Silicon is by far the most widely used semiconductor material and is likely to remain so for the foreseeable future, although from the perspective of an integrated circuit (IC) designer silicon is hardly a perfect semiconductor. Compared with other semiconductors, it is relatively poor in terms of how fast the charge carriers can move through the crystal lattice, which limits the speed at which silicon devices can operate. ‘Why is silicon still dominant?’ The answer to this question is economics. Silicon is abundant in nature, non-toxic, strong and an excellent conductor of heat. It can be grown to a very high purity and very large diameter (with 12 inch now being contemplated) wafers, and it readily forms a stable insulating ﬁlm (SiO2 or Si3N4) of high quality. Properties of this kind make silicon a natural choice for IC manufacturing and, in fact, over the past 40 years or so, the performance of silicon ICs and the density of devices per unit area have soared, while the cost per function has plunged (see ﬁgure 1.1). ICs are more diﬃcult and more expensive to fabricate from III-V compound semiconductors such as GaAs/AlGaAs or InP. High-quality oxides are scarce in the III-V semiconductors, impeding device integration. Highpurity, large diameter crystals are diﬃcult to grow and yield is poor because of more defect density.