Comprehensive Study Notes on Intrinsic and Extrinsic Semiconductors

Intrinsic Semiconductors

• Definition: An intrinsic semiconductor is defined as a semiconductor material in its extremely pure form, characterized by having negligible impurities.
• Behavior Under Electromotive Force: When an electric field is applied to an intrinsic semiconductor at a temperature greater than $0^{\circ}K$, specific charge carrier movements occur:
  • Conduction electrons migrate toward the anode.
  • Holes located within the valence band migrate toward the cathode.
• Equilibrium Condition: An intrinsic semiconductor is mathematically characterized as one in which the number of conduction electrons is exactly equal to the number of holes.

Extrinsic Semiconductors and the Doping Process

• Overview: The electrical conductivity of semiconductors can be significantly improved through the introduction of a small number of suitable replacement atoms.
• Impurities: These replacement atoms introduced into the crystal lattice are known as IMPURITIES.
• Doping: The specific process of adding these impurity atoms to a pure semiconductor is called DOPING.
• Classification of Doping Agents:
  1. Pentavalent Atoms: These atoms possess $5$ valence electrons. Examples include Arsenic ($As$), Antimony ($Sb$), and Phosphorus ($P$).
  2. Trivalent Atoms: These atoms possess $3$ valence electrons. Examples include Gallium ($Ga$), Indium ($In$), Aluminum ($Al$), and Boron ($B$).
• Functional Roles of Dopants:
  • Donor Atoms: A pentavalent doping atom is known as a donor atom because it contributes or "donates" $1$ electron to the conduction band of the pure Germanium ($Ge$).
  • Acceptor Atoms: A trivalent atom is called an acceptor atom because it accepts $1$ electron from the Germanium ($Ge$) atom.

Sub-division of Extrinsic Semiconductors

Depending on the specific type of doping material utilized, extrinsic semiconductors are sub-divided into two distinct classes:

• (i) N-type semiconductors.
• (ii) P-type semiconductors.

N-Type Semiconductors

• General Formation: This type of semiconductor is obtained when a pentavalent material, such as Antimony ($Sb$), is added to a pure Germanium ($Ge$) crystal.
• Covalent Bonding Mechanism:
  • Each Antimony atom forms covalent bonds with the surrounding $4$ Germanium atoms.
  • This bonding process utilizes $4$ of the Antimony's $5$ valence electrons.
• The Fifth Electron: The fifth electron remains surplus and is only loosely bound to the Antimony atom.
• Impurity Characterization: In this scenario, Antimony is termed a donor impurity, which transforms the pure Germanium into an N-type (where "N" stands for negative) extrinsic semiconductor.
• Carrier Dynamics and Conduction:
  • Majority Carriers: Electrons serve as the majority carriers in N-type semiconductors.
  • Minority Carriers: Holes constitute the minority carriers.
  • Excess Conduction: The resulting process of electrical flow in this material is referred to as 'excess' conduction.

Statistical and Numerical Context

• Percentage Marker: The material indicates a value of $42\%$.
• Temperature Threshold: The behavior of carriers is discussed relative to a temperature greater than $0\,^{\circ}K$.