Environmental Engineering Reference
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the base and reducing the lateral distance between the emitter and
base contacts. The thermal noise generated by other parasitic resis-
tances of bipolar junction transistors, such as emitter and collector
bulk resistances, also constitute the overall noise of the device. How-
ever, because the emitter is heavily doped, the associated thermal
noise is small [99]. The thermal noise originating from the collector
resistance is often surpassed by the noise of collector loads. The noise
equivalent circuit of BJTs in the forward active region is given in Fig.
11.25. When a BJT is operated in an ON/OFF mode, its equivalent
circuit can also be obtained by including the above identified noise
sources in the large-signal equivalent circuit of BJTs.
MOSFET Transistors
: MOSFET transistors are building elements
of switched capacitor and switched current circuits. Recent advance
in CMOS technology has also made CMOS a viable technology choice
for high-speed and RF applications. Noise generated by the intrinsic
part of a MOSFET transistor consists of (1) shot noise in the gate
leakage current, (2) thermal noise due to the random thermal motion
of mobile carriers in the inversion layer and (3) flicker noise due to
channel charge density fluctuation caused by the traps at the oxide-
silicon interface [100]. The shot noise of the gate leakage current is
usually negligible. To ensure a stable operation, MOSFET transistors
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