# Avalanche Photodiodes

**Avalanche Photodiodes**

Avalanche photodiodes also provide higher output currents than ordinary photodiodes by using avalanche multiplication (in electronics ‘avalanche’ is in fact defined as cascade multiplications of ions) to amplify the photocurrent Ip created by the junction photoeffect, as shown in eqn (3.5):

The value of the avalanche multiplication factor M, however, suffers from statistical variations (as does any avalanche multiplication device-see also Photomultipliers) and depends on the stability of the applied reverse voltage V_{R} at the avalanche voltage V_{a} (for V_{R} < V_{a} → M = 1 whereas for > V_{R} → M » 1). To achieve a high signal-to-noise ratio, therefore, a point on the characteristic must be found where M is sufficiently large (i.e. V_{R} as large as possible) but also where the avalanche current is due mainly to light generated carriers and not field induced carriers (i.e. V_{R} as small as possible). The balance between noise and gain is thus difficult to achieve, temperature stability is also poor and a highly stable bias voltage source is required (typically it needs to be stable to within 0.1% of its set value) thereby raising the cost and limiting its applications. The avalanche photodiode is, nevertheless, a very sensitive high frequency device (due to the high speed characteristic of the avalanche effect) and is therefore suitable for wide dynamic range, broadband light amplification such as is the case in the front-end of optical range finders (Chappel, 1976).

Optical transducers are based on the photoelectric effect and vary in cost and size from the subminiature photodiode to the wide angle photomul tiplier. The most popular optical transducers in the robotic field are usually spectrally matched Light Emitting Diode/Photodiode combinations, used mostly in proximity detectors, and the solid state array cameras, used for scene imaging and subsequent processing.