Junction field-effect transistors

Chapter 2.9 Junction field-effect transistors

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 2.9. JUNCTION FIELD-EFFECT TRANSISTORS65tegrated PNP transistors are not nearly a good as the NPN variety within the same integratedcircuit die. Thus, integrated circuits use the NPN variety as much as possible.• REVIEW:• Bipolar transistors conduct current using both electrons and holes in the same device.• Operation of a bipolar transistor as a current amplifier requires that the collector-basejunction be reverse biased and the emitter-base junction be forward biased.• A transistor differs from a pair of back to back diodes in that the base, the center layer, isvery thin. This allows majority carriers from the emitter to diffuse as minority carriersthrough the base into the depletion region of the base-collector junction, where the strongelectric field collects them.• Emitter efficiency is improved by heavier doping compared with the collector. Emitterefficiency: α = IC/IE, 0.99 for small signal devices• Current gain is β=IC/IB, 100 to 300 for small signal transistors.2.9Junction field-effect transistorsThe field effect transistor was proposed by Julius Lilienfeld in US patents in 1926 and 1933(1,900,018). Moreover, Shockley, Brattain, and Bardeen were investigating the field effecttransistor in 1947. Though, the extreme difficulties sidetracked them into inventing the bipolartransistor instead. Shockley’s field effect transistor theory was published in 1952. However, thematerials processing technology was not mature enough until 1960 when John Atalla produceda working device.A field effect transistor(FET) is a unipolardevice, conducting a current using only onekind of charge carrier. If based on an N-type slab of semiconductor, the carriers are electrons.Conversely, a P-type based device uses only holes.At the circuit level, field effect transistor operation is simple. A voltage applied to the gate,input element, controls the resistance of the channel, the unipolar region between the gateregions. (Figure 75,2.38) In an N-channel device, this is a lightly doped N-type slab of siliconwith terminals at the ends. The sourceand drainterminals are analogous to the emitter andcollector, respectively, of a BJT. In an N-channel device, a heavy P-type region on both sides ofthe center of the slab serves as a control electrode, the gate. The gate is analogous to the baseof a BJT.“Cleanliness is next to godliness” applies to the manufacture of field effect transistors.Though it is possible to make bipolar transistors outside of a clean room, it is a necessityfor field effect transistors. Even in such an environment, manufacture is tricky because ofcontamination control issues. The unipolar field effect transistor is conceptually simple, butdifficult to manufacture. Most transistors today are a metal oxide semiconductor variety (latersection) of the field effect transistor contained within integrated circuits. However, discreteJFET devices are available.A properly biased N-channel junction field effect transistor (JFET) is shown in Figure 75,2.38.The gate constitutes a diode junction to the source to drain semiconductor slab. The gate is