JPH0281480A - Semiconductor photodetector - Google Patents

Abstract

PURPOSE:To prevent a sudden change in a current value by a method wherein a low-concentration layer whose impurity concentration is low is formed to be adjacent to a light-absorbing layer and a variation in a width of a depletion layer by a variation in a bias voltage is absorbed. CONSTITUTION:A thickness of a light-absorbing layer is made a little thinner than that of a conventional layer so that a depletion layer can be extended easily up to a buffer layer; at the same time, an impurity concentration of an InP buffer layer to be grown on an InP substrate is lowered gradually according to a distance from a P-N junction. Such a concentration distribution of the InP buffer layer can be achieved easily when an amount of a dopant gas is increased gradually during an epitaxial growth operation by VPE, MOCVD or the like. During this process, the impurity concentration of the InP buffer layer may be at a definite low concentration without changing it continuously. In this case, the concentration of the buffer layer is set at 1X10<16>cm<-3> or lower; an extension of the depletion layer can be absorbed. Thereby, an increase in an electric current near a breakdown voltage of an APD becomes gentle; accordingly, a bias voltage can be adjusted easily and a peripheral circuit can be simplified.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) This invention provides an avalanche φ photodiode (A P
Regarding D).

(Prior Art) The concentration structure of a conventional APD and the electric field distribution in the operating state are shown in FIGS. 6(a) and 6(b). For example, when a reverse bias is applied to a PN junction formed in an InP layer, the depletion layer changes from the InP layer through the InGaAsP layer to the InGaAsP layer.
It extends to the As layer. When light is absorbed in the depleted InGaAs layer, electron-hole pairs are generated. These holes are accelerated by the electric field in the depletion layer and injected into the InP layer. The injected holes collide with the lattice in the InP crystal and generate new electron-hole pairs. When the electric field in the InP layer is sufficiently high, the acceleration of these holes and the generation of new electron-hole pairs occur in a chain reaction, resulting in multiplication of carriers. This is the operating principle of APD.

The IV characteristics of a typical APD are shown in FIG.

At low bias, the dark current and photocurrent gradually increase with increasing bias voltage. Breakdown voltage (vb
.

The multiplication factor of the APD is expressed as the ratio of the photocurrent at low voltage to the photocurrent at operating voltage, and a signal is output as the difference between the photocurrent and dark current at operating voltage. Therefore, the operating voltage of the APD is selected to be 0.9 to 0.95 Vb.

Here, there is a problem in that the operating voltage of the APD is in a region where the current value changes suddenly due to a slight voltage variation. That is, even a slight change in the bias voltage causes a sudden change in the multiplication factor. Furthermore, since the vb value varies depending on the temperature, the multiplication factor etc. also vary due to changes in the environmental temperature. Therefore, in order to set the APD's multiplication factor to an arbitrary value and maintain it, the bias voltage must be finely adjusted as needed, which makes the circuit configuration around the APD extremely complex. There was a problem.

(Issue 8 to be Solved by the Invention) In this manner, ordinary APDs attempt to solve the problem that the current value changes suddenly even with a slight variation in the bias voltage near the operating voltage.

[Structure of the Invention] (Means for Solving the Problems) The present invention provides a layer that absorbs variations in depletion layer width due to variations in bias voltage, so that even if there is a slight variation in voltage, the multiplication layer is By making sure that the electric field value does not change much,
Solve the above problem. One method is to provide a lightly doped layer with a low impurity concentration adjacent to the light absorption layer and extend the depletion layer to this layer. The impurity concentration of this layer is
This method of providing a low concentration layer, which is more effective if it is set so that the impurity concentration gradually decreases as it moves away from the PN junction, can also be realized by gradually decreasing the impurity concentration in the light absorption layer.

It can also be realized by gradually lowering the impurity concentration of the second conductivity type region forming the PN junction as the distance from the PN junction increases.

(Effect) Voltage is expressed as the integral of the electric field. When the degree of impurity in the layer is constant, the lower the impurity concentration, the longer the depletion layer becomes, so the maximum electric field value becomes lower. Therefore, if the impurity concentration in the region where the end of the depletion layer reaches is lowered, a larger voltage is required to extend the depletion layer, so the electric field value of the multiplication layer function will not change significantly depending on a slight change in the bias voltage. do not.

On the other hand, if the lIs is too low, the operating voltage itself becomes large, making it difficult to use it practically. Therefore, the impurity concentration of the low-concentration layer is made slightly higher near the PN junction, and gradually decreases as it moves away.By doing this, the extension of the depletion layer can be absorbed without increasing the operating voltage itself too much. can do.

(Example) A concentration structure diagram and an electric field distribution diagram of a first example of the present invention are shown in FIGS. 1(a) and 1(b). The structure is similar to conventional APD, but the thickness of the light absorption layer is made slightly thinner than before to make it easier to extend the depletion layer to the buffer layer. The impurity concentration is gradually decreased as the distance from the PN junction increases. Such a concentration distribution of the InP buffer layer can be easily achieved by gradually increasing the amount of dopant gas during the epitaxial growth by VPE, MOCVD, or the like. The IV characteristics at this time are shown in FIG. Unlike conventional APDs, a sudden increase in current near vb is not observed, and a gradual breakdown characteristic is exhibited.

The impurity concentration of the lnP buffer layer may be a constant low concentration instead of changing continuously like this. In this case, the effect of absorbing the extension of the depletion layer cannot be expected unless the concentration of the buffer layer is 1×10 16 cnI −3 or less.

The decrease in the impurity concentration of the InP buffer layer is shown in Figure 3(a).
, (b) may be performed in stages. In LPE growth, such a structure is formed by preparing several InP melts with different amounts of impurities and growing them in order from the one with the least amount of impurities.

In addition, the region where the impurity concentration gradually decreases and absorbs the elongation of the depletion layer is not limited to the InP buffer layer but also the InGaA
It may be applied to the s-light absorption layer, or it is also possible to absorb the elongation of the depletion layer only by the light absorption layer. The situation is shown in FIGS. 4(a) and 4(b).

Although the example above has been shown in which the elongation of the depletion layer is absorbed by the n-type layer, it is also possible to absorb the elongation of the depletion layer by the p-type layer as shown in FIGS. 5(a) and 5(b). This kind of structure is 8
It can be formed by ion-implanting an n-type impurity such as 1, for example, at 150 KeV, annealing at 700 to 750° C., and then diffusing Cd, Zn, etc. as in the conventional APD. Of course, it is also possible to combine the method of absorbing with the n-type layer shown above and the method of absorbing the elongation of the depletion layer with the p-type layer shown here.

As described above, in the invention, (i) the impurity temperature of the low concentration layer is set to PN
(ii) reduce the impurity temperature in the light absorption layer stepwise or continuously as you move away from the PN junction; (ii) reduce the impurity temperature in the light absorption layer stepwise or continuously as you move away from the PN junction; Alternatively, it is possible to implement a combination of methods of reducing the amount continuously.

Further, the present invention may be applied not only to InGaAs/InP APDs but also to any semiconductor system such as Sl, Ge, GaAs, GaSb, etc. without departing from the spirit of the present invention.

[Effects of the Invention] According to the present invention, the increase in current near the breakdown voltage of the APD becomes gradual, so the bias voltage can be easily adjusted and the peripheral circuitry can be simplified.

[Brief explanation of the drawing]

Figures 1 and 3 to 5 are diagrams showing the concentration distribution and electric field distribution of an APD that is an embodiment of the present invention, and Figure 2 is a diagram showing the ■V characteristics of an APD that is an embodiment of the present invention. , FIG. 6 is a diagram showing the concentration distribution and electric field distribution of the conventional APD, and FIG. 7 is a diagram showing the IV characteristics of the conventional APD.

Claims (2)

[Claims] (1) It has a light absorption layer of a first conductivity type, a carrier multiplication layer of the first conductivity type, and a semiconductor layer of a second conductivity type forming a PN junction in contact with the carrier multiplication layer. In the semiconductor light-receiving element, an impurity concentration of 1 x 10^1^6 cm is provided adjacent to the surface of the light absorption layer opposite to the PN junction.
1. A semiconductor light-receiving device having a structure in which the depletion layer extending from the PN junction penetrates into the low concentration layer when the device is in an operating state. (2) In the semiconductor light receiving element, the impurity concentration of the light absorption layer is 1×10^1^6 cm^-^3 or less,
2. The semiconductor light receiving element according to claim 1, wherein the semiconductor light receiving element has a thickness of 1.5 μm or less.