24.5 MOSFETs
731
(a)
(b)
Fig. 24.42 a Radial distribution function of ions in Ge 2 Sb 2 Te 5 (GST) for various temperatures (cmp. Fig. 3.14b).
Adapted from [2096]. b Atom arrangement in the amorphous phase of GST with square units highlighted that nucleate
crystallization. Adapted from [2097]
• a molecular configuration change (e.g. redox reaction) between crossed wire lines (molecular electronics [2106–2108].
24.5.8 Heterojunction FETs
Several types of field-effect transistors have been devised that use heterojunctions (HJFET).
HIGFET
As conducting channel, the two-dimensional electron gas at an undoped heterointerface is used. Such
a transistor is called a heterojunction insulating gate FET (HIGFET). With forward or backward gate
voltage, an electron or hole gas can be created (channel enhancement mode), as visualized in Fig. 24.43.
Thus, a complementary logic can be realized. However, the p-channel suffers from low hole mobility.
HEMT
If the top wide-bandgap layer is n-doped, a modulation-doped FET (MODFET) is made (see
Sect. 12.3.4). This structure is also called a HEMT (high electron mobility transistor) or TEGFET
(two-dimensional electron gas FET) (Fig. 24.44). A thin undoped AlGaAs spacer layer is introduced
between the doped AlGaAs and the undoped GaAs to reduce impurity scattering from carriers that tunnel into the barrier. With increasing gate voltage, a parallel conduction channel in the AlGaAs is opened.
The natural idea would be to increase the Al fraction in the AlGaAs to increase the quantum-well barrier height. Unfortunately, the barrier height is limited to 160 meV for an aluminum concentration of
about 20%. For Al content higher than about 22%, the DX center (cf. Sect. 7.7.6) forms a deep level
such that the apparent ionization energy increases drastically and no shallow donors can be used for
modulation doping. An improvement for the barrier conduction problem is the use of δ-doping [2109],
i.e. the introduction of a highly doped thin (mono-)layer (Fig. 24.45), which results in higher channel
carrier concentration.
731
(a)
(b)
Fig. 24.42 a Radial distribution function of ions in Ge 2 Sb 2 Te 5 (GST) for various temperatures (cmp. Fig. 3.14b).
Adapted from [2096]. b Atom arrangement in the amorphous phase of GST with square units highlighted that nucleate
crystallization. Adapted from [2097]
• a molecular configuration change (e.g. redox reaction) between crossed wire lines (molecular electronics [2106–2108].
24.5.8 Heterojunction FETs
Several types of field-effect transistors have been devised that use heterojunctions (HJFET).
HIGFET
As conducting channel, the two-dimensional electron gas at an undoped heterointerface is used. Such
a transistor is called a heterojunction insulating gate FET (HIGFET). With forward or backward gate
voltage, an electron or hole gas can be created (channel enhancement mode), as visualized in Fig. 24.43.
Thus, a complementary logic can be realized. However, the p-channel suffers from low hole mobility.
HEMT
If the top wide-bandgap layer is n-doped, a modulation-doped FET (MODFET) is made (see
Sect. 12.3.4). This structure is also called a HEMT (high electron mobility transistor) or TEGFET
(two-dimensional electron gas FET) (Fig. 24.44). A thin undoped AlGaAs spacer layer is introduced
between the doped AlGaAs and the undoped GaAs to reduce impurity scattering from carriers that tunnel into the barrier. With increasing gate voltage, a parallel conduction channel in the AlGaAs is opened.
The natural idea would be to increase the Al fraction in the AlGaAs to increase the quantum-well barrier height. Unfortunately, the barrier height is limited to 160 meV for an aluminum concentration of
about 20%. For Al content higher than about 22%, the DX center (cf. Sect. 7.7.6) forms a deep level
such that the apparent ionization energy increases drastically and no shallow donors can be used for
modulation doping. An improvement for the barrier conduction problem is the use of δ-doping [2109],
i.e. the introduction of a highly doped thin (mono-)layer (Fig. 24.45), which results in higher channel
carrier concentration.