Photodiodes And Photocells
Photodiodes And Photocells
These are photovoltaic devices based on the junction photoeffect, that is the creation of optically induced electron-hole pairs within a P-N junction. It is therefore useful at this point to retrace some of the basic steps of semiconductor physics.
Electron-hole pairs are generated by thermal agitation throughout a semiconductor material and are, by their nature, in continuous random movement. Upon formation of a P-N junction (e.g. by diffusion of an element of opposite type to that of the bulk dopant) some of the mobile carriers from one side cross the junction and recombine with some of the mobile carriers on the other side (i.e. electrons from the N region recombine with holes from the P region) leaving behind fixed positive and negative ions. The process continues and this thin region either side of their junction, now depleted of mobile carriers (and thus termed ‘depletion’ region), widens until the electrical potential created by the fixed ions is sufficient to prevent any further diffusion (i.e. junction crossing) by the mobile carriers. The junction is now in thermal equilibrium and has acquired a so called ‘barrier potential’ (this however cannot be measured since it is cancelled out by the external connections contact potential). If the material is now irradiated, any electron-hole pairs produced by the internal photoelectric effect within the depletion region are swept across the junction by the electric field associated with the barrier potential thereby producing a current flow, as illustrated in Figure 1.
This injection of optically induced majority carriers therefore reduces the barrier potential and, since the connections contact potential is still the same, it produces a measurable voltage across the junction. The device thus created is therefore an electrical generator capable, as shown in Figure 2, of driving a current through a suitably connected external load, with the P terminal becoming positive with respect to the N terminal.
The photocurrent which has been caused to flow across the junction (Le. the rate of change of the majority carriers crossing it) is approximately proportional to the rate at which the light quanta impinge on the photovoltaic device and therefore it increases with the intensity of the illumination. This is shown diagrammatically in Figure 3.6 under different lighting conditions.
When the device has no load connected to it the external photocurrent must, naturally, be zero and therefore all the current flows through the diode in the forward direction. This produces an ‘open circuit voltage’ V0c which is a logarithmic function of the photocurrent Ip and has an upper limit of 0.6 volts for silicon devices (a photovoltaic device used with a very large load resistor will therefore act as a logarithmic voltage source). When the device is short circuited, on the other hand, the maximum current that can flow from the P to the N region is the short-circuit current Isc which is equal to the photocurrent Ip (a photovoltaic device used with a small load resistor will therefore act as a linear current source). This behaviour is illustrated by re-drawing the fourth quadrant of the device I-V characteristic (shown in Figure 3) to highlight the open and short circuit operating points under different illumination conditions, as shown in Figure 4. This is the principle of operation of a silicon photocell.
The operation of a photodiode is very similar, indeed it differs only in that it is used with a reverse bias. This means that a larger depletion region is formed either side of the junction and only a small reverse current flows in the absence of any illumination (the so called ‘dark’ current). When the junction is irradiated by light of a suitable wavelength, the additional carriers thus created raise the reverse current by a value proportional to the incident radiation therefore yielding similar curves to those of the photocell (as shown previously in Figure 3, third quadrant).
A photo diode can therefore be used as a photocell in the absence of an external bias but exhibits a smaller junction capacitance under reverse bias (i.e. a wider gap between the virtual ‘plates’ of the stray capacitor) and is therefore suitable for high-speed applications such as light pulse measure ment, as required in optical rangefinder systems (see Chapter 6 for further details). The photo diode frequency response is in fact governed by its effective load resistor and the junction capacitance, as shown in eqn (3.2):As well as the basic junction photodetectors described so far, such as the photocell and photodiode, there are other more sensitive devices also based on the junction photo effect but provide a higher current output for the same illuminance input value. These devices, in other words, have an amplification function associated with the basic junction photoeffect and, as shown in Figure 5, are known as amplifying photodetectors. The most common examples of such devices are the photo transistor and the avalanchephotodiode.