This preview shows pages 1–2. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: ECE 323 Winter 2009 A Pocket Guide to MOSFETs January 29, 2009 There are several different symbols that are traditionally used to represent enhance- ment MOSFETs, depending on the applica- tion and the manufacturing process used for the transistor. In analog integrated circuits , the two symbols shown on the top line of Fig. 1 are commonly used. In this case, the arrow indicates which terminal is designated the source terminal and the direction of the arrow indicates whether the transistor is an integrated circuit symbols G D S G S D i D i D i D i D NMOS PMOS G NMOS PMOS discrete part symbols D S B i D i D i D i D G D S B logic circuit symbols D or S S or D G D or S S or D G NMOS PMOS Figure 1: Circuit symbols used to repre- sent enhancement MOSFETs. NMOS or PMOS 1 transistor. These symbols are most like the BJT npn and pnp symbols with which we are familiar. As reflected by the similarity in the sym- bols, the gate terminal serves a function very similar to the base terminal of a BJT and the source terminal serves a function very simi- lar to the BJT emitter. Continuing the anal- ogy, the gate-source voltage is used to control the drain current much in the same way the base-emitter voltage is used to control the collector current in a BJT. The primary dif- ferences between BJTs and MOSFETs are that no current flows through the gate of a MOSFET and the voltage control of the cur- rent is quadratic in nature instead of expo- nential. Because no gate current flows in a MOS- FET, the currents at the drain and source terminals are identical. Because the currents are identical in magnitude, they are both la- beled i D and called the drain current. The direction of positive current flow is indicated by the arrow shown on the 3 terminal sym- bols shown in the top row of Fig. 1. For ex- ample, the current flows into the drain termi- nal of an NMOS transistor, through the tran- sistor and out of the source terminal. On the other hand, the positive current flows into the source terminal of a PMOS transistor, 1 The term complementary MOS (CMOS), refers more to a technology than a specific type of MOSFET transistor. In fact, CMOS technology is defined by the ability for both NMOS and PMOS transistors to be incorporated on the the same integrated circuit chip....
View Full Document