I have great pleasure to present this book on Semiconductor Devices and Basic Electronic Systems F.Y.B.Sc. Electronic Science of B.Sc (Computer Science) Paper I Semester-I, ELC-111, CBCS (June-2019-20).It is written as per the 2019-20 patterns of Choice Base Credit System (CBCS) syllabus of Savitribai Phule Pune University, Pune-411007 implemented from June-2019.The Author and Co-Author have tried there level best to simplify the difficult concept in electronic science with neat diagram and simple mathematical equation.The basics of electronics which is required to understand the concept of Semiconductor, Conductor, Insulator, PN Junction Diode, The Light-Emitting Diodes (LEDs): Symbol and its use in circuit, IR transmitter and receiver applications, Photo diode circuit, Photo transistors, LDR and its use in street light controller and Opto- Isolators (MCT2E) and its use in isolation are made very simple. Application of PN Junction Diode in Power Supply such as half wave, full Wave and Bridge rectifier. Regulated Power Supply Unregulated Power supply, Automatic Voltage Regulator, SMPS, UPS are covered.Advance transistors such as JFET Transistor, MOSFET and application as switch is covered.  The oscillator such as RC Oscillator Wein bridge and Crystal Oscillator and Square wave generator that is Multivibrator circuit using IC-555 is covered. Digital to Analog converter Binary weighted and R-2R ladder type DAC  Circuit , Analog to Digital converters such as  Flash ADC, successive approximation ADC, parameters of DAC and ADC.