comprises a base layer (36) having a second conductivity type which is opposite to the first conductivity type, and the second contact compound semiconductor layer comprises an emitter layer (40) having the first conductivity type.

Inventors: FUJII, TOSHI, INATA, TSUGUO, MUTO, SHUNICHI
Applicants: FUJITSU LIMITED
Application year: 1987
Main classification: H01L29
IPC codes: H01L29