David R. Rosseinsky The University Exeter. England
I
An Electrochemist's Description of Rectification by a pnJunction
Interest in solid-state chemistry has recently been promoted hv dramatic advances such as the preparation of hiehlv . . . - conductivt: nun-metals (I),the burgeuning of solid-state electronics devices. rhe disrover\. of remarkable carbon-fibre (21 and other filamtrnt prqrrlirs. and thc hke. Recent articles in .I. ('HI.:.M. EDUC:. r.l.4, represent parallel edllcational mow itoring of the subject, and it is the purpose of the present note to continue this expository line with a totally chemical mechanism for one o? the basic processes of interest, rectification by semiconductor pn junctions. This represents not only a translation from the language of physics, hut apossihly more precise and more general description of the process: something like it is sometimes found in physics texts, but always with unnecessary apology. Silicon's low conductivity (semiconductivity) can he attributed simply to a charge-carrier route which is less accessible than in metals. A very dilute solid solution of boron in silicon can he juxtaposed to a very dilute solid solution of ohos~horus in silicon. their mutual boundarv beina the . . "junction." The redox properties of the atoms are such that the following electron transfer reactions take place B.
+ Si
Psi + Si
--
B,
+ Si+
(1)
Pi;
+ Si-
(2)
The B- and P+ are embedded in their respective regions. Being derived from the solid solvent, the Si species can exnerience charee mobilitv. ,. since neiehbor-to-neiehbor electron transfer r e u l ~ in s app:irrnt motion of Si-; and likewise 5 ' . Thus at the iunrtam the spontaneous annihilation of o ~ ~ o s i t e charges (ontwo otherwise identical atoms always highiy exoergic, if only from symmetry arguments) can proceed thus
-
Sit + Si- 2Si (3) Now this reaction can proceed to only a very small extent, because the remnant Pii ions trapped on the one side, and the B c trapped on the other, clearly will set up a powerful field to counter the motions of the Si ions depicted in (3). Reaction (3) thus attains a field-determined equilibrium. The field across the junction is then responsible for favoring the motion (perpendicular to the junction) of charges driven through the material in one direction, while disfavoring their reverse motion; i.e., for rectification. Now, with an external field applied, electrons migrate more readily from the P+ region throueh " the iunction to the B- reeion than in the reverse direction. In silicon, electrons are responsible for conduction, i.e. are the "maioritv carriers." and though the presence of boron effectivel; deireases their number in that ;egion, they are replenished by rapid injection through the junction. The rectifying field arises from the thermal distribution of electrons in response to the P + B distribution.
If electrons are liberated, or in oversimplified terms "almost ionized." bv. .ohoto-excitation in the reeion of the iunction. their u,iil llnrlergo rectiiicsrion, and the son;equt,nt flux in a determined ilirwtitm (on>titutesrhe rurrent from the u v erating solar cell. 111prmciplr the snmr merhanism would hold with posiriw charge carriers. ~ h preceding k appears for the first time to describe rectification and photoelectric enerw conversion without requiring . introductory solid-state phys& while heing entirely consistent with the usual exposition. Before relating the above to semiconductor terminoloev -.(. 5,. 6 )we note that the creation of the junction potential is a result of net transfer of an electron from P in one reeion to B in another via the silicon host acting as a sort of spa& catalyst for the redox reaction, which is the net sum of processes (1) to (3). The P and B atoms are "donor" and "acceptor'' doping species (the physicists' donor and acceptor heing the electrochemists' reductant and oxidant), giving rise to negative and positive charce carriers, resoectivelv, in the (therefore) n and p regions, ~espectively. he form"lation above could be applied to a semiconductor in which a hopping .. - mechanism rathk; than a band mechanism of conduction operated, and so is less restrictive than a hand formulation. If the concept of band conduction is introduced, then Si- would represent an electron in the conduction band, and might be more properly designated esi. The onset of reaction (3) and the subsequent restraint on it, can be ascribed to,respectively, the eaualization. and no more than the eaualization. of the Fermi 1e;els of the' two regions; likewise tKe displaced P+ and Bdis~ositionsmav be described as soace charees. With the additional elaboration, band bending at the junction as the rectifvine influence can now readilv be perceived as accordine with theiimple redox exposition. ~hes'electrochernicd vie& have been experimentally verified in novel svstems of mixed valence discs-of C U C ~ - C ; C(7). ~~ It is furthermore possible to extend the idea of sinale junctiun rectificatim il mnnifidtl j~mcrionrttrcr ari>ingin highly anisutropis material; withuut doping. Thia pro\,ide.; a pointer to theorigins of photovoltaismh such materials, a phenomenon of considerable interest just now ( 6 ) . Literature Cited ( 1 1 Ferrsris,.l.. Cowan. D. 0..Walalka. V.. Peilstein, J. H.. J. Amrr. Chpm Sot. 95. 948 (197:il. 121 Williams, W. C..Spcmg, P. L.. and Gibbons, D. J..d Ph.vs. C h m . Solids, 33, 1879
York. 1967.p. 695.
171 Rorseinrky, D.%and Malpar, R,E.,Noturr, 119751.
Volume 53.Number 10, October 1976 / 617
