JPS6140148B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in infrared sensing elements, particularly photovoltaic infrared sensing elements.

Generally, a narrow bandgap multi-component semiconductor such as mercury cadmium telluride (Hg ₁ -xCdxTe) is used as a constituent material of a semiconductor infrared sensing element. A photovoltaic infrared sensing element made of such a multi-component semiconductor, as shown in the conceptual diagram of FIG. An n-type semiconductor layer 2 having a conductivity type opposite to that of the substrate 1 is provided,
A diode is formed by forming a PN junction.
When infrared rays whose photon energy is larger than the energy gap of the element material are incident on the PN junction of this diode, the PN junction is caused by the internal photoelectric conversion effect.
A potential difference occurs between the junctions, and the diode electrodes 3, 4
A voltage Vo is generated between them.

This photovoltaic infrared sensing element can be represented by the equivalent circuit shown in FIG. Photocurrent I that is photoelectrically converted when infrared rays enter the PN junction of the sensing element
is I=Ri・φ(Ri: Responsibility, φ:
The response sensitivity (responsivity) Ri to the incident light is Ri = ηλq/nc (η: quantum efficiency, λ: wavelength, q: elementary charge). , h: Planck constant,
(c: speed of light), and quantum efficiency η of about 0.7 to 0.9 has already been achieved.

On the other hand, the voltage responsivity Rv of this sensing element is
is expressed as Rv=Ri·Zo (Zo: junction resistance), and for the same responsivity Ri, a sensing element with a larger junction resistance Zo will have a higher voltage responsivity.

Also, regarding sensitivity D ^* , if flicker noise is ignored, (Ad: light-receiving area, k: Boltzmann constant, T: element temperature, Isc: short-circuit current) Therefore, a larger junction resistance Zo gives a larger sensitivity D ^* .

It was found that the majority of the causes of the decrease in junction resistance Zo in current device structures are due to leakage current in the device.

In order to improve the above-mentioned drawback of lowering the _junction resistance, conventionally, as shown in FIG. After selectively thermally diffusing mercury (Hg) or the like to form an n-type semiconductor layer 2 having a conductivity type opposite to that of the substrate 1, the thermal diffusion process is performed to clean the exposed surface 6 of the PN junction. The extremely thin surface layer that has become rough due to an unnecessary increase in the mercury diffusion concentration, that is, the area indicated by the dotted line 5, is etched away, and the insulating layer is formed as shown in Figure 4, which is a cross-sectional view, and Figure 5, which is a top view. It is deposited over the insulating layer 7 of the electrode 3 so as to be connected to the peripheral edge 30 of the light-receiving surface 8 on which the light-receiving window is formed in step 7. Further, the other electrode 4 was formed by depositing gold (Au) or the like on the back side of the semiconductor substrate 1.

However, the effect of improving the bonding resistance by the above-mentioned etching process has hardly been seen in the past.

On the other hand, it is well known that a large surface inversion layer generally occurs at a portion where a PN junction is in contact with an insulating layer, such as a layer made of zinc sulfide (ZnS) or the like. This surface inversion layer is generated by an electric field based on negative ions in the insulating layer, similar to the surface depletion layer that occurs along with it, so the inversion layer, the depletion layer in contact with it, and the opposite conductivity type to the inversion layer. The part composed of these layers is also collectively called an electric field induced junction. In other words, in the vicinity of the PN junction on the surface of an actual semiconductor, an electric field-induced junction exists in addition to the PN junction.

FIG. 6 schematically represents this situation. The same parts as in Figure 3 are given the same symbols, but the other parts are: 14 is a metallurgical PN junction, 15 is a depletion layer extending on both sides, and 16 is distributed in the insulating layer. 17 is an accumulation layer generated by holes drawn to the semiconductor surface by the ions, 18 and 19 are an inversion region and a depletion layer generated in the N-type diffusion region, respectively, surrounded by dotted lines. The portion 20 is an electric field induced junction composed of the N type region 18 inverted to P type, the surface depletion layer 19, and the N type diffusion layer 2, or I _B and I _F are a PN junction and an electric field induced junction, respectively. This shows each current based on .

In other words, the leakage current is the component I _B based on the PN junction.
and the component I _F based on the electric field induced junction, I _B +
I _F , which is observed as a large value due to an effect equivalent to substantially increasing the junction area.

The reason why the junction resistance could not be improved even by etching the rough surface layer as described above suggests that the electric field-induced junction is the dominant source of leakage current.

The present invention has been made in view of these drawbacks, and it is an object of the present invention to provide a novel photovoltaic infrared sensing element.

An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 7 shows a cross-sectional structure of an infrared detecting element according to the present invention, and the steps up to the formation of the surface protection insulating layer 7 are the same as the conventional ones. The difference is that
When forming the insulating layer, refer to 7 in the figure.
A, 7b, and 7c have a three-layer structure, and when the formation of the first layer 7a is completed, an electrode having a cross section shown at 21a is provided, and then a second insulating layer 7b is formed.
21 again when this is completed.
The point is that an electrode having the cross section shown by b is provided, and a third insulating layer 7c is finally formed thereon. 21a,
The electrodes indicated by 21b are made of an alloy such as chromium, and terminals are drawn out from them so that they can be biased with respect to the substrate 1. The shape and planar arrangement of both electrodes 21a and 21b are as follows.
As shown in the figure, it is a loop with the same center,
The exposed surface 6 of the metallurgical joint is arranged in the gap between the two electrodes. Because of this structure, the electrode 21a will be called an internal gate, and the electrode 21b will be called an external gate.

FIG. 8 mainly shows the arrangement of the gates 21a, 21b and the current extraction electrode 3, and for the sake of understanding, the insulating film is omitted, and only the end image of the light-receiving surface opening is shown by the dotted line 25. It is shown as follows. Furthermore, a dashed dotted line 6 seen in the gap between both gates in the figure is an exposed portion of the PN junction. If represented on this plan view, the inversion layer 18 and the accumulation layer 17 shown in FIG. Corresponds to each position directly below. Note that since the current extraction electrode 3 is disposed on the uppermost layer of the insulating layer, it does not come into contact with both gate electrodes 21a and 21b.

Now, if an appropriate positive voltage of V ₂ is applied to the terminal Go, the electrode 21b will be positively biased with respect to the substrate, and due to this effect, the excess holes accumulated on the surface of the P-type semiconductor will be removed. It is driven into the bulk, and the surface becomes a flat band. Similarly, if a voltage of _V1 , also a positive value, is applied to the terminal Gi, the electrode 21a will be similarly biased positively, and the holes that have gathered on the surface of the N-type region directly under the electrode will be driven away. The P-type inversion layer disappears and the surface remains in a flat band state. However, the values of bias voltages V ₁ and V ₂ in this case must be determined depending on the degree of excess or deficiency of electrons and holes on both the N and P surfaces.

By applying bias voltages V ₁ and V ₂ of appropriate values to both gates in this way, the surface accumulation layer and surface inversion layer shown in FIG. can be returned to that of This means that the surplus junction area that had been increased substantially disappears, so the leakage current decreases due to the disappearance of the current I _F based on the field-induced junction, and as a result, this sensing element junction The value of the resistance Zo is increased and therefore the value of the sensitivity D ^* is also increased.

The reason why the gate is divided into outer and inner gates and the bias voltages V ₁ and V ₂ are set separately is for P-type and n-type gates.
In some cases, the voltage required to create a flat band is different, the ion distribution in the insulating film directly under each electrode is not uniform, the concentration is uneven or uneven, or there is a difference between the internal and external gates. This is based on the fact that fine voltage adjustment is required for each gate when each distance to the semiconductor surface cannot be ignored. If the ion distribution is uniform and the effect of ion concentration is dominant, the above two gates should be electrically connected and one bias voltage should be applied to them. is also possible.

In the photovoltaic infrared sensing element according to the present invention described above, the harmful and unnecessary surface inversion layer and surface accumulation layer near the exposed surface of the PN junction are replaced by two gate electrodes embedded in a protective insulating film and By adopting a structure that is destroyed by the applied bias voltage, it is possible to significantly reduce leakage current, thereby sufficiently increasing the resistance of the PN junction, and improving element sensitivity and responsivity. You get big benefits.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view for explaining the structure of a conventional infrared sensing element, Figure 2 is a principle diagram for explaining the equivalent circuit of a photovoltaic infrared sensing element, and Figures 3 and 4 are for conventional infrared sensing elements. Figure 5 is a cross-sectional view explaining the manufacturing process of the sensing element, Figure 5 is a top view explaining the structure of a conventional infrared sensing element, and Figure 6 is a schematic diagram showing the state of the inversion layer near the PN junction covered with an insulator. FIG. 7 is a diagram showing the cross-sectional structure of the infrared sensing element according to the present invention, and FIG. 8 is a diagram showing the structure and arrangement of the gate of the sensing element together with the current extraction electrode. 1: Semiconductor substrate, 2: N-type semiconductor layer, 3: Current extraction electrode, 8: Light-receiving surface, 16: Ions in insulator, 17: P-type accumulation layer, 18: P-type inversion layer, 1
9: surface depletion layer, 21a, 21b: voltage application gate, I _B : PN junction current, I _F : electric field induced junction current, Go, Gi: gate electrode terminal.

Claims (1)

[Claims] 1 On one surface of a semiconductor substrate made of multi-dimensional semiconductor,
In a photovoltaic infrared sensing element formed by forming a PN junction by forming a semiconductor layer of a conductivity type opposite to that of the substrate, an exposed surface line of the PN junction is formed in an insulating coating that covers and protects the surface of the PN junction. A loop-shaped electrode facing both the inside and outside of the exposed wire is embedded between the exposed wires, and a voltage application terminal connected to the loop-shaped electrode is led out, and is electrically separated from the loop-shaped electrode. A photovoltaic infrared sensing element, characterized in that one end of the conductor is connected to a part of the light-receiving surface of the reverse conductivity type semiconductor layer, and the other end is used as a detection current extraction terminal.