{"id":631,"date":"2018-11-26T06:14:40","date_gmt":"2018-11-26T06:14:40","guid":{"rendered":"http:\/\/phyp10.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=631"},"modified":"2018-11-26T06:38:55","modified_gmt":"2018-11-26T06:38:55","slug":"631","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/phyp10\/chapter\/631\/","title":{"rendered":"Physics of Semiconductors and semiconductor laser"},"content":{"raw":"

Learning Outcomes<\/strong>\r\n<\/span><\/p>\r\n

From this module students may get to know about the following:\r\n<\/span><\/p>\r\n

\u2022 Semiconductor Diode\r\n\u2022 Semiconductor Laser\r\n<\/span>\r\nPhysics of Semiconductors and semiconductor laser:<\/strong><\/span>\r\n<\/span><\/p>\r\n

Introduction<\/span><\/strong>\r\n<\/span><\/p>\r\n

In semiconductor laser theory, the optical amplification is achieved in a semiconductor material. The choice of material depends on the desired wavelength and properties such as modulation speed. It may be a bulk semiconductor, but more often a quantum hetero-structure. For creating the population inversion, pumping may be done electrically or optically.<\/span><\/p>\r\n

Light is generated in a semiconductor laser by radiative recombination of electrons and holes. In order to generate laser different models are assessed wherein many \u2013particles interactions are considered which otherwise in simple models are neglected. In Free carrier model carrier plasma is simply seen as a reservoir that relaxes the carrier distributions. Simple models for the gain coefficient are used to obtain a set of rate equations that enable us to dynamically calculate the time-dependent laser response.<\/span>\r\n<\/span><\/p>\r\n

The semiconductors materials can be broadly classified into two categories namely, Direct band gap and Indirect band gap materials .A general picture of the same is explained as:<\/span><\/p>\r\n\"\"\r\n

(Ref: No. 6)<\/strong><\/span>\r\n<\/span><\/p>\r\n

Crystal momentum p<\/strong> or the wave vector k<\/strong> for electron in a crystal is represented by\u00a0 the relation\r\nP<\/strong>=2\u03c0hk,h being the Plank\u2019s constant.\r\n<\/span>\r\nWhen\u00a0 electron\u00a0 makes\u00a0 a\u00a0 transition\u00a0 downwards\u00a0 from\u00a0 conduction\u00a0 band\u00a0 to\u00a0 valence\u00a0 band\u00a0 and recombines with hole, a photon is emitted with corresponding energy difference of E2-E1<\/span>\r\nSo that\r\n<\/span><\/p>\r\n

\u210ef<\/em>\u2212 ?2 \u2212 ?1\u00a0 = Eg\r\n<\/span><\/p>\r\n

However, in case of indirect band gap, the lowest energy conduction band electrons have a non zero momentum.<\/span><\/p>\r\n

\"\"\r\n(Ref: No. 6)<\/strong><\/span>\r\n<\/span><\/p>\r\n

Thus\u00a0 radiative\u00a0 recombination\u00a0 in\u00a0 this\u00a0 case\u00a0 requires\u00a0 momentum\u00a0 transfer\u00a0 due\u00a0 to\u00a0 photon participation .so frequency of emitted radiation is given by the equation\r\n<\/span><\/p>\r\n

\u210ef<\/em>= ?2 \u2212 ?1; \u210ef<\/em>p\u00a0<\/sub> \u2248 Eg\r\n<\/span><\/p>\r\n

Here fp is photon frequency. Thus recombination probability of an electron hole pair leading to the emission of radiation is much smaller in indirect band gap materials than in direct band gap\u00a0<\/span>materials .hence the direct band gap materials like gallium arsenide(GaAs) are used in the fabrication of semiconductor lasers.<\/span><\/p>\r\n

\r\nThe p-n junction<\/strong><\/span>\r\n<\/span><\/p>\r\n

The commonly used semiconductor material for fabricating laser diodes is gallium arsenide which emits light at a wavelength ~ 8400 Ao in the near infrared of optical Spectrum. The Wavelength of emission can be varied from 6300 A (visible) for the III-V mixed semiconductor gallium arsenide phosphide to 8.5 \u03bcm (infrared) for the IV-VI semiconductor lead selenide.<\/span>\r\n<\/span><\/p>\r\n

In intrinsic semiconductor materials, the ambient temperature raises some electrons from the valence band into the conduction band by thermal excitation, leaving behind an equal number of holes in the valence band. Photon emission due to the recombination of electron and holes in intrinsic semiconductors is too small. Therefore, to increase the photon emission, we need to increase the charge carrier density by adding dopnts .Thus the energy level diagram modifies accordingly, resulting in an n-type semiconductor material when donor materials having excess electrons are added to it . On the other hand, when acceptor material rich in holes are added to it, a p-type semiconductor material is formed.<\/span><\/p>\r\n

\"\"\r\n((Ref: No. 6)<\/strong>\r\n<\/span>\r\nWhen these two materials are joined to form a p-n junction, the electrons and holes tend to move in opposite directions across the physical boundary by a diffusion process, setting up a depletion layer sandwiched between the p-type and n-type materials .across the depletion layer, a barrier potential is set up in such a way that it inhibits further flow of charge carriers, thus establishing equilibrium condition. The electrons and holes which manage to find themselves in the depletion region may recombine with the emission of a photon corresponding to the energy gap Eg. In a normal p-n junction in electronic devices, these photons never see the light of the day since their numbers are small and efficiency of the production is small in Si and Ge materials typically used in electronic devices.<\/span><\/p>\r\n \r\n

\"\"\r\n(Ref: No. 6)<\/strong>\r\n<\/span>\r\nDegenerate Semiconductor Diode<\/strong><\/span>\r\n<\/span><\/p>\r\n

When heavy doping is done then the semiconductor material almost acts like a conductor and is referred to as degenerate semiconductor.<\/span>\r\n<\/span><\/p>\r\n

Degenerate p-type and n-type diodes of band gap semiconductor materials whose energy level diagram is.<\/span>\r\n<\/span><\/p>\r\n\"\"\r\n

(Ref: No. 6)<\/strong>\r\n<\/span><\/p>\r\n\r\n