A photon activated transistor could be used to carry even  more current than a photodiode. In this case, the light energy  is focused on a collector-base junction. This frees electrons  in the depletion area and starts a flow of electrons from the  base that turns on the transistor. Once on, heavier current  flows from the emitter to the collector.  [Figure 11-39]  In  practice, engineers have developed numerous ways to use  the energy in light photons to trigger semiconductor devices  in electronic circuits.  [Figure 11-40]  Other more specialized FETs exist. Some of their unique  characteristics are owed to design alterations and others to  material variations. The transistor devices discussed above  use silicon-based semiconductors. But the use of other  semiconductor materials can yield variations in performance.  Metal semiconductor FETs (MESFETS) for example, are  often used in microwave applications. They have a combined  metal and semiconductor material at the gate and are typically  made from gallium arsenide or indium phosphide. MESFETs  are used for their quickness when starting and stopping  current flows especially in opposite directions. High electron  mobility transistors (HEMT) and pseudomorphic high  electron mobility transistors (PHEMT) are also constructed  from gallium arsenide semiconductor material and are used  for high-frequency applications.  Light emitting diodes (LEDs) have become so commonly  used in electronics that their importance may tend to be  overlooked. Numerous avionics displays and indicators use  LEDs for indicator lights, digital readouts, and backlighting  of liquid crystal display (LCD) screens.  Light contains electromagnetic energy that is carried by  photons. The amount of energy depends on the frequency  of light of the photon. This energy can be very useful in  the operation of electronic devices since all semiconductors  are affected by light energy. When a photon strikes a  semiconductor atom, it raises the energy level above what is  needed to hold its electrons in orbit. The extra energy frees an  electron enabling it to flow as current. The vacated position  of the electron becomes a hole. In photodiodes, this occurs in  the depletion area of the reversed biased PN junction turning  on the device and allowing current to flow.  Figure 11-38  illustrates a photodiode in a coil circuit. In  this case, the light striking the photodiode causes current to  flow in the circuit whereas the diode would have otherwise  blocked it. The result is the coil energizes and closes another  circuit enabling its operation. 11-18 Light Emitting Diodes Photodiodes and Phototransistors B C E − + B C E Photo transistor symbol Simple coil circuit − + Photodiode symbol Simple coil circuit Figure 11-39.  A photo transistor in a simple coil circuit (bottom)  and the symbol for a phototransistor (top).  Figure 11-40.  Phototransistors.  Figure 11-38.  The symbol for a photodiode and a photodiode in a  simple coil circuit. 