1. A synapse for a spike timing dependent plasticity (STDP) function cell, the synapse comprising: a memory device having a variable resistance; and a transistor connected to the memory device, wherein a channel of the memory device is connected in series with a channel of the transistor, and wherein the synapse is configured to selectively operate in a communication mode in which the synapse transmits information from a pre-synaptic neuron to a post-synaptic neuron, and a learning mode in which a synaptic weight of the synapse changes in response to a voltage difference between voltages applied to both ends of the memory device.