A photodiode is a component Semi-conducteur having the capacity to detect a Rayonnement field Optique and to transform it into Electrical signal.

General information

Like all Diode in electronics, it consists of a Jonction PN. This basic configuration was improved by the introduction of an intrinsic zone (I) to constitute the photodiode PINE. In absence of polarization (called photovoltaic mode) it generates a tension. In opposite polarization by an external food (photoamperic mode), it generates a current. 3 distinct areas are located:

1. a Zone of space charge (ZCE)
2. a neutral area of Standard NR
3. a neutral area of Standard P.

This component concerns the Optoélectronique.

Operation

When a semiconductor is exposed to a Flux luminous, the Photon S are absorbed provided that the energy of the photon (E_ph) is higher than the prohibited bandwidth (E_g). This corresponds to energy necessary to the electron to release itself from the barrier of potential which maintains it in the solid. The existence of the forbidden band involves the existence of a threshold of absorption such as $h\; \backslash \; nu\_0\; =\; E\_g$. During the absorption of a photon, two phenomena can occur:

• the Photoemission: it is the exit of the electron out of photosensitive material. The electron cannot leave that if it is excited close to surface.
• the Photoconductivity: the electron is released inside material. The electrons thus released contribute to the electric conductivity of material.

When the photons penetrate in the semiconductor provided with a sufficient energy, they can create Photoporteur S in excess in material. An increase in the current then is observed. Two mechanisms intervene simultaneously:

• There is creation of minority carriers, i.e. electrons in the area P and of the holes in the area NR. Those are likely to reach the ZCE by diffusion and to be then propelled towards zones where they are in a majority. Indeed, once in the ZCE, polarization being opposite, one supports the passage of minority towards their zone of predilection. These carriers thus contribute to create the diffusion current.
• There is generation of pairs electron hole in the ZCE, which dissociates under the action of the electric field; the electron joining the zone NR, the hole the P. zone This current is called the current of transit or photocurrent of generation.

These two contributions are added to create the I_ph photocurrent which is added with the reverse current with the junction. The expression of the current crossing the junction is then: $I\_d\; =\; I\_s\; (e^\; \{E\_g\; \backslash \; over\; N\; U\_t\}\; -\; 1)\; -\; I\_\; \{pH\}$

Electric characteristics

A photodiode can be represented by a power source I_ph (depend on illumination), in parallel with the capacity of junction C_j and a resistance of R_sh shunt of a high value (characterizing the escape of current), the whole being in series with a resistance interns R_s:

• Resistance of shunt: the resistance of shunt of an ideal photodiode is infinite. Actually this resistance lies between 100 kΩ and 1 GΩ according to the quality of the photodiode. This resistance is used to calculate the leakage current (or noise) in photovoltaic mode, i.e. without polarization of the photodiode.
• Capacity of junction: this capacity is due to the zone of load; it is inversely proportional to the width of space charge (W): $C\_j\; =\; \{\backslash \; delta\_\; \{SC\}\; \backslash \; over\; W\}\; A$. Where has is the generated surface of the photodiode. W is proportional to polarization reverses and the capacity decreases if polarization increases. This capacity oscillates around 100 PF for weak polarizations with a few tens of PF for high polarizations.
• internal Resistance: this resistance is primarily due to the resistance of the substrate and resistances of contact. R_s can vary between 10 and 500Ω according to the surface of the photodiode.

Other characteristics:

• Response time: it is usually defined like the time necessary to reach 90% of the final current in the photodiode. This time depends on 3 factors:
• t_transit : time that course carriers in the zone of space charge.
• t_diffusion: time that course carriers in the neutral areas.
• the time-constant t_τ: time-constant of diagram are equivalent (of R_S resistance + R_C and C_j capacity + C_γ): $t\_\; \backslash \; tau\; =\; (R\_S\; +\; R\_C)\; (C\_j\; +\; C\_\; \backslash \; gamma)$. Thus the time-constant is equal to: $\backslash \; sqrt$. But each time is difficult to determine; only total time is taken into account. In general the time of diffusion is slower than the time of transit.
• Photosensitivity: it is defined by $S\_\; \{pH\}\; =\; \{\backslash \; Delta\; I\_\; \{pH\}\; \backslash \; over\; \backslash \; Delta\; E\}$ and determines the conditions of use (200nA/Lux for the photodiodes with germanium (Ge), 10nA/Lux for the photodiodes with silicon (Si)). The photodiodes Ge have a more important photosensitivity but their dark current is notable I₀ = 10 uA. It is thus preferable to use photodiodes If (I₀ = 10 Pa) for the detection of weak illuminations.
• Output of capture: it is the report/ratio of the number of elementary charges crossing the junction on the number of incidental photons. This output depends on the wavelength of the radiation and the parameters of construction of the component. It will define the spectral field of use of the detector.

Optimization

To have a better quantum effectiveness, the majority of the photoporteurs will have to be created in the ZCE, where the rate of recombination is weak. One gains there thus on the level of the response time of the photodiode. To carry out this condition, the photodiode must have a frontal zone as mean as possible. This condition limits the quantity of radiation however absorptive. It is thus a question of making a compromise between the quantity of radiation absorptive and the response time of the photodiode: generally $W\; \backslash \; geq\; \{1\; \backslash \; over\; \backslash \; alpha\}$. W being the width of the ZCE and α, the coefficient absorption.

We have just seen the interest to have a zone of sufficiently large space charge so that the photocurrent is primarily created in this zone and sufficiently mean so that the time of transit is not too important. One can however increase artificially by intercalating an intrinsic area I between the areas of the type NR and type P. This led to another type of photodiode: the photodiodes PINE. If the opposite polarization of the structure is sufficient, an important electric field exists in all the intrinsic zone and the photoporteurs reach their speed limit very quickly. Very fast photodiodes thus are obtained. Moreover, the electric field in the area of déplétion (the ZCE) prevents the recombination of the carriers, which makes the photodiode very sensitive.

Case of the phototransistors

A phototransistor is a bipolar Transistor whose Base is sensitive to the light radiation; the base is then known as floating since it is deprived of connection. When the base is not lit, the transistor is traversed by the I_CE0 leakage current. The illumination of the base led to a I_ph photocurrent which one can name current of ordering of the transistor. This one appears in the junction collector-bases in the form: $I\_c\; =\; \backslash \; beta\; I\_\; \{pH\}\; +\; I\_\; \{CE0\}$.

The current of illumination of the phototransistor is thus the photocurrent of the photodiode Collecteur - bases multiplied by the profit β of the transistor. Its photosensitive reaction is thus definitely higher than that of a photodiode (from 100 to 400 times more). On the other hand the dark current is more important.

One observes another difference between phototransistor and photodiode: the base of the phototransistor is thicker, which involves a more important time-constant and, therefore a Frequency cut-off lower than that of the photodiodes. One can possibly increase the cut-off frequency by decreasing photosensitivity by connecting the base to the transmitting .

See too

• Electric eye

External bonds

• File on the photodiodes on the electronic site ABC

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