Photodiode

A photodiode or photodiode is a semiconductor diode that visible light - in some embodiments, also IR, UV or X-rays - converts at a pn junction or pin junction by the internal photoelectric effect in an electric current, or - depending on the wiring - this provides an illumination-dependent resistance. It is used, inter alia, to convert light into an electrical voltage or an electrical current, or to receive light transmitted information.

Construction

Photo diodes are made, for example, silicon ( visible light wavelength to about 1 micron ), germanium (Ge; infrared to about 1.8 microns wavelength ), or other, by now also organic semiconductor detector materials. For a similar wavelength range as the germanium photodiodes are from the more suitable material indium gallium arsenide (InGaAs ) are common.

Photodiodes for the area of the mid-infrared (wavelength 5-20 microns ) are produced ( CdTe, Ge: Au diodes). However, you must then be cooled with liquid nitrogen, for example, because the thermal motion at room temperature is sufficient to raise electrons from the valence band to the conduction band. Thereby, the dark current of the photodiode at room temperature is so large that the signal to be measured goes down therein. A second reason for cooling is otherwise held superposition of the IR radiation of the sensor housing itself

Photodiodes for light measurement have a daylight filter that limits the sensitivity in the red and infrared spectral range and adapts the sensitivity curve of the eye. In contrast, photodiodes have to receive infrared signals (as in remote controls) a daylight - blocking filter. For example you are cast in dark colored resin and are thus protected from interference from visible light.

A typical silicon photodiode is composed of a weakly doped n- base material having a more heavily doped layer on the back, which forms the contact (cathode). The photosensitive surface is defined by a region with a thin p-doped layer at the front. This layer is thin enough so that most of the light can pass through the pn junction. The electrical contact is most often at the edge. On the surface of a protective layer as passivation and antireflection film. Also often located in front of the photodiode a light transmissive windows or it is in transparent potting material.

PIN photodiodes have the intrinsic layer between the p- and n-type layer generally has a higher permissible reverse voltage and reduced junction capacitance CS. Thereby the bandwidth is increased.

Unlike photoresistors ( LDR) photodiodes have much shorter response times. Typical cut-off frequencies of the photodiodes is at 10 MHz, wherein the PIN photodiode in excess of 1 GHz.

The lateral diode is a special design of a photodiode to detect, for example, the position of a laser beam.

Function

Make photons of sufficient energy to the material of the diode, then the charge carriers ( electron -hole pairs ) are generated. In the space charge region, the charge carriers drift fast against the diffusion voltage in the same way doped regions, and lead to a stream. Carriers generated outside the space charge zone can also contribute to the current. But you must go only by diffusion to the space charge region. A portion is lost by recombination and there is a small delay. Without external connection of the terminals formed in this measurable voltage of the same polarity as the forward voltage ( saturation). If the terminals are electrically connected to each other or are placed at a reverse voltage of the diode, passes a photocurrent which is proportional to the incident light.

The photon must have an energy higher than the band gap to cause this effect (in the case of silicon, for example, greater than 1.1 eV).

The photocurrent is linear to the light over many orders of magnitude, if no saturation occurs. Ideally transmits every photon having an energy that is greater than the characteristic energy gap ( band gap ) of the semiconductor, wherein the current. Practically, however, the value is smaller, and is referred to as quantum efficiency. The reaction time is very short, with suitable wiring; it may be as low as a fraction of a nanosecond.

When a voltage is applied in the reverse direction of the diode from the outside, a small current flows even in the dark. This is called the dark current (ID). It depends exponentially on the temperature of the photodiode. The dark current characteristic is an important quality characteristic of photodiodes.

An extension represents the phototransistor, consisting of a combination of a photodiode and a bipolar transistor in principle, and in contrast to the photo-diode has a higher sensitivity.

Modes

Photodiodes can be used in the following three modes:

Operating as a photo element

Photodiode provides electrical energy. In this role, she is a picture element in large-scale production of the photodiode is called a solar cell. Without load it is in saturation and the voltage approaches a limit to ( open-circuit voltage UL), which depends little on the light intensity. With increasing load ( RL is smaller) decreases the voltage and the current aims of his hand a limit ( short-circuit current IK ) to. At the bend of this characteristic is present power adjustment - the desired operating point for photovoltaic systems (English Maximum Power Point). In this mode, the photodiode is relatively slow and is not suitable for detection of fast signals. This type of circuit is used to measure the brightness, such as lighting instruments ( light meter, lux meter ) used.

In contrast to the photo resistor ( LDR) No external power source is needed. CCD sensors in a large part of the sensor surface is filled with photo- diodes, each charging a capacitor connected in parallel. If the stored charge is removed well before the saturation voltage of the photodiode is achieved, the load is proportional to the brightness. The cutoff frequency is low.

Operating in the quasi - short circuit

If the photodiode operated in short circuit ( U = 0), it provides a linearly dependent over many orders of magnitude of the irradiance power in the reverse direction ( I ≤ 0). This is often connected to a transimpedance amplifier - a circuit that provides a virtual short-circuit of the photocurrent generates a proportional voltage signal and to the diode terminals. This irradiance can be measured very accurately. Because the voltage does not change at the photodiode, no capacity is reloaded. This high cutoff frequencies are possible.

Operating in the stopband

If you put on the photodiode at a reverse voltage (U ≤ 0), then flows linearly dependent on the light-blocking power, that is, upon irradiation they also conducts in the reverse direction ( I ≤ 0). This mode is usually selected for photodiodes in CMOS integrated sensors. The following effects are still characteristic of the stopband:

• The barrier layer capacitance C decreases with the applied voltage, so that the response time decreases with increasing voltage. Thus, high cut-off frequencies can be achieved.
• May be an avalanche effect occurs, which amplifies the photo-current by the avalanche effect. ( See also avalanche photodiode )
• The leakage current ( dark current ID) increases with the applied voltage and the temperature; he superimposed the photocurrent and determined at low irradiation significantly the noise.
• Since the differential resistance is very large, the current hardly depends on the operating voltage.

Characteristics and Applications

Exemplary characteristics are used to describe a photodiode, in brackets as an example the values ​​of the silicon photodiode BP 104:

• Permissible reverse voltage (20 volts)
• Spectral range of photosensitivity ( 400 to 1100 nm)

Applications are in light meters with a large-area selenium - photodiode, which directly feed a moving coil instrument sensors in digital cameras, receiving elements for optical waveguides.

Worldwide research activities focus in particular on developing cheaper solar cells, improved CCD and CMOS image sensors as well as faster and more sensitive photodiodes for fiber-optic communication networks.