Intrinsic and Extrinsic Semiconductors- p and n type,

Types of Semiconductors – Intrisic semiconductors and Extrinsic Semiconductors

Intrinsic Semiconductors
- Also called as pure semiconductors as they do not contain any impurities
- Normally made of single-element materials like pure silicon or germanium with a crystalline structure
- They have small energy gap (forbidden energy). At room temperature, it is 1.1 eV for Silicon and 0.7 eV for Germanium.
- At room temperature, thermal energy causes electrons to jump from the valence band (covalent bond) to the conduction band, creating electron-hole pairs. Number of electrons ($n_e$) = number of holes ($n_h$. \[n_e = n_h\]
- Electrons in conduction band and holes in valence band are responsible for conductivity of semiconductors i.e. they are carrier charges.
- Intrinsic carrier concentration, $n_i = n_e =n_h$
- At absolute zero temperature, no electron jumps to conduction band. Hence, the the conductivity of semiconductors is zero. With increase in temperature, conductivity of the semiconductors increases as it causes formation of higher number of electron-hole pairs.
- Net current through a semiconductor, $I$, under the influence of an external electric field is sum of electron current, $I_e$, and hole current, $I_h$.
$$ I = I_e + I_h $$
- Collisions of electrons with holes cause their recombination. It happens simultaneously with generation of electron-hole pairs. However, at equilibrium at given temperature, the rate of generation is equal to the rate of recombination of the electron-hole pairs. This keep the charge carriers and hence the conductivity of the semiconductors constant.
- However, it has low conductivity at room temperature.

Extrinsic Semiconductors
- Definition and Types
  - Extrinsic semiconductors are the semiconductor materials which are intensely modified by adding impurity atoms to it to change its electric properties.
  - The impurity atoms are called as dopants.
  - The process of adding the dopants is called as doping.
  - Extrinsic semiconductors are also called as impurity semiconductors.
  - Doping helps in precisely controlling the electric conductivity of the semiconductors by adjusting the concentration of the dopant.
  - These are fundamental to modern electronics.
  - Necessary condition for doping is that the sizes of the dopant and semiconductor atoms should be nearly the same size.
  - There are two types of extrinsic semiconductors, p-type semiconductor and n-type semiconductor.
Number of negative charge carriers (electrons) and positive ones (holes) are different.

$$ {n_i}^2 = n_h \times n_e $$

n-type semiconductor
- Extrinsic semiconductor formed by doping of pentavalent (5 valence electrons) impurities such as Phosphorus (P), Arsenic (As), or Antimony (Sb) to the pure semiconductors such as Silicon or Germanium.
- Atom with five valence electrons occupies the position of atom with four valence electron in the crystal lattice. Its four electrons participate in the covalent bond formation while fifth electron does not. It remains weakly bound in the lattice.
- Weakly bound electrons have higher energy level, called as Donor Energy Level, $E_D$. Its value is slightly less than the energy level of the conduction band. Thus, these electrons need less energy to jump to the conduction band. The energy difference between $E_C$ and $E_D$ is called as Donor Activation Energy, $E_{CD}$.
$$ E_{CD} = E_C – E_D $$

As production of electron-hole pairs increase weakly with the temperature, electrons become majority carriers while holes are minority carriers . $n_e >> n_h$

p-type semiconductor
- Extrinsic semiconductor formed by doping of trivalent (3 valence electrons) impurities such as Boron, Aluminum, Gallium, or Indium to the pure semiconductors such as Silicon or Germanium.
- Atom with three valence electrons occupies the position of an atom with four valence electrons in the crystal lattice. Three out of the four covalent bonds the atom forms are complete while the fourth remains incomplete due to shortage of one valence electron.
- The electron shortage at incomplete covalent bond works as electron deficiency and is named hole. The incomplete covalent bond has tendency to pull neighboring electrons in its holes.
- When electron moves to the hole, it appears that hole is moving opposite to the electron. Thus, there is a current due to movement of holes (electrons) in valence band.
- However, it needs slightly more energy to pull electrons from neighboring bonds. It means that the holes have additional energy states, called as Acceptor Energy Level ($E_A$). It is slightly more than energy level of valence band.
- Under the influence of external electric field, electrons with higher energy moves to the acceptor energy level replacing the holes that sink (fall down) to the valence band.
- Movement of electrons to trivalent atoms neighboring covalent bonds, results in formation of negatively charged ions.