Theory of the Swept Intrinsic Structure

Applications have been suggested for semiconductor structures having both extrinsic and intrinsic regions. Examples are the "swept intrinsic" structure, in which a region of high resistivity is set up by an electric field that sweeps out the mobile carriers, and the analogue transistors, where the intrinsic region is analogous to the vacuum in a vacuum tube. However, the junction between an intrinsic region and an N or P region 1239 1240 T H E B E L L SYSTEM T E C H N I C A L J O U R N A L , N O V E M B E R 1 9 5 6 is considerably less well understood than the simple NP junction. Most of the assumptions that make the NP case relatively simple to deal with do not apply to junctions where one side is intrinsic. Specifically, the space charge is that of the mobile carriers; thus the flow and electrostatic problems cannot be separated as they can in PN junction under reverse bias. The following sections analyze the iV-intrinsic - P structure under reverse bias. For a given material with fairly highly doped extrinsic regions, the problem is defined by the length of the intrinsic region and the applied voltage. Taking the intrinsic region infinitely long gives the solution for a simple ^-intrinsic or P-intrinsic structure. The results are given and plotted in terms of the electric field distribution. From this the potential, space charge and carrier concentrations can be found; so also can the current-voltage curve. The final section considers the case where the middle layer contains some fixed charge but where the carrier charge cannot be neglected.