As Figure 1 shows, the PCM storage element
consists of two electrodes separated by
a resistive heater and a chalcogenide (the
phase-change material). Ge2Sb2Te5 (GST)
is the most commonly used chalcogenide,
but others offer higher resistivity and improve
the device’s electrical characteristics. Nitrogen
doping increases resistivity and lowers programming
current, whereas GS offers lowerlatency
phase changes.5,6 (GS contains the
first two elements of GST, germanium and
antimony, and does not include tellurium.)
Phase changes are induced by injecting
current into the resistor junction and heating
the chalcogenide. The current and voltage
characteristics of the chalcogenide are identical
regardless of its initial phase, thereby lowering
programming complexity and latency.7
The amplitude and width of the injected current
pulse determine the programmed state.
PCM cells are one-transistor (1T), oneresistor
(1R) devices comprising a resistive
storage element and an access transistor
(Figure 1). One of three devices typically controls
access: a field-effect transistor (FET), a
bipolar junction transistor (BJT), or a
diode. In the future, FET scaling and large
voltage drops across the cell will adversely affect
gate-oxide reliability for unselected wordlines.8
BJTs are faster and can scale more
robustly without this vulnerability.8,9 Diodes
occupy smaller areas and potentially enable
greater cell densities but require higher operating
voltages.