SEMICONDUCTORS

Semiconductors Semiconductors have a resistivity/resistance between that of conductors and insulators Their electrons are not free to move but a little energy will free them for conduction Their resistance decreases with increase in temperature The two most common semiconductors are silicon and germanium M V V K Srinivas Prasad

INTRINIC SEMICONDUCTORS M V V K Srinivas Prasad

The Silicon, Si, Atom Silicon has a valency of 4 i.e. 4 electrons in its outer shell Each silicon atom shares its 4 outer electrons with 4 neighbouring atoms These shared electrons – bonds – are shown as horizontal and vertical lines between the atoms This picture shows the shared electrons M V V K Srinivas Prasad

Silicon – the crystal lattice If we extend this arrangement throughout a piece of silicon… We have the crystal lattice of silicon This is how silicon looks when it is 0K It has no free electrons – it cannot conduct electricity – therefore it behaves like an insulator M V V K Srinivas Prasad

Electron Movement in Silicon At room temperature An electron may gain enough energy to break free of its bond… It is then available for conduction and is free to travel throughout the material M V V K Srinivas Prasad

Hole Movement in Silicon M V V K Srinivas Prasad

Hole Movement in Silicon This hole can also move… M V V K Srinivas Prasad

Heating Silicon M V V K Srinivas Prasad

Intrinsic Conduction Take a piece of silicon… And apply a potential difference across it… This sets up an electric field throughout the silicon – seen here as dashed lines M V V K Srinivas Prasad

Intrinsic Conduction M V V K Srinivas Prasad

Intrinsic Conduction M V V K Srinivas Prasad

Intrinsic Semiconductors Consider nominally pure semiconductor at T = 0 K There is no electrons in the conduction band At T > 0 K a small fraction of electrons is thermally excited into the conduction band, “leaving” the same number of holes in the valence band M V V K Srinivas Prasad

This hole is positive, and so can attract nearby electrons which then move out of their bond etc. Thus, as electrons move in one direction, holes effectively move in the other direction Electron moves to fill hole As electron moves in one direction hole effectively moves in other

Intrinsic Semiconductors at T >0 K Electrons and holes contribute to the current when a voltage is applied M V V K Srinivas Prasad

Carrier Concentrations at T >0 K The number of electrons equals the number of holes, ne = nh The Fermi level lies in the middle of the band gap ne = nh increase rapidly with temperature M V V K Srinivas Prasad

M V V K Srinivas Prasad

Electron and hole conductivity • In a semiconductor, there can be electrons and holes: • Total Electrical Conductivity thus given by: # electrons/m 3 electron mobility # holes/m hole mobility M V V K Srinivas Prasad

Intrinsic carriers With intrinsic systems (only), for every free electron, there is also a free hole. # electrons = n = # holes = p = ni --true for pure Si, or Ge, etc. Holes don’t move as easily (mobility of holes is always less than for electrons), but still there are so many that they will contribute at least an extra 10-20% to the intrinsic conductivity. μh is ~20% of μe M V V K Srinivas Prasad

M V V K Srinivas Prasad

EXTRINIC SEMICONDUCTORS M V V K Srinivas Prasad

Prepared by adding (doping) impurities to intrinic semiconductors Doping is the incorporation of [substitutional] impurities (trivalent or pentavalent) into a semiconductor according to our requirements In other words, impurities are introduced in a controlled manner Electrical Properties of Semiconductors can be altered drastically by adding minute amounts of suitable impurities to the pure crystals M V V K Srinivas Prasad

Doping Pentavalent Group VA elements Trivalent Group III A elements Phosphorous Arsenic Antimony Trivalent Group III A elements Boron Gallium Indium M V V K Srinivas Prasad

The Phosphorus Atom Phosphorus is number 15 in the periodic table It has 15 protons and 15 electrons – 5 of these electrons are in its outer shell M V V K Srinivas Prasad

Doping – Making n-type Silicon We now have an electron that is not bonded – it is thus free for conduction M V V K Srinivas Prasad

Doping – Making n-type Silicon As more electrons are available for conduction we have increased the conductivity of the material Phosphorus is called the dopant If we now apply a potential difference across the silicon… M V V K Srinivas Prasad

Extrinsic Conduction – n-type Silicon M V V K Srinivas Prasad

M V V K Srinivas Prasad

M V V K Srinivas Prasad

M V V K Srinivas Prasad

This crystal has been doped with a pentavalent impurity. The free electrons in n type silicon support the flow of current. M V V K Srinivas Prasad

M V V K Srinivas Prasad

M V V K Srinivas Prasad

Donor electrons Unlike for intrinsic semiconductors, free electron doesn’t leave a mobile free hole behind. Instead, any holes are trapped in donor state and thus will not contribute substantially to conductivity as for intrinsic semiconductors (thus p~0). M V V K Srinivas Prasad

The Boron Atom Boron is number 5 in the periodic table It has 5 protons and 5 electrons – 3 of these electrons are in its outer shell M V V K Srinivas Prasad

Doping – Making p-type Silicon Notice we have a hole in a bond – this hole is thus free for conduction M V V K Srinivas Prasad

Doping – Making p-type Silicon Boron is the dopant in this case If we now apply a potential difference across the silicon… M V V K Srinivas Prasad

Extrinsic Conduction – p-type silicon M V V K Srinivas Prasad

M V V K Srinivas Prasad

M V V K Srinivas Prasad

M V V K Srinivas Prasad

This crystal has been doped with a trivalent impurity. The holes in p type silicon contribute to the current. Note that the hole current direction is opposite to electron current so the electrical current is in the same direction M V V K Srinivas Prasad

M V V K Srinivas Prasad

M V V K Srinivas Prasad

Extrinsic conductivity—p type Every acceptor generates excess mobile holes (p=Na). Now holes totally outnumber electrons, so conductivity equation switches to p domination. M V V K Srinivas Prasad

Ef=Edonor= Ec-0.05eV Ef=Eacceptor= Ev+0.05eV M V V K Srinivas Prasad

• N-type Extrinsic: (n >> p) • P-type Extrinsic: (p >> n) • Intrinsic: # electrons = # holes (n = p) --case for pure Si • Extrinsic: --n ≠ p --occurs when DOPANTS are added with a different # valence electrons than the host (e.g., Si atoms) • N-type Extrinsic: (n >> p) • P-type Extrinsic: (p >> n) M V V K Srinivas Prasad

Variation of carrier concentration with temperature in intrinsic semiconductors

Variation of carrier concentration with temperature in extrinsic semiconductors