Nonalloyed ohmic contacts for high-electron-mobility transistors based on AlGaN/GaN heterostructures

A microwave field-effect transistor with nonalloyed ohmic contacts is fabricated using the technique of regrowing a heavily doped region under the contact metallization by molecular beam epitaxy through a preliminarily formed dielectric mask. The fabricated field-effect transistor with a gate length of 0.18 µm and a total width of 100 µm has a current–amplification cutoff frequency of 66 GHz and ohmic contact resistivity of 0.15-0.18 Ω mm.     

Prospects for the development of high-power field-effect transistors based on heterostructures with donor-acceptor doping

We report the first results on the development of high-power field-effect transistors on gallium-arsenide heterosrtuctures with a quantum well and additional potential barriers, formed from layers with different doping types, optimized to reduce transverse spatial electron transport and enhance quantum confinement. The transistors yield a doubled output power at a trapezoidal gate length of 0.4–0.5 μm and a total gate width of 0.8 mm at a frequency of 10 GHz in the continuous mode of operation. The gain exceeds 9.5 dB at a specific output power above 1.6 W/mm and a power-added efficiency of up to 50%. Prospects for the development of such devices are presented.     

Four-terminal field-effect transistors

A derivation is presented for the transconductances of a four-terminal transistor of arbitrary channel doping profile. The derivation takes into account finite barrier potential and is specifically applied to the symmetrical, abrupt-junction, four-terminal, field-effect transistor. Curves are presented showing variations of normalized transconductance with applied gate-source EMF.     

Organic field-effect transistors for biosensing applications

Organic field-effect transistors (OFETs) with regioregular poly(3-hexylthiophene) (P3HT) have been designed and fabricated aiming at the lowest possible working point (i.e. the adjusted values of gate voltage and drain–source voltage) for the use as sensor in electrolytes. Using thermally grown silicondioxide with a thickness of 45 nm it has been possible to dramatically lower the gate potential. Even around one volt the channel current and its modulation are still large enough for detection with simple operational amplifiers.The experimental results indicate a strong dependence of the transistor performance on the solvent used for spin coating the organic film. A theoretical analysis based on an analytical model allowed us to relate the different behavior of the transistor to their mobility, which is in turn dependent on the density of traps. In the context of this paper the leakage currents – as a non-zero gate current – have been analyzed. The observed gate leakage currents of the electrode structures themselves as well as of the complete transistors can be well described by Fowler’s and Nordheim’s field enhanced tunneling.     

Four-terminal field-effect transistors for amplitude modulation

An amplitude modulator using a field-effect tetrode transistor (FETT) is investigated. The operation of the modulation circuit is based on the `linear mode' where the transconductance from one gate to the drain is a linear function of the bias signal at the other gate. Experimental result shows that the modulator has good linearity for RF application.     

Medium-power GaAs field-effect transistors

Results on medium-power GaAs m.e.s.f.e.t.s. are described. Output powers as high as 300 mW at 9 GHz at 1 dB gain compression with a linear gain of 5.2 dB and drain efficiency of 30% have been obtained with single-cell m.e.s.f.e.t.s. At 4 GHz, a power output of 665 mW at 1 dB gain compression, a linear gain of 8 dB and a drain efficiency of 44.5% were realised with a 3-cell m.e.s.f.e.t. Two-tone intermodulation characteristics at 4 GHz are also described. A major innovation has been the use of a high-resistivity chromium-doped epitaxial GaAs buffer layer to isolate the device active region from the bulk-grown substrate.     

Amorphous-silicon silicon-nitride field-effect transistors

n-channel and p-channel amorphous-silicon field-effect transistors have been fabricated on a glassy substrate using undoped and impurity-doped a-Si films as the semiconductor and silicon nitride deposited from an SiH4-N2 mixture as the gate insulator. A change in the source-drain conductance of greater than four orders of magnitude is realised by changing the gate potential from 0 to 5 V.     

InP Schottky-gate field-effect transistors

MESFET's with gates 1 µm long were fabricated in LPE layers of InP on Cr-doped InP substrates. The quality of the LPE material is characterized by an electron concentration of 4 × 10¹⁵cm^-3with a mobility of 2.6 × 10⁴cm²V^-1s^-1at 77 K for growths from undoped melts. The devices have current gain cutoff frequencies of 20 GHZ or somewhat larger. This value is greater than that of the best analogous GaAs MESFET bya factor of 1.5. A factor of 1.3 is predicted on the basis of a simple theory. The highest power gain cutoff frequency, fmax, for the InP device is 33 GHz which is somewhat smaller than that of the best analogous GaAs device. The lowest minimum noise figure at 10 GHz for these first InP devices is 3.9 dB with an associated gain of 4.8 dB. The best result for the GaAs counterpart is 3.2 dB with an associated gain of 7.8 dB. The power gain of the InP device suffers compared to the GaAs device because of degenerative feedback resulting from a large gate-to-drain capacitance and because of a small output resistance. If the magnitudes of these two equivalent circuit elements were the same for MESFET's in the two materials, fmaxfor the InP device would be more than double its present value.     

Nonplanar power field-effect transistors

This paper reviews the criteria involved in the design of silicon power field-effect transistors. Particular emphasis is placed on recent nonplanar structures which will, in the near future, present a serious challenge to bipolar power transistors as linear amplifiers and high-speed switches.     

Electrical characteristics of AlGaN/GaN metal-insulator semiconductor heterostructure field-effect transistors on sapphire substrates

The electrical performance of AlGaN/GaN metal-insulator semiconductor, heterostructure field-effect transistors (MISHFETs) were studied and compared to passivated and unpassivated HFETs. Record MISHFET current densities up to 1,010 mA/mm were achieved, and the devices exhibited stable operation at elevated temperatures up to 200°C. Higher maximum-drain current, breakdown voltage, and a lower gate-leakage current were obtained in the MISHFETs compared to unpassivated HFETs. The breakdown voltage of these devices exhibited a negative temperature coefficient of 0.14 VK⁻¹, suggesting that a mechanism other than impact ionization may be responsible. Different structures of MIS diodes also reveal that the high-field region at the gate edge dominates the breakdown mechanism of these devices. Gate-pulse measurements indicate the presence of current collapse in the MISHFETs, despite the expected passivation effect of the insulator. However, a striking feature observed was the mitigation of these effects upon annealing the devices at 385°C for 5 min under N₂ ambient.     

The transfer-impedance method for noise in field-effect transistors

The open-circuited voltage fluctuations, both for thermal noise, generation-recombination noise and 1/f noise in field-effect transistors, is derived with the help of the transfer-impedance method. Full equivalence with the short-circuited noise generators, derived by other methods, is obtained.     

Non-conventional metallic electrodes for organic field-effect transistors

We study micrometer-sized organic field-effect transistors with either Pd or NiFe metallic electrodes. Neither of these materials is commonly used in organic electronics applications, but they could prove to be particularly advantageous in certain niche applications such as organic spintronics. Using organic semiconductors with different carrier transport characteristics as active layer, namely n-type C60 fullerene and p-type Pentacene, we prove that Pd (NiFe) is a very suitable electrode for p- (n-) type semiconductors. In particular, we characterized devices with channel lengths in the order of the micrometer, a distance which has allowed us to evaluate the electronic behavior in a regime where the interfacial problems become predominant and it is possible to reach elevated longitudinal electric fields. Our experimental results agree well with a simple model based on rigid energy levels.     

Piezoelectric effect in CdSe field-effect transistors

The strain-induced variation of the CdSe TFT characteristics was measured. The experimental and theoretical values of the generated piezoelectric voltage agree within 25 percent. The strain sensitivity of the fabricated devices was in the order of 8000 volts per unit strain.     

Physical processes in insulated-gate field-effect transistors

The current-voltage characteristics of insulated-gate field-effect transistors have been calculated for arbitrary ratios of the gate insulator-semiconductor thickness. Comparison of the calculations to experimental results with CdS thin film transistors shows a good correspondence for both enhancement and depletion type units. It is found that the failure of experimental devices to show current saturation for drain voltages beyond pinch-off can be attributed to the presence of partially ionised donors at the CdS-SiO interface.     

Thermal noise in junction-gate field-effect transistors

Measurements are reported on the device parameters and the noise properties of junction-gate field-effect transistors and the results are compared with theory. It is found that the high-frequency input conductance g11and the high-frequency gate-drain conductance g12vary as the square of the frequency and show practically full thermal noise. The high-frequency transconductance decreases with increasing frequency, as expected theoretically. The high-frequency output noise of the FET for short-circuited input is the sum of the low-frequency noise of the FET and the thermal noise of |g12|. A small correlation effect between the short-circuit gate noise and the short-circuit drain noise exists and agrees with theory. An approximate expression for the noise figure is given that is reasonably correct up to the cutoff frequency of the FET.     

Interface states in metal-oxide-semiconductor field-effect transistors

The effect of Co⁶⁰-γ-radiation on MOS field-effect transistors manifests itself as a shift of the characteristics, a decrease in the transconductance and the appearance of long time instabilities.

In this paper the results of an investigation of the instabilities produced by charge exchange associated with states near the oxide-silicon interface are reported. A method for determining, in nearly the whole energy gap, the distribution of energy levels and time constants of interface states is presented. The sensitivity is sufficient to allow an investigation of interface states even in unirradiated transistors.

The measured distribution, P(τ), of the time constants of the slow interface states, τ, is approximately of the form 1/τ and thus explains the appearance of a 1/ƒ noise power spectrum. Interpreting the observed phenomena with a semi-classical tunnelling model leads to the conclusion that the spatial distribution of the slow interface states is fairly constant throughout the first 20 Å from the interface into the oxide layer.     

Ion-implanted microwave field-effect transistors in GaAs

Microwave field effect transistors have been fabricated in gallium arsenide by using sulfur ion implantation directly into semi insulating Cr doped substrates to produce the channel region, eliminating the need for growth of an epitaxial layer. This implantation method has been used to produce 0·25 μm thick, n-type layers with uniform thickness and carrier concentration, and carrier mobility ranging from 2410 to 3620 cm²/V sec in different samples. Because of the uniformity, FET's fabricated in these layers have exhibited reproducibility of transconductance and pinchoff voltage from device to device on a wafer to better than ±10 per cent. Cr doped GaAs of commonly available quality was found to be satisfactory for FET fabrication, although minimum Cr compensation is desirable to obtain highest mobility. S parameter measurements of microwave characteristics indicated a projected f_max = 20 GHz but transducer gain cutoff occurred at approximately 7 GHz because of impedance mismatch and package parasitics.     

Subthreshold slope for insulated gate field-effect transistors

An explicit expression has been derived for the subthreshold slope of an insulated gate field-effect transistor. This expression is used to explore the influence of surface band-bending, gate insulator thickness, substrate doping, substrate bias, and temperature.