Avalanche-Photodiode

Avalanche photodiodes or avalanche photodiodes (English avalanche photodiode, APD), are highly sensitive, fast photodiodes and are among the avalanche diodes. They use the internal photoelectric effect for charge carrier generation and the avalanche breakdown ( avalanche effect ) to the internal reinforcement. They can be regarded as the equivalent of the semiconductor photomultiplier and find application in the detection of very low radiation powers, down to single photons with attainable intrinsic frequencies up to the gigahertz range. The spectral sensitivity is depending on the material used in a range of about 250-1700 nm, from a diode type is always only a partial area can be covered.

• 2.1 VIS -NIR Si and Ge - APD
• 2.2 IR APD heterostructure of III-V compound semiconductors

• 3.1 Proportional radiation mode
• 3.2 Single Photon Counting

Design and function

Avalanche photodiodes are designed for a controlled avalanche, and similar in its construction PIN photodiodes. Unlike the pin - layer construction of these diodes is an additional narrow and highly doped p- or n-type layer, the space charge distribution modeled ( see middle picture) that following the intrinsic i-or π - layer is a region of very high electric field strength distribution is generated (see lower figure). This area acts as a so-called multiplication zone and generates the internal gain of the avalanche photodiode. A typical Si - APD having a p n IP doping profile, wherein the weakly p -doped or intrinsic i- π layer serving as the absorption area, as in the PIN diode. Upon application of a reverse voltage generated by the photons where free electrons drift in the multiplication region, that is generated by the space charge region of the p- n junction. The charge carriers are then greatly accelerated due to the prevailing high electric field strength and produce by impact ionization secondary charge carriers, which are then accelerated and in turn generate more charge carriers ( see bottom picture). Si avalanche photodiodes are operated with reverse blocking voltage, close to the breakdown voltage of several 100 V to achieve a gain of M = 100 ... 500 (multiplication factor). Above the breakdown voltage, it comes to an avalanche -like progression of the process described ( avalanche ), resulting in a ( short-term ) amplification factor of several million (see below: single photon counting ).

Multiplication and addition of noise factor

The gain as described above caused by impact ionization of the free charge carriers, wherein, depending on the material of both electrons and holes for reproduction in the multiplied zone, are used. The critical Ionisationskoeffizienten the electrons and the holes? P? N depending exponentially on the electric field strength. The charge carrier with the larger Ionisationskoeffizient be injected into the multiplication zone to achieve optimum and low noise amplification (eg for silicon? N > and? N? P for germanium and indium phosphide

Dependent on the applied voltage UR multiplication factor M is obtained below the breakdown voltage UBD approximately as follows (I · RS is the voltage drop across the series resistance of the diode):

Due to the statistical nature of the charge carrier multiplication, the gain is not constant and it is in addition to the thermal noise (Johnson - Nyquist noise) to an increased shot noise ( shot noise Sheet ). This may mean that, for large reinforcements degrade the signal -to-noise ratio. The additional noise ( engl. excess noise) is given with the addition of noise factor F ( M) as follows:

Where k is the ratio of the electrons and holes Ionisationskoeffizienten ( for? n >? p k =? p /? n or? n for? p < = k? n /? p ). It follows that in order to minimize the noise of the difference between the Ionisationskoeffizienten should be as large as possible. For Si k ≈ 0.02 and for Ge and III -V compound semiconductors such as InP is k ≈ 0.5.

Timing and gain-bandwidth product

The time behavior of an avalanche photodiode is determined by the drift processes in the depletion region and the setting up and dismantling of the charge carrier avalanche multiplication in the zone, which APDs are slower than pin or Schottky photodiodes. The transit or duration of the charge carriers in the multiplication zone largely determines the time constant Mk, which is directly proportional to the gain M, and the ratio of Ionisationskoeffizienten k (typical values ​​are of the order of ≈ 1 ps ). The constant gain-bandwidth product ( engl. gain bandwidth product, GBP, GBW or the UK) is given by:

It can be a gain-bandwidth product of 200 GHz for Si and Ge of about 30 GHz, and for InGaAs based APDs of> 50 GHz achieved. Furthermore, also in this case, the largest possible difference in Ionisationskoeffizienten of the charge carriers of beneficial as well as a narrow as possible multiplication zone.

Materials, structural and spectral sensitivity

VIS-NIR Si and Ge - APD

Silicon is the most commonly used material, since due to the large difference of the charge carriers Ionisationskoeffizienten particularly low-noise avalanche photodiodes can be produced. The spectral sensitivity ranges depending on the version of 300-1000 nm maximum sensitivity achieved NIR Si - APDs (500-1000 nm ) with a maximum spectral sensitivity at 800-900 nm optimized for the short-wave frequency range types can be used up to about 300 nm ( maximum spectral sensitivity at about 600 nm ), which is made possible by near-surface localized absorption zone. This is required because the depth of penetration of the photon decreases with decreasing wavelength. On the other hand, by the band gap Eg, the maximum detectable wavelength limited, and for the cutoff wavelength? G is given by:

For Si ( Eg = 1.12 eV at 300 K), a value of 1100 nm ( for λ >? g the corresponding material is transparent).

Avalanche photodiode in a wavelength range of over 1000 nm, as they are needed in the optical fiber telecommunications, germanium may be used. Due to the lower energy of the band gap of Eg = 0.67 eV (at 300 K), a spectral sensitivity range of 900-1600 nm is achieved. However, a disadvantage of Ge - APDs is the high additive noise factor ( the Ionisationskoeffizient the holes? P is only slightly larger than that of the electrons? N ) and the existing high dark current.

IR APD heterostructure of III-V compound semiconductors

For the optical fiber transmission technology in the second and third window (1300 and 1550 nm) have been developed avalanche photodiodes made ​​of III -V compound semiconductors, which have better properties than Ge APDs, but in the production are significantly more expensive. Lower additive noise factors and dark current are achieved through the combination of III -V compound semiconductors with different band gaps, the main representative of InGaAs / InP APDs represent. In so-called SAM structures (English separate absorption and multiplication ) is used as a multiplication zone indium (InGaAs ) as the absorption zone and indium phosphide (InP ). Typical layer structure is:

InP has due to its large band gap of Eg = 1.27 eV (at 300 K) a lower dark current and due to a more favorable ratio of the Ionisationskoeffizienten (? N

Set (at 300 K) from 0.4 to 1.4 eV.

For the absorption zone is, for example, In0, 53Ga0, 47As used having a band gap of Eg = 0.75 eV, thus, a similar spectral sensitivity range such as germanium can be reached ( 900-1600 nm). An extension of this range above 1600 nm ( L-band ) was achieved by increasing the In fraction in the absorption zone to In0, 83Ga0, 17As, wherein these APDs additional In0, 52Al0, 48AS - layer is used as a multiplication zone use.

Due to the discontinuity of the energy band at the border of the hetero- structure, a potential step that lead to the accumulation of the holes in the valence band, and a delay in the timing and for limiting the bandwidth of the APD. The remedy here is so-called SACM structures ( engl. separate absorption, degree ding and multiplication ), where between the absorption and multiplication zone InGaAsP ( degree thing ) is inserted layer with a band gap between the InGaAs and InP ( 0 75 to 1.27 eV) is located. A typical layer structure of a SAGM APD is after as follows:

Further developments are SAGCM structures ( engl. separate absorption, degree thing, chargesheet and multiplication ) and superlattice avalanche photodiodes, with further improvements in noise and gain characteristics.

In recent years, special avalanche photodiodes have been developed for the ultraviolet wavelength range of 250-350 nm, and the gallium nitride (GaN ) or ( 4H) silicon carbide based. Due to the large band gap of 3.37 eV and Eggan = Eg4H -SiC = 3.28 eV these APDs are relatively insensitive in the solar spectrum (german solar blind ) and in the visible spectral range. Thus you do not need expensive optical filters to suppress unwanted background radiation, as they are necessary for the photomultipliers or Si - APDs are typically used in this area. Further, they exhibit better properties than PMTs in harsh environments and high temperature applications, such as the detection or monitoring of flames ( inter alia, for gas turbines ) or to gamma ray detection with deep drilling the oil and gas exploration.

By means of metal-organic vapor phase epitaxy ( MOVPE ), can APDs in pin and SAM structure of gallium nitride and aluminum gallium nitride ( AlGaN ), for B. Al 0 36Ga0, 64N as absorption zone on sapphire substrates are produced ( with an AlN interface). It may be the quantum efficiencies of up to 45% ( at 280 nm) and could achieve the detection of individual photons are shown in the so-called Geiger mode.

Far superior in their properties are APDs of 4H -SiC. They are durable and exhibit low excess noise, due to the favorable ratio of Ionisationskoeffizienten of the charge carriers of k ≈ 0.1. In contrast to the direct band gap of GaN, the decrease in sensitivity to the visible spectral range is not as sharply defined. It can be quantum efficiencies of up to 50 % can be achieved ( at 270 nm) and also the detection of individual photons in the Geiger mode could be shown.

Operating mode

Radiation proportional operation

Below the breakdown voltage occurs in a reverse voltage -and temperature- dependent gain and avalanche photodiodes may be used to construct a highly sensitive photoreceptor to radiation power - proportional output voltage, the APD itself functions as a radiation power proportional to the power source. Although silicon APDs have higher equivalent noise power than, say, pin photodiodes (since the reinforcing effect is subjected to stochastic mechanisms ), but it can still lower noise photoreceiver be built with them, as with conventional photodiodes currently the noise contribution of the following amplifier is much higher than that the pin photodiode. APD it offered amplifier modules that compensate for the temperature-dependent gain factor of the APD by adjusting the reverse bias voltage.

Single photon counting

Avalanche photodiodes (APD ), which have been developed specifically for operation above the breakdown voltage in the so-called Geiger mode are referred to as SPAD ( for eng. Single- photon avalanche diode) or Geiger -mode APD (G- APD). They reach a temporary reinforcement of up to 108, as a signal generated by a single photon, electron-hole pair due to the acceleration in the multiplication region ( caused by the high electric field strength ) can generate several million carriers. By a corresponding wiring must be prevented that the high current through the diode is conductive ( self-preservation of the charge carrier avalanche ), which is realized by a series resistance in the simplest case. By the voltage drop across the series resistor, the reverse voltage across the APD, which thereby returns to the locked state ( passive quenching ) reduces. The process repeats itself automatically and the current pulses can be counted. The active quenching the reverse voltage is lowered active upon detection of a breakdown current in a few nanoseconds, with a special electronics. Thereafter re- activated by raising the reverse bias over the breakdown voltage the SPAD again. By the signal processing electronics of the dead times of approximately 100 nsec arise and it can therefore be of counting rates up to 10 MHz to realize.