JPH0351115B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an n-i-p-i-n structure (p-i-n
Although the present invention is similarly applicable to the -i-p structure, the present invention relates to a light-receiving element of the n-i-p-i-n structure for simplicity.

The n-i-p-i-n element has a thin p-layer of around 100 Å formed in the i-layer of the n-i-n structure.
Since the majority carrier is an element mainly involved in conduction, it is expected to be a high-speed element, and recently, three-terminal elements using this structure have also been proposed.

Furthermore, this element can also be used as a highly sensitive photodetector element without excessive noise, and its range of applications is wide.

First, the operation of the n-i-p-i-n element will be explained. Figure 1 shows a conventional n-i-
This shows the thermal equilibrium state of the pin element and the band structure when voltage V is applied, and φ _BO is 2
represents the height of the potential barrier between two n layers.
The density J of the current that flows when voltage V is applied is expressed by the thermionic emission formula, J=A ^* T ² exp (-qφ _BO /kT) {exp (qα ₂ V/kT) − exp (-qα ₁ V/kT)} (1). Here, A ^* is the effective Richardson constant, T is the absolute temperature, k is the Boltzmann constant, q is the elementary quantity of electrons, and when α ₁ and α ₂ are the thicknesses of the two i layers, α ₁ = d ₁ / (d ₁ + d ₂ ), α ₂ = d ₂ / (d ₁ + d ₂ )
is given by If d ₁ ≠ d ₂ , asymmetry appears in the voltage-current characteristics. For this reason, like a normal pn junction diode, it can be called forward direction or reverse direction.

Next, the operation when this n-i-p-i-n element is applied to a light receiving element will be explained.

When there is no light irradiation, no holes are injected into the light receiving element, so the response speed of the light receiving element is extremely fast, and it is expected to be an ultra-high-speed element. On the other hand, when light irradiation is performed, holes that are photoexcited minority carriers gather in the p-layer. This works to reduce the value of the potential barrier φ _BO , so
The electron current flowing through the element increases. That is, a photocurrent flows due to light irradiation, and it has been reported that the sensitivity at this time is about 700 A/W, and the gain is about 1000 times as high. On the other hand, response speed values of 50 to 500 ps have been reported, and there are great expectations as a high-gain, high-speed light-receiving device.

However, in conventional structures, e.g. GaAs
In the case of an n-i-p-i-n photodetector using
Most of them had a barrier height of around 0.6 eV, and therefore the dark current tended to be relatively large. The potential barrier φBO is obtained by solving Poisson's equation, and is given by φ _BO =d ₁ ·d ₂ /d ₁ +d ₂ ·N _A ·X _A /ε _S ·q (2). Here, N _A is the acceptor concentration in the p layer, X _A is the thickness of the p layer, and ε _S is the dielectric constant.
To increase φ _BO , you can increase the difference between d ₁ and d ₂ (d ₁ < d ₂ ), increase d ₁ , increase N _A , and increase X _A. .
However, if _d1 is increased, the lifetime of the hole becomes longer and high-speed response is adversely affected.Increasing NA too much is problematic for crystal growth.
The layer only has to act as a barrier, and there were problems such as a loss of high-speed response when _XA was increased.

The present invention aims to reduce dark current and prevent fall degradation of photoresponse by increasing the potential barrier to majority carriers without causing such problems.

The present invention will be explained in detail below.

FIG. 2 shows a mesa-type n-i-p-i-
FIG.
is n ⁺ -GaAs substrate, 2 is n-GaAs layer (n10 ¹⁸ cm
^-3 , thickness approximately 1 μm), 3 is i-GaAs layer (p10 ¹⁴ cm
^-3 , about 2 μm thick), 4 is a p-G-Ga _0.7 Al _0.3 As layer (p10 ¹⁸ cm ^-3 , 100 Å thick), 5 is an i-GaAS layer (p 10 ¹⁴ cm ^-3 , about 1000 Å thick) ), 6 is n-GaAs
The layer (n10 ¹⁸ cm ^-3 , thickness approximately 1 μm), 7 and 8 are electrodes. FIG. 3 shows the band structure of this light-receiving element in a thermal equilibrium state. The height of the potential barrier seen from the n-layer 6 is n-i-p-i for GaAs only.
- Compared to the case where an n-structure is formed, it is almost as strong as GaAs.
Ga _0.7 Al _0.3 The difference in conduction band energy units between Ga 0.7 Al 0.3 As increases by ΔEc. The difference in forbidden band width ΔEg is ~0.4eV in this case, and assuming ΔEc0.85ΔEg,
ΔEC is 0.34eV. If other conditions are the same,
The current density is the value obtained by setting φ _BO to φ _BO + ΔEc in equation (1), and when ΔEc is 0.34 eV, it becomes approximately 10 ^-6 times at room temperature, and a significant reduction in dark current can be achieved.

〔Example〕

In FIG. 2, all semiconductors except the p-layer have the same composition. This structure reduces the dark current caused by electrons, but has little effect on the dark current caused by holes. However, when used as a hole receiving element, since the ratio of electron dark current to hole dark current is related to gain, it is also necessary to reduce the hole dark current. Examples according to the present invention show a structure in which dark current due to holes is also reduced. FIG. 4 shows a cross-sectional view of the semiconductor photodetector of this example, and this device has a wavelength of 0.9.
It is designed as a light-receiving element in the ~1.7 μm band. 10 is an n ⁺ -InP substrate, 11 is an n-InP layer (n
10 ¹⁸ cm ^-3 , thickness approximately 2 μm), 12 is n-AlAs _0.4
Sb _0.6 layer (n10 ¹⁸ cm ^-3 , thickness about 100 Å), 13 is i
-In _0.53 Ga _0.47 As layer (n10 ¹⁵ cm ^-3 , thickness approx. 1000
14 is a p-Als _0.4 Sb _0.6 layer (p10 ¹⁸ cm ^-3 , about 100 Å thick), 15 is an i-In _0.53 Ga _0.47 As layer (n
10 ¹⁸ cm ^-3 , thickness approximately 1000 Å), 16 is an n-InP layer (n
10 ¹⁸ cm ⁻³ and a thickness of approximately 1 μm), and 17 and 18 are electrodes. Here, the forbidden band width of AlAs _0.4 Sb _0.6 is approximately 1.9 eV
is larger than that of In _0.53 Ga _0.47 As and InP. FIG. 5 shows the band structure of this light receiving element in a thermal equilibrium state. The p-AlAs _0.4 Sb _0.6 layer 14 increases the potential barrier similarly to the light receiving element shown in FIG. 2, and the dark current due to electrons is significantly reduced. On the other hand, by forming the n-AlAs _0.4 Sb _0.6 layer 12, holes generated in a small number in the n-InP layer 11 are prevented from being diffused and injected into the i-In _0.53 Ga _0.47 As layer 13, Therefore, dark current due to holes is reduced. That is, two AlAs _0.4 Sb _0.6 layers are
-by forming in the structure of p-i-n,
Dark current caused by both electrons and holes can be reduced. Furthermore, when this structure is irradiated with light of 0.9 to 1.7 μm,
Light is absorbed only by the two i-layers, and is hardly absorbed by the n-layers on either side. If light absorption occurs in the layers on both sides and electron-hole pairs are generated, there is almost no electric field here, so they will mainly migrate to the i-layer by diffusion, and therefore the carrier movement due to this diffusion will be This results in deterioration of the falling edge of the optical response of the light receiving element. According to the present invention, by configuring the structure in which light absorption occurs only in the i-layer portion, it is possible to prevent such falling degradation of the photoresponse caused by current. Furthermore, even if a very small number of carriers are generated, the diffusion of minority carriers (holes in this case) in particular is prevented from being injected into the i-layer by the layer 12, so it is possible to sufficiently prevent the degradation of the photoresponse from falling. That is, by constructing the structure as shown in FIG. 4, a light-receiving element with low dark current and high-speed response gain can be realized.

In the explanation of the above embodiments, GaAs/
Two combinations of GaAlAs and InP/InGaAs/AlAsSb were used, but of course other semiconductor combinations,
For example, GaPSb, AlGaAsSb, AlInAsP,
Semiconductors such as AlPSb may be combined.
Furthermore, it is not limited to mesa type elements, but can also be applied to planar type elements.
Of course, it may be used as a light receiving element.

Such a structure can be sufficiently produced by molecular beam epitaxial growth for crystal growth and by conventional techniques for other processes.

As explained in detail above, according to the present invention, it is possible to fabricate a n-i-p-i-n photodetector that has low dark current and can prevent fall degradation of photoresponse, and has ultra-high speed and high sensitivity. It can be widely applied to light receiving elements.

[Brief explanation of drawings]

Fig. 1 is a band structure diagram of a conventional n-i-p-i-n element (a) at thermal equilibrium and (b) when a voltage V is applied. Fig. 2 is a band structure diagram for explaining the principle of the present invention. 3 is a band structure diagram of the semiconductor device of the present invention shown in FIG. 2 at thermal equilibrium; FIG. 4 is a sectional view of an embodiment of the present invention; FIG. 5 is a diagram of the embodiment of the invention shown in FIG. 4. The band structure diagrams at thermal equilibrium are shown for each. 1...n ⁺ -GaAs substrate, 2...n-GaAs layer,
3...i-GaAs layer, 4...p-Ga _0.7 Al _0.3 As layer,
5...i-GaAs layer, 6...n-GaAs layer, 7,
8...electrode, 10...n ⁺ -InP substrate, 11...n
-InP layer, 12... n-AlAs _0.4 Sb _0.6 layer, 13...
...i-In _0.53 Ga _0.47 As layer, 14...p-AlAs _0.4
Sb _0.6 layer, 15...i-In _0.53 Ga _0.47 As layer, 16...
... n-InP layer, 17, 18... electrode.

Claims (1)

[Claims] 1. A first semiconductor layer with a carrier concentration of 10 ¹⁷ cm ^-3 or more, a second semiconductor layer with a carrier concentration of 10 ¹⁶ cm ^-3 or less, and a third semiconductor layer with a carrier concentration of 10 ¹⁷ cm ^-3 or more and a thickness of about 100 Å. and the fourth below the carrier concentration 10 ¹⁶ cm ^-3
and a fifth semiconductor layer having a carrier concentration of 10 ¹⁷ cm ^-3 or higher are sequentially stacked, and the conductivity types of the first semiconductor layer and the fifth semiconductor layer are the same, and the conductivity type of the third semiconductor layer is the same as that of the third semiconductor layer. In a semiconductor element formed to have a conductivity type different from the conductivity type of the fifth semiconductor, the forbidden band width of the second semiconductor layer and the fourth semiconductor layer is the same as that of the third semiconductor layer. , and the forbidden band widths of the first semiconductor layer and the fifth semiconductor layer are larger than the forbidden band widths of the second semiconductor layer and the fourth semiconductor layer, and The first semiconductor layer is composed of two semiconductor layers having different forbidden band widths, and of the two semiconductor layers, the semiconductor layer having a larger forbidden band width and a thickness of 1000 Å or less is in contact with the second semiconductor layer. A semiconductor light-receiving element characterized by: