The band gap (of any semiconductor material) is the energy difference between the conduction band minimum and the valence band maximum. To move from the valence to the conduction band, an electron needs minimum energy equal to the band gap value. Only an electron in the conduction band can conduct electric current—true for any semiconductor material. The electron exiting the valence band leaves behind a vacant location in the crystal lattice—a ‘hole’. An electron has factors of higher mobility than a hole. Band gap engineering is a set of semiconductor material processing techniques to increase|decrease the band gap of a semiconductor material, making it is easier|harder for an electron (in that “band gap engineered” material) to move from the valence to the conduction band. A “band gap engineered” semiconductor device satisfies specifications (e.g., ultra high switching frequency, large current|voltage etc.,) for specific target applications. All state-of-art digital integrated circuits (microprocessors, memory, field programmable gate arrays, programmable logic arrays etc.,) would not be able to satisfy their stringent performance characteristic specifications without band gap engineered transistors. This chapter explains in detail how and why built-in band gap engineering features boost a semiconductor device’s performance characteristics. There are two varieties of band gap engineering—band gap broadening|widening and band gap narrowing.