Laser Diode (GaAlAs-Laser)

LASER DIODE (GaAlAs-Laser)

Laser diode is of two categories viz.

1. Homojunction semiconductor diode

2. Heterojunction semiconductor diode

Let us discuss in detail about hetero junction semiconductor laser diode.

Characteristics

Principle:

The electron in conduction band combines with a hole in the valence band and hence the recombination of electron and hole produces energy in the form of light. This photon, in turn may induce another electron in the conduction band (CB) to valence band (VB) and thereby stimulate the emission of another photon.

Construction

It consists of five layers as shown in Fig. 4.25. A layer of GaAs p-type layer (3^rd layer) which has a narrow band gap will act as the active region. This layer layer (3^rd layer) is sandwitched between the two layers having wider band gap viz. GaAlAs ‒ p-type (2^nd layer) and GaAlAs ‒ n-type (4^th layer).

A contact layer made of GaAs p-type (1^st layer) is made to form at the top of the 2^nd layer for necessary biasing. All these four layers are grown over the substrate (5^th layer) made of GaAs-n-type.

The junctions of GaAs - p-type (3^rd layer) and GaAlAs - n-type (4^th layer) are well polished and hence it act as an optical resonator. The upper and lower electrodes helps in forward biasing the diode.

Working

The working of a heterojunction laser is similar to that of the working of a homojunction laser.

1. The diode is forward biased with the help of upper and lower electrodes.

2. Due to forward biasing the charge carriers are produced in the wide band gap layers (2 and 4).

3. These charge carriers are injected into the active region (layer 3).

4. The charge carriers are continuously injected from 2^nd and 4^th layer to the 3^rd layer, until the population inversion is achieved.

5. At this state some of the injected charge carriers recombines and produces spontaneously emitted photons.

6. These spontaneously emitted photons stimulates the injected charge carriers to emit photons.

7. As a result more number of stimulated emission arises and thus large number of photons are produced.

8. These photons are reflected back and forth at the junction and hence an intense, coherent beam of LASER emerges out from the P-N junctions of active region i.e., between layer-3 and layer-4 as shown in Fig. 4.25.

9. The wavelength of the emitted radiation is given by

This wavelength lies in IR region.

Advantages

(i) Power output is very high.

(ii) It produces continuous wave output.

(iii) It has high directionality and high coherence.

(iv) It has low threshold current density compared to homojunction laser.

(v) These diodes are highly stable and has longer life time.

Disadvantages

(i) Cost is higher than homojunction laser.

(ii) Practical difficulties arises while growing the different layers of p-n junction.