Presence of negative resistance in FET output characteristics

Abstract

for inverted population, it is of interest to point out the following. If the ruby were ideal one would expect a maximum photobeat to occur whenever the ruby was a t a position where two cavity modes were 90" out of space phase. When the ruby was located at positions where the modes were in space phase, a minimum photobeat should occur.Precisely the opposite effect was observed for the nearest and next nearest neighbor axial mode separations. For higher order mode separations there was little positive or negative correlation with such theory except that the number of maxima or minima observed was equal to the mode number as mentioned previously. Presumably this was due to the fact that in our experimental contjguration the ruby length became comparable to, or large with respect to, the distance over which spatial phase changes between modes occurred.Several papers1-5 have discussed the variation in drain current for silicon fieldeffect transistors due to changes in temperature. Two opposing effects are present and, depending upon the relative pinchoff voltage of the FET, the net temperature coefficient a of the drain current may be either positive or negative.If a is negative and large enough in relation to the drain current and the drainambient thermal resistance, then it is possible that the drain or output characteristic curves for the FET may exhibit a voltagestable negative resistance slope.A typical example is shown in Fig. 1 obtained by sweeping the VDS very slowly such that the internal temperature could follow the variation in power dissipation and, thus, represent the characteristic curve for thermal equilibrium a t every point.Careful analysis yields the following equation for the resulting output resistance rds*. rda* [ I / d Z D ' -~D S / f D ] / r d a ( 1 )where a is the fractional temperature coefficient of the drain current, e is the drainambient thermal resistance, and rd, is the small-signal instantaneous value of the output resistance.The value of a is a function of the pinchoff voltage for the FET as well as the drain Manuscript Conditions for a temperature compensated silicon field effect transistor, P m IEEE (Carcspondencc). vol 51. Jul 1963. pp 1058-1059. a Sevm, L.. Effect of temperature on FET characteristics, Elccfrc-Techndogy, voi 13. Apr 1964. pp 103-107.Fig. 1. Slow-sweep drain characteristics for a typical 2N3368 FET illustrating the display of negative resistance.current, and may be either positive, negative, or zero. Thus, the value of rd** may also be either positive or negative and may even be infinite if the quantity within the brackets of (1) is negative and exactly equal in magnitude to the rda parallel with it.It should be realized that even though the negative contribution of the output resistance may be too high in magnitude to dominate the positive rd8, the basic effect is still present and will result in a value of r&* which will still be positive but higher than an instantaneous small-signal measurement might indicate. The negative resistance effect must be considered wh...