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P-I-N diode vs Schottky Barrier Photodiode vs Avalanche Photodiode-Difference between P-I-N diode,Schottky Barrier Photodiode,Avalanche Photodiode

This page compares P-I-N diode vs Schottky Barrier Photodiode vs Avalanche Photodiode and mentions basic difference between P-I-N diode, Schottky Barrier Photodiode and Avalanche Photodiode.

All these diodes function as optical detectors or photodetectors. Function of photodiode is to convert light signal into either voltage or current based on mode of operation. Photodiode is designed to operate in reverse bias condition. It has two modes of operation viz. photoelectric effect and photocurrent.

Different type of materials are used in the manufacturing of photodiodes based on wavelength of operation as mentioned in the table below. Refer Photodiode vs Phototransistor➤ for more information.

Material EM spectrum Wavelength(nm)
Silicon 190 to 1100
Germanium 400 to 1700
Indium Gallium Arsenide 800 to 2600
Lead(II) Sulfide < 1000 to 3500

The diodes designed to use as photodiode will have P-I-N junction rather than P-N junction. They are packaged with window or connection with fibre so that light will reach the sensitive part of the device.

P-I-N Diode

P-I-N diode structure

The figure-1 depicts P-I-N diode structure. As shown it has very lightly doped layer referred as intrinsic zone between P and N doped layers. Hence device is known as P-I-N diode instead of P-N diode.

I-layer has very small amount of dopent and it acts as very wide depletion layer.

Typically P-I-N diode operates at any wavelength shorter than cutoff wavelength. When light falls, energy of absorbed photon must be sufficient enough to promote electron across the bandgap. Otherwise it will not get absorbed. Material will absorb photons of any energy which is higher than the bandgap energy. P-I-N diodes operate at different wavelengths with different materials used in the construction. The wavelenght bands are 500 to 1000 nm, 1250 to 1400 nm and 1500 to 1600 nm.

Schottky Barrier Photodiode

Schottky Barrier Photodiode structure

Sometimes it is impossible to realize P-I-N diodes for given wavelength band. Moreover performance of such diodes are not par to be used as optical detectors. In these situations, Schottky barrier photodiode is used.

The figure-2 depicts Schottky Barrier Photodiode structure. As shown thin metal layer replaces either P-region or N-region of the diode. Hence it is known as "metal-semiconductor diode".

Depending upon semiconductor and metal, a barrier is formed at the interface of these two materials. This barrier results into bending of the bands. Due to application of voltage, the bands can be bended more or less. In this region of band bending, electron hole pairs can easily be separated.

Avalanche Photodiode

Avalanche Photodiode structure

One way to increase sensitivity of the optical receiver is amplification. Avalanche Photodiode is used to amplify the signal in addition to optical detection process. The device operation is based on "Avalanche Effect". In the avalanche effect, highly accelerated electron will excite another electron with the use of "impact ionization".

Avalanche Photodiode regions

As shown in figure-3 and figure-4, Avalanche Photodiode structure consists of n+, p, π and p+ regions. Here there are two main regions. In region-1 electron hole pairs are generated and separated. In region-2 carriers are accelared and impact ionized.

• Let us understand opeartion of Avalanche Photodiode.
• When photons arrive, it will pass through thin n+p junction. The carriers will get absorbed in π-region. This absorption results into generation of electron-hole pairs in this n+p region.
• The electric field in π region is high enough which separates the carriers, but it is not high enough for charge carriers to achieve the energy required for multiplication to occur.
• The electric field in n+p region is sufficiently higher. Due to this charge carriers are strongly accelerated and will pick up energy.

As we know that carrier mobility of holes is significantly lower compare to electron mobility in silicon. Moreover impact ionized holes need to travel all way from n+p region to p+ region on right side while electron only need to travel upto n+ region only. Hence here probability of electron multiplication is comparatively much higher than probability of hole multiplication. Hence in Avalanche Photodiode electron mainly contribute for overall current.

Let us understand difference between Avalanche Photodiode(APD) and P-I-N diode:

• APD is basically a P-I-N diode with very high reverse bias voltage. APD will have about 50volt as reverse bias compare to P-I-N diode reverse biased to 3 Volt or less (in photoconductive mode).

• i-region in P-I-N diode is lightly n-doped. i-region in Avalance photodiode is renamed as π region and it is lightly p-doped.

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