An Investigation of A Pressure-Sensitive Transistor
A theoretical analysis is given of the dependence of the parameters of a transistor on a force F applied to the emitter and distributed nonuniformly across its surface. The analysis is carried out on the assumption that the current is due to diffusion of the minority carriers. The changes in the transistor parameters are due to the influence of mechanical stress σ on the effective width of the forbidden band. The dependence of the minority carrier lifetime on σ is ignored. The transistor is represented by two parallel transistors with equivalent areas SF and S0 — SF, the first of which is assumed to be subjected to a uniaxial compression and the second is taken to be free of mechanical stresses. The two limiting cases of isotropic and anisotropic distribution of the normal stresses σ z are considered. In the first case, the gradient ∇ zσ z in the region of the p—n junction is practically equal to zero whereas in the second case the gradient is very large. It is shown that in the isotropic case the static current gain B = Ic /Ib is independent of the force F. However, the collector (Ic) and base (Ib) currents should depend most strongly on F when the input is short-circuited. In the anisotropic case, the factor B decreases as F increases, and the pressure sensitivity Γc = ∂Ic / ∂F passes through a maximum; the dependence of Ic on F is strongest under the open-circuit conditions. The main conclusions of the theory are compared with the results of an experimental investigation of germanium planar mesa transistors subjected to pressure concentrated at a point. The principal experimental and theoretical results are in good agreement. However, the presence of a wide zero-sensitivity region and of hysteresis of the dependence of Ic on F are not explained by the theory. It is suggested that these effects may be due to plastic deformation of a crystal near the tip of the needle used to apply the pressure to the transistor.
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