24.2 Bipolar Transistors
703
Fig. 24.11 Schematic
band diagram of a
heterojunction bipolar
transistor
emitter
base
collector
E V
E C
F n
F n
F p
n-Al Ga As
x
1-x
p-GaAs
n-GaAs
and the base from the lower-bandgap material. The schematic band diagram is shown in Fig. 24.11
(see Fig. 21.59c for the emitter–base diode).
The higher discontinuity in the valence band, compared to a homojunction with the base material,
provides a higher barrier for hole transport from the base to the emitter. Thus, the emitter efficiency is
increased. Another advantage is the possibility for higher doping of the base without loss of emitter
efficiency. This reduces the base series resistance and leads to better high-frequency behavior due to
higher current gain and a smaller RC time constant. Also, operation at higher temperature is possible
when the emitter has a larger band gap. Current InP/InGaAs-based HBTs have cutoff frequencies
beyond 30 GHz, SiGe-HBTs beyond 80 GHz. The high-frequency performance is influenced by the
velocity-overshoot effect (cf. Sect. 8.4.3) [2054].
In Fig. 24.12, an InAlAs/InGaAs HBT is shown [2056]. The cutoff frequency is 90 GHz. For the
layer design, a fairly thick collector with low doping was chosen. This design allows a broad depletion
layer with fairly small maximum electric field and thus a high breakdown voltage of BV CE0 > 8.5 V.
The base is not too thin (80 instead of maybe 60 nm) to reduce the series resistance. A graded region
between emitter and base was chosen to avoid a spike occurring in the conduction band (Fig. 21.59b)
and keep the turn-on voltage low.
24.2.8 Light-Emitting Transistors
The base current has two components. One is the recombination current in the neutral region of the
emitter; this current can be suppressed in the HBT. The other is the recombination in the base region
itself.
3 If quantum wells are introduced into the base region, this recombination can occur radiatively
between electrons and holes captured into the quantum well (Fig. 24.13). The spectrum exhibits two
peaks from the QWs and the GaAs barrier.
24.3 Field-Effect Transistors
Next to the bipolar transistors, the field-effect transistors (FET) are another large class of transistors.
FETs were historically conceptualized first but due to technological difficulties with semiconductor
3 Also, a recombination current in the emitter–base depletion region is possible. However, since in normal operating
conditions this diode is forward biased, the depletion layer is short and the associated recombination current is small, cf.
p. 614.
703
Fig. 24.11 Schematic
band diagram of a
heterojunction bipolar
transistor
emitter
base
collector
E V
E C
F n
F n
F p
n-Al Ga As
x
1-x
p-GaAs
n-GaAs
and the base from the lower-bandgap material. The schematic band diagram is shown in Fig. 24.11
(see Fig. 21.59c for the emitter–base diode).
The higher discontinuity in the valence band, compared to a homojunction with the base material,
provides a higher barrier for hole transport from the base to the emitter. Thus, the emitter efficiency is
increased. Another advantage is the possibility for higher doping of the base without loss of emitter
efficiency. This reduces the base series resistance and leads to better high-frequency behavior due to
higher current gain and a smaller RC time constant. Also, operation at higher temperature is possible
when the emitter has a larger band gap. Current InP/InGaAs-based HBTs have cutoff frequencies
beyond 30 GHz, SiGe-HBTs beyond 80 GHz. The high-frequency performance is influenced by the
velocity-overshoot effect (cf. Sect. 8.4.3) [2054].
In Fig. 24.12, an InAlAs/InGaAs HBT is shown [2056]. The cutoff frequency is 90 GHz. For the
layer design, a fairly thick collector with low doping was chosen. This design allows a broad depletion
layer with fairly small maximum electric field and thus a high breakdown voltage of BV CE0 > 8.5 V.
The base is not too thin (80 instead of maybe 60 nm) to reduce the series resistance. A graded region
between emitter and base was chosen to avoid a spike occurring in the conduction band (Fig. 21.59b)
and keep the turn-on voltage low.
24.2.8 Light-Emitting Transistors
The base current has two components. One is the recombination current in the neutral region of the
emitter; this current can be suppressed in the HBT. The other is the recombination in the base region
itself.
3 If quantum wells are introduced into the base region, this recombination can occur radiatively
between electrons and holes captured into the quantum well (Fig. 24.13). The spectrum exhibits two
peaks from the QWs and the GaAs barrier.
24.3 Field-Effect Transistors
Next to the bipolar transistors, the field-effect transistors (FET) are another large class of transistors.
FETs were historically conceptualized first but due to technological difficulties with semiconductor
3 Also, a recombination current in the emitter–base depletion region is possible. However, since in normal operating
conditions this diode is forward biased, the depletion layer is short and the associated recombination current is small, cf.
p. 614.