Hermann Statz

GaAs FET device and circuit simulation in SPICE

We have developed a GaAs FET model suitable for SPICE Circuit simulations. The dc equations are accurate to about 1 percent of the maximum drain current. A simple but accurate interpolation formula for drain current as a function of gate-to-source voltage connects the square-law behavior just above pinchoff and the square-root law for larger values of the drain current. The ac equations, with charge-storage elements, describe the variation of the gate-to-source and gate-to-drain capacitances as the drain-to-source voltage approaches zero and when this voltage becomes negative. Under normal operating conditions the gate-to-source capacitance is much larger than the gate-to-drain capacitance. At zero drain-to-source voltage both capacitances are about equal. For negative drain-to-source voltages the original source acts like a drain and vice versa. Consequently the normally large gate-to-source capacitance becomes small and acts like a gate-to-drain capacitance. In order to model these effect it is necessary to realize that, contrary to conventional SPICE usage, there are no separate gate-to-source and gate-to-drain charges, but that there is only one gate Charge which is a function of gate-to-source and gate-to-drain voltages. The present treatment Of these capacitances permits simulations-in which the drain-to-source voltage reverses polarity, as occurs in pass-gate circuits.

Signal and Noise Properties of Gallium Arsenide Microwave Field-Effect Transistors

Publisher Summary This chapter examines the signal and noise properties of gallium arsenide (GaAs) microwave field-effect transistors (FET). High frequency gallium arsenide field-effect transistors (GaAs FETs) have demonstrated remarkably low noise figures and high power gains at microwave frequencies. A practical microwave GaAs FET is usually fabricated by deposition or diffusion of source, gate, and drain contacts on the surface of an appropriately doped thin epitaxial n-type layer. This layer, in turn, is grown on a semi-insulating wafer by either a vapor or liquid epitaxial technique. The apparent minor role played by the negative resistance region in practical short-gate FETs suggests that radiofrequency instabilities due to this region, if they exist, occur at frequencies far above the normal frequency regime of microwave FETs. The small-signal equivalent circuit of the FET, valid up to moderately high frequencies is elaborated. It is found that noise in a microwave GaAs FET is produced both by sources intrinsic to the device and by thermal sources associated with the parasitic resistances.

Noise characteristics of gallium arsenide field-effect transistors

Small signal and noise characteristics for GaAs field-effect transistors are derived with the saturated drift velocity of the carriers underneath the gate taken into account. The noise contributed by the saturated carriers is nonnegligible and in most cases, exceeds the noise generated by the unsaturated region. Parasitic elements contribute importantly by preventing the full cancellation of the correlated noise of the intrinsic transistor and by adding their own Johnson noise. The theory predicts the experimentally observed trend of noise figure dependence on drain current and on source-to-drain voltage. The present theory doesnot take into account the effects of a possible short negative resistance region underneath the gate.

Noise in gallium arsenide avalanche Read diodes

The noise and signal properties of Read-type avalanche diodes under large-signal levels are examined. In contrast to most other previous theories, we include the saturation current in the equations rigorously from the beginning. We find that the noise performance is a strong function of the saturation current such that high saturation currents lead to lower noise performance. We compare the findings of our model with measurements on two very different Read-type avalanche diodes with a low-high-low profile. In agreement with theory, the lower noise diode has a higher saturation current. We also find experimentally that the noise measure of the diodes used as oscillators decreases with increasing power output. This feature is explained by the rising reverse saturation current with temperature which in some diodes more than compensates the normally increasing noise measure with power output.

The Multioscillator Ring Laser Gyroscope

Abstract The multioscillator (or four-frequency) ring laser gyroscope is discussed from both a theoretical and a practical point of view. Fundamentals of device operation are presented, important nonideal behaviors (error sources) are discussed and analyzed from first principles, typical multioscillator gyroscopes are described, and samples of representative data from developmental instruments in our laboratories are reviewed. A key to the development of practical multioscillator instruments has been the introduction of nonplanar ring resonators. The theoretical formalisms (geometric and wave optic) necessary for understanding the properties of nonplanar ring resonators, and nonplanar gyroscopes, are derived and discussed in detail. Much of the material presented is new.

On charge nonconservation in FET's

It is shown that source and drain charges are not state variables in an FET, especially for source-drain voltages near zero. This behavior, observed in the model proposed in [2], is a genuine manifestation of the physics of FETs and is not a sign of improper behavior in a circuit simulator. However, if necessary, one may construct a model having these charges as state variables by introducing a current generator that transfers charge from source to drain internally without producing extra currents in the leads of the device.

Velocity fluctuation noise in metal-semiconductor-metal diodes

A noise source arising from velocity fluctuations of drifting carriers in the depletion region of the metal-semiconductor-metal (MSM) diode is identified. This source of noise adds to the shot noise and becomes prominent at high current densities, when the shot-noise contribution is small because of space-charge smoothing.

Noise in Gunn oscillators

We estimate the ultimate noise of Gunn oscillators in the absence of 1/ f noise. The basic noise source considered is thermal or Johnson noise augmented by intervalley noise of carriers hopping between the high and low mobility bands. For example, for Q ext = 102and P out =10-1W we estimate δf rms ≈ 1-2 Hz, and AM noise relative to the carrier of -156 dD, both measured in 1-kHz bandwidths.

Large-signal dynamic loss in gallium arsenide read avalanche diodes

In this paper we show that very large displacement currents may flow in high power Read diodes, especially when they are operated in the pulsed mode. These displacement currents may have peak amplitudes which are larger than the maximum possible conduction currents in the undepleted material. When this occurs, then the conduction current in the undepleted material must be augmented by a displacement current in that region. This displacement current is accompanied by a large field, in turn, causes abnormally large dissipation, since losses are proportional to the product of the conduction current in the undepleted material and the electric field. For practical diodes, this effect tends to limit the maximum modulation index to about 0.45. By special design of diodes, one may limit this anomalous loss, resulting in higher efficiency and higher power output.

Frequency increase in pulsed avalanche diodes

We have tested many GaAs IMPATT diodes under high power pulsed conditions and found that the best power output and efficiency occur at higher frequencies in the pulsed mode than in the CW mode. This phenomenon can be understood by considering the effect on the avalanche region field produced by the space charge of the carriers injected into the drift zone. Qualitative agreement is demonstrated between theory and experiment.