Cadmium Sulfide Is a Very Unusual Model Semiconductor

Abstract

CdS has three states that are thermodynamically stable, one at low fields and two at high fields when stationary high-field domains are initiated in a range of negative differential conductivity, one when the domain is attached to the cathode then the CdS is n-type. And the other one when the domain is attached to the anode and the CdS is turned p-type. When the photoconductivity of CdS is field quenched stronger than linearly, a band of lowest conductivity is introduced between the electrodes, the field in this band is increased to maintain current continuity. This is the high-field domain, that limits the current to a low, constant value. With increased bias the domain expands, but current and domain field remain constant. As long as the domain is attached to the cathode, stationarity is achieved by the limited supply of electrons from the blocking cathode. When the domain is expanded to reach the anode, then a new, higher field domain is generated at the anode that expands toward the cathode and stability is maintained because of the limited supply of holes from the blocking anode. The Minimum Entropy principle forces the current to remain constant by adjusting the width of the higher field, anode-adjacent domain: the transition from the cathode to the anode adjacent domain is not visible in the current that remains saturated, but the conductivity changes from n- to p-type, and now attains the third stable thermodynamic state. This is the first time CdS is p-type that can never be achieved by doping (where the CdS is self-compensating by a strong intrinsic donor). This is very unusual since the current in both cases is carried by drift alone: j = e n μn Fc = e p μp Fa and is forced to remain the same, even though carrier density and mobility are substantially different. All of this is accomplished by adjusting the width and the field of the high-field domain. There is one more unusual coincidence, because both domains require that they are created in a range of overcritical negative conductivity. That needs to have a defect distribution of donors and acceptors for providing this negative differential conductivity at a similar field for electron- and for hole-quenching. It is given by field excitation from Coulomb attractive traps that all lie in the kV cm−1 range. Cadmium sulfide which has an extensive distribution of Coulomb attractive electron and hole traps that can produce such conditions that no other known semiconductor can provide. This is believed to make CdS such an unusual model semiconductor that can perform the many applications, foremost in combination with p-type solar cells without pn-junction that with a thin layer of CdS create highly efficient solar cells.