SIMPLIFIED THEORY A N D TRANSDUCER POTENTIALITIES OF THE “ PINCH EFFECT ” I N SEMICONDUCTORS

The recently reported "electric pinch effect" in anisotropic semiconductors is analyzed theoretically with emphasis on the underlying physical phenomena. The theory is generalized (a) to apply to extrinsic semiconductors and (b) to encompass effects in the direction transverse to stress and electric field. In ex­ trinsic material the magnitude of the pinch effect is reduced by the ratio of minority to majority carriers. In the transverse direction a potential difference is generated by two essentially independent mechanisms: (1) highly surface-dependent diffusion caused by a stress-dependent concentration gradient of free car­ riers, and (2) surface-independent transverse piezoresistance. The potential and limitations in the utilization of the pinch effect for electromechanical transducers are examined. SIMPLIFIED THEORY AND TRANSDUCER POTENTIALITIES OF THE "PINCH EFFECT" IN SEMICONDUCTORS By Ernest E. Pittelli and Wilhelm Rindner Electronics Research Center SUMMARY The recently reported "electric pinch effect" in anisotropic semiconductors is analyzed theoretically with emphasis on the underlying physical phenomena. The theory is generalized (a) to apply to extrinsic semiconductors and (b) to encompass effects in the direction transverse to stress and electric field. In extrin­ sic material the magnitude of the pinch effect is reduced by the ratio of minority to majority carriers. In the transverse direc­ tion a potential difference is generated by two essentially inde­ pendent mechanisms: (1) highly surface-dependent diffusion caused by a stress-dependent concentration gradient of free carriers, and (2) surface-independent transverse piezoresistance. The potential and limitations in the utilization of the pinch effect for electromechanical transducers are examined. INTRODUCTION A newly reported electromechanical interaction in intrinsic semiconductors, the so-called pinch effect (PE), has yielded resistance changes two orders of magnitude larger than those obtainable from piezoresistance (ref. 1). An effect of such magnitude obviously suggests intriguing new electromechanical transducer potentialities; however, the detailed physical origins of this PE, depicted in Figure 1, are quite involved, and a real­ istic assessment of device possibilities and limitations requires a good understanding of the underlying physics beyond mere recog­ nition of the impressive magnitude of the effect. Presently, one objective, therefore, is to present a sim­ plified, yet realistic, analytical description of the PE with emphasis on the physics underlying the mathematical formalism. Beyond this, the theory will be extended to extrinsic material, a new transverse mode (TPE) of the pinch effect will be proposed and shown to possess several advantages compared to the original longitudinal mode (LPE), and a general, albeit limited, character­ ization of PE device requirements, capabilities, and limitations, will be derived. To anticipate the conclusions of this report: the PE offers the potential for transducers of high sensitivity and permits the use of materials not suited for conventional piezoresistive devices. These advantages are obtained at the expense of stringent I I I I 1 1 1111 I I