ITT Technical Institute ET215 Devices 1 Unit 8 Chapter 4, Sections 4.4 4.5
Chapter 4 Section 4.4 MOSFET Characteristics A Metal-Oxide semiconductor field-effect transistor is the other major category of field-effect transistors. It differs from the JFET in that it has no pn junction structure; instead, the gate of the MOSFET is insulated from the channel by a very thin silicon dioxide (SiO 2 ) layer. The two basic types of MOSFETs are depletion (D) and enhancement (E).
MOSFET Characteristics Depletion MOSFET (D-MOSFET) The drain and source are diffused into the substrate material and then connected by a narrow channel adjacent to the insulated gate. P-channel D-MOSFETs are not widely used. D-MOSFETs can operate in one of two modes: depletion mode or enhancement mode. Also called depletion-enhancement MOSFET
MOSFET Characteristics Depletion MOSFET (D-MOSFET) The n-channel D-MOSFET operates in the depletion mode when a negative gate-to-source voltage is applied and in the enhancement mode when a positive gate-to-source voltage is applied. Depletion Mode: when a negative gate-tosource voltage is applied, the gate repels conductive electrons and thereby decreasing the electrons in the channel, leaving positive ions in their place. The greater the negative voltage, the greater the depletion of n-channel electrons. As sufficient negative gate-to-source voltage is applied, V GS(off), the area is totally depleted and drain current is zero.
MOSFET Characteristics Depletion MOSFET (D-MOSFET) Enhancement Mode: with an n-channel device and with a positive gate voltage, more conduction electrons are attracted into the channel, thus increasing (enhancing) the channel conductivity.
MOSFET Characteristics D-MOSFET Symbols & Transfer Characteristic
MOSFET Characteristics Enhancement MOSFET (E-MOSFET) This type of MOSFET operates ONLY in the enhancement mode and has no depletion mode. It differs in construction from the D-MOSFET in that it has no physical channel. For an n-channel device, a positive gate voltage above the threshold value, V GS(th), induces a channel by creating a thin layer of negative charges in the substrate region adjacent to the SiO 2 layer.
The gate of an n-channel device must be made positive in order to cause conduction. MOSFET Characteristics E-MOSFET Symbols & Transfer Characteristic Notice: broken lines symbolize the absence of a physical channel.
MOSFET Characteristics Enhancement MOSFET (E-MOSFET) Because the channel is closed unless a voltage is applied, to the gate, an E-MOSFET can be thought of as a normally off device. Handling Precautions To avoid ESD and possible damage, precautions should be taken: 1) Ship and store in conductive foam 2) Use proper ESD bench grounding techniques 3) Use proper ESD wrist-straps 4) Never remove a MOS device while power is on 5) Do not apply signals to a MOS device while the dc power supply is off
Chapter 4 Section 4.5 D-MOSFET Biasing Biasing a D-MOSFET An n-channel E-MOSFET can be with either a positive or negative voltage applied to the gate. It is the only transistor that can do this. Zero Bias: set V GS = 0 to the gate. This is the preferred way to bias a D-MOSFET. Drain-to-Source voltage is expressed as: V DS = V DD - I DSS R D
D-MOSFET Biasing Example: Determine the drain-to-source voltage in the circuit. The MOSFET data sheet gives I DSS = 12mA. Since I D = I DSS = 12 ma V DS = V DD I DSS R D = 18 V (12 ma)(560ω) = 11.28 V
D-MOSFET Biasing Other Bias Arrangements: Because the D-MOSFET can operate in depletion or enhancement mode, we can bias with any technique studied with BJTs or other JFETs.
E-MOSFET Biasing E-MOSFETs must have a V GS greater than the threshold value V GS(th). Any bias used for BJTs (except base bias) could be used with appropriate values for E-MOSFETs.
E-MOSFET Biasing Example: Determine the amount of drain current in the circuit. The E-MOSFET has a V GS(in) = 3 V. The meter indicates that V GS = 8.5 V. I D = (V DD V DS ) / R D = (15 V 8.5 V) / 4.7Ω = 1.38 ma