Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Semiconductor Devices Metal-semiconductor junction Rectifier (Schottky contact or Schottky barrier) Ohmic contact p – n rectifier Zener diode Photodiode (solar cell) Tunnel diode Transistor Other devices based on semiconductors (for hybrid circuits) Resistor Isolator Capacitor 1 Negative/Positive Charged Surface Band structure of an n-type semiconductor with negatively charged surface Band structure of a p-type semiconductor with positively charged surface Near the surface, the concentration of free electrons is lower – the negative charge of the surface represents a potential barrier for electrons Near the surface, the concentration of “free holes” is lower – the positive charge of the surface represents a potential barrier for free holes Custom: the edges of energy bands are diagramed distorted, not the Fermi energy 2 Contact: Metal and n-type Semiconductor M S EFM EFS 0 Potential barrier Energy bands of a metal and a n-type semiconductor (without contact) Fermi energies are different Electrons Energy bands of a metal and a n-type semiconductor (contact) Electrons flow into the metal until the Fermi energies are equalized. The surface of the metal charges negative. Simultaneously, a potential barrier is formed. In equilibrium, only one diffusion current exists (equal in both directions). 3 Contact: Metal and p-type Semiconductor M S EFM EFS 0 Potential barrier Energy bands: The Fermi energies are different Electrons Energy bands of a metal and a p-type semiconductor Electrons flow into the semiconductor until the Fermi energies are equalized. The surface of the metal charges positive. Simultaneously a “negative” potential barrier is formed. In the Equilibrium only one diffusion current exists (equal in both directions). 4 Work functions Metals Material Ag Al Au Be Ca Cs Cu Fe K Li Na Ni Zn [eV] 4,7 4,1 4,8 3,9 2,7 1,9 4,5 4,7 2,2 2,3 2,3 5,0 4,3 Semiconductors Material Diamond Ge Si Sn [eV] 4,8 4,6 3,6 4,4 * Work function = vacuum electron affinity = vacuum ionization energy 5 Electrical Currents Diffusion current Metal Drift current Metal Semiconductor Semiconductor – 𝐼 = 0 + 𝐼 > 0 U 6 Drift Current Reverse bias Forward bias An external electric field increases the potential barrier An external electric field decreases the potential barrier Barrier for electrons Acceleration of electrons 7 Drift Current Metal Semiconductor Φ χ I MS AC T 2 exp M k T B Semiconductor Metal Φ Φ S eV I SM AC T 2 exp M k BT 𝐴 … area 𝐶 … Richardson constants 𝑇 … temperature 𝜒 … affinity Φ… work function 𝑘B … Boltzmann constant 𝑉 … external voltage 𝑒 … elementary charge Total current: Φ Φ S eV I I SM I MS AT 2 C exp M k BT Φ χ eV exp 1 I ACT 2 exp M k T k T B B saturation current Φ χ C exp M k BT voltage dependency enhanced 8 Ohmic Contact Electrons Example: Al / Ge : Al < Ge the contact Al / Ge exhibits good electrical conductivity Technological example: Al / Si or Al / SiO2 Al > Si the contact Al / p-Si shows good electrical conductivity the contact Al / n-Si can be used as a rectifier 9 Ohmic Contact: Al / n-Si n-semiconductor n+-film metal Electron current Quantum tunneling The n+ slab has to be very thin. Problem: electromigration Material transport at high electric currents, due to the momentum transfer between conducting electrons and atoms or ions of the solid Solution: Al Al + Cu, Al Al + Si Coating with gold 10 p-n Junction (Diode) In equilibrium (without external voltage) Diode with external voltage 11 Electrochemical Potential Diffusion current Electrochemical potential in equilibrium state: jfield eEn jdiff eD eE eD dn μn dx eE e Field current U ; μ dn ; dx j D grad c eD k BT dU k BT dn d k BT ln n dx n dx dx k BT ln n const. e … The electrochemical potential of electrons is everywhere the same in state of equilibrium (without a current) 12 p-n Junction (Diode) Electrons U Holes k BT ln n const. e U k BT ln p const. e Potential difference (potential jump) U 0 left right p k BT nright k BT ln ln left e nleft e pright With external voltage Without external voltage jdiff jfield j0 j jfield jdiff 0 U eU k BT ln n ; n exp e k T B eU ; jfield nev j0 exp k BT eU 1 j j0 exp k T B jdiff j0 13 Semiconductor Diode (Rectifier) I U Abb. 14.61. Current-voltage-characteristic of a rectifier diode 14 Zener Diode Used with reverse bias Ionization process: Avalanche-like increase of the electric current Generation of free electrons 15 Photodiode (Solar Cell) 𝐸g E hν E g 1 Eg hc λ λ hc Eg 𝐸g [eV] Ge 0.7 Si 1.1 GaAs 1.5 𝜆 [m] 1.8 1.1 0.83 16 Tunnel Diode 17 Transistor (without external voltage) B E C Two potential barriers 18 Transistor (with external voltage) n Potential barrier p n Acceleration in the electric field Amplifier 19 Devices in Hybrid Circuits Resistor: electrical conductivity as function of the doping in the p-zone Capacitor: different electrical charges in p- and n-zone, separated by an insulator (dielectric) Technology Source material: SiO2 Si Czochralski method (monocrystalline silicon) Diffusion process: diffusion of phosphorus (n) or boron (p) in silicon. Mask – SiO2. 20