The evolution of computers has opened up a wide range of potential improvement in the characteristics and efficiency of semiconductor devices design. The use of these computers allows the development of mathematical models of these devices and enables a better understanding of their thermal, electrical and optical behaviours. It also reduces the cost of growing and testing. The current research has been directed towards the objective of providing the semiconductor laser diodes with a standard complete thermal model. It incorporates the laser diode (of reduced and real dimensions) and laser amplifier. On the other hand, the optical/electrical properties of the same diodes were the subjects of another thesis [1] where the TransmissionLine Modelling (TLM) method was used to model these lasers. Therefore, in order to complete the study of the DH laser diode, it was more convenient to use the same numerical method termed the Transmission-Line Modelling (or Matrix) (TLM) technique. The first thermal model of the OH laser developcd solves a steady state problem. The rate equations are solved under a continuous current input in the time domain assuming that both the carrier and the photon densities are uniformly distributed along the laser cavity. When the solution reaches the steady state, the optical output power, opticallosses within the diode, the power dissipated due to the nonradiative recombination and Joule heating in aIl the layers are calculated. The optical losses consist of (i) free carrier absorption of stimulated emission in both the active region of the laser diode and the cladding surrounding layers, (ii) free carrier absorption of spontaneous radiation in the active, capping and the substrate layers and (iii) scattering loss in the active-cladding interface. Incorporating aIl the heat sources in the TLM model of the diode, the two-dimensional temperature distribution in the plane of the laser mirror is examined. These temperature distributions are also obtained for different combinations of these powers. The magnitude of these temperatures and their distributions are comparable to those obtained in the thermal models iIlustratcd in the literature review. This thermal analysis has demonstrated that the TLM model of the laser diode can be very useful for predicting the temperature rise of the laser and it can also be efficient for the design of these lasers. In addition, it is also important to note that the TLM models developed in this work are standart and can be applied to any homogeneous or heterostructure device providing that its thermal properties are knoun .