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In an epitaxial transistor operating in saturation, charge is stored not only in the active and passive base regions but also in the region bounded between the collector junction and n - n + interface. The saturation operation of a transistor switch is characterized by a single most fundamental parameter, the saturation time constant, which may be related to the intrinsic physical structure of the transistor. In this paper, an attempt has been made to correlate the saturation behaviour of an epitaxial planar transistor with its physical structure. The analysis takes into account the effects of n - n + interface in the collector and the retarding-accelerating built in field for the minority carriers in the base, both of which so far have been subject to gross simplifications. The theoretical conclusions have been verified experimentally for some typical epitaxial planar transistors.

The Raman scattering spectrum of V${\mathrm{O}}_{2}$ is reported between 4.2 and 600\ifmmode^\circ\else\textdegree\fi{}K. In the semiconducting monoclinic phase only sharp phonon structure is observed. The metallic rutile phase exhibits a pronounced softening and damping in the observed phonon spectrum. This evidence supports a recently proposed phenomenological mechanism for the semiconductor-to-metal transition.

The Raman spectra of single crystals of CrO 2 and MoO 2 have been studied from 77°K to 500°K for a comparison with the heavily damped phonon features in metallic VO 2 . The spectrum of MoO 2 is very similar to that of monoclinic VO 2 whereas CrO 2 exhibits phonon spectra similar to rutile with no appreciable change at the ferromagnetic c curie temperature. The results are discussed with respect to the various models proposed for the semiconductor-to-metal transition in VO 2 .

Organic Chemistry Laboratories, University of Rajasthan, Jaipur 302004 Manuscript received 24 January 1974, revised 28 June 1974; accepted 1 July 1974 Chemical Investigation of the Bark and Heartwood of Grewia asiatica Linn. &nbsp