Noise Studies of Ceramic Encapsulated Junction Field Effect Transistors (JFETs)

In low noise preamplifiers for nuclear and X-ray spectroscopy, a major source of electronic noise is the lossy glass insulator in the header of the input junction field effect transistor. Substitution of low loss ceramic headers for glass headers and encapsulation with unselected Union Carbide and Texas Instruments 2N 4416 JFET chips decreases total preamplifier electronic noise from 330 eV to 200 eV (FWHM)Si at room temperature. Further reduction in noise is accomplished by cooling the JFET. Preamplifier noise decreases to 125 eV for the TI JFET and to 160 eV for the UC JFET at their optimum temperatures. These noise values were obtained with a feedback resistor and with 4 μsec RC main amplifier pulse shaping time constants. The TI JFET exhibits a narrow minimum in its noise vs temperature curve, therefore it is essential that temperature be controlled quite precisely. A JFET temperature control system is described. Additional results concerning chemical contamination of JFET silicon chips by glass headers are presented. Finally, a method of decreasing noise still further by eliminating the feedback resistor and by using a pulsed light feedback system is discussed.