The document discusses heterojunctions and p-n junctions. It defines a heterojunction as the interface between two dissimilar semiconductors with different band gaps. There are three types of heterojunctions based on band alignment: type I where bands straddle, type II where bands are staggered, and type III where there is a broken gap. A p-n heterojunction diode forms when a p-doped and n-doped semiconductor meet; electrons flow from the higher to lower Fermi level side and holes in the opposite direction.

1. 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)