Photo Diodes

The photodiode is another light-sensitive device which utilises a PN junction.  It is constructed in a manner similar to the photovoltaic cell, but it is used in basically the same way as the photoconductive cell.  In other words it is used essentially as a light-variable resistor.

The photodiode is a semiconductor device (usually made from silicon) and may be constructed in basically two ways.  One type of photodiode utilises a simple PN junction as shown below.  A P-type region is diffused into an N-type substrate as shown.  This diffusion takes place through a round window that is etched into a silicon dioxide layer that is formed on top of the N-type substrate.  Then a metal ring or window is formed over the silicon dioxide layer (through an evaporation process) as shown.  This window makes electrical contact with the P-type region and serves as an electrode to which an external lead can be attached.  however, the window also accurately controls the area that will receive or respond to light.  A metal base is then formed on the bottom N-type layer.  This metal layer serves as a second electrode to which another lead is attached

  

When used as a photovoltaic cell, the device is said to be operating in the photovoltaic mode and it will generate an output voltage (across its electrodes) that varies with the intensity of the light striking it’s P-type layer.  However, the photodiode is most commonly subjected to a reverse bias voltage as shown in Fig 1A.  In other words its P-type region is made negative with respect to it’s N-type region. Under these conditions a wide depletion region forms around the PN junction.  When photons enter this region to create electron-hole pairs, the separated electrons and holes are pulled in opposite directions because of the influence of the charges that exist on each side of the junction and the applied reverse bias.  The electrons are drawn toward the positive side of the bias source (the N-type region) and the holes are attracted toward the negative side of the bias voltage (the P-type region).  The separated electrons and holes therefore support a small current flow in the reverse direction through the photodiode.  As the light intensity increases, more photons produce more electron-hole pairs which further increase the conductivity of the photodiode resulting in a proportionally higher current.  When a photodiode is used in this manner it is said to be operating in the photoconductive or photocurrent mode.

PN junction and PIN photodiodes are often mounted on an insulative platform or substrate and sealed within a metal case as shown in Fig 2.  A glass window is provided at the top of the case, as shown, to allow light to enter and strike the photodiode.  The two leads extend through the insulative base at the bottom of the case and are internally bonded (with fine wires) to the photodiode’s electrodes.