GaAs Permeable Base Transistor Research Articles

Characterization of traps in GaAs/W Schottky diodes by optical and electrical deep-level transient spectroscopy methods

Both the electrical and optical deep-level transient spectroscopy (DLTS) measurement methods were used to investigate the deep traps in GaAs that originate from the impurities included in the Schottky metal on GaAs/W Schottky diodes. Two distinct electron traps (E1 and E2, whose activation energies EA were, respectively, 0.65 and 0.58 eV) were detected by electrical DLTS in samples annealed at 650 °C. The variation of EA and trap density with changes in the Schottky barrier height indicates that the trap E1 can be identified with the EL2 usually observed in the GaAs grown by metalorganic chemical-vapor deposition. In addition, the trap E2 is found to be due to impurities that diffused from the W film into the GaAs layer during annealing. Four hole traps with activation energies of 0.76, 0.55, 0.43, and 0.30 eV were detected by optical DLTS in samples annealed at 650 °C. Comparison of the EA for each hole trap with data previously reported indicates that the first three traps are due to the transition metals Cr, Fe, and Cu, and that the last one is due to a native defect. These metals are confirmed from secondary-ion-mass spectroscopy analysis to be included in the as-deposited W film. Furthermore, these traps are also observed in GaAs permeable base transistors with the buried W gate.

GaAs permeable base transistors fabricated with 240-mm-periodicity tungsten base gratings

The microwave performance of the first GaAs permeable base transistors (PBTs) fabricated with 240-nm-periodicity tungsten base gratings is described. These PBTs have demonstrated a maximum stable gain of 20.1 dB at 26.5 GHz, which is believed to be the largest small-signal power gain for any microwave transistor at this frequency. Extrapolation of this maximum stable gain using a 6 dB/octave rolloff yields a maximum frequency of oscillation (f/sub max/) of 265 GHz. The breakdown voltage is approximately 20 V, which should make these devices excellent microwave power sources. Extrapolation microwave measurements on these devices from 3 GHz using a 6 dB/octave rolloff yields a unity-current-gain frequency (f/sub T/) of 38 GHz, which matches the best prior result for a PBT. These 240-nm-periodicity PBTs that have given the best f/sub T/ have been depletion-mode devices, suggesting that 240-nm-periodicity depletion-mode PBTs should have f/sub T/ values above 38 GHz. >

Small-signal gain performance of the permeable base transistor at EHF

The small-signal amplifier performance at 40.5 GHz is reported for a GaAs permeable base transistor (PBT) having a base grating of 3200-A periodicity and 1-mm periphery. The device was embedded in a microstrip circuit with appropriate matching networks and biased for optimal small-signal gain with V be = 0.2 V, V ce = 2.0 V, and I ce = 59 mA. The prototype amplifier achieved 11 dB of stable gain at 40.5 GHz. The circuit design was facilitated by the moderate impedance levels and highly unilateral nature of the PBT.

Monte Carlo simulation of the GaAs permeable base transistor

A two-dimensional multiparticle Monte Carlo (MC) method for the solution of the Boltzmann transport equation has been implemented and the results compared with the conventional drift-diffusion equation solution obtained for both a uniformly doped and an n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -n-n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> GaAs permeable base transistor structure. Improved high-frequency performance is predicted by the MC simulation. Two-dimensional boundary conditions for a "regional" MC analysis have been applied to reduce the computer time that would be spent largely in analyzing the device retarding field region and the neutral regions of the device. The dc parameters, I-V characteristics, and unity current gain-frequency (f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</inf> ) are discussed. In the n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -n-n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> doped structure, a cooling effect was found that significantly enhances the device frequency performance by reducing the satellite valley population of electrons.

Analysis of Optically Controlled Microwave/Millimeter-Wave Device Structures

Light-induced voltage and the change in the source-to-drain channel current under optical illumination higher than the semiconductor band gap for GaAs MESFET, InP MESFET, Al/sub 0.3/Ga/sub 0.7/As/GaAs high electron mobility transistor (HEMT), and GaAs permeable base transistor (PBT) are analytically obtained. The GaAs PBT and GaAs MESFET have higher sensitivity than the InP MESFET. However, the Al/sub 0.3/Ga/sub 0.7/As/ GaAs HEMT is observed to have the highest sensitivity. Variations in the small-signal parameters, such as channel conductance, gate-to-source capacitance, and transconductance, as well as transient parameters, such as switching time and power-delay product, of GaAs MESFET with illumination are computed. The computed capacitance and transconductance are compared with the experimentally obtained values and are found to be in fair agreement. Based on these results, the design considerations for an optically controlled MESFET switch are discussed. Finally, variation in device parameter due to optical illumination and its effect on the cutoff frequencies f/sub T/ and f/sub max/ are also investigated.

Current prospects for the permeable base transistor

The permeable base transistor (PBT) is a new device which is predicted to operate at higher frequencies than any other three-terminal device. In addition to its applications for microwave amplification, the PBT may be useful in very high speed, digital logic circuits. The very small dimensions of the base structure and the requirement for embedding the base metallization in a semiconductor have necessitated the development of a great deal of new technology over the past six years. The greatest effort on the GaAs device has been to achieve controlled impurity levels and low defect densities in the epitaxial process used to embed the metal. A number of different embedding techniques have been used and extensively analyzed using secondary ion mass spectrometry (SIMS). The best device result achieved with the GaAs PBT is an extrapolated maximum frequency of oscillation ( f max) exceeding 100 GHz, and that achieved with a silicon PBT is an f max of 30 GHz. Both of these results are comparable with the best results obtained with any high-frequency transistor.

Current prospects for the permeable base transistor

Abstract The permeable base transistor (PBT) is a new device which is predicted to operate at higher frequencies than any other three-terminal device. In addition to its applications for microwave amplification, the PBT may be useful in very high speed, digital logic circuits. The very small dimensions of the base structure and the requirement for embedding the base metallization in a semiconductor have necessitated the development of a great deal of new technology over the past six years. The greatest effort on the GaAs device has been to achieve controlled impurity levels and low defect densities in the epitaxial process used to embed the metal. A number of different embedding techniques have been used and extensively analyzed using secondary ion mass spectrometry (SIMS). The best device result achieved with the GaAs PBT is an extrapolated maximum frequency of oscillation (fmax) exceeding 100 GHz, and that achieved with a silicon PBT is an fmax of 30 GHz. Both of these results are comparable with the best results obtained with any high-frequency transistor.

18.5-dB Gain at 18 GHz with a GaAs permeable base transistor

Permeable base transistors have recently been fabricated using organometallic chemical vapor deposition (OMCVD) with a gain of 18.5 dB at 18 GHz. This extrapolates to an f_{\max} of 150 GHz which is comparable to the best previous result. Secondary ion mass spectrometry (SIMS) analysis shows that there is a large concentration of undesired impurities in the grating region of these recent devices. It is quite probable that better performance can be achieved in future devices if those impurities can be eliminated.

Nonstatic effects in the GaAs permeable base transistor

Nonstatic transport effects in the permeable base transistor (PBT) are included in a two-dimensional multiparticle Monte Carlo (MC) analysis. This approach predicts charge depletion between the gate electrodes as V GS is increased over 0.5 V, as opposed to the accumulation of a net negative space charge predicted by the conventional static mobility model. More than a 60-percent increase in unity-current gain-frequency f T , of the PBT was predicted compared with the conventional result.

A self-aligned dual-grating GaAs permeable base transistor

A self-aligned technique has been developed for embedding two submicrometer-period W gratings, one above the other in single-crystal GaAs. This technique has been used to fabricate permeable base transistor structures with independently contacted base gratings that permit either vertical or planar integration. Initial device results are presented.

Transmission electron microscopy of GaAs permeable base transistor structures grown by vapor phase epitaxy

Transmission electron microscopy has been employed to characterize GaAs permeable base transistor structures. Submicrometer-period tungsten base gratings fabricated on GaAs substrates by using either liftoff or etching techniques have been studied. Fabrication-related problems such as electrically isolated tungsten debris and poor tungsten adhesion to GaAs have been identified. The crystal quality of GaAs layers grown by vapor phase epitaxy over the tungsten base gratings has also been investigated. Both stacking faults and voids have been found in these layers. The concentration of voids, which have an elongated-bubble shape, depends on the orientation of the base grating on the GaAs surface. The effect of these structural imperfections on device performance is discussed.