the effective mass of the electrons in the third band is higher than those in the usual conduction band, so the mobility or drift velocity of the electrons in that band is lower. As the forward voltage increases, more and more electrons can reach the third band, causing them to move slower, and current through the device decreases. This creates a region of negative differential resistance in the voltage/current relationship. When a high enough potential is applied to the diode, the charge carrier density along the cathode becomes unstable, and will develop small segments of low conductivity, with the rest of the cathode having high conductivity. Most of the cathode voltage drop will occur across the segment, so it will have a high electric field. Under the influence of this electric field it will move along the cathode to the anode. It is not possible to balance the population in both bands, so there will always be thin slices of high field strength in a general background of low field strength. So in practice, with a small increase in forward voltage, a low conductivity segment is created at the cathode, resistance increases, the segment moves along the bar to the anode, and when it reaches the anode it is absorbed and a new segment is created at the cathode to keep the total voltage constant. If the voltage is lowered, any existing slice is quenched and resistance decreases again. The laboratory methods that are used to select materials for the manufacture of Gunn diodes include angle-resolved photoemission spectroscopy. Applications Because of their high frequency capability, Gunn diodes are mainly used at microwave frequencies and above. They can produce some of the highest output power of any semiconductor devices at these frequencies. Their most common use is in oscillators, but they are also used in microwave amplifiers to amplify signals. Because the diode is a one-port (two terminal) device, an amplifier circuit must separate the outgoing amplified signal from the incoming input signal to prevent coupling. One common circuit is a reflection amplifier which uses a circulator to separate the signals. A bias tee is needed to isolate the bias current from the high frequency oscillations. Sensors and measuring instruments Gunn diode oscillators are used to generate microwave power for: airborne collision avoidance radar, anti-lock brakes, sensors for monitoring the flow of traffic, car radar detectors, pedestrian safety systems, "distance travelled"