Elemental and Compound Semiconductor

Based on the composition of semiconductors, there are two types of Semiconductors namely Elemental semiconductors and Compound Semiconductors.

Elemental semiconductors

These are made from single element and also known as indirect band gap semiconductors.

In which the recombination of free electron from the conduction band with the hole in the valence band takes place via traps. During recombination phonons [lattice vibrations] are produced and they heat the crystal lattice [position of the atom].

Example: Ge, Si.

Electrical Properties of Elemental semiconductors (gERMANIUM and siLICON)

S. No	Germanium (Ge)	Silicon(Si)
1.	In Ge the electrons can be easily excited to conduction band due to less gap energy (0.67 eV).	In Si the electrons cannot be easily excited to conduction band due to high gap energy (1.1 eV).
2.	Operating temperature is less 100°C.	Operating temperature is high 200°C.
3.	The increase in electrical conductivity due to doping less.	The increase in electrical conductivity due to doping more.
4.	Germanium is less stable on temperature and applied voltage.	Silicon is more stable on temperature and applied voltage.
5.	The desired carrier life time by doping the Gold is difficult to obtain in Ge.	The desired carrier life time by doping the Gold is easy to obtain in Si.
6.	In the case of thermistor, the electrical conductivity of Ge is increased 6% per degree increase in temperature.	In the case of thermistor, the electrical conductivity of Si is increased 8% per degree increase in temperature.
7.	The dielectric constant is more.	The dielectric constant is less.

S. No	Property	Germanium	Silicon
1.	Atomic Number	32	14
2.	Atomic weight	72.6	28.1
3.	Dielectric constant	16.1	11.8
4.	Band gap energy [eV]	0.67	1.08
5.	Intrinsic charge density/m³	2.5×10¹⁹	1.38×10¹⁶
6.	Electron diffusion constant [m²/s]	0.0099	0.0034
7.	Hole diffusion constant [m²/s]	0.0047	0.0013
8.	Electron mobility [m²/V-s ]	0.38	0.13
9.	Hole mobility [m²/V-s ]	0.18	0.05
10.	Intrinsic resistivity [ohm -m]	45	2300

Due to the above properties, Silicon is widely used to manufacture the electronic devices, IC circuits, detectors and solar cells.

Compound Semiconductors

The Compound Semiconductor is a semiconductor compound composed of elements from two or more different groups of the periodic table. i.,e III – V group, II – VI group and IV – VI group.

The elements can form binary (two elements, e.g. GaAs), ternary (three elements, e.g. InGaAs) and quaternary (four elements, e.g. AlIn $GaP)$ .

These are called as direct band gap semiconductor ie., electron-hole recombination takes place directly. Photons are emitted during recombination.

III – V group semiconductors have large forbidden energy gap with covalent bonding. These are used in photovoltaic cell, Laser, LED, Gauss meters. Some of them are,

Al P = 3eV

In Sb = 0.18eV

AlAs = 2.3eV

GaAs = 1.34eV

II – VI group semiconductors have ionic and covalent bonding and they are sensitive to radiations of visible region. These are used in light meters, Photoconductive cells and LDR. Some of them are,

Cd S = 2.45 eV

CdSe = 1.74 eV

CdTe = 1.45 eV

IV – VI group semiconductors have low band gap and sensitive to radiations of Infrared region. Their bonding is partially ionic. These are used in infrared detectors, thermo electro applications like thermo electro cooling, and thermo electric generators. Some of them are,

Pb S = 0.37 eV

PbSe = 0.27 eV

PbTe = 0.33 eV

Ternary Compounds: These are group of materials composed of three kinds of atoms and which are used as photo voltaic materials.

Example:

Cu ln S, Cu ln Se, Cu ln Te

Difference between Elemental and Compound Semiconductors

S. No	Elemental Semiconductors	Compound Semiconductors
1.	These are made from single element.	These are made from compound (mixed) element.
2.	These are made from IV group elements.	These are made from III and V [or] II and VI group of elements.
3.	These are called as indirect band gap semiconductor (electron – hole recombination takes place through trapes )	These are called as direct band gap semiconductor (electron – hole recombination takes place directly )
4.	Heat is produced in the recombination.	Photons are emitted during recombination.
5.	Life time of charge carriers is more due to indirect recombination.	Life time of charge carriers is less due to indirect recombination.
6.	Current amplification is more.	Current amplification is less.
7.	These are used for making diodes, transistor , etc. Examples: Ge , Si	These are used for making LED, laser diodes, etc. Examples: GaA, GaP , CdS , Mgo