Another family of transistors is known as metal-oxide semiconductor field-effect transistors (MOSFETs) of which there are two basic types called n-channel and p-channel; once again these names relate to the way in which the silicon is doped (Figure 10.7).
Figure 10.7 Metal-oxide semiconductor field-effect transistors (MOSFETs) (a) NMOS field-effect transistor; (b) PMOS field-effect transistor
In the case of these devices, the drain and source form the data terminals and the gate acts as the control terminal. Unlike bipolar devices, the control terminal is connected to a conducting plate, which is insulated from the silicon by a layer of non-conducting oxide. In the original devices the conducting plate was metal—hence, the term metal-oxide. When a signal is applied to the gate terminal, the plate, insulated by the oxide, creates an electromagnetic field, which turns the transistor ON or OFF—hence, the term field-effect.
Now this is the bit that always confuses the unwary, because the term channel refers to the piece of silicon under the gate terminal, that is, the piece linking the drain and source regions. But the channel in the n-channel device is formed from P-type material, while the channel in the p-channel device is formed from N-type material.
At first glance, this would appear to be totally counterintuitive, but there is reason behind the madness. Let’s consider the n-channel device. In order to turn this ON, a positive voltage is applied to the gate. This positive voltage attracts negative electrons in the P-type material and causes them to accumulate beneath the oxide layer where they form a negative channel—hence, the term n-channel. In fact, saying “n-channel” and “p-channel” is a bit of a mouthful, so instead we typically just refer to these as NMOS and PMOS transistors, respectively.
This chapter concentrates on MOSFETs, because their symbols, construction, and operation are easier to understand than those of bipolar junction transistors.
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