The Haynes-Shockley Experiment
The basic principles of the Haynes-Shockley experiment are as follows:
- A pulse of holes is created in an n-type bar (for example) that contains an electric field that is shown in figure as the pulse drifts in the field and spreads out by diffusion, the excess hole concentration is monitored at some point down the bar; the time required for the holes to drift a given distance in the field gives a measure of the mobility; and the spreading of the pulse during a given time is used to calculate the diffusion coefficient.
- In this fig a pulse of excess carriers is created by a light flash at some point x = 0 in an n-type semiconductor (n0 > p0). We assume that the excess carriers have a negligible effect on the electron concentration but change the hole concentration significantly.
- The excess holes drift in the direction of the electric field and eventually reach the point x = L, where they are monitored.
- By measuring the drift time td, we can calculate the drift velocity vd and, therefore, the hole mobility:
- Thus the hole mobility can be calculated directly from a measurement of the drift time for the pulse as it moves down the bar.
- In contrast with the Hall effect, which can be used with resistivity to obtain the majority carrier mobility, the Haynes-Shockley experiment is used to measure the minority carrier mobility.
- we use an experimental setup such as fig given below, which allows us to display the pulse on an oscilloscope as the carriers pass under a detector.
- A forward-biased p-n junction serves as an excellent injector of minority carriers, and a reverse biased junction serves as a detector.
- The measured quantity in Fig below is the pulse width Δt displayed on the oscilloscope in time.
- It is related to Δx by the drift velocity, as the pulse drifts past the detector point