DE2732040A1 - PHOTODETECTOR CIRCUIT ARRANGEMENT - Google Patents

Description

273204Ü

M.Chown 17-9

PHOTODETECTOR CIRCUIT ARRANGEMENT

The invention relates to a circuit arrangement for biasing an avalanche photodiode.

State of the art

The internal gain of an avalanche photodiode or S "avalanche photodiode (APD)" can vary greatly from the bias and the temperature. Therefore it is difficult »to find the required gain to be observed. One solution to the problem is to provide a constant current supply for the avalanche photodiode to be provided when the mean signal photocurrent is large compared to the leakage current. In this operating mode the receiver has a built-in automatic gain control. The gain changes to Compensating for fluctuations in the optical signal level, however, remains constant in the event of temperature fluctuations. This operating mode is suitable, for example, in digital systems when these are most unfavorable Conditions should be designed and if additional protection is provided. It consists of it however, the disadvantage that if the received signal exceeds the intended value, the signal-to-noise

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Ratio is only slightly improved, well the Gain remains well below the optimal value.

A constant reinforcement (without the internal reinforcement effect) can be achieved by applying a bias voltage due to the temperature fluctuation is matched to the bias voltage Vm required for a given gain M. One The possibility of doing this is to use a second, adapted Use avalanche photodiode which is masked so that it is not illuminated. The breakdown voltage in the reverse direction can be used as a reference voltage for the circuit for generating the bias voltage will. Receivers using this method are commercially available, but they are expensive. It is difficult to maintain tracking over large temperature ranges.

task

It is the object of the invention to provide a circuit arrangement _for biasing an avalanche photodiode which avoids the disadvantages mentioned.

solution

The object is achieved with the means specified in claim 1. Further training results from the Subclaims.

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In the solution according to the invention, the ratio of the two photocurrents is the gains of the two Photodiodes proportional. The constant of proportionality of the photo tones depends on the relative primary efficiencies of the two photodiodes and v> n the proportions of the optical signals, each on a photodiode fall. This factor is determined by the type and arrangement of the two photodiodes. To this Heise an electrical monitoring of the amplification is given, which in the form of a feedback the bias supply and thus the amplification of the controls the entire arrangement.

Because the avalanche photodiode is more sensitive than the second »non-amplifying one. Photodiode and in any case Receiving a stronger signal, one would generally get a good signal-to-noise ratio at the output of the expect second «non-amplifying photodiode.

In fact, it is quite possible that it is much smaller than one. In principle, this is not a problem, though the changes in the preload (e.g. to compensate for temperature fluctuations) are slow compared to the signal are. To limit the bandwidth for a better signal-to-noise ratio, the two signals are filtered. Low pass filtering might be suitable, but would be difficult prepare when the leakage current is a concern. This can be fixed if so instead is filtered so that the direct current component is suppressed. In this way you will be able to control the

Bias only uses signal components.

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description The invention will now be exemplified with reference to the drawings

explained in more detail.

Show it:

1 shows a photodetector arrangement according to the invention with discrete photo diodes,

2 shows a circuit arrangement for controlling the

Biasing an avalanche photodiode with the photodetector arrangement according to the invention,

FIG. 3 shows a connection to the circuit arrangement according to FIG alternative circuit arrangement,

4 shows a photodetector arrangement with an integrated Photodiode structure, and

Figures 5 and 6 show other integrated photodiode structures.

In the arrangement shown in Figure 1, there is a light distributor 1 in front of the avalanche photodiode 2, that it transfers a small but sufficient part of the received optical signal to a second, non-amplifying one Photodiode 3 throws. The electrical output signals of the avalanche photodiode APD and the second photodiode SPD can be used in a circuit 6, as shown in FIG. 2, to generate a control voltage which changes only slowly compared to the signal voltage. The output signal of the avalanche

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Photodiode is applied to a signal amplifier of a standard receiver. The output signals of the avalanche photodiode APD and the non-amplifying photodiode SPD are fed to a filter arrangement 5 by two Obtain control voltages V1 and V2, where V1 is proportional to α and the gain and V2 is proportional α and the mean value of the received signal. α is the portion of the optical signal that is directed to the non-amplifying photodiode SPD arrives. These two voltages VI and V2 are the input signals of the Biasing device 6 which biases to control the gain of the avalanche photodiode APD surrenders. This gain can thus either be set to a predetermined constant value or to a value which depends in a predetermined manner on the power of the received signal. Regulation to a value that is dependent on the received signal is preferred, since the optimum gain is achieved by Signal level is dependent.

if you opt for the constant gain method decides, you can supply the avalanche photodiode APD with a current that a Fixed multiple of the output current of the non-amplifying Photodiode is SPD. A simpler circuit arrangement can be used to keep the gain constant used, which is shown in Fig.3. With this circuit, the output current of the photodiode is SPD filtered, for example in a low-pass filter 7, and the

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filtered photocurrent I _{pD of} the photodiode SPD on it in an amplifier 8, which has a fixed gain factor k. The avalanche photodiode is supplied with this amplified current kI _pD.

The circuit arrangement according to Figure 3 is well suited for Integration of a complete optical receiver, with both photodiodes »the transistor amplifier and The connection lines are arranged on a single semiconductor chip that only has a reversed bias supply and some passive external components are required to preset the gain. Such a structure is suitable for integrating the two photodiodes, that of a so-called "penetration ^ Avalanche photodiode (English: "reachthrough avalanche photodiode") goes out. Such a structure, which is shown in Figure 4, consists of a body 9 of intrinsic semi-material, which at its one Surface a p> zone 1O and on the opposite Side has an n + - zone 11, through which a p-zone is diffused, so that a "buried" pn-junction consists. This known structure is changed for the implementation of the invention in that in the p + -zone a small n-zone 13 is diffused. The pn junction between zones 10 and 13 forms the non-amplifying one Photodiode SPD, the area and depth of this transition determines the proportion of the optical signal received by this photodiode SPD. If the absorption coefficient of the n-zone 13 is equal to τ ~ μm, a diffusion would occur

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up to a depth of 3 pm an absorption of 20% of the incident light. Only a single additional contact (on the photodiode SPD) is necessary, and the avalanche photodiode APD bias is between zones 10 and 12 in the usual manner applied, whereas the bias of the photodiode SPD between the zones 10 and 13 is applied.

FIG. 5 shows a structure that is alternative to FIG with a non-amplifying photodiode SPD, which is a has pin structure. The well-known "punch-through" avalanche photodiode has an intrinsic zone 14 within the p + zone 10 and an n-zone 15 within the intrinsic zone 14. If one assumes an efficiency of the pin structure, if the zones 15, 14 and 10 successively about 0.5 µm, 1.5 µm and 0.5 µm thick, approximately 85% of the incident light is irradiated into the area of the avalanche photodiode.

Another structure shown in Figure 6 goes

from a so-called "guard ring" - avalanche photodiode (English "guard ring avalanche photodiode"). The basic structure this avalanche photodiode contains a p-doped semiconductor body 16 with a diffused at the center n + zone 17 and a circular guard ring structure. Outside the guard ring structure there is a further circular η-doped zone 19 which forms the pn junction of the non-amplifying photodiode SPD. The circular n-zone 19 is covered with an opaque metallization layer 2O, which is covered by the p-doped semiconductor body 16 by an over

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this lying insulation layer 21 is isolated. However, the metallization layer 20 has contact with the n-zone 19. The entire structure is on a coating 22, which has a scattered reflection of the light that is not picked up by the avalanche photodiode. Consequently the non-amplifying photodiode SPD only receives light passing through the area of the avalanche photodiode has passed through, and is sufficiently strong, do reflected on the underside of the η-doped zone 19 to become. The metallization layer does not only serve as a contact for the non-reinforcing one Photodiode SPD, but also shields this from the incident light.

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Claims (6)

Dlpl.-Phys. Leo Thul
Short Street 8
7 Stuttgart 30 M.Chown 17-9 INTERNATIONAL STANDARD ELECTRIC CORPORATION, NEW YORK Claims / 1.) Circuit arrangement for biasing an avalanche photodiode, characterized in that, in addition to the avalanche photodiode (APD), a second, non-amplifying photodiode (SPD) is arranged in such a way that the greater part of the received optical signal is transmitted to the avalanche photodiode ( APD) and a smaller constant portion of the received optical signal on the second, non-amplifying photodiode (SPD) and that means (5,6; 7,8) are provided which from the electrical output signal of the second photodiode (SPD) a bias voltage for derive the avalanche photodiode (APD). 2. Arrangement according to claim 1, characterized in that the means are a low-pass filter (7) and an amplifier (8) with a constant gain (k) (Fig.3). 3. Arrangement according to claim 1, characterized in that in addition to the output signal of the second, non-amplifying photodiode (SPD), the output signal of the avalanche photodiode (APD) is used to obtain the bias voltage for the avalanche photodiode (APD) (Fig.2 ). 7? I! I977 709884/0860 ORIGINAL INSPECTED 273204Ü M.Chown 17-9 4. Arrangement according to one of the preceding claims, characterized / that the second non-amplifying photodiode (SPD) is arranged in the light incidence area (13, 15) of the surface of a "penetration" avalanche photodiode (Fig.4, Fig.5). 5. Arrangement according to claim 4, characterized in that the second, non-amplifying photodiode (SPD) is a pin photodiode (10, 14, 15, Fig.5). 6. An arrangement according to one of claims 1 to 3, characterized in that the second, non-reinforcing photodiode (SPD) in an annular structure (19) to the guard ring area of a guard ring avalanche photodiode (17, 18) and from the light incident on this Metallization layer (20) is shielded, which forms an electrical contact for the second, non-amplifying photodiode, and that the guard ring avalanche photodiode is arranged on a reflective surface (22) (Figure 6). 709884/0860