Double Injection Field Effect Transistor a New Type of Solid State Device
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DOUBLE INJECTION FIELD EFFECT TRANSISTOR A NEW TYPE OF SOLID STATE DEVICE M. HACK, M. SHUR* AND W. CZUBATYJ Energy Conversion Devices, Inc., 1675 West Maple Road, Troy, Michigan 48084 *Department of Electrical Engineering, University of Minnesota, Minneapolis, Minnesota 55455 ABSTRACT We propose a new general principle of operation for solid state devices, and demonstrate a novel transistor which we call a Double Injection Field Effect Transistor, based on this principle. We have fabricated amorphous silicon alloy double injection transistors, operating on the modulation of a double injection current by a gate field covering the complete path of the current channel. Using these amorphous silicon alloy double injection transistors, we have achieved currents over fourteen times those theoretically possible for conventional amorphous silicon field effect transistors operating under similar conditions. This new principle, applicable to both thin film amorphous and crystalline devices, offers the potential of high current, high speed field effect transistors. INIRODUCTION Most semiconductor devices existing today are either field effect transistors (FETs) or bipolar junction transistors (BJTs). Field effect transistors are essentially unipolar devices based on a capacitive coupling between the gate and the channel. The capacitively induced (or depleted) charge directly contributes to (or deducts from) the channel conductance. By contrast, we propose a device which uses the capacitive modulation of the quasi-neutral plasma of electrons and holes created in semiconductor by double injection. For amorphous silicon alloys, the process of double injection has been described previously [1]. The device presented in this work is schematically shown, in the insert, in Figure 1. In a double injection field effect transistor (DIFET), the capacitively induced charge, in the conducting channel, leads to an increased plasma density so as to maintain plasma quasi-neutrality. In contrast to a conventional field effect transistor, the charge densities of both electrons and holes induced into the channel may be much larger than the total net induced charge. This leads to a greatly increased transconductance. The electron-hole plasma in a DIFET will never be neutral. Carriers of the same sign as the net space charge will respond according to the sign of the gate field and the second carrier will adjust so as to preserve quasi-neutrality. This enables us to construct devices where the double injection current of the p-i-n diode is either considerably enhanced or depleted by the gate field. For enhancement (desirable for high current, high speed performance) the gate must act over the complete channel so that the concentration of both carriers can be controllably increased by orders of magnitude enhancing the conductance of the semiconductor region all the way from anode to cathode. For depletion mode operation, it is necessary only to reduce both carrier concentrations in one portion of the source-drain region and so a gate need not cover
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