Measurement of equivalent circuit parameters of transistors at v.h.f.

Abstract

A technique is presented for measuring the current gain of transistors in magnitude and phase under near-short-circuit conditions at frequencies up to some 180Mc/s. The basic method of measurement involves the determination of three relative voltage magnitudes, from which the magnitude and phase of current gain can be deduced. An important feature of the technique is its simplicity: no specialized equipment is required, signal amplification and voltage magnitude assessment being carried out by means of a standard communication receiver together with a suitable calibrated attenuator and/or indicator.Using the technique of current-gain measurement in conjunction with other measurements, the parameters of the complete high-frequency equivalent circuit of the transistor are determined for diffusion and drift transistors of essentially one-dimensional geometry. In the case of the drift transistor, an important step in this process of measurement is the determination of the base field parameter, m, defined as the ratio of the ‘built-in’ base-region p.d. to the thermal p.d., kT/e. It is shown that m may be deduced from the phase shift of the terminal common-base short-circuit current gain at its 3 dB cut-off frequency, the emitter depletion-layer capacitance having also been measured. Results of equivalent circuit determination are presented for a diffusion transistor and several types of drift transistor, the dependence of the terminal and intrinsic current gains on frequency being shown as polar plots. The properties of the intrinsic current gain of base transport, as deduced from measurements, are compared with the theoretical behaviour assuming for the latter a constant electric field strength throughout the base region for the drift transistors. It is concluded that the commonly used constant-field theory is adequate for analysis purposes in most practical applications of the drift transistors studied, although certain discrepancies are observed between theory and measurement.