Junction field effect transistors (JFET) at cryogenic temperatures can be employed as almost perfect charge detectors with leakage currents of less than 1 aA. The electrical input and output characteristics of the commercial n-channel silicon JFET BF545B is studied as a function of temperature in the range between 30 and 300 K. As long as the charge carrier concentration is constant an increasing drain current is observed for reduced temperatures and low gate voltages. Using a constant mobility model for the device this behaviour can be explained with the higher electron mobility in the source-drain channel. The mobility is found to increase with T⁻¹ at lower temperatures which corresponds to a dopant concentration of 7 ⋅ 10¹⁶/cm³. For larger negative gate voltages a source-drain voltage is found at which the drain current is almost temperature independent. As soon as the charge carriers freeze out the input characteristics changes significantly due to the exponential decrease of the carrier concentration. The effective reduction of the gate leakage current is crucial for ultimate sensitive charge detection. Hence, the current is measured through the entire temperature range in an open gate configuration. The leakage current decreases exponentially with lower temperatures across more than six orders of magnitude and reaches values of 10⁻²⁰ A below 160 K. It is exclusively due to the generation of electron-hole pairs in the depletion layer since the data are in full agreement with the Shockley-Read-Hall model of the generation current.