Heterojunction

A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction. It is often advantageous to engineer the electronic energy bands in many solid state device applications including semiconductor lasers, solar cells and transistors to name a few. The combination of multiple heterojunctions together in a device is called a heterostructure although the two terms are commonly used interchangeably. The requirement that each material be a semiconductor with unequal band gaps is somewhat loose especially on small length scales where electronic properties depend on spatial properties. A more modern definition may be to say that a heterojunction is the interface between any two solid state materials including crystalline and amorphous structures of metallic, insulating and semiconducting material.

In 2000, the Nobel Prize has been awarded with one half jointly to Zhores I. Alferov (A.F. Ioffe Physico-Technical Institute, St. Petersburg, Russia) and Herbert Kroemer (University of California at Santa Barbara, California, USA), "for developing semiconductor heterostructures used in high-speed- and opto-electronics"

The three types of semiconductor heterojunctions organized by band alignment.

Energy Band Offsets

The important variables for heterojunction characterization and analysis are defined for two semiconductors physically separated (top) and joined in chemical equilibrium (bottom).

Semiconductor interfaces can be organized into three types of heterojunctions: straddling gap (type I), staggered gap (type II) or broken gap (type III) as seen in the figure above. There are three relevant material properties for classifying a given junction and understanding the charge dynamics at a heterojunction. The energy difference between the valence band (VB) and conduction band (CB) called the band gap is anywhere from 0eV for a metal (there is no gap) to over 4eV for an insulator. The work function of the material is the energy difference between the fermi energy (chemical equilibrium energy) and the vacuum level (where electron removal occurs). Finally the electron affinity of each material is needed which is the energy difference between the CB and the vacuum level.

Calculating energy band offsets is very straightforward given the material properties using the Anderson's rule. The conduction band offset depends only on the electron affinity difference between the two semiconductors:

$\Delta E_{C}=\chi _{1}-\chi _{2}=\Delta \chi \,$

Then using the change in band gap:

$\Delta E_{G}=E_{G2}-E_{G1}\,$

The valence band offset is simply given by:

$\Delta E_{V}=\Delta E_{G}-\Delta \chi \,$

Which confirms the trivial relationship between band offsets and band gap difference:

$\Delta E_{G}=\Delta E_{C}+\Delta E_{V}\,$

In Anderson's idealized model these material parameters are unchanged when the materials are brought together to form an interface, so it ignore the quantum size effect, defect states and other perturbations which may or may not be the result of imperfect crystal lattice matches (more on lattice considerations below). When two materials are brought together and allowed to reach chemical/thermal equilibrium the fermi level in each material aligns and is constant throughout the system. To the extent that they are able, electrons in the material leave some regions (depletion) and build up in others (accumulation) in order to find equilibrium. When this occurs a certain amount of band bending occurs near the interface. This band bending can be quantified with the built in potential given by:

$V_{bi}=\phi _{1}-\phi _{1}=(E_{G1}+\chi _{1}-\Delta E_{F1})-(E_{G2}+\chi _{2}-\Delta E_{F2})\,$

Where $\Delta E_{F}\,$ is the energy difference between the fermi level and the intrinsic fermi level which is located in the middle of the band gap. In most cases, if the materials are undoped this term is zero. Otherwise it can be calculated with typical solid state device calculations and depends on dopant concentrations and temperature.

In such a structure, the implementable diode characteristics can closely approach those of an idealized diode. Furthermore, the diode model parameters that define the diode current vs. voltage response can be tuned by adjusting the thicknesses and band gaps of the layers.

Applications

Semiconductor diode lasers used in CD & DVD players and fiber optic transceivers are manufactured using alternating layers of various III-V and II-VI compound semiconductors to form lasing heterojunctions.

When a heterojunction is used as the base-emitter junction of a bipolar junction transistor, extremely high forward gain and low reverse gain result. This translates into very good high frequency operation (values in tens to hundreds of GHz) and low leakage currents.

Fabrication

Heterojunction manufacturing requires the use of molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD) technologies into order to precisely control the deposition thickness and alternating band gap value of the semiconductor. MBE tends to be very production uneconomical compared to traditional silicon device fabrication. Gallium arsenide (GaAs) is a typical semiconductor used in heterojunctions.

Bibliography

• Template:Harvard reference, ISBN 0134956567

• Template:Harvard reference, ISBN 0-12-498050-3. A somewhat dated reference respect to applications, but always a good introduction to basic principles of heterojunction devices.

See also

• Anderson's rule
• Band gap
• Heterojunction bipolar transistor
• Homojunction
• pn junction