Heterojunction

Band energies of two materials with different doping with different band gaps between valence band energy E _v and conduction band energy E _c , without contact .

Heterojunction of these materials with contact . Diffusion voltage (see pn junction ). The energy difference to the vacuum energy level corresponds to the ionization energy . ${\displaystyle \psi _{D}}$

The boundary layer between two different semiconductor materials is referred to as heterojunction (also called heterostructure , English heterojunction ) . In contrast to a pn junction , not (only) the type of doping , but the type of material is different. The semiconductors therefore have i. A. a different energy of the band gap .

Heterojunctions are found in III-V semiconductors or II-VI semiconductors .

The Nobel Prize in Physics 2000 was awarded to Herbert Kroemer and Schores Iwanowitsch Alfjorow for semiconductor heterojunctions.

calculation

In the case of a pn heterojunction, an irregularity occurs in the energy bands of the materials. The extent of this irregularity, a bending of the band edges, can be calculated using the Poisson equation. Assuming the transition from the negatively doped material 1 to the positively doped material 2 with the relative dielectric constants and impurity concentrations , or on, presents the diffusion voltage with applied external electric field of the voltage band bending the following size a: ${\displaystyle X}$ ${\displaystyle \varepsilon }$ ${\displaystyle N_{\mathrm {D} }}$ ${\displaystyle N_{\mathrm {A} }}$ ${\displaystyle \psi _{\mathrm {D} }}$ ${\displaystyle U}$

${\displaystyle X_{1}={\sqrt {{\frac {2}{q}}{\frac {\varepsilon _{1}\varepsilon _{2}\cdot N_{\mathrm {A} }(\psi _{\mathrm {D} }-U)}{N_{\mathrm {D} }(\varepsilon _{2}N_{\mathrm {D} }+\varepsilon _{1}N_{A})}}}}}$, ${\displaystyle X_{2}={\sqrt {{\frac {2}{q}}{\frac {\varepsilon _{1}\varepsilon _{2}\cdot N_{\mathrm {D} }(\psi _{\mathrm {D} }-U)}{N_{\mathrm {A} }(\varepsilon _{2}N_{\mathrm {D} }+\varepsilon _{1}N_{\mathrm {A} })}}}}}$

application

Application find heterojunctions u. a. in laser diodes : If, during optical recombination, radiation is emitted in the area with the smaller band gap, it cannot be absorbed by electrons in the area of ​​the larger band gap. The probability that the radiation leaves the semiconductor material is therefore greater.

Another area of ​​application is in solar cells : Here undesired minority charge carriers can be shielded from the contacts by using the higher band gap as a potential barrier for these charge carriers. In this way, the recombination and thus the loss of charge carriers on the defect-rich metal-semiconductor contact and in the highly doped layers on the contacts can be reduced, since the recombination partners in the form of minority charge carriers have been withdrawn from the majority charge carriers by the step in the heterojunction. With the help of this concept, a record efficiency of 25.6% was achieved for silicon solar cells in 2014 by using amorphous silicon as a material with a large band gap on crystalline silicon .

See also

• High Electron Mobility Transistor (HEMT)
• Heterojunction bipolar transistor (HBT)
• Gallium arsenide
• Gallium nitride

Individual evidence

1. ^ Nobel Prize 2000, Nobelprize.org
2. ↑ Panasonic HIT (R) Solar Cell Achieves World's Highest Energy Conversion Efficiency of 25.6% at Research Level: Panasonic News