Topics we will cover include: Thermal motion of carriers, Carrier motion under electric field, Drift current, Mobility and conductivity, Velocity saturation, Diffusion of carriers, General expression for currents in semiconductor, Carrier concentration and mobility, and the Van der Pauw technique. The rate at which diffusion occurs depends on the velocity at which carriers move and on the distance between scattering events. It is termed diffusivity and measured in cm 2 s -1 . 2.10.1 Introduction: Carrier diffusion Carrier diffusion is due to the thermal energy which causes the carriers to move at random even when no field is applied. Lifetime τ p and diffusion length L p of holes in n-type Si vs. donor density. We can generalize the answer for conductivity. In physics (specifically, the kinetic theory of gases) the Einstein relation (also known as Wright-Sullivan relation) is a previously unexpected connection revealed independently by William Sutherland in 1904, Albert Einstein in 1905, and by Marian Smoluchowski in 1906 in their works on Brownian motion.The more general form of the equation is =, where D is the diffusion coefficient; This expression states that the current is the product of the electronic charge, q, a velocity, v, and the density of available carriers in the semiconductor located next to the interface.The velocity equals the mobility multiplied with the field at the interface for the diffusion current and the Richardson velocity (see section 3.4.2) for the thermionic emission current. • Drift current: produced by electric field • Diffusion current: produced by concentration gradient • Diffusion and drift currents are sizeable in modern Traditional diffusion, LPCVD (Low Pressure Chemical Vapor Deposition), and other batch semiconductor applications require the thermal and purity properties of advanced ceramics. Drift velocity = Mobility * Electric Field. T = 300 K. For 10 12 cm -3 < N d ≤ 10 17 cm -3 - from numerous experimental data for good quality industrial produced n -Si. is the electron drift velocity and the electron mobility for this semiconductor ? Starting with Equation 3.24 (with slightly modified notation from your text to be consistent with our discussions): J =q(pµp +nµn)E (Equation 3.24: modified) where: J is the current density (in A/m2=C/m2s=V/Ωm2) q is the elemental charge (≈1.602x10-19 C) n is the number of free electrons in the medium p is the number of free holes in the medium In general, the drift velocity is given by v = E, where is the mobility of charge carriers and E is the applied electric field. 6.012 Spring 2007 Lecture 3 16 What did we learn today? There is no generic answer for drift velocity for comparison between semiconductors and conductors. Solution The average drift velocity of the electrons v n is given by v n = L t (1) where L is the length of the semiconductor and t is the transit time. This chapter focuses on atom diffusion in crystalline semiconductors , where diffusing atoms migrate from one lattice site to adjacent sites in the semiconductor crystal. In an intrinsic semiconductor the flow of current is due to movement of both electrons and … Values for silicon, the most used semiconductor material for solar cells, are given in the appendix . Batch Diffusion & LPCVD Components. • Electrons and holes in semiconductors are mobile and charged – ⇒Carriers of electrical current! A steady drift velocity is thus achieved giving rise to a steady flow of current. Diffusion describes the movement of atoms through space, primarily due to thermal motion, and it occurs in all forms of matter.

diffusion velocity in semiconductor

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