Haynes Shockley Experiment
Haynes–Shockley experiment (1948) demonstrated that diffusion of minority carriers in a semiconductor could result in a current. The experiment can measure drift mobility, carrier lifetime, diffusion coefficient and relaxation time of electrons and holes. It allows us to investigate the drift velocity, the diffusion process and the recombination of excess charge carriers.
In this experiment, excess carriers are optically injected (using laser) in a slab of P-doped semiconductor bar, of length l. An electric field Es is temporarily produced in the sample by a pulsed generator (shown as a battery in series with a switch) in above figure. Two partially rectifying (Schottky) contact electrodes (E and C) at a distance d from each other are made by using metal needles. So the figure shows them as diodes DE and DC.
Let us apply a –ve pulse of large amplitude to the electrode E (emitter). It forward biases the diode DE and injects electrons in the region under the emitter. The pulse drifts with velocity vd because of the electric field Es. The pulse also spreads out by diffusion. Thus after a time t, it will reach the region underlying the collector electrode C. When the excess electron pulse reaches the point contact C, the minority charge carrier density is locally increased. Thus the inverse current increases and it produces a voltage drop across the resistance R.
Let us apply a –ve pulse of large amplitude to the electrode E (emitter). It forward biases the diode DE and injects electrons in the region under the emitter. The pulse drifts with velocity vd because of the electric field Es. The pulse also spreads out by diffusion. Thus after a time t, it will reach the region underlying the collector electrode C. When the excess electron pulse reaches the point contact C, the minority charge carrier density is locally increased. Thus the inverse current increases and it produces a voltage drop across the resistance R.
It produces a short –ve pulse on the oscilloscope. The amplitude of this pulse is comparable to that of the injected pulse. After a time delay t, another (second) negative pulse, which is wider and much smaller than the first one, is found to appear.
The first peak occurs simultaneously with the injected pulse. It is caused by the em signal propagating across the sample at the speed of light. The second pulse corresponds to the excess electron distribution passing under the collector contact. Its shape is Gaussian and its amplitude and width are determined by diffusion and recombination processes. The shape of actual pulse depends on the drift time t, on distance d, on the drift velocity μEs, and also on the diffusion constant D.
While drifting towards the collector, the injected electrons diffuse out and broaden their spatial distribution. Thus the width of the collected pulse increases with the time of flight t. The electrons recombine with holes and so their number decreases exponentially with time t as given by n(t) = no exp(-t/τ), where τ is excess charge carriers lifetime. By measuring the distance d and the time of flight t, we get the drift velocity, vd = d/t. By measuring pulse amplitude VS and the sample length l, we get the sweep field ES =VS/l and therefore of the electron mobility μ = vd/Es = (d l)/(t VS). The measurement of the pulse width at half-height Δt gives the diffusion constant D.
The first peak occurs simultaneously with the injected pulse. It is caused by the em signal propagating across the sample at the speed of light. The second pulse corresponds to the excess electron distribution passing under the collector contact. Its shape is Gaussian and its amplitude and width are determined by diffusion and recombination processes. The shape of actual pulse depends on the drift time t, on distance d, on the drift velocity μEs, and also on the diffusion constant D.
While drifting towards the collector, the injected electrons diffuse out and broaden their spatial distribution. Thus the width of the collected pulse increases with the time of flight t. The electrons recombine with holes and so their number decreases exponentially with time t as given by n(t) = no exp(-t/τ), where τ is excess charge carriers lifetime. By measuring the distance d and the time of flight t, we get the drift velocity, vd = d/t. By measuring pulse amplitude VS and the sample length l, we get the sweep field ES =VS/l and therefore of the electron mobility μ = vd/Es = (d l)/(t VS). The measurement of the pulse width at half-height Δt gives the diffusion constant D.
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