EC-6.1 Carriers, Doping and Concentration

Work through these

• Adding an atom with one electron too many, and what it donates

  An impurity with five valence electrons in a four-bonded lattice has one left over, held so loosely that thermal energy frees it. The material now has far more electrons than holes and is called n-type.

  NPTEL: Introduction to Semiconductor Devices · Course

• Adding an atom with one electron too few, and what it accepts

  An impurity with three valence electrons leaves a bond incomplete, and a neighbouring electron fills it, moving the vacancy along. The material now has far more holes than electrons and is called p-type.

• Majority and minority carriers, and why the small number still matters

  Doping fixes which carrier is abundant, but the rare one is what a junction's behaviour actually depends on. A transistor is a device built almost entirely on what the minority carrier does.

  NPTEL: Introduction to Semiconductor Devices · Course

• The product of the two concentrations is a constant

  In equilibrium the electron and hole concentrations multiply to a value fixed by the material and the temperature, so pushing one up pushes the other down. This one relation answers most concentration questions in a line.

• The Fermi level, and what it says about a doped region

  The Fermi level marks where the probability of a state being occupied is one half, and doping moves it toward whichever band has the abundant carrier. On a band diagram its position is how you read the doping at a glance.

  NPTEL: Solid State Devices · Course

• Charge neutrality, and the check it gives you for free

  A doped region has no net charge, so the mobile carriers must balance the fixed ionised impurities. Writing that balance is the fastest way to find an unknown concentration and to catch an arithmetic slip.