Q6Analog Electronics
Question
6 marks
(A) What are the main constructional differences between a MOSFET and a BJT? What effect do they have on the current conduction mechanism of a MOSFET? Also describe the types of MOSFET.
Answer
MOSFET uses a metal-oxide-semiconductor gate insulated by SiO2 (unlike BJT's p-n junction base), conducts via field-induced channel with majority carriers only, and exists in depletion and enhancement types.
- Gate insulation: MOSFET gate is separated from the semiconductor by a thin SiO2 insulating layer; BJT base is a p-n junction directly in contact.
- Terminals: MOSFET has Gate (G), Drain (D), Source (S), and Body (B); BJT has Base (B), Collector (C), Emitter (E).
- Channel: MOSFET current flows through an induced or pre-implanted channel parallel to the surface; BJT current flows through bulk semiconductor perpendicular to junctions.
- Current control: MOSFET — voltage-controlled (gate voltage); BJT — current-controlled (base current).
Since the gate is insulated, no gate current flows. The electric field from penetrates the oxide and creates an inversion layer (channel) of carriers. MOSFET is a unipolar device — only majority carriers (electrons for N-channel) flow from source to drain. No minority carrier injection occurs, unlike in BJT.
- N-channel Enhancement MOSFET (NMOS): No channel at ; channel induced for (Normally OFF).
- P-channel Enhancement MOSFET (PMOS): Channel induced for (Normally OFF).
- N-channel Depletion MOSFET: Channel pre-implanted; conducts at (Normally ON), depleted by negative .
- P-channel Depletion MOSFET: Channel pre-implanted; conducts at (Normally ON), depleted by positive .