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The effect of fast neutron and gamma radiation on the electrical characteristics of commercial 1 ?m and 4 ?m low noise GaAs MESFET has been evaluated. The change in the microwave scattering parameters at 2 to 4 GHz (S-band), 1 MHz equivalent circuit parameters, DC characteristics and low frequency input noise equivalent voltage from 2 KHz to 1.5 MHz was determined at neutron fluences between 5 × 1013 to 8 × 1014 n/cm2 and gamma doses of up to 8 × 107 rads (Si). These radiation induced changes are described and causes for these changes are discussed.

The electrical characteristics of guarded Au- and Al-n GaAs Schottky diodes with free carrier concentration in the range of 3 x 1015 to 8 x 1016 cm -3 have been determined after neutron irradiation. Low neutron fluences producing carrier removal rates less than 20% cause a slight change in the C-V and the forward I-V characteristics but increase significantly the reverse current. Assuming Levine's model for a Schottky barrier, the slight changes in the I-V characteristics at forward bias and small reverse bias voltage have been attributed to a change in the distribution of interface states at the metal-semiconductor interface. The excess, non-thermionic reverse current observed after low fluence neutron irradiation is not due to surface leakage or classical generation-recombination but appears to be due to a high field process. Consideration of several high field effect mechanisms indicates that field emission from a deep level is the likely cause.

The electrical characteristics of GaAs MIS Schottky barrier diodes using a thin (~60 A) native oxide layer were evaluated before and after ionizing ?-ray irradiation and fast-neutron irradiation. No change in electrical behavior could be observed with a Cobalt-60? - irradiation level as high as 1.5 × 107 rads (Si) absorbed dose. The MIS diodes together with reference MS diodes were progressively irradiated with neutrons from a fluence of 5 × 1013 n/cm2 to 8 × 1014 n/cm2. Carrier compensation in GaAs is negligible at these radiation levels, but significant changes were observed in I-V characteristics. The low-voltage forward I-V exhibited an appreciable increase in depletion region recombination current resulting from a decrease in carrier lifetime, while the diode ideality factor was found to increase at higher forward voltages. The reverse current showed orders of magnitude increase with irradiation, and its strong field dependence and relatively weak temperature dependence suggests field emission through traps as possible mechanism of current flow. The attenuation in reverse current caused by the oxide layer is seen to be reduced after neutron irradiation. Finally, the increase in reverse current is found to vary approximately in proportion to the neutron fluence. The implications of radiation results on practical devices are briefly discussed.

The effect of fast neutron and gamma radiation on the electrical characteristics of low noise microwave GaAs MESFET amplifiers has been evaluated. The change in the noise figure and gain at S- and X-band was determined at neutron fluences between 3×1012 to 1×1015 n/cm2 and gamma dose of 2×107 rads (Si). The radiation induced changes are described and the causes for them are discussed.

A nonlinear MESFET model that explains the degradation in power GaAs MESFET amplifier third order intermodulation distortion (IM3) characteristics following neutron irradiation is presented. The model verifies the experimental data and shows that the significant increase in IM3 observed at a neutron fluence of 1.4 × 1015 n/cm2 and higher is mainly due to the non-linearity in device transconductance gm and output conductance go. The model also indicates that IM3 is dominated by the device gm or go non-linearity depending on applied gate bias.

The effect of fast neutron radiation on the electrical characteristics of power GaAs MESFETs and power GaAs MESFET amplifiers has been evaluated. The changes in amplifier power output and intermodulation and harmonic distortion start to occur at neutron fluence of 2 × 1014 n/cm2 and significant degradation is observed at 1 × 1015 n/cm2.

Aftereffects in IMPATT oscillators under transient ionizing radiation were evaluated using half watt Si and GaAs X-band IMPATT diodes in a resonant cap waveguide cavity. During the radiation testing, the RF impedance was varied by changing the length of a sliding short behind the diode and the bias impedance was varied independently by inserting lengths of coaxial cable between a fixed R-C bias network and the diode. From the experimental. results we conclude that: 1. the aftereffects are clearly related to a device-circuit interaction, which occurs in the resonant cap waveguide circuit only if the sliding short is one guide wavelength, rather than one-half guide wavelength, from the resonant cap diode structure; 2. the principal cause of the aftereffects is not improper bias circuit impedance resulting in bias circuit oscillations, although bias circuit oscillations can cause diode failure if they are allowed to occur; and 3. the GaAs Schottky diode is more prone to the aftereffects, occurring at power levels above 200 mW and dose rates above 2 x 109 rads/sec for the diode tested. The aftereffects appear to be triggered by a looping RF circuit impedance locus combined with the change in large signal IMPATT impedance with enhanced leakage current.

The performance of a variety of IMPATT diode oscillators has been measured under transient ionizing radiation conditions, and the results of a large signal model which agrees with experiment are presented. Five hundred milliwatt cw diode oscillators (silicon and gallium arsenide with various types of avalanching junctions) were exposed to 100 nanosecond pulses of 10 MeV electrons at dose rates between 2 × 108 and 8 × 109 rads/sec. With these oscillators, the RF power is reduced at increasing dose rates and is quenched entirely during the radiation pulse at a dose rate dependent upon the DC bias current (greater than 5 × 109 rads/sec for 500 mW oscillators in vacuum). With diodes open to air, oscillators in low Q cavities were quenched during the radiation pulse with a total dose of 500 rads. The results of a large signal theory including the effects of leakage current are presented that agree well with the RF power decrease during irradiation.

The performance of complementary (n+pp+) silicon TRAPATT diodes has been measured under transient ionizing radiation conditions in both evacuated and airfilled coaxial cavities. Twenty-five watt pulsed, 2.5 GHz oscillators were exposed to 100 nanosecond pulses of 10 MeV electrons at dose rates between 2 × 107 and 4 × 109 rads/sec. The RF power is unaffected below a critical dose rate, which is typically between 8 × 107 and 2 × 108 rads/sec. Above this critical dose rate the RF power is almost quenched entirely during the radiation pulse, but recovers to the original level within 50 nanoseconds afterwards for all dose rates used. Rapid recovery of original RF power levels occurred when the radiation pulse occurred at the initiation of the 300 nanosecond RF TRAPATT pulse, in the middle of the TRAPATT pulse, or during the decay of the TRAPATT pulse. The RF power during the radiation pulse is in agreement with a first order model of TRAPATT operation, in which the principal effect of the radiation pulse is a reduction in mobile charge generated by the avalanche shock front. The decreased mobile charge reduces the time required to remove the trapped plasma and the time necessary for recovery of the diode voltage, thereby decreasing TRAPATT power.

The performance of 500 milliwatt X-band injection locked IMPATT oscillators, exposed to 100 nanosecond pulses of 10 MeV electrons at dose rates between 1 × 108 and 3 × 109 rads/sec, is compared to free running IMPATT oscillators. The RF power reduction during the radiation pulse remained the same for the locked and free running oscillators, while the RF frequency shift depended upon the power of the locking signal. The locking figure of merit remained constant over the range of dose rates used, and original RF performance was regained within 50 nanoseconds following the radiation pulse.

Guarded Al-and Au-nGaAs Schottky barrier diodes were subjected to Co60 ?-ray irradiation and their electrical characteristics evaluated. These GaAs Schottkys did not exhibit significant change in their I-V and C-V characteristics up to an absorbed dose as high as 1.5 × 107 rads. Diodes that were previously neutron-irradiated with consequent degradation were also subjected to Co60 irradiation, but no synergistic changes were observed.