Application of JFET-Mesfet devices to bucket-brigade circuits

Abstract

Electronically-controlled delay lines based on the bucket-brigade concept have been made in the past by using MOSFET and bipolar structures. To the present time, depletion mode JFET or MESFET structures have not been used for such circuits. In this paper, the operating characteristics of JFET devices are discussed with reference to their use in a bucket brigade delay line, and it is shown that by using JFET or MESFET switches a significant improvement in performance can be expected from such circuits. The high frequency performance of a bucket brigade circuit is to a large extent governed by the current flow capability of the switching devices. For a MOSFET this is given by: <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_{DS} \frac{dQ}{dt} = \frac{g_{m}}{2} (V_{gs} = Vth)</tex> for a JFET we have: <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_{DSS} = \frac{dQ}{dt} = \frac{g_{m}}{2} V_{p} (1- \frac{V_{gs}}{V_{p}})^{2}</tex> The bandwidth of these devices is therefore directly related to the value of g <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</inf> , which in a p-channel MOSFET device is of the order of 24 µmhos/ square and for a JFET structure of the order of 150 µmhos/square. It follows, therefore, that data rates far in excess of those already reported for MOSFET Brigades (5 - 10 MHz) can be expected from JFET structures. For low frequency operation, the bipolar brigade is severely handicapped by relatively high leakage currents which are primarily due to the enlarged base area required for the storage capacitor. With JFET brigades (using MOS capacitors) the leakage currents are considerably reduced. A ten stage JFET brigade has been operated successfully at 100°C at a clocking frequency of 100 Hz, and at 10 MHz with a 2 volt clock.