Typical applications for pulse generators include preventing device
heating, exposing devices to time-controlled stressing or charging,
generating clock signals, testing fuses, and setting and resetting
memory devices. A pulse generator can output a voltage in a
time-controlled, time-accurate manner, allowing the user to tailor the
amount of voltage (pulse height) and the duration of the pulse (pulse
width), as well as the voltage ramp rate (rise and fall time). This type
of instrument also provides the ability to control the number of pulses
output and even to synchronize multiple pulses. Fortunately for memory
device manufacturers, a growing number of parametric test systems offer
pulse generators as optional equipment (see figure 3).
Figure 3. The pulse generator for the Keithley S530 offers two to six
channels of pulse outputs, each of which is capable of outputting a
maximum of ±40 VDC with pulse durations from 100 ns to 1 s.
NAND flash cells
fall into two categories: single bit (logical 0/1) and multi bit. As the
names imply, in a single-bit cell, each storage location can hold only
one bit; in a multi-bit cell, each storage location can hold multiple
bits. Single-bit cells require a two-level pulse to set or reset the
device, which results in two distinct VT
values (see figure 4). Multi-bit
cells need multilevel pulses to place the cell in each of its possible
states, which requires pulses with from four to eight possible voltage
Figure 4. A single-bit cell (top), which can be
in one of two states,
requires two distinct voltage levels VT for set or
cells (bottom) can be in one of four to
eight states, requiring a
pulse generator that can generate four to
eight values of VT.
To program a single-bit cell using FN
tunneling, a positive pulse is applied to the gate, while the drain,
source, and bulk voltages are set to 0 V (grounded). This causes charge
to be pushed into the floating gate. To erase the cell via FN tunneling,
a negative pulse is applied to the gate (drain, source, and bulk
terminals set to 0 V or connected to ground).
To program the cell
using HCI, simultaneous pulses are applied to the gate and drain
(source and bulk grounded or set to zero). This causes a field to appear
in the transistor channel, thereby creating the necessary hot carriers.
The pulse height and polarity of the gate pulse determine whether
charge is applied to or removed from the floating gate.
threshold voltage is usually measured afterward to ensure that the cell
has indeed been programmed or erased. If one programs and erases the
cells thousands of time, one can monitor its lifetime. (For the sake of
simplicity, this article focuses only on single-bit flash memory cells.)
parametric test system is oriented primarily toward performing accurate
DC measurements. As a result, the switch matrix and relays typically
used are designed and optimized for characteristics that ensure good DC
performance, such as low leakage current, minimal offset voltages and
currents, and low resistances. The optimization of DC performance
usually comes at some cost to the system’s AC performance.
contrast, pulses are essentially AC signals. A square pulse train can be
represented by the Fourier expansion as an infinite series of
is the period, τ the pulse width, and t
is the total time.