chapter  7
Organic Semiconductors
WithRoderick C. I. MacKenzie
Pages 28

The majority of this book focuses on classical crystalline semiconductors such as silicon and gallium arsenide. Indeed, almost all electronic devices we use today from transistors to quantum well lasers (Lim 2007) are based on crystalline semiconductors, and it was the development of these materials and devices made from them that has laid the foundation of the Internet and information age. When one thinks of the atomic structure of silicon, one thinks of a highly ordered, regular arrangement of atoms spanning the physical dimensions of the sample. One could visualize this arrangement of atoms on a more human scale, as marbles of the same size neatly packed in a tin. Bloch's theorem (Bloch 1928) tells us that this regular arrangement of atoms will lead to a well-defined conduction and valance band, with a forbidden region neatly positioned between them called the band gap. Another important characteristic of crystalline semiconductors used in semiconductor devices is that they are highly pure with typical impurity concentrations of one part in ten million (Delannoy 2012). This high purity means that conducting electrons and holes are unlikely to scatter off defects such as oxygen molecules embedded in the lattice. This results in very 192high-charge carrier mobility values (typically ~1000 cm2 V–1 s–1 at room temperature [Canali et al. 1975]). Practically, this high mobility means that very little waste heat is generated as the device is used; this in turn enables millions of transistors to be integrated into tiny chips and the development of highly complex information processing units. From the theoretical perspective, this lack of defect states means that we can fairly successfully imagine conducting electrons and holes in these materials as quasi-free particles that obey classical Newtonian mechanics.