1 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

2 Negative/Positive Charged Surface Band structure of an n-type semiconductor with negatively charged surface Near the surface, the concentration of free electrons is lower – the negative charge of the surface represents a potential barrier for electrons Band structure of a p-type semiconductor with positively charged surface 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

3 Contact: Metal and n-type Semiconductor Energy bands of a metal and a n-type semiconductor (contact) Potential barrier Energy bands of a metal and a n-type semiconductor (without contact) Fermi energies are different  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). Electrons

4 Contact: Metal and p-type Semiconductor Energy bands of a metal and a p-type semiconductor Potential barrier Energy bands: The Fermi energies are different  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). Electrons

5 Work functions Metals Material  [eV] Ag4,7 Al4,1 Au4,8 Be3,9 Ca2,7 Cs1,9 Cu4,5 Fe4,7 K2,2 Li2,3 Na2,3 Ni5,0 Zn4,3 Semiconductors Material  [eV] Diamond4,8 Ge4,6 Si3,6 Sn4,4 * Work function = vacuum electron affinity = vacuum ionization energy

6 Electrical Currents Diffusion currentDrift current MetalSemiconductor MetalSemiconductor U –+

7 Drift Current Reverse bias An external electric field increases the potential barrier Barrier for electrons Forward bias An external electric field decreases the potential barrier Acceleration of electrons

8 Drift Current Metal  Semiconductor Semiconductor  Metal Total current: enhanced

9 Ohmic Contact Electrons Example: Al / Ge :  Al <  Ge  the contact Al / Ge exhibits good electrical conductivity Technological example: Al / Si or Al / SiO 2  Al >  Si  the contact Al / p-Si shows good electrical conductivity the contact Al / n-Si can be used as a rectifier

10 Ohmic Contact: Al / n-Si metal n + -film n-semi- conductor The n + slab has to be very thin. Quantum tunneling Electron current 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

11 p-n Junction (Diode) In equilibrium (without external voltage) Diode with external voltage

12 Electrochemical Potential Electrochemical potential in equilibrium state: … The electrochemical potential of electrons is everywhere the same in state of equilibrium (without a current) Diffusion current Field current

13 p-n Junction (Diode) ElectronsHoles Potential difference (potential jump) Without external voltage With external voltage

14 Semiconductor Diode (Rectifier) U I Abb Current-voltage-characteristic of a rectifier diode

15 Zener Diode Used with reverse bias Ionization process: Avalanche-like increase of the electric current Generation of free electrons

16 Photodiode (Solar Cell)

17 Tunnel Diode

18 Transistor (without external voltage) EC B Two potential barriers

19 Transistor (with external voltage) npn Potential barrier Acceleration in the electric field Amplifier

20 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: SiO 2  Si  Czochralski method (monocrystalline silicon) Diffusion process: diffusion of phosphorus (n) or boron (p) in silicon. Mask – SiO 2.