JP6489701B2 - Detection diode - Google Patents

Description

  The present invention relates to detection diodes, and more particularly, to a low noise, high speed semiconductor detection diode that receives RF electrical signals in the THz frequency band (0.1 to 10 THz) and operates with zero bias.

Conventionally, a non-linear current-voltage characteristic (I-V characteristic) of a diode is generally used as a detection means in a radio system of the THz frequency band. Typically, Schottky Barrier Diode (SBD), Backward Diode, Planar Doped Barrier (PDB) Diode, etc. are used. For example, in envelope detection using SBD, when detecting a weak signal, it is desirable to operate at zero bias in order to suppress the influence of power supply noise, and in order to obtain good sensitivity, saturation current It is necessary to increase the value Is. Further, when configuring a receiver operating in the THz frequency band, it is difficult to obtain impedance matching because the impedance Z ₀ of the pure resistance antenna formed on the semiconductor substrate is as low as about 75 Ω. At this time, in order to ensure the coupling efficiency, it is necessary to increase the saturation current value Is in order to bring the differential resistance value R _D of the SBD close to Z ₀ .

In order to increase the saturation current value Is, the SBD using InGaAsP lattice-matched to InP is known as a semiconductor material in which the barrier height φ _B decreases, that is, the saturation current density Jc (A / cm ² ) increases. There is. However, when high frequency operation of several hundred GHz or more is performed, the required junction area is extremely small. Therefore, as long as an InGaAsP-based material lattice-matched to InP is used, the barrier can be obtained to a desired saturation current value Is. The height φ _B can not be reduced. Therefore, Hetero Barrier Barrier (HBD) capable of arbitrarily controlling the barrier height φ _B has been proposed (see, for example, Non-Patent Document 1).

FIG. 1 shows the structure of a conventional HBD. High concentration n ⁺ -InP cathode layer 3 (third semiconductor layer), undoped (= low concentration) InP barrier layer 2 (second semiconductor layer), high concentration n ⁺ -InGaAs anode layer 1 (first) The semiconductor layers are sequentially stacked, the cathode electrode 5 is formed on the cathode layer 3, and the anode electrode 4 is formed on the anode layer 1. The barrier layer 2 has smaller electron affinity than the anode layer 1, and an energy barrier (heterobarrier) for electrons is formed on the barrier layer 2 side of the interface (InGaAs / InP interface) of the two. Since the barrier height φ _B is an amount measured from the Fermi level of the n ⁺ -InGaAs anode layer 1, the barrier height φ _B can be arbitrarily reduced to φ _B = 0 by adjusting the doping level of the anode layer 1.

The relationship between each parameter and the differential resistance value R _D of the diode serving as the focal point will be described below. The nonlinear I-V characteristics of the HBD can be calculated using the saturation current value Is, the thermal voltage V _T , and the ideal coefficient n of the quasi-directional current:
I _D (V) = Is {exp [V x (nV _T ) ^-1 ]-exp [-V x (n-1) x (nV _T ) ^-1 ]} (1)
It can be expressed as (for example, non-patent document 2). Furthermore, assuming that the saturation current value Is is the junction area of the diode S _j , the material of the barrier is Richardson's constant A ^* , and the temperature is T (K),
Is (S _j , T) = S _j × A ^* × T ² × exp (−qφ _B / V _T ) (2)
Given by

For example, assuming that S _j = 1 μm ² , T = 300 K, A ^* = 9.2 A / cm ² / K, and qφ _B = 85 meV, n = 1.2, then the differential resistance value R _{D of the} diode (= dV) / DI) is calculated as 84 Ω from the equation (1). It is understood that this differential resistance value R _D is close to the impedance Z ₀ (75 Ω) of the self-complementary antenna formed on the Si hemispherical lens. Even with a small junction area required for operation in the THz frequency band, such as S _j = 1 μm ² , a sufficiently low differential resistance value R _D can be realized. Therefore, this HBD operates in the THz frequency band and can be applied as a low noise detection means operating at zero bias.

H. Ito and T. Ishibashi, “Fermi-level managed barrier diode for broadband and low-noise terahertz-wave detection,” Electronics Letters, Vol. 51, Issue 18, pp. 1440-1442, 2015. M. S. Tyagi et al., “Metal-Semiconductor Schottky Barrier Junctions and Their Applications,” edited by B. L. Sharma _ Plenum, New York, 1984, Chap. 1, p.19. S. M. Sze, “Physics of Semiconductor Devices,” John Wiley and Sons, 1981, Chap. 5, p. 250.

However, HBD using a low concentration InP barrier layer, by reducing the barrier height Qfai _B, has the following problems.

FIG. 2 shows an example of the profile of the conduction band edge of the conventional HBD. It is the result of calculating the profile (band profile) of the conduction band edge of the HBD having a heterostructure at the InGaAs / InP interface in consideration of the nonparabolicity of the band and the energy distribution of the electronic charge. In the structure of the HBD, the thickness of the undoped InP barrier layer 2 is 1000 A, and the carrier concentration of the n ⁺ -InP cathode layer 3 is 1 × 10 ¹⁸ / cm ³ . The energy of the electron is the conduction band edge E _C1 = 0 of the n ⁺ -InGaAs anode layer 1, and the position of the Fermi level (Ef) of the anode layer 1 is indicated by a broken line.

The conduction band edge (E _C2 ) of the barrier layer 2 when the Fermi level (Ef) is changed is shown, with the barrier height qφ _B as a parameter:
(A) Ef−E _C1 = 140 meV, qφ _B = 100 meV
(B) Ef−E _C1 = 114 meV, qφ _B = 126 meV
(C) Ef−E _C1 = 88 meV, qφ _B = 152 meV
Shows the case.

When the barrier height qφ _B is high (C), the InP barrier near the InGaAs / InP hetero interface has a triangular potential shape, but the Fermi level is raised to increase the barrier height qφ _B It can be seen that as the size decreases (B) to (A), the band shape near the heterointerface is flattened. The bending of this band is due to the negative charge of electrons distributed and distributed in the undoped InP barrier layer 2, and the smaller the barrier height qφ _B , the more all electrons whose energy is distributed above the Fermi level. As the amount of charge increases, the influence of planarization becomes strong.

As described above, by adjusting the doping level of the first semiconductor layer, it is possible to adjust the barrier height of the hetero barrier by raising and lowering the position of the Fermi level. However, as apparent from the calculation results, as the Fermi level Ef rises, the band shape tends to be convex upward and to be flat. Since the optimum barrier height qφ _B is about 100 meV or less, in the conventional HBD, there is a problem that the band shape in the vicinity of the hetero interface is flattened.

The problem that the InP barrier near the InGaAs / InP hetero interface becomes flat is, first, deterioration of the ideal coefficient n value (= increase of n value). The peak position xm of the "effective" potential for electrons is not at the heterointerface. This is because image power acts on electrons near the InGaAs / InP interface.
Electric field strength near the xm 界面 interface- ^1/2
By moving away from the interface to the InP side. As the barrier height qφ _B decreases, the electric field strength decreases, so xm increases and the n value of the HBD increases. As a result, the non-linearity of the IV characteristic of the HBD is weakened, and the detection performance is degraded.

Second, when the barrier shape near the InGaAs / InP hetero interface becomes flat, the diode current itself is modulated and lowered. This is because the parameter that governs the electron thermal emission current near the interface shown in equation (2) changes from the state limited by heat emission to the state limited by diffusion as the electric field strength of the barrier decreases. It is because there is a tendency (for example, nonpatent literature 3). When the electron transport mechanism tends to be diffusion limited, the effective I _S (S _j , T) decreases and the HBD current itself decreases, leading to an increase in the differential resistance value R _D and a decrease in the detection current. When the diode area is increased to lower the differential resistance value R _D , the junction capacitance is increased, and the frequency characteristics are degraded.

  An object of the present invention is to ensure good detection characteristics by suppressing deterioration of IV characteristics in an HBD operated at zero bias in a THz frequency band.

In order to achieve the object of the present invention, in one embodiment, the second semiconductor layer has a high concentration of n-type first semiconductor layer and a second electron affinity smaller than that of the first semiconductor layer. And a high concentration n-type third semiconductor layer are sequentially stacked, and a hetero barrier is formed on the second semiconductor layer side of the interface between the first semiconductor layer and the second semiconductor layer. And adjusting the doping level of the first semiconductor layer to adjust the barrier height of the hetero barrier, forming an anode electrode on the first semiconductor layer, and the third semiconductor layer A semiconductor laminated structure in which a cathode electrode is formed on the sub-cathode layer on which the second semiconductor layer is formed, the second semiconductor layer is an undoped barrier layer in contact with the first semiconductor layer, and the third semiconductor layer Including n-type barrier layer in contact And wherein the door.

  In order to solve the problem of "bending of the barrier layer due to the negative charge of electrons" in the conventional typical HBD, the present invention provides means for realizing an appropriate barrier shape. That is, the band profile of the hetero barrier can be improved by providing a region where n-type doping (introduction of donor) is performed on a part of the second semiconductor layer and the cathode (third semiconductor layer) side. In the THz frequency band, it is possible to solve the problem of the deterioration of the I-V characteristics of the HBD operated at zero bias and to ensure good detection characteristics.

It is a figure which shows the structure of the conventional HBD. It is a figure which shows an example of the profile of the conduction band edge of the conventional HBD. It is a figure showing the structure of HBD concerning one embodiment of the present invention. FIG. 5 shows a first example of the profile of the conduction band edge of the HBD according to an embodiment of the present invention. It is a figure which shows the 2nd example of the profile of the conduction band edge of HBD concerning one Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 3 shows the structure of the HBD according to an embodiment of the present invention. High concentration n + -InGaAs sub-cathode layer 13B, High concentration n ⁺ -InP cathode layer 13A (third semiconductor layer), n-InP barrier layer 12B, Undoped (= low concentration) InP barrier layer 12A (second Semiconductor layer), high concentration n ⁺ -InGaAs anode layer 11 (first semiconductor layer) are sequentially stacked, cathode electrode 15 is formed on sub cathode layer 13 B, and anode electrode 14 is formed on anode layer 11. There is.

  In order to manufacture this HBD, the sub-cathode layer 13B to the anode layer 11 are sequentially epitaxially grown on a semi-insulating InP substrate by MO-VPE method or MBE method, and the substrate is mesa-etched. Thereafter, the anode electrode 14 and the cathode electrode 15 are patterned to form an ohmic contact. In the conventional HBT, the InP barrier layer is undoped (= low concentration), whereas in the present embodiment, a region where n-type doping (introduction of donor) is applied to a part of the InP barrier layer and the cathode side Provide The donor distribution of this n-type doping region may be uniform or may be shaped to increase toward the cathode side.

In the present embodiment, as described above, in order to suppress the flattening of the band shape due to the “negative electron charge” whose energy is distributed above the Fermi level, the bending of the band by the “positive donor charge” To compensate. More specifically, the balance of the energy dependent electron charge distribution n (x) and the donor charge Nd (x) uniquely determines the band profile (potential change), so the n ⁺ -InP cathode of the InP barrier layer A band profile is adjusted by providing a doped n-InP barrier layer 12B on the layer 13A side. With such a design, the conduction band on the cathode side is lowered, and the band profile is linearly lowered to the cathode side, so that the InP barrier near the InGaAs / InP hetero interface has a triangular shape. Since the peak position xm of the potential for electrons approaches the n ⁺ -InGaAs anode layer as the electric field in the vicinity of the interface rises, the deterioration of the n value can be suppressed.

  Thus, according to the present embodiment, by compensating for the “band bending due to electron charge” in the conventional HBD and improving the problem of “band flattening near the hetero interface”, good I− V characteristics can be secured, and the reception sensitivity in the THz frequency band can be improved.

FIG. 4 shows a first example of the profile of the conduction band edge of the HBD according to an embodiment of the present invention. A barrier height qφ _B = 100 meV measured from the Fermi level, and a doped n-InP barrier layer 12B is provided on the n ⁺ -InP cathode layer 13A side. The n-InP barrier layer 12B, the carrier concentration and comparable barrier layer in the vicinity of the interface between the n ⁺ -InGaAs anode layer 11 and the InP barrier layer 12A, the doping of a constant concentration (N _D) used, doped regions Is a calculation result of the band profile when the is placed at the optimum position. The position of the n-InP barrier layer 12B is adjusted by the barrier height qφ B and the width of the barrier layers (12A, 12B), but the width is about 1/2 to 2/3 on the cathode side of the InP barrier layer. By doing this, a good band profile can be obtained. In the example of the structure of the HBD of this embodiment, the carrier concentration of the n ⁺ -InGaAs anode layer 11 is 5 × 10 ¹⁸ / cm ^3, and the width 700 A, N _D = on the cathode side of the n-InP barrier layer 12 B. The n-type impurity is introduced at 1 × 10 ¹⁷ / cm ³ . In consideration of the barrier height qφ _B = 100 meV, the carrier concentration in the barrier layer near the interface can be estimated to be approximately 1/50 of the carrier concentration of the n ⁺ -InGaAs anode layer 11, which corresponds to the n-InP barrier layer. It is almost the same as the doping concentration of 12B.

Compared with the case of the profile (A) of FIG. 2 (qφ _B = 100 meV) which is the calculation result in the conventional example, in FIG. 4, the InP barrier portion in the vicinity of the InGaAs / InP hetero interface is restored to a triangular shape. I understand. The electric field intensity at the interface is calculated to be 7.1 kV / cm, and assuming that the mobility of InP is 4,000 cm ² / Vs, the effective diffusion rate can be estimated to be v _D = 2.8 × 10 ⁷ cm / s. it can. On the other hand, since the heat release rate is about v _th = 1 × 10 ⁷ cm / s and v _th <v _D , it can be understood that the electron transport mechanism of the HBD is limited by the heat release. Therefore, according to the present embodiment, good IV characteristics can be obtained by compensating for the “band bending due to electron charge” in the conventional HBD and improving the problem of “band flattening near the hetero interface”. Thus, the reception sensitivity in the THz frequency band can be improved.

FIG. 5 shows a second example of the profile of the conduction band edge of the HBD according to an embodiment of the present invention. It is the calculation result of the band profile when the doping of the n-InP barrier layer 12B is more appropriately adjusted and disposed. On the cathode side of the undoped InP barrier layer 12A, as the n-InP barrier layer 12B, a width 800A from the barrier layer 12A to the cathode layer 13A N _D (x) = 4 × 10 ¹⁶ / cm ³ to 4 × 10 ^{3 An} n-type impurity is introduced to increase the concentration exponentially to ¹⁷ / cm ³ .

  It can be seen that, compared to the band profile shown in FIG. 4, the InP barrier portion near the InGaAs / InP hetero interface has a better triangular shape. Since the energy distribution of electrons in the n-InP barrier layer 12B changes exponentially, a better band profile can be obtained than the above-described barrier layer 12 of constant concentration. Thus, according to the present embodiment, by compensating for the “band bending due to electron charge” in the conventional HBD and improving the problem of “band flattening near the hetero interface”, good I− V characteristics can be secured, and the reception sensitivity in the THz frequency band can be improved.

1, 11 high concentration n ⁺ -InGaAs anode layer 2, 12A undoped (= low concentration) InP barrier layer 3, 13A high concentration n ⁺ -InP cathode layer 4, 14 anode electrode 5, 15 cathode electrode 12B n- InP barrier layer 13B high concentration n ⁺ -InGaAs sub cathode layer

Claims (5)

A high concentration n-type first semiconductor layer,
A second semiconductor layer having a smaller electron affinity as compared to the first semiconductor layer;
A high concentration n-type third semiconductor layer is sequentially stacked,
A hetero barrier is formed on the second semiconductor layer side of the interface between the first semiconductor layer and the second semiconductor layer, and the barrier level of the hetero barrier is achieved by adjusting the doping level of the first semiconductor layer. The height is adjusted,
The semiconductor laminate structure includes an anode electrode formed on the first semiconductor layer, and a cathode electrode formed on the sub-cathode layer on which the third semiconductor layer is formed .
The detection diode characterized in that the second semiconductor layer includes an undoped barrier layer in contact with the first semiconductor layer and an n-type barrier layer in contact with the third semiconductor layer.   The detection diode according to claim 1, wherein the n-type barrier layer is doped with n-type impurities at a constant concentration.   The n-type barrier layer is introduced from the undoped barrier layer toward the third semiconductor layer such that the concentration of n-type impurity increases exponentially. Detection diode described in.   4. The detection diode according to claim 1, wherein the first semiconductor layer is made of InGaAs, and the second semiconductor layer is made of InP. High concentration n-type InGaAs anode layer,
An InP barrier layer having a smaller electron affinity as compared to the anode layer;
High concentration n-type InP cathode layers are sequentially stacked,
A hetero barrier is formed on the barrier layer side of the interface between the anode layer and the barrier layer,
The semiconductor laminate structure includes an anode electrode formed on the anode layer , and a cathode electrode formed on a sub-cathode layer on which the cathode layer is formed.
The detection diode characterized in that the barrier layer includes an undoped InP barrier layer in contact with the anode layer, and an n-type InP barrier layer in contact with the cathode layer.