Masumi Fukuta

GaAs microwave power FET

A design consideration for an X-band GaAs power FET, features of the fabrication process, and electrical characteristics of the FET are described. Interdigitated 53 source and 52 drain electrodes and an overlaid gate electrode for connecting 104 Schottky gates in parallel have been introduced to achieve a 1.5-µm-long and 5200-µm-wide gate FET. A sheet grounding technique has been developed in order to minimize the common source lead inductance (L8= 50 pH). The resulting devices can produce 0.7-W and 1.6-W saturation output power at 10 GHz and 8 GHz, respectively. At 6 GHz, a linear gain of 7 dB, an output power of 0.85 W at 1-dB gain compression and 30-percent power added efficiency can be achieved. The intercept point for third-order intermodulation products is 37.5 dBm at 6.2 GHz.

GaAs microwave MOSFET's

GaAs microwave metal-oxide-semiconductor field-effect transistors (MOSFETs) with plasma-grown native oxides as gate insulator have been fabricated using a low-temperature magnetically controlled plasma-oxidation technique. A small-signal enhancement device with the gate length of 2.0 µm has demonstrated useful unilateral power gains in the 2-8-GHz frequency range. A maximum frequency of oscillation in the enhancement device is 13 GHz. This is the highest in all enhancement-mode GaAs devices reported up to this time. A medium-power depletion device with the gate length of 1.8 µm has the maximum frequency of oscillation of 22 GHz. This value is 10 percent larger than that of the best analogous metal-semiconductor field-effect transistor (MESFET). The intrinsic current-gain cutoff frequency for the depletion MOSFET is 4.5 GHz which is 22 percent higher than that of the MESFET. The superiority of the depletion MOSFET in the small-signal microwave performance over the MESFET results from the smaller gate parasitic capacitance in the MOSFET as compared to the MESFET. The depletion MOSFET has produced 0.4-W output power at 6.5 GHz as a Class A amplifier. Quite a large frequency dispersion of transconductance is observed in the enhancement MOSFET at a frequency range between 10 and 100 kHz and attributed to interface states. The effect of the interface states does not severely restrict the microwave-frequency capabilities of the enhancement MOSFET as well as the depletion MOSFET since the interface states are unable to follow the input-signal variations at high frequencies.

Normally-off type GaAs MESFET for low power, high speed logic circuits

wave amplifiers, but for high speed switching circuits’. Some of the logic using normally-on type GaAs MESFETs have large power dissipation and complicated circuit construction. The normally-off type GaAs MESFET logic has not as yet been reported, even though it is expected to have some attractive features such as low power dissipation and simple circuit configuration. Figure 1 is a microphotograph of the buffered output 13-stage ring oscillator consisting of normally-off type GaAs MESFETs and epitaxial resistors. A cutaway view of the inverter used in the ring oscillator is shown in Figure 2. The devices were fabricated on a sulfur-doped N-type epitaxial layer grown by VPE onto a semi-insulating Cr-doped substrate. The do ing density and thickness of the epitaxial layer was 1 x 101’cm-3 and 0.1 pm, respectively. The N-type layer outside the active area was etched down to the semiinsulating substrate to isolate inverters from each other. A dual-metal system was used. 0.04-pm thick Au-Ge eutectic alloy and 0.4-pm thick Au were continuously deposited as the first metal layer and were alloyed at 450’C for 120 seconds to make ohmic contact. Next, O.5pm-thick Si02 film which was used for the isolation of the dual metal layers was deposited by Chemical Vapor Deposition (CVD). This film was etched selectively to open the gate windows and the contact holes to the first metal layer. The second metal layer made with Cr-Pt-Au was used for the Schottky gate and crossing over or connecting to the GaAs MESFETs are substantially useful not only for micro-

Low‐temperature plasma oxidation of GaAs

A low‐temperature plasma oxidation of GaAs (lower than 100 °C) has been realized. The oxidation apparatus mainly consists of a quartz tube chamber, a low‐power rf oscillator and an electrical magnet. The oxidation rate can be controlled in the range 100–600 A/min by changing the magnetic field perpendicularly applied to the sample. The interface state density between p‐type GaAs and its oxide film is the order of 1010 cm−2 eV−1 around 0.5 eV from the top of the valence band. This low state density suggests that the oxide film can be applied to various GaAs MOS devices. For the oxide film of n‐type GaAs, an anomalous frequency dispersion in the MOS capacitance is found in the accumulation region. This anomaly is very similar to that observed in anodic oxidation.

A self-aligned source/drain planar device for ultrahigh-speed GaAs MESFET VLSIs

A self-aligned source/drain planar GaAs MESFET fabrication technique using high temperature stable Ti-W mixed metal gates as implantation masks will be reported. Propagation delays of 50ps have been attained from fully implanted1.5/mum-gate normally-off GaAs MESFET logic.

4-GHz 15-W power GaAs MESFET

High-field behavior of GaAs MESFETs such as drain-source breakdown characteristics and visible light emission and a model explaining these phenomena are described. An FET structure with a high drain-source breakdown voltage in excess of 26 V has been developed following an analysis of the high-field behavior of the device. Typical characteristics of the fabricated devices at 4 GHz are as follows: P<inf>out</inf>= 9.6 W G<inf>a</inf>= 5 dB η<inf>add</inf>= 33.6 percent at 18 V from single chip (W<inf>G</inf>= 13 mm) P<inf>out</inf>= 15 W G<inf>a</inf>= 5 dB η<inf>add</inf>= 28.3 percent at 22 V from two chip (W<inf>G</inf>= 26 mm) where P<inf>out</inf>, G<inf>a</inf>, η<inf>add</inf>, and W<inf>G</inf>indicate the output power, associated power gain, power added efficiency, and total gate width of the FETs, respectively.

Planar GaAs MOSFET integrated logic

Selective and multiple ion implantations directly into a semi-insulating GaAs substrate were utilized to fabricate planar integrated circuits with deep-depletion plasma-grown native oxide gate GaAs MOSFETs. 1.2-µm gate 27-stage enhancement/depletion (E/D) type ring oscillators, with the circuit optimized to reduce parasitic capacitance, were fabricated (using conventional photolithography) to assess the speed-power performance in digital applications. A minimum propagation delay of 72 ps with a power-delay product of 139 fJ was obtained, making these devices the fastest among current GaAs and Si logic fabricated by conventional photolithography. A minimum power-delay product of 36 fJ with a propagation delay of 157 ps was obtained. The power-delay product is comparable with that of 1.2-µm gate GaAs E-MESFET logic, and the speed is more than twice as great. This paper includes a comparison of the theoretical cut off frequency of MESFET and MOSFET logic devices operating in depletion mode. Results indicate that MOSFET logic has superior potential for high-speed operation.

MP-A2 Low-power GaAs digital integrated circuits with normally-off MESFET's

Direct writing electron-beam lithography has been utilized to fabricate GaAs FET inverters. The fabrication process used t o realize 0.5-pm FET integrated circuits on ion implanted channels is described. The bandwidth of the small-signal gi:in, the large-signal switching characteristics, and ring oscillator results are presented and compared with a computer simulation of the inverter.

Design and performance of GaAs normally-off MESFET integrated circuits

DC and transient analyses of GaAs normally-off MESFET integrated circuits are described. The design tradeoffs between device parameters and logic characteristics are discussed for an inverter with a resistive load. By increasing the supply voltage to several times that of the built-in voltage, the propagation delay time can be lowered similar to that when using an active load (current source). To investigate the speed-power performance of the ICs, ring oscillators with different fan-in and fan-out configurations were fabricated. A binary frequency divider which uses a master-slave flip-flop was tested. The maximum counting frequency of the divider was 610 MHz at a supply voltage of 1.5 V. This coincides with the results obtained from the ring oscillators with fan-in/fan-out = 2/2. Comparing the experimental results with the theory, the effective electron mobility in the thin channel layer is expected to be very low. By improving the mobility and shortening the gate length to half a micrometer, practical functioning circuits should operate with an average propagation delay time of less than 100 ps.

Visible light emission from GaAs field-effect transistor

Visible light radiation has been observed at the drain of a Schottky-barrier-gate GaAs microwave field-effect transistor (FET). The radiation takes place at the region where the drain current increases after saturation. The origin of the radiation has been attributed to impact ionization within the stationary high-field domain caused by negative differential mobility of n-GaAs.