如果半导体材料中导带和价带中的电子和空穴的动能相同的话，则该半导体可以称之为直接带隙半导体。其导带最小值（导带底）和满带最大值在k空间中同一位置，电子要跃迁到导带上产生导电的电子和空穴（形成半满能带）只需要吸收能量。

In semiconductor physics, the band gap of a semiconductor is always one of two types, a direct band gap or an indirect band gap. The minimal-energy state in the conduction band and the maximal-energy state in the valence band are each characterized by a certain crystal momentum (k-vector) in the Brillouin zone. If the k-vectors are the same, it is called a "direct gap". If they are different, it is called an "indirect gap". The band gap is called "direct" if the momentum of electrons and holes is the same in both the conduction band and the valence band; an electron can directly emit a photon. In an "indirect" gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice.