Depletion and enhancement mosfets with electrically trimmable threshold

Abstract

A switching device having an electrically trimmable threshold voltage comprises a control transistor having favorable programming and erasing characteristics and a sensing transistor suited for stability and high drain voltages. The control transistor includes a floating gate for storing a charge. The control transistor receives an input voltage to vary the charge. The sensing transistor, which has a threshold voltage, includes the floating gate, which is formed from a single, contiguous layer of polysilicon or from separate polysilicon layers connected by metallization, such that the floating gate is shared by the control transistor and the sensing transistor. The control transistor has a tunnel oxide layer between a semiconductor layer and the floating gate having a thickness that is conducive to injection or tunneling of electrons through the tunnel oxide layer. The sensing transistor has a gate oxide layer between the semiconductor layer and the floating gate having a thickness greater than the thickness of the tunnel oxide layer, such as to substantially inhibit injection or tunneling of electrons through the gate oxide layer. Applying the input voltage to the control transistor varies the charge on the floating gate and thereby changes the threshold voltage of the sensing transistor.