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.

Multi Segment LED

A further development of the standard LED package is the seven segment numerical indicator and the sixteen segment alpha-numeric indicator. In these devices, the PN junctions are elongated into a rectangular format and the light is emitted in a bar shape. The letter or number which a multi-segment display is required to produce is formed from a combination of illuminated segments.
Below shows the layout of the constituent light emitting diodes which are used in seven segment displays and  shows the layout for sixteen segment indicators. A sample of the letters or figures which may also be produced by some of the possible combinations of illuminated diodes is also illustrated.

Because these displays are composed of linear segments (that is, there are no curls or twists which can be produced), some anomalies could exist between similarly formed letters or numbers. Any combination which may introduce a misinterpretation is usually not specified in the equipment manual which covers the interpretation of the display.
As an example, the number 1 and the letter I could easily be read one for the other, and the distinction will be shown in the display dictionary.


The diode junctions which form the segments of the display require both an anode and a cathode connection. For a sixteen segment display this would result in thirty two connectors. Fortunately, most situations allow for a common connection to all the cathodes (or, alternatively, all the anodes). The displays are then referred to as common anode connected or common cathode connected and the number of leads reduces to eight or seventeen for the two types of display.

Light Emitting Diodes

Light emitting diodes are available in a wide variety of shapes and sizes, and they can be manufactured to fulfil a specific purpose or for general use. In size, they range from a miniature decimal point for use in multi-number configuration to a three-inch high single character display.


The A diagram shows the PN junction encapsulated in clear plastic with the light from the device focused by a lens.  The lens is usually made of self-colored plastic to match, or modify slightly, the colour being emitted by the junction.  The lens can be made to focus the light to a small pinpoint or to spread it over a wide area.

More detailed drawing of the PN wafer of the light emitting diode is shown in B, from this diagram, it can be seen that one face of the silicon is left clear of any obstructions or connecting wire which may impede the path of the light rays.  For a single indicator diode, the junction uses a square section of silicon.  During manufacture it is considerably easier to slice the silicon die into squares than it is to cut and trim complicated shapes; the desired outward appearance of the LED can be more easily obtained by shaping the lens of the device.  Thus, from a square section of silicon, plastic lens caps of square, rectangular, circular or star shape can achieve the shapes required for a particular indicator.