Patent GB-843467-A (GB)
Title: Improvements in and relating to semiconductor p-n junction units
Filed: 1956-03-28
Abstract: <PICT:0843467/III/1> <PICT:0843467/III/2> In a method of making PN junctions a crystal is grown at a cyclically varying rate from a melt consisting of semiconductor material containing traces of both donor and acceptor impurities in such proportion that at predetermined growth rate "intrinsic" material is formed, the impurities having segregation coefficients whose variations with growth rate are different, so that at growth rates above and below the intrinsic rate the material formed is of different conductivity type.  The growth rate is varied by altering the melt temperature, and by varying the character of the heating cycle sharp or broad junctions can be produced, while the number of conduction carriers and hence the conductivity, is determined by the absolute amounts of impurity present.  The melt is held in a crucible mounted in a graphite heater pot (Fig. 2) the sides of which are split into two parts by a cut in a plane containing the axis of the pot.  The power input to terminals connected to the two halves of the pot is varied cyclically to produce the desired temperature variations.  In one embodiment a seed crystal 41 (Fig. 1) mounted in a stainless steel chuck 7 is rotated in a melt consisting mainly of germanium or silicon doped with antimony and gallium.  The antimony and gallium are added to the melt either individually, or as an alloy formed from a ground mixture by sintering or from an alloy melt by quenching.  The melt is heated by heater 4 to a temperature slightly below that for intrinsic growth, and then temperature variations about this intrinsic temperature are built up to the desired amplitude by gradually increasing the power variations or by decreasing the frequency of the variations and keeping the amplitude constant, until a steady cycle consisting of alternate periods of high and low power is established.  A complete growth cycle is as follows. The growth rate reaches a maximum immediately before a power step up occurs, after which it falls with the rising temperature and the P type impurity concentration increases relative to the N type on account of its smaller rate of change of segregation constant with growth rate until below the intrinsic temperature the material crystallising is P type.  Further rise in temperature results in melting back of at least part of the P type material.  When growth recommences after a power reduction step it is initially of P type material but if the intrinsic growth rate is very small N type material soon begins to grow thereby ensuring that the P type region is thin.  The cycle is repeated over a long period to give a series of alternate PN junctions.  In an alternative method the donor/acceptor ratio is chosen to give a high intrinsic growth rate and the power cycle is such that the growth rate is always positive.  In the above described methods in which a constant withdrawal rate is used argon is circulated through the chamber 6 containing the melt and nitrogen through the heater chamber via duct 13a.  The selection of a power input cycle which will give rise to the desired type of junction is effected by growing a trial ingot from the melt with power input variations which progressively increase in selected increments.  The resulting ingot is then examined and the PN junctions located by applying an alternating voltage of 500 across the ingot and pouring a benzene or carbon tetrachloride suspension of barium titanate over it.  The barium titanate collects at the PN junctions.  The thickness of the P type layers may thus be correlated with the power amplitude variations and a power cycle suitable for a particular application chosen.  After initial adjustment the power amplitude variation is maintained constant throughout the growth of the ingot in the above cases so that successive pairs of zones will be identical.  Specification 738,231 is referred to.