702
24 Transistors
24.2.5.3 Common Collector Circuit
In Fig. 24.10c, the collector is connected to mass for alternating currents. Input and output current
flow through the load resistance at which part of the input voltage drops. The input voltage is divided
between the load resistor R L and the emitter–base diode. At the transistor, the voltage V BE = V 1 − V RL
is applied. If the input voltage (V 1 = V BC ) is increased, I 2 increases. This leads to a larger voltage drop
at the load resistor and therefore to a decrease of V BE , working against the original increase. The input
resistance R 1 is large despite a small load resistance, R 1 ≈ β 0 R L . The input voltage is larger than V RL ,
thus no voltage gain occurs (actually it is a little smaller than 1). The current amplification is (β + 1).
The output resistance R 2 is small, R 2 = V 2 /I 2 = R L ≈ R 1 /β 0 . Therefore, this circuit is also called an
impedance amplifier that allows high-impedance sources to be connected low-impedance loads. Since
an increase of the input voltage leads to an increase of the output voltage that is present at the emitter,
this circuit is a direct amplifier and is also called an emitter follower.
24.2.6 High-Frequency Properties
Transistors for amplification of high-frequency signals are typically chosen as npn transistors since
electrons, the minority carriers in the base, have higher mobility than holes. The active area and parasitic
capacitance must be minimized. The emitter is formed in the shape of a stripe, nowadays in the 100 nm
regime. The base width is in the 10 nm range. High p-doping of GaAs with low diffusion of the dopant
is accomplished with carbon. Defects that would short emitter and collector at such thin base width
must be avoided.
An important figure of merit is the cutoff frequency f T for which h FE is unity in the common emitter
configuration. The cutoff frequency is related to the emitter–collector delay time τ EC by
f T =
1
2π τ EC
.
(24.32)
The delay time is determined by the charging time of the emitter–base depletion layer, the base capacitance, and the transport through the base–collector depletion layer. It is favorable if all times are short
and similar. It does not help to minimize only one or two of the three processes since the longest time
determines the transistor performance.
Another important figure of merit is the maximum frequency with which the transistor can oscillate
in a feedback circuit with zero loss. This frequency is denoted by f max . Approximately,
f max
f T
8π R B C CB
,
(24.33)
where R B is the base resistance and C CB is the collector–base capacitance. f max is larger than f T , by a
factor of the order of three.
24.2.7 Heterojunction Bipolar Transistors
In a heterojunction bipolar transistor (HBT), the emitter–base diode is formed with a heterostructure
diode. The desired functionality is obtained when the emitter is made from the higher-bandgap material
24 Transistors
24.2.5.3 Common Collector Circuit
In Fig. 24.10c, the collector is connected to mass for alternating currents. Input and output current
flow through the load resistance at which part of the input voltage drops. The input voltage is divided
between the load resistor R L and the emitter–base diode. At the transistor, the voltage V BE = V 1 − V RL
is applied. If the input voltage (V 1 = V BC ) is increased, I 2 increases. This leads to a larger voltage drop
at the load resistor and therefore to a decrease of V BE , working against the original increase. The input
resistance R 1 is large despite a small load resistance, R 1 ≈ β 0 R L . The input voltage is larger than V RL ,
thus no voltage gain occurs (actually it is a little smaller than 1). The current amplification is (β + 1).
The output resistance R 2 is small, R 2 = V 2 /I 2 = R L ≈ R 1 /β 0 . Therefore, this circuit is also called an
impedance amplifier that allows high-impedance sources to be connected low-impedance loads. Since
an increase of the input voltage leads to an increase of the output voltage that is present at the emitter,
this circuit is a direct amplifier and is also called an emitter follower.
24.2.6 High-Frequency Properties
Transistors for amplification of high-frequency signals are typically chosen as npn transistors since
electrons, the minority carriers in the base, have higher mobility than holes. The active area and parasitic
capacitance must be minimized. The emitter is formed in the shape of a stripe, nowadays in the 100 nm
regime. The base width is in the 10 nm range. High p-doping of GaAs with low diffusion of the dopant
is accomplished with carbon. Defects that would short emitter and collector at such thin base width
must be avoided.
An important figure of merit is the cutoff frequency f T for which h FE is unity in the common emitter
configuration. The cutoff frequency is related to the emitter–collector delay time τ EC by
f T =
1
2π τ EC
.
(24.32)
The delay time is determined by the charging time of the emitter–base depletion layer, the base capacitance, and the transport through the base–collector depletion layer. It is favorable if all times are short
and similar. It does not help to minimize only one or two of the three processes since the longest time
determines the transistor performance.
Another important figure of merit is the maximum frequency with which the transistor can oscillate
in a feedback circuit with zero loss. This frequency is denoted by f max . Approximately,
f max
f T
8π R B C CB
,
(24.33)
where R B is the base resistance and C CB is the collector–base capacitance. f max is larger than f T , by a
factor of the order of three.
24.2.7 Heterojunction Bipolar Transistors
In a heterojunction bipolar transistor (HBT), the emitter–base diode is formed with a heterostructure
diode. The desired functionality is obtained when the emitter is made from the higher-bandgap material