J. Laskar

On the speed and noise performance of direct ion-implanted GaAs MESFETs

Recent advances in high-speed and ultra-low-noise performance of GaAs MESFETs are reviewed. Experimental results showing that the current gain cutoff frequency and noise figure achieved by direct ion-implanted GaAs MESFETs are equal to or better than those achieved by GaAs HEMTs are presented. Detailed cryogenic-temperature microwave measurements of F/sub t/ on HEMTs and MESFETs are reported, showing a similar dependence of the effective velocity with temperature. It is concluded that the transport properties of the high electron mobility in the two-dimensional electron gas in HEMTs have been misinterpreted for high-speed device operation, and that the high-field velocity is the most important parameter for high-speed device operation. It is the fundamental Gamma -L valley separation of the material and the associated effectiveness, either GaAs or InGaAs, that limit the high-field velocity and thus the speed of the devices. >

Three‐terminal delta‐doped barrier switching device with S‐shaped negative differential resistance

A molecular beam epitaxial grown GaAs three‐terminal device with a delta‐doped barrier and GaInAs quantum well exhibiting controllable S‐shaped negative differential resistance and switching voltages has been fabricated and tested. The device has a large potential barrier between the anode and cathode regions which can be modulated via a third terminal. The modulation of the potential barrier has a substantial effect on the switching behavior of the device. For the devices having a cathode contact area of 50 μm2, a spectrum analyzer reveals unstable oscillation up to the system measurement limit of 21 GHz. The output power signal for the best device is greater than −10 dBm which is 20 dB above the noise floor at 20.8 GHz. The results show this device to be a potentially useful and promising high‐frequency oscillator.

Gate-controlled negative differential resistance in drain current characteristics of AlGaAs/InGaAs/GaAs pseudomorphic MODFETs

The observation of negative differential resistance (NDR) and negative transconductance at high drain and gate fields in depletion-mode AlGaAs/InGaAs/GaAs MODFETs with gate lengths L/sub g/ approximately 0.25 mu m is discussed. It is shown that under high bias voltage conditions, V/sub ds/>2.5 V and V/sub gs/>0 V, the device drain current characteristic switches from a high current state to a low current state, resulting in reflection gain in the drain circuit of the MODFET. The decrease in the drain current of the device corresponds to a sudden increase in the gate current. It is shown that the device can be operated in two regions: (1) standard MODFET operation for V/sub gs/<0 V resulting in f/sub max/ values of >120 GHz, and (2) a NDR region which yields operation as a reflection gain amplifier for V/sub gs/ >0 V and V/sub ds/>2.5 V, resulting in 2 dB of reflection gain at 26.5 GHz. The NDR is attributed to the redistribution of charge and voltage in the channel caused by electrons crossing the heterobarrier under high-field conditions. The NDR gain regime, which is controllable by gate and drain voltages, is a new operating mode for MODFETs under high bias conditions.<<ETX>>

Selective reactive ion etching for short‐gate‐length GaAs/AlGaAs/InGaAs pseudomorphic modulation‐doped field‐effect transistors

Selective reactive ion etching of GaAs on AlGaAs in SiCl4/SiF4 plasma is reported. A selectivity ratio of 350:1 has been obtained at low power. A small decrease in the saturation current of gateless MODFET structures has been observed after etching the GaAs cap layer and has been ascribed to be due to low‐power ion damage of the AlGaAs layer. This process was applied to the fabrication of 0.2 μm T‐gate pseudomorphic MODFET’s. The dc and microwave performance of reactive‐ion‐etched devices and wet‐etched devices were identical. However, for these short‐gate‐length devices a threshold voltage standard deviation of 30 mV was obtained for the reactive‐ion‐etched devices as compared to 230 mV for the wet‐etched devices.

Experimental and theoretical investigation of the DC and high-frequency characteristics of the negative differential resistance in pseudomorphic AlGaAs/InGaAs/GaAs MODFET's

The authors investigated the negative differential resistance (NDR) in the I-V characteristics of pseudomorphic AlGaAs/InGaAs/GaAs modulation doped field-effect transistors (MODFETs) with gate lengths of 0.3 mu m. They experimentally verified the existence of abrupt multiple NDR in both the input circuit and the output circuit. The NDR occurs over a short range of drain voltage (less than 200 mV) and gate voltage (less than 5 mV) for NDR induced by thermionic emission. The authors provide a general interpretation of the measured DC results based on tunneling real-space transfer (TRST) which occurs because of the formation of hybrid excited states across the InGaAs channel and AlGaAs donor layer. The existence of stable reflection is verified in both the input and output circuits with stable broadband frequency response in the output circuit to at least 49 GHz. These results show that NDR via TRST in pseudomorphic MODFETs can provide wideband frequency response not limited by the electron transit time from source to drain. >

Realizing Gbps wireless personal area networks - guest editorial

The 19 papers in this special issue focus on the push beyond giga-bit-per-second (Gbps) data rates, in order to more rapidly access data on personal devices, as well as potentially replace all the cables going into a device, including the video cable. This issue beings together the state-of-the-art, across multiple disciplines, for achieving Gbps WPAN capability.

Characteristics of GaAs/AlGaAs-doped channel MISFET's at cryogenic temperatures

High-frequency measurements at cryogenic temperatures to 125 K of 0.3- mu m gate length GaAs-Al/sub 0.3/Ga/sub 0.7/As metal-insulator-semiconductor field-effect transistors (MISFETs) with a doped channel are discussed. Experimental results demonstrate significant improvement in performance including an increase in the maximum frequency of oscillation from 70 to 81 GHz and an increase in the unity current gain cutoff frequency from 46 to 57 GHz. Independently determined decreases in electron mobility and increases in electron velocity under similar conditions lead to the conclusion that carrier velocity and not mobility controls transport in these devices. These results show the high-speed potential of doped channel MISFETs at both room temperature and cryogenic temperatures.<<ETX>>

Cryogenic small-signal model for 0.55 mu m gate-length ion-implanted GaAs MESFET's

The cryogenic microwave performance of 0.5*300- mu m gate ion-implanted GaAs MESFETs is presented. The devices studied have been fabricated as part of a process control monitor chip (PCM) which uses comparable industry standard design rules. Detailed small-signal element modeling has been performed to determine the temperature dependence of important physical parameters over a lattice temperature range from 300 K to 115 K. The authors find appreciable improvement in cut-off frequency (f/sub T/) and well behaved temperature dependence of transconductance (g/sub m/) and gate-source capacitance (C/sub gs/). Empirical relations for the temperature dependence of f/sub T/, the maximum frequency of oscillation (f/sub max/), g/sub m/ and C/sub gs/, that should provide accurate temperature-dependent device and circuit models are presented.<<ETX>>

Reduced lattice temperature high‐speed operation of pseudomorphic InGaAs/GaAs field‐effect transistors

This letter presents a detailed study of the temperature dependence of submicron pseudomorphic InGaAs on GaAs substrate modulation doped field‐effect transistors (MODFETs), doped‐channel metal insulator field effect transistors (MISFETs), and metal semiconductor field‐effect transistors (MESFETs). We determine similar variation in the measured extrinsic current gain cutoff frequency, FT, and similar dependencies of the effective electron velocity, veff, with reduced lattice temperature for the different field‐effect transistors. The veff∼T−b where 0.15<b<0.20 over the temperature range of 300 to 110 K. These results provide direct experimental evidence that the saturated velocity of electrons is the most important parameter for high‐speed operation and with proper design these different pseudomorphic InGaAs/GaAs field‐effect transistors provide similar potential for high‐speed operation.

Low-power performance of 0.5- mu m JFET for low-cost MMIC's in personal communications

The low-power microwave performance of an enhancement-mode ion-implanted GaAs JFET is reported. A 0.5- mu m*100- mu m E-JFET with a threshold voltage of V/sub th/=0.3 V achieved a maximum DC transconductance of g/sub m/=489 mS/mm at V/sub ds/=1.5 V and I/sub ds/=18 mA. Operating at 0.5 mW of power with V/sub ds/=0.5 V and I/sub ds/=1 mA, the best device on a 3-in wafer achieved a noise figure of 0.8 dB with an associated gain of 9.6 dB measured at 4 GHz. Across a 3-in wafer the average noise figure was F/sub min/=1.2 dB and the average associated gain was G/sub a/=9.8 dB for 15 devices measured. These results demonstrate that the E-JFET is an excellent choice for low-power personal communication applications.<<ETX>>