24.6 Thin-Film Transistors
737
(a)
(b)
Fig. 24.52 a Schematic cross section of a top-gate amorphous silicon (a-Si) thin-film transistor (MISFET) on glass
substrate. b Schematic cross section of a transparent ZnO thin-film transistor (MESFET)
(a)
(b)
Fig. 24.53 a Field effect mobility and on/off current ratio for oxide channel transistors. Filled squares represent MISFET
transistors; open squares are for MESFETs from [2135]. The shaded area indicates best performance. b Voltage swing
for MISFETs (filled squares, subthreshold voltage swing) and MESFET (empty square, above turn-on voltage from
[2135]) with TSO channels. The dashed line is guide to the eye for the trend of best performance. The dash-dotted line
indicates the thermodynamic limit of about 60 meV/decade for the swing [2138]. Adapted from [2136]
talline silicon has a mobility of typically less than 10 cm
2 /Vs. With the use of laser irradiation or thermal
annealing, amorphous or small-grain polycrystalline silicon layers can be recrystallized, increasing the
mobility up to several 100 cm
2 /Vs, improving transistor performance [2128, 2130, 2131]. However,
for display applications a mobility of 10 cm
2 /Vs is large enough.
The main optimization criteria for thin-film transistors are high on-off ratio, long-term stability,
good uniformity and reproducibility, and low cost. Recently, flexible (on polymer substrate) and transparent TFTs (TFET, transparent FET), e.g. with polycrystalline ZnO or GaInZnO (GIZO) channel
(Fig. 24.52b), are being investigated for advanced applications such as all-transparent electronics and
displays [151, 2132–2135]. A compilation of recent results on transparent semiconducting oxide (TSO)
channel FETs can be found in [2136]. In Fig. 24.53 performance data for various TSO channel FETs
are visualized. In Fig. 24.54 a transparent inverter based on ZnO-MESFETs is depicted [2137].
24.6.3 Transistors with 2D Materials
The ultimate thin film transistor is one with a single monolayer as channel. Various 2D materials (cf.
Sect. 13) as mono- or multi-layers have been investigated in transistors [2140, 2141]. The channel
mobility for conventional layers decreases typically for thickness d below 10 nm and strongly below
5 nm. The idea to use 2D materials is to avoid a rough surface and interface defects towards the gate. A
single layer MoS 2 transistor with HfO 2 gate and Au ohmic contacts has been reported in [2142] with a
(field effect) mobility above 200 cm
2 /Vs, low sub-threshold voltage swing and high on/off ratios above
737
(a)
(b)
Fig. 24.52 a Schematic cross section of a top-gate amorphous silicon (a-Si) thin-film transistor (MISFET) on glass
substrate. b Schematic cross section of a transparent ZnO thin-film transistor (MESFET)
(a)
(b)
Fig. 24.53 a Field effect mobility and on/off current ratio for oxide channel transistors. Filled squares represent MISFET
transistors; open squares are for MESFETs from [2135]. The shaded area indicates best performance. b Voltage swing
for MISFETs (filled squares, subthreshold voltage swing) and MESFET (empty square, above turn-on voltage from
[2135]) with TSO channels. The dashed line is guide to the eye for the trend of best performance. The dash-dotted line
indicates the thermodynamic limit of about 60 meV/decade for the swing [2138]. Adapted from [2136]
talline silicon has a mobility of typically less than 10 cm
2 /Vs. With the use of laser irradiation or thermal
annealing, amorphous or small-grain polycrystalline silicon layers can be recrystallized, increasing the
mobility up to several 100 cm
2 /Vs, improving transistor performance [2128, 2130, 2131]. However,
for display applications a mobility of 10 cm
2 /Vs is large enough.
The main optimization criteria for thin-film transistors are high on-off ratio, long-term stability,
good uniformity and reproducibility, and low cost. Recently, flexible (on polymer substrate) and transparent TFTs (TFET, transparent FET), e.g. with polycrystalline ZnO or GaInZnO (GIZO) channel
(Fig. 24.52b), are being investigated for advanced applications such as all-transparent electronics and
displays [151, 2132–2135]. A compilation of recent results on transparent semiconducting oxide (TSO)
channel FETs can be found in [2136]. In Fig. 24.53 performance data for various TSO channel FETs
are visualized. In Fig. 24.54 a transparent inverter based on ZnO-MESFETs is depicted [2137].
24.6.3 Transistors with 2D Materials
The ultimate thin film transistor is one with a single monolayer as channel. Various 2D materials (cf.
Sect. 13) as mono- or multi-layers have been investigated in transistors [2140, 2141]. The channel
mobility for conventional layers decreases typically for thickness d below 10 nm and strongly below
5 nm. The idea to use 2D materials is to avoid a rough surface and interface defects towards the gate. A
single layer MoS 2 transistor with HfO 2 gate and Au ohmic contacts has been reported in [2142] with a
(field effect) mobility above 200 cm
2 /Vs, low sub-threshold voltage swing and high on/off ratios above