Avalanche multiplication

Ikonopisov284 has conducted a systematic study of breakdown mechanisms in growing anodic oxides. He has enumerated factors significantly affecting the breakdown (nature of the anodized metal, electrolyte composition and resistivity) as well as those of less importance (current density, surface topography, temperature, etc.). By assuming a mechanism of avalanche multiplication of electrons injected into the oxide by the Schottky mechanism, Ikonopisov has correctly predicted the dependence of Ub on electrolyte resistivity and other breakdown features. 

Avalanche multiplication, in compound semiconductors, 22.T51-152 Avalanche photodiodes (APDs), 24 619 29 153 22 182... 

The formation of pores during anodization of an initially flat silicon electrode in HF affects the I-V characteristics. While this effect is small for p-type and highly doped n-type samples, it becomes dramatic for moderate and low doped n-type substrates anodized in the dark. In the latter case a reproducible I-V curve in the common sense does not exist. If, for example, a constant potential is applied to the electrode the current density usually increases monotonically with anodization time (Thl, Th2]. Therefore the I-V characteristic, as shown in Fig. 8.9, is sensitive to scan speed. The reverse is true for application of a certain current density. In this case the potential jumps to values close to the breakdown bias for the flat electrode and decreases to much lower values for prolonged anodization. These transient effects are caused by formation of pores in the initially flat surface. The lowering of the breakdown bias at the pore tips leads to local breakdown either by tunneling or by avalanche multiplication. The prior case will be discussed in this section while the next section focuses on the latter. 

Avalanche Photodiode (APD)—A photodiode designed to take advantage of avalanche multiplication of photocurrent. As the reverse-bias voltage approaches the breakdown voltage, hole-electron pairs created by absorbed photons acquire sufficient energy to create additional hole electron pairs when they collide with ions thus a multiplication or signal gain is achieved. 

A detector element in which amplification of photon generated charge takes place by avalanche multiplication is disclosed in US-A-4912536. 

Crystalline silicon devices take advantage of avalanche multiplication to enhance the gain without the problems of the secondary photoconductor. Carriers are accelerated under the field until their energy is high enough to cause impact ionization. The requirement is roughly that... 

Fig. 14.4. Schematic illustration of hot-electron effects (a) avalanche multiplication in the channel and (b) hot-electron-induced channel shortening...

Although there is yet no clear understanding of why a threshold electric field at a given region of the sample causes initiation, one plausible explanation is that the field produces an avalanching multiplication of charge carriers [30]. This is... 

TTS exists also in single photon avalanche photodiodes (SPADs). The source of TTS in SPADs is the different depth at which the photons are absorbed, and the nonuniformity of the avalanche multiplication efficiency. This results in differing delays in the build-up of the carrier avalanche and in different avalanche transit times. Consequently the TTS depends on the wavelength and the voltage. Moreover, if a passive quenching circuit is used, the reverse voltage may not have completely recovered from the breakdown of the previous photon. The result is an increase of the TTS width or a shift of the TTS with the count rate. 

The three curves for amplifier-limited operation have been calculated assuming typical amplifier normalized noise currents of 0.3,1, and 3 pA/a/Hz for the respective band-widths 1 kHz, I MHz, and I GHz. The applicable value depends upon the specifications of the chosen amplifier. Generally, the best performance is obtained by integrating the amplifier and detector on the chip. Avalanche multiplication can supply pre-amplification current gain and reduce the expected NEP by a factor from 10 to 100. At 1 GHz bandwidth, this can bring performance to within an order of magnitude of photon noise-limited behavior. 

Internal amplification of the photocurrent can be achieved with avalanche diodes, which are reverse-biased senuconductor diodes, where the free carriers acquire sufficient energy in the accelerating field to produce additional carriers on collisions with the lattice (Fig. 4.101). The multiplication factor M, defined as the average number of electron-hole pairs after avalanche multiplication initiated by a single photoproduced electron-hole pair, increases with the reverse-bias voltage. The multiplication factor... 

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