Impact Avalanche Transit Time (IMPATT) device has emerged as the most suitable solid-state source to generate sufficiently high power in microwave, millimeter wave and terahertz frequencies. At high frequencies such as at 94 GHz InP IMPATT diodes in general have higher DC-to-RF conversion efficiency than all other well known material used for IMPATT diode such as, GaAs, Si due to some special reasons. The illumination of the active area of a microwave diode by an optical signal effectively acts as an additional terminal to the device through which the internal operation of the device can be controlled. Different DC Properties and the Small Signal Properties for different Illuminated levels have been discussed in this book. All the results are simulated based. This book comprises of four different chapters.