The four-layered m.i.s.s. device (metal-tunnel insulator-n-psemiconductor) displays a current-controlled negative resistance in itsI/Vcharacteristics. This switching mechanism is the result of a regenerative feedback interaction between thep-njunction and the metal-tunnel insulator-semiconductor parts of the device.The modes of operation of the m.i.s.s. can be classified into avalanche or punch-through mode devices; thin- or thick-tunnel oxide devices; direct-tunnel (e.g. (100)-oriented Si) or indirect tunnel (e.g. (111)-oriented Si) devices. A detailed model of the m.i.s.s. is developed to account for both the thick-tunnel oxide m.i.s.s. and the indirect-tunnel m.i.s.s.The latter two modes exhibit a higher holding voltage than that of the thin-tunnel oxide m.i.s.s. The holding voltage of the thick-tunnel oxide m.i.s.s. increases monotonically with the oxide thickness.