What are the basic characteristics of silicon photodiodes?

Silicon photodiodes are the simplest and most representative photovoltaic devices. Among them, PN junction silicon photodiodes are the most basic photovoltaic devices. Due to their high response speed, low dark current, junction capacitance and other characteristics, they are widely used in various fields such as photoelectric detection, sensing, and security inspection.

1. Structure

Photodiodes can be divided into two structural forms: 2DU type with P-type silicon as substrate, and 2CU type with N-type silicon as substrate.

Figure (a) is the principle structure diagram of the 2DU type photodiode.

Figure (b) is the working principle diagram of the photodiode

Figure (c) shows the circuit symbol of a photodiode.

2CU type silicon photodiode is made of N-type silicon single crystal. According to the size of the appearance, it can be divided into 2CU1, 2CU-2, 2CU3 and other models. Among them, 2CU1 and 2CU2 are larger in size, and 2CU3 is slightly smaller (see Figure 1 (a)). This type of photodiode is mostly packaged in a metal tube shell with a lens window. There are positive and negative electrode leads at the bottom, which are connected to the photosensitive surface (P-type layer) and N-type substrate in the tube core respectively. After the light enters from the window, it is focused on the tube core by the lens. Due to this focusing effect, the light intensity is enhanced, so that a large photocurrent can be generated.

2DU silicon photodiodes are made of P-type silicon single crystals. They are divided into 2DUA, 2DUB and other types in appearance. Among them, 2DUA tubes are smaller in size (see Figure 1 (b)). 2DU silicon photodiodes are currently mostly packaged in ceramic resin, and the window for incident light does not have a lens. There are three leads for this type of tube, namely the front electrode, the back electrode, and the ring electrode (see Figure 1 (b)). The front electrode is the lead of the photosensitive area (N-type area); the back electrode is the lead of the substrate (P-type area); the ring electrode is another electrode designed to reduce the dark current of the phototube and improve the stability of the tube. The dark current of the phototube refers to the reverse leakage current of the photodiode under the condition of no light and the highest operating voltage. We require that the dark current is as small as possible, so that the tube has stable performance and stronger ability to detect weak light.

2. Basic characteristics

The output characteristic curve of the photodiode under different bias voltages can be obtained from the current equation of the photodiode.

The working area of ​​the photodiode should be in the third and fourth quadrants of the figure. In optoelectronic technology, the method of redefining the positive direction of current and voltage is often used to rotate the characteristic curve to the following. The positive direction of the redefined current and voltage is the same as the direction of the built-in electric field of the PN junction.

(1) Spectral response characteristics

The current sensitivity of the peak response wavelength is usually used as the current sensitivity of the photodiode. The current response rate of silicon photodiodes is usually 0.4~05mA/mW. The spectral response range of Si photodiodes is 0.4~1.1mm and the peak response wavelength is about 0.9 mm

(2) Volt-ampere characteristics

As can be seen from the figure, the current is very sensitive to the change of photovoltage under low reverse voltage. This is because the increase of reverse bias widens the depletion layer and enhances the junction electric field, which has a great influence on the absorption rate of light in the junction area and the collection efficiency of photogenerated carriers. When the reverse bias is further increased, the collection of photogenerated carriers has reached the limit, and the photocurrent tends to saturation. At this time, the photocurrent has almost nothing to do with the external reverse bias, but only depends on the incident light power.

The photodiode has a good linear relationship between the incident light power and the photocurrent under a small load resistance. The figure shows the photocurrent output characteristics of the photodiode under a certain negative bias voltage.

(3) Frequency response characteristics

The frequency response of a photodiode is mainly determined by three factors:

(a) Diffusion time of photogenerated carriers near the depletion layer;

(b) Drift time of photogenerated carriers in the depletion layer;

(c) The circuit time constant determined by the junction capacitance Ci in parallel with the load resistance RL.

The frequency characteristics are better than those of photoconductive detectors and are suitable for detecting rapidly changing optical signals.