Avalanche photodiode

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention overcomes the disadvantages of the conventional avalanche photodiode.
The avalanche photodiode of the present invention comprises: a photo-electric conversion region made of a semiconductor having an energy band gap that is smaller than the photon energy; and a carrier multiplying region made of a semiconductor that differs from the semiconductor of the photo-electric region and has .
alpha.
/.
beta.
>>1 or .
alpha.
/.
beta.
<<1 as an impact ionization coefficient ratio, whereby an avalanche photodiode having a high quantum efficiency and a low noise index in the multiplying step is obtained.
Referring to the figures in the drawing, the present invention will be illustrated.
In FIG.
1, the reference letter (A) designates a N type high impurity content semiconductor layer and (B) designates a P type impurity semiconductor layer having .
alpha.
/.
beta.
>>1 of an impact ionization coefficient ratio.
The semiconductor layer (B) forms the carrier multiplying region and the semiconductor layers (A) and (B) are connected by P-N junction (1).
The semiconductor layers (A) and (B) are usually formed by using the same crystalline substrate, however, it is not always necessary to use the same crystalline substrate.
The reference (C) designates a semiconductor layer having an energy gap that is smaller than the photon energy upon exposure to the incident light and having a higher specific resistance than layer (B).
The semiconductor layer (C) forms a photo-electric conversion region made of a P type impurity semiconductor substance that differs from the semiconductor layers (A), (B).
The reference (2) designates a heterojunction for connecting the layer (B) and the layer (C).
The reference (D) designates a P type high impurity content semiconductor layer connected to the semiconductor layer (C).
The layer (D) can be made of the same semiconductor substance as that of the layer (C).
The reference numerals (3) and (4) designate electrodes and the arrow line designates the incident light