Doping – “The deliberate addition of impurity atom (3rd group or 5th group) element atoms into an otherwise pure Si or Ge crystal, to increase its conductivity, is called doping”.
The impurity atoms are called dopants. The material that is formed after doping is called a doped semiconductor or an extrinsic semiconductor.
[The dopant has to be such that it does not distort the original pure semiconductor lattice. It occupies only a very few of the original semiconductor atom sites in the crystal. A necessary condition to attain this is that the sizes of the dopant and the semiconductor atoms should be nearly the same.]
n – type extrinsic semiconductor:
When 5th group element atoms [like Arsenic (As), Antimony (Sb), Phosphorous
(P), etc.] are introduced (dropped) into an otherwise pure semiconductor, the crystal thus formed is called n-type semiconductor.
When an antimony (or arsenic) atom is introduced in an otherwise pure semiconducting crystal, the antimony atom replaces one of the Si atom from its lattice point. The antimony atom shares four of its valence electrons with four neighboring Si atom. Due to this the fifth valence electron of the Sb atom becomes very weakly bound with its parent atom.
The semiconductor’s energy band structure is affected by doping. In the case of n –type extrinsic semiconductors, additional energy states due to donor impurities (ED) , called ‘donor level’, is formed at slightly below the bottom EC of the conduction band and electrons from this level can move into the conduction band with very small supply of energy (about 0.01eV energy is required).
When such a crystal is brought to the room temperature all these fifth electrons of Sb atoms, after getting a small energy (0.01 electrons volt), leaves the atom and becomes free. Due to this the antimony atoms become positively charged immobile atoms (ions). In the energy band diagram this process in shown by extrinsic transition between the donor level and the conduction band. So the conduction band will have most electrons coming from the donor impurities.
Besides this there are a few electrons, originally present in the bond, which after getting sufficient energy breaks the bonds & causes generation of free electron – hole pair. This is shown by the intrinsic transition between the valence band & the conduction band.
So at room temperature in a n-type semiconductor we get
- A large number of free electrons – (majority charge carriers)
- A few holes – (minority charge carriers)
- Negatively charge immobile doped atoms fixed at their lattice points.
So at any instant in a n-type semiconductor the free electron density is much higher than hole density i.e. ne >> nh
P- type semiconductor
When the third group element atom (Indium, Boron, Al) are doped in an otherwise pure Si or Ge crystal, the crystal thus formed is known as p-type semiconductor.
When an Aluminum (Al) atom is introduced in Si crystal, it replaces one of the Si atoms and gets its position in the lattice. The Al atom shares all its three valance electrons with three neighboring Si atoms & there by formed three covalent bonds. But it doesn’t satisfy the Al atom and also one neighboring Si atom. These two atoms together form a positively charged region, which attracts the electrons from the neighboring band. Due to this the electrons present in the neighboring atoms become weekly bound.
In the case of p-type extrinsic semiconductors, additional energy state due to acceptor impurities (EA), called ‘acceptor level’ is formed. The acceptor energy level EA is slightly above the top EV of the valence band as shown in Fig. With very small supply of energy an electron from the valence band can jump to the level EA and thereby ionise the acceptor atom negatively.
When such a crystal is brought to the room temperature every Al atom present in it receives one electron from some neighboring bonds. Due to this transfer of electrons a hole is created in the bond. In this process Al atom becomes negatively charged immobile atom. In energy band diagram this process is shown by extrinsic transition between the valence band and the acceptor level, which is present just above the valence band at an energy difference of about 0.01 – 0.05 eV.
Besides this there are a few electrons originally present in the bond, after receiving sufficient energy breaks the bonds and generate electrons -hole pair.(Intrinsic transition). Thus at room temperature in a p-type semiconductor we get
- A large number of holes – (majority charge carriers.)
- A few free electrons – (minority charge carriers)
- Negatively charged immobile Al atoms.
So, for p-type semiconductors nh >> ne
Note that the crystal maintains an overall charge neutrality as the charge of additional charge carriers is just equal and opposite to that of the ionised cores in the lattice.
Extrinsic Semiconductors: n-type and p-type semiconductors.
(In Hindi + English mix Language)
Note:
- Donor level – The additional energy level is generated by the donor atoms (fifth group element atoms) in a n-type semiconductor is known as donor level. This level is present just below the conduction band & thereby it is present in forbidden gap. At low temperature it contains all the fifth electrons (additional electrons) of the donor atoms.
- Acceptor level – The additional energy level formed by the acceptor atoms (3rd group element atoms) in a p-type semiconductor is known as acceptor level. This level is present in the forbidden gap just above the V.B. At low temperature this energy level is empty & it accepts electrons from the valence band whenever the crystal is brought to the room temperature.
- At any temperature, a semi conducting crystal is always electrically neutral this is because the number of free electrons and the number of holes & the immobile charges is such that the crystal is electrically neutral,
- At any temperature in a semi conducting crystal continuously the process of recombination of electrons and hole as well as the generation of electron hole pair take place simultaneously. In equilibrium the rate of recombination of electron- hole pair is equal to the rate of generation of electro- hole pair.
