i. The silicon ‘reach through’ APD (RAPD) consists of p+–π–p–n+ layers as shown in Figure18.

ii. The high-field region where the avalanchemultiplication takes place is relatively narrow and centered on the p–n+ junction. Thusunder low reverse bias most of the voltage is dropped across the p–n+ junction.

iii. When the reverse bias voltage is increased the depletion layer widens across the p- region until it ‘reaches through’ to the nearly intrinsic (lightly doped) π-region.

iv. Since theπ-region is much wider than the p-region the field in the π-region is much lower than that at the p–n+ junction (see Figure 18(b)).

v. This has the effect of removing some of the excess applied voltage from the multiplication region to the π-region giving a relatively slow increase in multiplication factor with applied voltage.

Why it is called reach through APD?

i. To ensure carrier multiplication without excess noise for a specific thickness of multiplication region within the APD it is necessary to reduce the ratio of the ionization coefficients for electrons and holes k.

ii. Hence for minimum noise, the electric field at avalanche breakdown must be as low as possible and the impact ionization should be initiated by electrons.

iii. To this end a ‘reach through ‘structure has been implemented with the silicon APD.Hence called reach through APD.

The p–i–n photodiode:

i. In order to allow operation at longer wavelengths where the light penetrates more deeply into the semiconductor material, a wider depletion region is necessary.

ii. To achieve this the n-type material is doped so lightly that it can be considered intrinsic, and to make a low resistance contact a highly doped n-type (n+) layer is added.

iii. This creates a p–i–n (or PIN) structure, as may be seen in Figure 19 where all the absorption takes place in the depletion region.

iv. There are two junctions each between intrinsic and extrinsic material.

v. Forward resistance varies as forward current hence can be used as current controlled resistance.

vi. The junction capacitance in reverse biased is constant.

vii. It can be used as microwave switch operated by rapid change in bias.

viii. An APD differs from a PIN photodiode by providing internal photo-electronic signal gain.Therefore, output signal current, $I_s,$ from and APD equals $I_s= MR_0(1)P_s,$ where $R_0(1)$ is theintrinsic responsivity of the APD at a gain M=1 and wavelength l, M is the gain of the APD,and $P_s$ is the incident optical power.