Types of Semiconductor Laser

Laser action can also be produced in semiconductors. The most compact of all lasers is the semiconductor laser. It is also called injection laser. A semiconductor laser is a specially fabricated p-n junction device that emits coherent light when forward – biased.

Types of semiconductor laser

There are two types of semiconductor lasers:

(i) Homojunction laser

Homojunction means that a $p-n$ junction is formed by a single crystalline material.

E.g., : Gallium arsenide (Ga-As).

(ii) Heterojunction laser

Heterojunction means that the junction has one material on one side and a different material on the other side. It is also called modern laser.

E.g., : Gallium aluminium arsenide (Ga-Al-As).

Homojunction laser

The most compact of all lasers is the semiconductor laser. It is produced by stimulated emission by recombination of holes and electrons. These holes and electrons are produced in the depletion region by the direct flow of current. The population inversion in the depletion region is responsible for laser action. This kind of laser is called as homojunction semiconductor laser or injection laser or $p-n$ junction laser.

Characteristics

Type	Homojunction semiconductor laser.
Active medium	P-N junction diode.
Active centre	Recombination of electrons and holes.
Pumping method	Direct pumping ( electrical energy to optical energy)
Optical resonator	Junction of diodes – polished.
Power output	1mW
Nature of Output	Pulsed (or) continuous wave form.
Wavelength of the output	8400 Å – 8800 Å
Band gap	1.44 eV

Principle

When a forward bias is applied to the heavily doped p-n junction diode the electrons are injected into the $p$ – side of the junction and holes are injected into the $n$ -side. The recombination of holes and electrons with in the junction region results in recombination radiation. This region is called depletion region.

If the concentration of recombination ions is more than the concentration of majority carrier in the p-type or n-type, then it is called ‘population inversion’. During recombination process, light radiation (photons) is released from direct band gap semiconductors like gallium arsenide. It stimulate other charges and as a result, stimulated emission takes place which leads to laser light.

Construction

The active medium is a p-n junction diode made of crystalline Gallium arsenide. This p-region is doped with Ge and n-region is doped with Tellurim. The p-n junction layer is very thin like a few microns.

Electric current is applied to the p-n junction diode through strip electrode. The end faces of the junction diode are well polished and parallel, so that they act as an optical resonator.

Fig. 1.1 Homojunction semiconductor laser

Working

In a p-n junction diode the p-type and n-type regions are heavily doped.
Under large applied forward bias, electrons and holes are injected into the junction and as a result, the region around the junction contains large number of electrons within conduction band and large number of holes with in the valence band.
When the population density is high enough, a condition of population inversion is achieved and recombination may be stimulated resulting in laser action as shown in the energy level diagram. (Fig.1.2). Hence the wavelength of light emitted is $8400Å$ .

Fig. 1.2 Energy level diagram of Homojunction semiconductor laser

Advantages

Efficiency is high.
It is highly economical.
It is small and compact.
It can have a continuous wave output (or) pulsed output.
The modulation of the output is possible.

Disadvantages

Laser output has large beam divergence.
Coherence of laser beam is very poor.
Threshold current density is very large.

Applications

It is mostly used in fibre optic communication.
It is used in printers for computer printouts, writing and reading CD’s.
It can be used as a pain killer.
It is used to produce laser diodes which are more powerful and coherent than LED.

Heterojunction laser

A p-n junction made up of different material with two regions, $n$ -type and $p$ -type is known as heterojunction. Two types of heterojunction are available.

Isotype heterojunction (made up of two different materials having same type of conductivity).
Anisotype heterojunction (made up of two different materials having different conductivity).

Characteristics

Type	It is a Heterojunction semiconductor laser.
Active medium	p-n junction made from different layers.
Pumping method	Direct conversion method.
Power output	The power output of laser beam is 10mW
Nature of Output	Continuous wave form.
Wavelength of the output	Nearly 8000Å
Band gap	1.55 eV

Principle

When p-n junction diode is forward biased, electrons from $n$ region and holes from $p$ -region recombine with each other at the junction. During recombination process light is released from certain. specified direct band gap semiconductors.

Construction

Heterojunction semiconductor laser consists of five layers as shown in (Fig.1.3).

Fig. 1.3 Heterojunction semiconductor laser

A layer of Ga-As- p-type ( $3 rd$ layer) will act as the active region. This layer is sandwitched between the two layers having wider band gap, GaAlAs – p-type (2nd layer) and GaAlAs n-type (4^th layer). Electric current is applied to the crystal through the electrode which is fixed on the top and bottom layers. The end faces of the junctions of $3 rd$ and $4 th$ layer are well polished and parallel to each other. They act as the optical resonator.

Working

The energy level diagram is shown in (Fig.1.4). When the $p-n$ junction is forward biased, the electrons and holes are injected into the junction region.
The region around the junction contains large amount of electrons in the conduction band and holes in the valence band.
The population inversion is achieved. At this state, some of the injected charge carriers recombines and produce radiation in the form of light.
When the forward biased voltage is increased more and more light photons are emitted and the light intensity is more.
These photons can trigger a chain of stimulated recombinations resulting in the release of photons inphase.
The photons moving at the plane of the junction travels back and forth by reflection between two sides, and grow its strength coherent beam of laser having wave length nearly $8000Å$ that emerges out from the junction region.