3.2.2. Threshold Voltage
The threshold voltage V
T of a FET can be defined as the voltage at which the FET turns to the off state from the on state
[72,77][56][57] as shown in
Figure 6. In an ambipolar FET, the threshold voltage will be the voltage at which the current I
ds minimum occurs. In other words, it is the voltage at which the carrier inversion occurs. The region between the turn-on voltage and the threshold voltage is known as the sub-threshold regime in the transfer characteristics curve and the reciprocal slope of the log (I
ds) − V
g curve gives the sub-threshold swing, which reflects the speed of switching ability of the device.
3.2.3. Mobility Calculations in FETs
The most important intrinsic electrical properties of a semiconductor are its charge carrier mobility and diffusion length
[51][40]. A good proportion of the perovskite-based thin film FET studies were carried out to measure the mobility of the semiconducting materials
[78][58]. In a perovskite thin-film FET, the field effect mobility
μ
is considered as the average drifting velocity of the charge carriers in the active semiconducting layer with an applied electric field
[79][59]. The efficiency of the FET devices is increased with the charge carrier mobility in the channel, as the drain-source current primarily depends on it
[79][59]. The calculation of charge carrier mobility in a semiconducting layer of a perovskite FET is carried out based on the transfer characteristics curve with two important assumptions: (a) mobility does not depend on carrier density, and (b) the transverse gate electric field is much greater than the applied source and drain voltage
[80][60].
3.3. Early Works (Before 2012) on Perovskite FETs
Based on tThe
literature, the first research paper on perovskite FET was published in 1999 using a layered bulk (C
6H
5C
2H
4NH
3)
2 SnI
4 thin film as the semiconducting channel with Pd as the source and drain electrodes from the IBM T. J. Watson Research Center
[15]. The same research group reported (C
6H
4FC
2H
4NH
3)
2SnI
4 perovskite thin-film FETs in 2001
[16] and (phenethyl ammonium)
2SnI
4 in 2002
[84][61]. A group in Kyushu University of Japan continued the work based on (PEA)
2SnI
4 channel FETs by using different fabrication methods of deposition techniques in 2003 and 2004
[85,86][62][63]. In 2006, the same group turned to the hybrid perovskite of MASnI
3 as channel materials in FET
[87][64]. Apart from those two groups’ works, the perovskite of PbZr
0.52Ti
0.48O
3 (PZT) was used as channel materials in ferroelectric FETs on two occasions
[14,88][14][65].
3.4. Recent Works (After 2012) on Perovskite FETs
After the introduction of perovskite materials in solar cells
[12[12][66],
93], a huge number of perovskite materials have been used as semiconducting active materials in the fabrication of solar cells. In consequence, there is a substantial increase in the number of these since the first MAPbI
3 channel-based perovskite FET
[94][67]. All the reported perovskite-based thin-film FET studies in the literature after 2012 are classified into three different categories based on the perovskite material types.
3.4.1. Hybrid (Organic Inorganic) Perovskite FETs
The hybrid halide perovskites were used in most of the thin film FET studies. Optoelectronic and material properties of tin- and lead-based hybrid perovskites with different organic cations were analysed and MAPbI
3 was found as the most promising hybrid perovskite semiconductor than MASnI
3, HC(NH
2)
2SnI
3 and HC(NH
2)
2PbI
3 [95][68]. Having organic cations makes the device fabrication easier with single hybrid perovskite materials
[84][61].
3.4.2. All-Inorganic Perovskite FETs
CsPbBr
3 is the most dominant of all inorganic perovskite materials, which was studied in
[89][69] many different crystal structures in FET research works. All-inorganic perovskites loaded with CsPbBr
3 quantum dots and organic rubrene semiconducting sheets were used, and optical and electronic characteristics were analyzed by Youn et al.
[132][70]. In 2019, a phototransistor was fabricated with the semiconducting channels of all inorganic CsPbBr
3, and the hole mobility was found to be 0.02 cm
2s
−1V
−1 and 0.34 cm
2s
−1V
−1 in dark and illuminated conditions, respectively, with an excellent ambipolarity
[133][71]. The high carrier mobility in the all-inorganic perovskite is improved as the charge carrier mobility is enhanced by the incorporation of inorganic cations of Rb and Cs
[134][72].
3.4.3. Double and Triple Perovskite FETs
However, the charge carrier mobility is affected by the interlayer distance in these types of perovskites, and the number of FET studies based on double or triple perovskites has increased recently on account of their greater stability than the other two types
[146][73]. Perhaps the earliest study of FET was performed based on the double perovskite of (PEA)
2SnI
4 [15]. Then, the various double and triple perovskite materials were used in different forms for thin-film FET applications. Cs
2AgBiBr
6 is one of the most popular materials for solar PV applications these days.
3.5. Single-Crystalline Perovskite FETs
In a polycrystalline semiconductor material, the presence of grains and grain boundaries causes the screening effect in the field effect mobility
[37][25]. However, this effect, which is generated by the heat in the polycrystalline material, is denied in the device applications of single crystalline FET
[37][25]. The trap density of single-crystalline perovskites was seen to be lower than polycrystalline perovskites
[164][74]. The tunnel junction formations occur due to the passive charge accumulations in the grain boundaries
[97][75] in the polycrystalline materials. So, the grain boundary effects are eliminated in single crystalline perovskite FETs, which are less defective than the polycrystalline FETs. However, the poor stability of the single crystalline perovskite materials is a drawback in commercial device applications. The charge transport mechanism in MAPbI
3 in single crystalline semiconductors was compared with the polycrystalline FET in
[11]. The effect of grain boundaries was reduced in the single crystalline transistor, and the electrical characteristics were recorded to be better in single crystalline FET
[165][76]. Electrochemical reactions of Au electrodes in a CH
3NH
3PbBr
3 single-crystal-based FET were investigated in
[108][77]. The single MaPbI
3 crystal’s photo-generated carrier diffusion was found to be lesser at lower temperatures in
[114][78].
3.6. Perovskite FETs with Nanostructured Channel
Perovskite nanomaterials are immensely used in device applications. In FETs, the -perovskite nanomaterial is used in two types: (1) single nanostructure FETs and (2) thin-film FETs made up of perovskite nanostructures. The single nanostructure FETs consist of a single nanostructure material as a channel. CsPbX
3 nanowires have received significant interest as a material for optoelectronic applications, including flexible light detectors
[166,167][79][80].