An Impatt diode has n+ - p - i - p + structure and is used with reverse bias. It exhibits negative resistance and operates on the principle of avalanche breakdown. Impatt diode circuits are classified as broadly tunable circuit, low Q circuit and high Q circuit. The impedance of Impatt diode is a few ohms. The word Impatt stands for Impact Avalanche Transit Time diode. The features of Impatt diode oscillator are : frequency 1 to 300 GHz, Power output (0.5 W to 5 W for single diode circuit and upto 40 W for combination of several diodes), efficiency about 20%. Its applications include police radar systems, low power microwave transmitter etc. Avalanche diode can also be operated in large signal high efficiency mode called Trapped Avalanche Transit Time mode. The Trapatt oscillations depend on the delay in the current caused by avalanche process. The avalanche delay makes it possible to increase the diode voltage well above the breakdown voltage. Therefore a very rapid multiplication of charge carriers occurs. A Trapatt diode is also a negative resistance device. The features of Trapatt diode oscillator are : Frequency 3 to 50 GHz, Power output 1-3 W, efficiency about 25%. Its applications are low power doppler radar, microwave beacon landing system etc.