Hetero junction

PN HETERO-JUNCTION UNDER EQUILIBRIUM AND HIGH FORWARD BIAS

1. HETERO-JUNCTION Energy band diagrams of a Hetero-junction under equilibrium and a p-n junction under High forward bias J.YOGAPRABHA

2. HETERO-JUNCTION 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 . 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 of heterojunction is the interface between any two solid-state materials, including crystalline and amorphous structures of metallic, insulating,  fast ion conductor  and semiconducting materials.

3. ENERGY BAND ALIGNMENT The behaviour of a semiconductor junction depends crucially on the alignment of the  energy bands  at the interface. 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 [9] . Away from the junction, the  band bending  can be computed based on the usual procedure of solving  Poisson's equation .

4. The three types of semiconductor heterojunctions organized by band alignment.

5. Introduction: Most interesting semiconductor devices usually have two or more different kinds of semiconductorsThere are four different kinds of commonly encountered heterostructures : a) pn heterojunction diode b) nn heterojunctions c) pp heterojunctions d) Quantum wells, quantum wires, and quantum dots A pn Heterojunction Diode : Consider a junction of a p-doped semiconductor (semiconductor 1) with an n-doped semiconductor (semiconductor 2). The two semiconductors are not necessarily the same, e.g. 1 could be AlGaAs and 2 could be GaAs . We assume that 1 has a wider band gap than 2. The band diagrams of 1 and 2 by themselves are shown below.

6. Semiconductor P-N Heterojunction Once a junction is made: • Electrons will flow from the side with higher Fermi level (1) to the side with lower Fermi level (2) • Holes will flow from the side with lower Fermi level (2) to the side with higher Fermi level (1)