Fundamental concepts of integrated-circuit technology M. Rudan University of Bologna

Fundamental concepts of integrated-circuit technologyM. Rudan SILICON PLANAR TECHNOLOGY 10 in = cm 12 in = cm  300 μm = 0.3 mm Wafer Chip

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan SILICON PLANAR TECHNOLOGY  300 μm = 0.3 mm  10 ÷ 20 μm Bulk Devices

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan N-TYPE DOPING (I)  10 ÷ 20 μm Bulk Adding suitably-chosen atoms increases the local concentration of free charge n+n+

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan N-TYPE DOPING (II)  10 ÷ 20 μm Bulk Typically, the wafer is also lightly doped (from factory) n+n+ n silicon (n - silicon)

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan N-TYPE DOPING (III)  The conductivity of pure (intrinsic) silicon (Si) is low: the material is a semiconductor. It is a IV-type material.  The addition of a small amount of V-type material, like phosphorus (P), arsenic (As), or antimony (Sb) increases the local concentration of free electrons.  The added materials are called dopants or impurities, of donor type.  The doped silicon is called extrinsic of n-type. It is not a chemical compound, e.g., of Si and P.

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan N-TYPE DOPING (IV)  The larger concentration of free electrons makes the local electrical conductivity to increase.  In an intrinsic semiconductor the electrical conductivity increases as temperature increases (opposite to conductors).  In a doped semiconductor the conductivity becomes independent of temperature at least in a range of temperatures.  The inclusion of dopants makes it possible to externally control the electrical conductivity.

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan P-TYPE DOPING (I)  The semiconductors allow for another type of doping, “dual” with respect to the n-type.  The addition of a small amount of III-type dopants, like boron (B), aluminum (Al), gallium (Ga), or Indium (In) increases the local concentration of free positive charges, called holes.  The doped silicon is called extrinsic of p-type. The dopants are of the acceptor type.  Holes are fictitious particles. The particles that actually carry the current are always electrons.

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan P-TYPE DOPING (II)  10 ÷ 20 μm Bulk Adding suitably-chosen atoms increases the local concentration of free charge p+p+

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan P-TYPE DOPING (III)  The larger concentration of free holes makes the local electrical conductivity to increase.  Also in a p-type doped semiconductor the conductivity becomes independent of temperature at least in a range of temperatures.  The inclusion of n-type or p-type dopants makes it possible to externally control the electrical conductivity and also to obtain regions with a different type of dominant free charge.

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan N- AND P-TYPE DOPING  The electrons and holes are also collectively called carriers.  In an n-type region, the electrons are called majority carriers. A few holes are also present, that are called minority carriers.  In a p-type region, the holes are called majority carriers. A few electrons are also present, that are called minority carriers.  Under the influence of a given electric field, electrons and holes are accelerated in opposite directions.

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan P-TYPE DOPING (IV)  10 ÷ 20 μm Bulk p+p+ Typically, the wafer is also lightly doped (from factory) p silicon (p - silicon)

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan P-N JUNCTION (I)  10 ÷ 20 μm Bulk p+p+ Two adjacent regions with opposite doping form a p-n junction. n silicon (n - silicon)

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan P-N JUNCTION (II)  10 ÷ 20 μm Bulk n+n+ p silicon (p - silicon) Two adjacent regions with opposite doping form a p-n junction.

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan P-SUBSTRATE MOS CAPACITOR p silicon (bulk) V G = 0 SiO 2 (insulator) Gate and bulk contacts V G > 0 Layer of electrons Layer of missing electrons The drawing is not in scale.

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan N-SUBSTRATE MOS CAPACITOR n silicon (bulk) V G = 0 SiO 2 (insulator) Gate and bulk contacts V G < 0 Layer of holes Layer of electrons The drawing is not in scale.

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan N-CHANNEL MOS TRANSISTOR (MOSFET) p silicon (bulk) V GB = 0 Gate and bulk contacts V GB > 0 Layer of electrons Layer of missing electrons The drawing is not in scale. SiO 2 (insulator) Source Drain Conductive channel

University of Bologna Fundamental concepts of integrated-circuit technologyM. Rudan P-CHANNEL MOS TRANSISTOR (MOSFET) n silicon (bulk) V GB = 0 Gate and bulk contacts V GB < 0 Layer of holes Layer of electrons The drawing is not in scale. SiO 2 (insulator) Source Drain Conductive channel