Heterojunction bipolar transistor with a specific graded base structure

An article comprising a heterojunction bipolar transistor comprising first, second and third semiconductor regions, to be referred to as emitter, base and collector, respectively, and further comprising means for electrically contacting said emitter, base and collector, respectively, the base being intermediate the emitter and collector and having a width W.sub.B, the emitter and collector each comprising semiconductor material of a first conductivity type, the base comprising material of a second conductivity type that differs from the first conductivity type and having a base bandgap;

• CHARACTERIZED IN THAT
• the base is selected such that the base bandgap comprises N(N.gtoreq.2) regions (designated 1st, 2nd, . . . , j-th, . . . , Nth) of substantially constant bandgap width, with a substantially step wise change (.DELTA..sub.j) in bandgap width between the j-th region and the (j+1)-th region, the bandgap width decreasing monotonically, without increasing anywhere, in the direction from emitter towards collector, with .DELTA..sub.j >kT, where k is Boltzmann's constant and T is an absolute transistor operating temperature, and furthermore, .DELTA..sub.j >.DELTA..sub.inelastic, where .DELTA..sub.inelastic is an inelastic minority carrier scattering threshold energy associated with the material of the j'th region.