In recent years, silicon carbide has received increased attention because of its potential for high-power devices. The unique material properties of SiC, high electric breakdown field, high saturated electron drift velocity, and high thermal conductivity are what give this material its tremendous potential in the power device arena. 4H-SiC Schottky barrier diodes (1400 V) with forward current densities over 700 A/cm/sup 2/ at 2 V have been demonstrated. Packaged SITs have produced 57 W of output power at 500 MHz, SiC UMOSFETs (1200 V) are projected to have 15 times the current density of Si IGBTs (1200 V). Submicron gate length 4H-SiC MESFETs have achieved f/sub max/=32 GHz, f/sub T/=14.0 GHz, and power density=2.8 W/mm @ 1.8 GHz. The performances of a wide variety of SiC devices are compared to that of similar Si and GaAs devices and to theoretically expected results.

Silicon carbide high-power devices

The influence of material parameters upon the characteristics of vertical channel power field effect transistors is examined. It is demonstrated that for devices with the same breakdown voltage and device structure, the on‐resistance is inversely proportional to the third power of the energyband gap and inversely proportional to the mobility. In addition the frequency response of these devices increases in proportion to the mobility and the energyband gap. Calculated device parameters for III–V semiconductor compounds, as well as their alloys, have been compared to those of a silicondevice with the same breakdown voltage. It is found that devicesfabricated from GaAs,InP, and GaP are expected to have a current handling capability which is a factor of 12.7, 5, and 1.85 better than that of the silicondevice with the same breakdown voltage. In addition, the current handling capability of devicesfabricated from the alloy semiconductors GaAlAs, GaAsP, and InGaP are even superior to those of a GaAsdevice with the same breakdown voltage.

Semiconductors for high‐voltage, vertical channel field‐effect transistors

Organic thin film transistor (OTFT) based device modeling and circuit application is a rapidly emerging research area. Taking cognizance of this fact, our paper reviews various basic to advanced OTFT structures, their performance parameters, materials of individual OTFT layers, their molecular structures, OTFT charge transport phenomena, and fabrication techniques. The performance of p- and n-type conducting polymer and small molecule organic semiconductors are reviewed primarily in terms of field effect mobility, current on/off ratio, and operating voltage for various OTFT structures. Moreover, different organic/inorganic materials for realizing the dielectric layer, electrodes, and the substrate in an OTFT are analyzed. Some of the compact models that are essential for predicting and optimizing the device performance are described that takes into account the mobility enhancement factor and channel length modulation. A detailed study of the single gate, dual gate, vertical channel, and cylindrical gate O...

Organic Thin Film Transistors: Structures, Models, Materials, Fabrication, and Applications: A Review

A new transistor structure has been reported in which a thin tungsten grating has been embedded inside a single crystal of gallium arsenide. The embedded metal grating, which forms a Schottky barrier with the gallium arsenide, is the base of the transistor and can be used to raise and lower a potential barrier in the semiconductor between the grating lines. The name given to this device is the permeable base transistor (PBT). Devices have been fabricated with a noise figure of 3.5 dB, an associated gain of 9 dB at 4 GHz, and a maximum frequency of oscillation of 17 GHz. This transistor structure is numerically modeled over a wide range of metal grating thicknesses, periodicities, and carrier concentrations. The results from these simulations have been condensed into a unified equation for the base-to-collector transfer characteristic which is valid for the PBT, FET's, and bipolar transistors, and simplifies the comparison between different device structures. A new iterative technique has been used to approximate the nonequilibrium electron velocity, leading to a predicted f T above 200 GHz, a maximum frequency of oscillation near 1000 GHz and a power-delay product below 1 fJ for devices with small grating dimensions and large carder concentrations.

Fabrication and numerical simulation of the permeable base transistor

A hybrid-mode device based on a standard submicrometer CMOS technology is presented. The device is essentially a MOSFET in which the gate and the well are internally connected to form the base of a lateral bipolar junction transistor (BJT). At low collector current levels, lateral bipolar action with a current gain higher than 1000 is achieved. No additional processing steps are needed to obtain the BJT when the MOSFET is properly designed. n-p-n BJTs with a 0.25- mu m base width have been successfully fabricated in a p-well 0.25- mu m bulk n-MOSFET process. The electrical characteristics of the n-MOSFET and the lateral n-p-n BJT at room and liquid nitrogen temperatures are reported. >

High-gain lateral bipolar action in a MOSFET structure

The reason why the usual FET shows the saturated characteristics has been shown that with increasing drain voltage, the effect of the negative feedback action, increases through a marked increase of the Series channel resistance in the neighborhood of the pinch-off voltage, under which condition the apparent transfercon-ductance G_{m'}= G_{m}/(1 + r_{s}.G_{m}) becomes G_{m'} \simeq r_{s}^{-1} . It is also pointed out that a transistor in analogy to the vacuum type proposed by Watanabe and Nishizawa in 1950, exhibits the nonsaturated build-up character only When the internal negative feedback action is as little as G_{m'} \simeq G_{m} . In this case, when the channel has not yet pinched off, the characteristics are ohmic and then the transistor can operate as a good variable resistor; on the other hand, when the channel has already pinched, the transistor shows the build-up characteristics similar to those of a vacuum tube triode as a result of the static induction from the drain. The transistor similar to that of the vacuum tube triode type is named "Static Induction Transistor," because its output character is based on the static induction as well as input characteristics. The SIT has the exponential characteristics in contrast with the "Analog Transistor" which is expected by Shockley to follow the space-charge conduction law. The SIT has already been ascertained to have low noise, low distortion, and high-power capability, and its fabrication has been already realized in the form of a high-power transistor (2 kW, 8 MHz), a high-frequency transistor (a few watts, UHF), and a high-speed thyristor. Microwave transistors and very high-speed integrated circuits are being constructed, as well as variable resistors.

J. Shibata

Papers

Field-effect transistor versus analog transistor (static induction transistor)