Study of Underlapped Finfets Behavior for a Radiation Sensing Purpose
Authors: Fonseca Willliam da Silva , Agopian Paula Ghedini Der 
Published: May 2020

The use of MOSFET transistors as radiation sensors has been intensively studied in recent years. Since the main effects caused by the total ionization dose (TID) are well known, the application of this technology as a dosimeter for many applications has greatly increased. One example is the application of MOSFETs as dosimeter of Radiotherapy (1). Knowing that SOI FinFETs have high immunity to soft errors and thin FinFET devices also presents a good radiation hardness for TID effects, the Underlapped FinFET structure can be applied as an accurate radiation sensor since only the oxide in the underlap region will be responsible for the drain current (ID) variation.
The experimental behavior of self-aligned FinFET transistor with fin width (WFIN) of 20nm before and after proton irradiation are presented in figure 1. As already reported in (2, 3), the FinFET with WFIN=20 nm shows a high immunity to TID effects of proton radiation even considering the accumulated charges in the front and buried oxides due to the high coupling between gates.
From now on, it was considered the underlap regions for both source and drain (S/D) sides. The simulation was calibrated with experimental FinFET behavior for self-aligned structure and it was extrapolated for other underlapped structures varying the underlap lengths (LSP). The underlap region was varied from 0nm (self-aligned structure) to 50nm and the permittivity (k) of the oxide in the underlap region was varied from 1 (no oxide in the underlap region) to 25 (hafnium oxide – HfO2).
Figure 2 presents the drain current as a function of gate voltage (VG) for underlapped FinFETs, varying the underlap length and the permittivity of the oxide in the underlap region. The increase of the LSP results in a strong degradation of ID due to the loss charge control over the underlap, caused by lower electric field effect (fringing field), which increases the series resistance and the current spreading in the spacer.
Besides the underlap length, the impact of permittivity on the on-state current level (ION) can also be observed in figure 2. The permittivity increase results in a higher fringing fields from the gate to the source/drain regions and consequently a higher current density, that in turns, causes the improvement on ION. The electric field and the electron density were extracted from numerical simulations for devices with k=1 and k=25 and a higher fringing field and consequently the higher electron density over the underlap region, for devices with k=25, can be observed in figure 3.
The sensitivity (S), that was calculated by ID (k=X)/ID (k=1), was evaluated in order to select the better oxide and length to the underlap region. From figure 4, it is possible to observe that the greater the LSP and permittivity, the better is the sensitivity of the device. Although devices with longer underlap present a high series resistance and consequently a smaller drain current level, the ION enhancement caused by the higher influence of fringing field becomes more pronounced when using the higher permittivity, resulting in a more sensitive device. However, for high drain bias (VDS=1V) the underlap length plays a hole until LSP=30 nm, for longer underlap no significant sensitivity variation was obtained.
Considering that the device with longer underlap region (LSP=50nm) and higher k (HfO2 – k=25) presents a better sensitivity, it was considered to evaluate its charges sensing ability.
Aiming to analyze the underlapped FinFET as a radiation sensor (TID), fixed charges density (Qox) with carrier concentration of 0, 1011, 5x1011 and 1012 cm-2 were considered in the underlap region, where Qox=0 cm-2 characterizes the pre-radiated transistor. Figure 5 shows that despite the low on-state current caused by the high series resistance, when the oxide charges are considered, there is an increase of the potential in the underlap region causing an enhancement of ION. The radiation sensitivity was calculated by post-radiated ION over pre-radiated ION ratio.
Although narrow FinFETs are well known as a radiation hardness transistor, from figure 6 it is possible to notice that a long underlap region with a high k dielectric allows this device to have a good sensing behavior. When considering a relatively low trapped charge density (Qox= 5x1011cm-2), the drain current increases about 60%, i.e., sensitivity equal 1.6. Increasing the trapped charge density to Qox=1x1012 cm-2, the obtained sensitivity values are still higher, reaching an increment of 140% on the ION current level (S=2.4), confirming the excellent characteristic of the device working as a radiation sensor.

Figure 1