Insulated-gate field-effect transistors (MOSFET)

Chapter 2.10 Insulated-gate field-effect transistors (MOSFET)

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

  • 70CHAPTER 2. SOLID-STATE DEVICE THEORYThe Metal semiconductor field effect transistor (MESFET)is similar to a JFET except thegate is a schottky diode instead of a junction diode. A schottky diodeis a metal rectifyingcontact to a semiconductor compared with a more common ohmic contact. In Figure 78,2.44 thesource and drain are heavily doped (N+). The channel is lightly doped (N−). MESFET’s arehigher speed than JFET’s. The MESET is a depletion mode device, normally on, like a JFET.They are used as microwave power amplifiers to 30 gHz. MESFET’s can be fabricated from sil-icon, gallium arsenide, indium phosphide, silicon carbide, and the diamond allotrope of carbon.• REVIEW:• The unipolar junction field effect transistor (FET or JFET) is so called because conductionin the channel is due to one type of carrier• The JFET source, gate, and drain correspond to the BJT’s emitter, base, and collector,respectively.• Application of reverse bias to the gate varies the channel resistance by expanding thegate diode depletion region.2.10Insulated-gate field-effect transistors (MOSFET)The insulated-gate field-effect transistor(IGFET), also known as the metal oxide field effecttransistor(MOSFET), is a derivative of the field effect transistor (FET). Today, most transis-tors are of the MOSFET type as components of digital integrated circuits. Though discreteBJT’s are more numerous than discrete MOSFET’s. The MOSFET transistor count within anintegrated circuit may approach hundreds of a million. The dimensions of individual MOSFETdevices are under a micron, decreasing every 18 months. Much larger MOSFET’s are capableof switching nearly 100 amperes of current at low voltages; some handle nearly 1000 V at lowercurrents. These devices occupy a good fraction of a square centimeter of silicon. MOSFET’sfind much wider application than JFET’s. However, MOSFET power devices are not as widelyused as bipolar junction transistors at this time.The MOSFET has source, gate, and drain terminals like the FET. However, the gate leaddoes not make a direct connection to the silicon compared with the case for the FET. TheMOSFET gate is a metallic or polysilicon layer atop a silicon dioxide insulator. The gate bearsa resemblance to a metal oxide semiconductor(MOS) capacitor in Figure 80,2.45. When charged,the plates of the capacitor take on the charge polarity of the respective battery terminals.The lower plate is P-type silicon from which electrons are repelled by the negative (-) batteryterminal toward the oxide, and attracted by the positive (+) top plate. This excess of electronsnear the oxide creates an inverted (excess of electrons) channel under the oxide. This channelis also accompanied by a depletion region isolating the channel from the bulk silicon substrate.In Figure 80,2.46 (a) the MOS capacitor is placed between a pair of N-type diffusions in a P-type substrate. With no charge on the capacitor, no bias on the gate, the N-type diffusions, thesource and drain, remain electrically isolated.A positive bias applied to the gate, charges the capacitor (the gate). The gate atop the oxidetakes on a positive charge from the gate bias battery. The P-type substrate below the gate takeson a negative charge. An inversion region with an excess of electrons forms below the gate