# Si photodiode

### What factors determine photodiode response speed?

Photodiode response speed is determined by factors such as the CR time constant, carrier diffusion time, and carrier transit time in the depletion layer. Compared to short-wavelength light, long-wavelength light makes the carrier diffusion time longer, so the photodiode response is slower. The following methods improve the response speed.

1. Select photodiodes with lower terminal capacitance.
3. Lower terminal capacitance by applying reverse voltage. (Note that dark current increases with increasing reverse voltage.)

### Are areas other than the photosensitive area sensitive to light?

Yes, areas (such as gaps between elements) other than the photosensitive area are sensitive to light. That sensitivity does not match the sensitivity of the effective photosensitive area.

### What effects occur from light incident on areas other than the photosensitive area?

Areas other than the photosensitive area are sensitive to light, so light incident there can generate noise signals and delay the response speed. This problem can be eliminated by focusing the light to a small spot so that it does not enter the areas other than the photosensitive area or light-shielding the areas other than the photosensitive area. As an alternative, forming an aluminum light-shielding film or black light-shielding film on the areas other than the photosensitive area on the chip will also solve this problem.

### What is the value of the photodiode's series resistance (Rs)?

A resistance of several ohms.

### What is the photodiode's shunt resistance (Rsh)?

The shunt resistance of Hamamatsu photodiodes is given calculated by the following equation from the dark current (ID) measured when 10 mV is applied:

Rsh [Ω]=0.01 [V]/ID [A]

### How does spectral response depend on temperature?

In a wavelength region shorter than the peak sensitivity wavelength, the spectral response has almost no dependence on temperature. In a region longer than the peak sensitivity wavelength, however, it has a positive temperature coefficient.

### How does spectral response depend on temperature?

One factor that determines Si photodiode response speed is "carrier diffusion speed." When detecting long-wavelength light which penetrates into deep sections of Si photodiodes, the effect of the carrier diffusion speed becomes more significant, resulting in a slow response speed. When detecting short-wavelength light, however, the response speed is less affected by the carrier diffusion speed.

### What is the refractive index of borosilicate glass?

The refractive index is n=1.51.

### What is the refractive index of plastic packages?

The refractive index is n=1.53.

### What is the refractive index of epoxy resin used for ceramic packages?

The refractive index is n=1.53.

### What is the refractive index of silicone resin used for ceramic packages?

The refractive index is n=1.41.

### What method should be used to clean the photosensor after soldering it?

Resin is frequently used to seal photosensors (window adhesion and resin filling). So using a cleaning solvent may cause the resin to swell or melt, and in some cases the solvent may creep inside the package. So using no-clean solder that does not require cleaning after soldering is recommended.

### Do some photodiode products support lead-free solder?

We provide products that support lead-free solder. Soldering methods vary according to the product, so contact our sales office.

### Explain the precautions when wiping the photosensitive area.

The glass and resin sealant on the light input window is easily scratched and damaged. To clean it, wipe gently with a cotton swab (cotton bud) moistened with ethanol. Using the same cotton swab over and over may cause window scratches due to debris or foreign matter on the cotton swab.