InGaAs Photodiode
A silicon photodiode is incapable of detecting the wavelength we intend to emit our data with. Silicon photodiodes can typically detect a maximum of 1000nm. Our system will be using a wavelength of at least 1300nm.
Other materials such as Germanium (Ge) and Indium Gallium Arsenide Phosphide (InGaAsP) were considered. Ge photodiodes have a high dark current – this is the major noise factor in reverse bias systems and hence this material will not be used.
InGaAsP is a modified Indium Gallium Arsenide Phosphide (InGaAs). It has a smaller spectral response range (1000-1350nm). InGaAsP are rare since InGaAs photodiodes cover the spectral range of InGaAsP and are of similar prices.
Indium Gallium Arsenide (InGaAs) can detect wavelengths from 900 – 1700 nm with peak responsivity at 1550nm. This makes it ideal for our optical system.
PIN or APD
PIN:
PIN (P-type Intrinsic N-type sandwich diode) is a P-N diode with an intrinsic layer in between the P and N regions.
The image above shows the basic structure of a PIN diode.
The P-type region consists of ‘holes’ and electrons are in the N-type region. In a reverse bias system, the depletion region decreases.
This means it is easier for the electrons from the N-region migrate to P-region in the diagram to the right. (note: diagram to the right is for P-N junction and is included to aid help understand the hole-electron equilibrium)
In a PIN junction the holes and electrons both travel into the I-layer. The applied voltage helps speed up the transfer of charge carriers, hence speeding up the diode’s operation time. Once the electrons and number of holes reach equilibrium the PIN diode will conduct a current.
APD:
An APD (avalanche photodiode) is similar to PIN except that it uses an internal gains system which requires a high voltage. This consequently means that APD has a higher dark current and overall noise current.
The very high voltages (100+ volts) causes the electrons initially generated to rapidly accelerate. As these electrons travel through the APD’s active region they collide into other electrons present in the semiconductor material. A fraction of these electrons become a part of the photocurrent because of the collision.
APDs are in general more expensive than PIN diodes when comparing like for like models. Another problem was that mass produced InGaAs APDs had a lower cut-off frequency. The lowest one I have found had 600MHz lower cut off frequency. Our LED source is capable of supplying a maximum of 100MHz. It is possible to custom order an InGaAs APD, however that is an unnecessary addition to the cost of the system as we do not require an APD for our system to function. The InGaAs PIN diode is able to detect a frequency below 400MHz.
An APD would provide a higher sensitivity. However we are not required to transfer the signal received into any form of output and therefore our only priority is to detect the signal emitted by the LED transmitter. An InGaAs PIN diode is perfectly capable of doing so.
A PIN diode is cheaper, has a lower noise and is compatible with our spectral and frequency. Therefore, our fibre communications system will use an InGaAs PIN diode.
Noise
In a photodiode there are two types of noise that have to be considered. These are ‘shot noise’ and ‘Johnson noise’.
The shot noise is dependent on dark noise. Dark noise is present in every photodiode with a reverse bias (voltage) applied across it. The dark noise increases with the voltage, it is also linearly proportional to the temperature of the detector.
The Johnson noise includes shunt, series and load resistance.
The total noise current is the mean square root of shot and Johnson noise.
Noise current = (Ij2 + Is2)1/2
Units of noise current is A/Hz1/2 (amps per square root hertz)
This can now be used to calculate Noise Equivalent Power (NEP). NEP is the power of the noise in the system. In other words, the detector must receive more power than the NEP to be able to generate a photocurrent which is higher than noise current.
The units of NEP is W/Hz1/2.
In reverse bias systems, dark current (therefore shot noise) is the dominant factor of the noise current.
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