Question 5: (a) The parameter Hamb, is the ambipolar mobility parameter. If we assume low level injection, show that the

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Question 5 A The Parameter Hamb Is The Ambipolar Mobility Parameter If We Assume Low Level Injection Show That The 1 (170.29 KiB) Viewed 31 times

Question 5: (a) The parameter Hamb, is the ambipolar mobility parameter. If we assume low level injection, show that the electron mobility, n = Mamb for the given experiment. [5] After a delay time, t, excess carriers that have survived recombination will reach the sample region underneath the point probe C. The contact (diode) Dc, which is reverse biased by the resistor Rb, acts as a collector for minority carriers. This results is a weak current of electrons which flows through Rb; only when the burst/pulse of excess carriers reaches the collector region does this current increase - due to the larger density of free electrons, and a negative pulse across R₁ can be detected by an oscilloscope. Two peaks will be obtained on the oscilloscope, see Figure 1: the first peak (with amplitude and width similar to those of the pulse applied to the emitter) is due to the injection pulse propagating from E to C at the speed of light in the semiconductor material; therefore, it is practically simultaneous with the injection pulse. [Understanding this peak requires a deeper understanding of a concept of relaxation dielectric relaxation time e/o, where & is the dielectric constant and o the electrical conductivity of the sample (ε/o ~10-¹2 s) and will be taught in an electromagnetic course.] The second peak, which is of interest to us, is due to the excess carriers flowing past the collector and it is much smaller and broader, owing to diffusion and recombination processes. Equations Used to Determine - drift velocity (VD), electron mobility (₂), diffusion constant (D₂) and the lifetime of excess carriers, Tno. The drift velocity, vp is obtained experimentally from knowing the distance, d, between the two electrodes and the time of flight, t: VD = ²/1 (3) Question 6: