A proposed space-charge-limited dielectric triode

Abstract

Recent experiments have shown that large, controllable, space-charge-limited (s.c.l.) current can be obtained in insulating crystals of cadmium sulphide if these are grown sufficiently free from lattice defects arid provided with a suitable cathode contact. A dielectric diode made in this way is analogous to the thermionic vacuum diode; in particular there is he possibility of adding a control electrode to form the dielectric triode which, together with the vacuum triode and the semi-conductor transistor will complete the basic trio of charge-control devices. This paper anticipates the construction of this triode and deduces the characteristics which it should possess.The basic equations of the triode are derived and it is shown that its d.c. characteristics should be very similar in form to those of the vacuum triode. Frequency characteristics are determined by electron trapping effects and equations are derived describing the response of the triode to time-varying signals. These equations are solved for the case of a sinusoidal signal voltage and the frequency dependence of mutual conductance and anode resistance calculated. It is shown that a triode relatively free from electron traps should have quite uniform characteristics up to frequencies at which transit times of the order of 1mμsec become significant and should therefore be useful as a wide-band video amplifier. However, if the triode possesses a high density of shallow traps its parameters are markedly dependent on frequency. It is shown that in this case the mutual conductance should rise and the anode resistance should fall, possibly by very many times, as the operating frequency increases. The high frequency values should be maintained up to frequencies at which transit times of the order of 0.1mμsec become significant; the device should thus be a useful v.h.f. amplifier.Because of its necessarily small size there are many technological problems involved in the construction of a practical triode. The development of the device is a worthwhile proposition, however, because it should possess a high input resistance, be relatively insensitive to temperature changes, and in its applications should be able to utilize the vast amount of circuit design accumulated for the vacuum triode.