# A Review of Cell Operation Algorithm for 3D NAND Flash Memory
## 3 Read Algorithm
### Read Failure Improvement
#### 3.3.2 Poly-Silicon Channel Effect  
Several methods have been proposed to improve the abnormal read failure phenomenon caused by the grain boundary nature of the poly-silicon channel. Most abnormal read failure characteristics reported to date for poly-Si channels have been mainly caused by grain boundary traps (GBT). First, the problem caused by GBT is that the lower the temperature of the operating condition of the chip, the lower the channel current, which affects the sensing operation, deteriorating the cell distribution. In 2D NAND, using single crystalline silicon, phonon scattering decreases as the temperature decreases, and thereby increasing channel current [77]. However, when poly-silicon is used as a channel material, electrons are trapped at the grain boundary of poly-silicon and these trapped electrons forms a potential barrier, degrading the channel conductance. As the temperature is much lowered, the potential barrier for electrons becomes higher and higher and the channel current is further reduced, which deteriorates the sensing margin, causing read fail. As a way to improve this, a scheme to uniformly compensate for the channel current using an analog temp sensor that modulates the BL voltage in proportion to the external temperature was proposed [78]; it was demonstrated that cell distribution degradation was improved at low temperatures. However, in this method, there is a risk that the cell-to-cell interference is deteriorated during the read operation due to the neighboring gate-induced barrier lowering (NIBL) phenomenon [79]. On the other hand, as another method of compensating channel current at low temperature, a method of compensating for the pass voltage of unselected WL according to temperature has been proposed [80]. It was shown that read failure by Vt variation across temperature can be improved by increasing the pass voltage of the select transistor, which has a large effect on the channel current reduction. Compared to the previous BL voltage compensation scheme, in this new method, the NIBL phenomenon can be alleviated; however, if too high a pass voltage is applied at low temperature, it may cause a soft programming read disturbance, although the Fowler Nordheim (FN) tunneling current across the oxide is reduced at low temperature. The second problem caused by GBT is the BL transient current phenomenon. In general, when a read voltage is applied between a channel and a gate during a read operation, band bending occurs in the poly-Si, and the poly-Si grain boundary trap below the fermi level tries to fill electrons with a high probability [81]. Therefore, as the read voltage is applied and electrons fill into these GBTs for a time of 1 msec, the channel current continuously decreases. To solve this phenomenon, [PERSON] et al. proposed the 'pre-condition' voltage method to improve the transient current that occurs during the 1 msec immediately after the read voltage is applied to the selected WL [82]. However, becasue this method requires an additional pulse phase, there is a problem that the overall read performance deteriorates. On the other hand, [PERSON] et al. proposed a method to improve the read failure phenomenon that occurs only when the first selected WL is read after programming operation [83]. The physical mechanism is as follows. This phenomenon occurs during the idle period after the program operation is performed. During this idle time, electrons in the GBTs gradually discharge, because the gate voltage changes from program bias to 0 V [84], as shown in Figure 12. During the first read operation, the previous discharged GBTs cause lower cell Vt. After the first read operation, previous discharged GBTs are refilled due to the applied read bias, and then return to the original Vt state.