Heterostructures explained

Heterostructures exploit the proper­ties of a semiconductor's band gap, which is the energy required to move an electron from the valence band to the conduction band. The structures are built from several thin layers of different semi­conductors with differing band gaps.

In a single-material semiconductor, the band gap is the same throughout. When an electric field, E, is applied, the valence and conduction bands tilt; the slope of the tilt supplies the force on the charge-carriers, the electrons, Fe, or holes, Fh The forces on electrons and holes are opposite in direc­tion.

In a heterostructure, the band gap varies. Typically, a layer of a higher-bandgap semicon­ductor, like aluminum gal­lium arsenide, is placed next to a lower-bandgap semiconductor, like galli­um arsenide itself. The transitional region be­tween the two materials is the heterojunction; it may be graded or abrupt.

Because the change in the material means a variation in the electron band gap, the valence and conduction band edges can no longer be parallel edges in the heterojunc­tion. The slopes of the band edges create the equivalent of an electric field and act as forces on electrons and holes. This Kroemer named a quasi-electric field. It even becomes possible - and is in fact very com­mon - to have the forces on the electrons and holes act in the same direction, something that is fundamentally impossible to achieve with ordinary electric fields alone.

Kroemer considers this disconnection of the forces from the true electric field the fundamental design principle of all heterostructures, an idea first explicitly spelled out in his 1957 RCA Review paper.

If the compositional variation of the heterostructure is compressed right at the emitter-to-base junction of a bipolar tran­sistor, such that carriers are injected from a wider-gap emitter into a narrower-gap base, the quasi-electric fields become quasi-electric potential barriers. In the case of a pnp transistor (the kind of device dominating transistor technology at the time Kroemer first developed his ideas), the transition in band gap bars the escape of electrons from the base into the emitter; consequently, the base can be doped more heavily, reduc­ing its resistance and greatly increasing device speed.

In the double-heterostructure laser two wider-gap semicon­ductors sandwich a lower-gap semiconduc­tor between them, so as to create wells for both the electrons and the holes. When a voltage is applied, the electrons and holes are trapped in the well, recombine, and emit energy as photons.

Today, heterostructures and devices based on them employ not just GaAs and AIGaAs, but essentially all III-V semicon­ductors, including the nitrides, as well as II-VI semiconductors and even the com­bination of silicon with a silicon-germa­nium alloy.

Task I

Speak on Kroemer’s scientific interests, researches and investigations.

Task II

Tell about his teaching experience and achievements.

Task III

Discuss Kroemer’s attitude to Nobel Prize and his discoveries.

Task IV

Speak on heterostructures.

Appendix 1