Negative differential resistance (NDR) memory device with reduced soft error rate
An active negative differential resistance element (an NDR FET) and a memory device (such as an SRAM) using such elements is disclosed. Soft error rate (SER) performance for NDR FETs and such memory devices are enhanced by adjusting a location of charge traps in a charge trapping layer that is responsible for effectuating an NDR behavior. Both an SER and a switching speed performance characteristic can be tailored by suitable placement of the charge traps.
1 . A method of forming a semiconductor field effect transistor (PET) for a memory device, the PET having a control gate, a source region, a drain region, the method comprising the steps of:
forming a channel for carrying a current between the source and drain regions; and
forming a trapping layer located proximate to and forming an interface with said channel, said trapping layer including trapping sites adapted for trapping at least warm carriers from said channel so as to effectuate a negative differential resistance mode for the FET;
wherein an operational switching steed for the FET is directly related to a distance which said trapping sites are located from said interface, such that locating said trapping sites at a distance D 1 results in a maximum operational switching speed S 1 , and such that locating said trapping sites at a distance D 2 (D 2 >D 1 ) results in a minimum operational switching speed S 2 (S 2 <S 1 ); and
distributing said trapping sites within said trapping layer at an approximate distance D (D 2 >D>D 1 ) in accordance with a target operational switching speed S for the FET (S 1 >S>S 2 ) and a target soft error rate for the memory device.
2 . The method of claim 1 , wherein D 1 is about 0.5 nm, and D 2 is about 1.5 nm.
3 . The method of claim 1 wherein S 2 is about 1 nanosecond and S 1 is about 1 picosecond.
4 . The method of claim 1 , wherein the target soft error rate is about 1000 failures in time per Mbit.
5 . The method of claim 1 , further including a step of forming an additional set of trapping sites at an approximate distance D′ from the interface where (D 2 >D′>D 1 ).
6 . The method of claim 1 , wherein said trapping sites are distributed at a particular distance by adjustment of an implant energy and dosage.
7 . The method of claim 1 , wherein a concentration of said trapping sites in a bulk region of said trapping layer is controlled by adjustment of a temperature and/or time characteristic of a heat treatment process.
8 . The method of claim 1 , wherein said trapping layer consists of a first dielectric layer and a second dielectric layer, and said trapping sites are located only within said first dielectric layer.
9 . The method of claim 1 , wherein said trapping sites are located along only a limited portion of the interface.
10 . The method of claim 9 , wherein said portion is nearer said source region than said drain region.
11 . The method of claim 1 , wherein said trapping sites are distributed so that substantially all of said trapping sites are within 1-1.5 nm of said interface.
12 . A memory cell comprising:
a data transfer element adapted to facilitate a read operation or a write operation involving a storage node of the memory cell;
a first negative differential resistance (NDR) element coupled to said data transfer element, said storage node and a first voltage potential, wherein said first NDR element is adapted to operate with a first NDR characteristic between said storage node and said first voltage potential;
a second NDR element coupled to said first NDR element, said data transfer element, said storage node and a second voltage potential wherein said second NDR element is adapted to operate with a second NDR characteristic between said storage node and said second voltage potential;
said first NDR element and said second NDR element both including a trap layer in which charge traps are used to effectuate said first NDR characteristic and said second NDR characteristics; and
wherein said charge traps are distributed in said trap layer so as to cause the memory cell to achieve a soft error rate of approximately 1,000 failures-in-time (FITs)/Mbit or less.
13 . The memory device of claim 12 , wherein said memory device is a static random access memory (SRAM) cell.
14 . The memory device of claim 12 , wherein said first NDR element and said second NDR element are NDR-capable FETs.
15 . The memory device of claim 14 , wherein said charge traps are distributed in said trap layer so as to NDR-capable FETs to switch with a switching speed between 1 picosecond and 10 nanoseconds.
16 . In a memory cell including a transfer field effect transistor (FET), a first negative differential resistance (NDR) element and a second NDR element that are operably interconnected to store a data value, the improvement comprising:
at least one of the first NDR element and the second NDR element being implemented as an NDR-capable FET, said NDR-capable FET using a charge trapping mechanism to achieve an NDR behavior suitable for storing the data value; and
wherein charge traps are distributed in a charge trapping layer of said NDR capable FET so as to cause the memory cell to achieve a switching speed between 1 picosecond and 10 nanoseconds and a soft error rate of approximately 1,000 failures-in-time (FITs)/Mbit or less.
17 . The memory cell of claim 16 , wherein said traps are formed by a doping impurity such as Boron.
18 . The memory cell of claim 16 , wherein said traps have a trap density of approximately 1 to 5*10 14 traps/cm 2 at a distance of about 0.5 nm from an interface of said trapping layer with a channel of said NDR-capable FET.
19 . The memory cell of claim 16 , wherein said traps have an energy level of about 0.5 eV about a conduction band edge of a channel of said NDR-capable FET.
20 . The memory cell of claim 16 , wherein said trapping layer is comprised of two separate layers, including a first dielectric layer with a high concentration of said charge traps, and a second dielectric layer with a substantially smaller concentration of said charge traps.
21 . In a process for making a memory cell having three elements, including a transfer field effect transistor (FET), a first negative differential resistance (NDR) element and a second NDR element that are operably interconnected to store a data value, the improvement comprising the steps of:
forming at least one of the first NDR element and the second NDR element as an NDR-capable PET, said NDR-capable FET including a charge trapping layer for effectuating an NDR characteristic; and
distributing charge traps in said charge trapping layer of said NDR capable FET in accordance with a target switching speed and a target soft error rate for the memory cell.
22 . The process of claim 21 wherein said memory cell as formed achieves a switching speed between 1 picosecond and 10 nanoseconds and a soft error rate of approximately 1,000 failures-in-time (FITs)/Mbit or less.
23 . The process of claim 21 , wherein said trapping layer is formed as two separate layers in two separate processing steps, including a first dielectric layer with a high concentration of said charge traps, and a second dielectric layer with a substantially smaller concentration of said charge traps.
24 . The process of claim 21 , wherein said charge traps are implanted into a channel region of said NDR capable FET, and a rapid thermal anneal step is performed before said charge trapping layer is formed at an interface with said channel region.
25 . The process of claim 21 , wherein said charge traps are impurities that are directly implanted into said trapping layer after said trapping layer is formed.