This paper describes a novel technique of charge restoration - the drain feedback, based on the characteristics of the gate-to-drain junction of the input FET. The restoration charge required at the input of the preamplifier is generated by impact ionization in high-field regions of the FET. Actually the feedback is obtained by regulating the drain voltage according to the input energy count-rate product and consequently adjusting the electric field for the necessary charge generation. In the first part of the paper the principles of impact ionization in semiconductors at cryogenic temperatures are outlined and applied to junction FET's (JFET's) under saturation conditions. Then, the properties of FET gate leakage current generated by impact ionization are analyzed. Finally, the continuous mode of operation of the new feedback method is presented, and results from its application in x-ray spectroscopy with silicon and germanium detectors are given. Superior noise and count-rate performance coupled with simplicity and reliability are the outstanding features of the drain feedback method. It is the first feedback method in which the cryogenic input stage comprises the detector and the FET without the parasitic increases of stray capacitance or light-induced leakage currents characteristic of the other resistorless configurations.

Drain Feedback - A Novel Feedback Technique for Low-Noise Cryogenic Preamplifiers

An analytical approximation to the field distribution in the channel portion between gate and drain of the junction field-effect transistor is derived, assuming an infinitely small channel width-to-height ratio, and modified for finite channel widths by introducing an effective impurity concentration which depends on drain current. The approximation is applicable also in the limiting case of zero gate edge curvature, i.e., for Schottky-barrier gate. The theoretical field distribution is used to extract impact-ionization coefficients from published experimental data on gate current enhancement at large drain voltages. These impact-ionization coefficients agree with published data derived from bulk impact ionization.

Field distribution in junction field-effect transistors at large drain voltages

Measurements are reported on the noise resistance and the noise conductance of the junction-gate FET in the temperature range 77°K-400°K. At low temperatures anomalous noise behavior has been observed. The measurements are discussed in the light of existing theories and, when necessary, the theoretical model has been extended. The agreement is satisfactory. Generally the extra noise is caused by mobility saturation, increased free-carrier temperature, free-carrier trapping and multiplication effects in the pinched-off region. Finally, several applications are discussed in relation to the limiting noise sources.

Anomalous noise behavior of the junction-gate field-effect transistor at low temperatures

In this paper the significance of various noise sources in field-effect transistors is discussed. The intrinsic thermal noise of the conducting channel can be reduced by reducing the channel length. A lower limit to the channel length is set by the carrier velocity saturation effects, which are the hot carrier channel noise and the gate leakage current due to impact ionization. Devices with a ratio of the transconductance to the input capacitance of 3 x 109 sec-1 have been made, and a further improvement appears to be possible.

Field Effect Transistors for Charge Amplifiers

Ionization rates in semiconductors can be measured at low values of electric field using a new method involving a field-effect transistor (FET) structure which offers greater sensitivity than reverse biased p-n junction diode methods. Carriers of only one polarity cause ionization in the FET and no correction is required for ionization caused by carriers of the opposite polarity. Since secondary carriers resulting from ionization are attracted to a different terminal (gate or substrate) than that used to collect the primary carriers (drain), very small ionization currents can be detected. Values of electron ionization rate α n and hole ionization rate α p as low as 10-3cm-1have been obtained for silicon. The approximate relationship \alpha = \alpha_{\infin}e^{-b/E} is observed and the values of α at high fields correspond to those obtained conventionally. Values of α p from 0.04-1to 0.4 cm-1have been obtained for germanium. Analytical determination of electric field was provided by a solution of Poisson's equation for the field-effect structure. Difficulty in accurately determining the FET channel doping introduces a ± 30 percent uncertainty in electric field values. The method is applicable to any semiconductor material where junction, MOS, or Schottky barrier techniques can be used to construct field-effect transistors.

Avalanche ionization rates measured in silicon and germanium at low electric fields

The gate currents of Ge and Si junction FET's operating beyond pinch-off show a component which increases as the temperature decreases and increases rapidly with drain voltage. A model is presented which explains these currents in terms of impact ionization by majority charge carriers, accelerated to energies above 1 eV in the high field region of the pinched-off channel. The probability of carriers reaching these energies is thought to be determined by their interactions with the lattice and thus is a function of lattice temperature, carrier type, and semiconductor material.

R.D. Ryan

Papers

The gate currents of junction field-effect transistors at low temperatures