The two most common types of transistors are the bipolar junction transistor (BJT) and the field effect transistor (FET). Most digital logic and memory products are designed using FETs. There are both N-type and P-type FETs. The N-type FET (NFET) consists of a source and drain, each consisting of a n-doped diffusion within a p-type bulk, the channel, which is the area between the source and the drain, and the gate, which is above but separated from the channel by a thin dielectric. If a positive voltage is applied between the drain and the source of an NFET, and the gate and bulk is held at the lower voltage (same as the source), current cannot flow between the drain and the source because the drain is a reverse-biased NP junction. If the gate voltage is increased sufficiently, however, electrons are drawn from the bulk to the surface of the channel, beneath the gate. If the voltage is high enough, the p-type material near the surface of the channel will become n-type. This removes the reverse-biased NP junction and allows current to flow from the drain to the source (electrons flow from the source to the drain). In this way, an NFET acts like a switch which is open when the gate voltage is low and closed when the gate voltage is high.