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The human brain is capable of short- and long-term memory with retention times ranging from a few seconds to several years. Electrolyte-gated transistors have drawn attention for their potential to mimic synaptic behaviors in neuromorphic applications, but they generally operate at low voltages to avoid instability and, hence, offer limited tunability. Sputtered silicon dioxide electrolytes are utilized in this work to gate indium-gallium-zinc-oxide thin-film transistors, which offer robust operation at much higher voltages. The synaptic memory behavior is studied under single and multiple pulses and under mild (1 V) and strong stimuli (up to 8 V). The devices are found to be capable of providing an extremely wide range of memory retention time from ∼2 ms to ∼20 000 s, over seven orders of magnitude. Furthermore, based on the experimental data on individual transistors, pattern learning and memorizing functionalities are conceptually demonstrated.