Chapter 2 Field-Effect Transistors (FETs) SJTU Zhou Lingling

Outline Introduction Device Structure and Physical Operation Current-Voltage Characteristics MOSFET Circuit at DC The MOSFET as an amplifier Biasing in MOS Amplifier Circuits Small-signal Operation and Models Single-Stage MOS amplifier The MOSFET Internal Capacitance and High-Frequency Model The depletion-type MOSFET SJTU Zhou Lingling

Introduction Characteristics Far more useful than two-terminal device. Voltage between two terminals can control the current flows in third terminal. Quite small. Low power. Simple manufacturing process. SJTU Zhou Lingling

Introduction Classification of MOSFET Widely used in IC circuits P channel Enhancement type Depletion type N channel JFET Widely used in IC circuits SJTU Zhou Lingling

Device Structure and Physical Operation Device structure of the enhancement NMOS Physical operation p channel device SJTU Zhou Lingling

Device Structure of the Enhancement-Type NMOS Perspective view Four terminals Channel length and width SJTU Zhou Lingling

Device Structure of the Enhancement-Type NMOS Cross-section view. L = 0.1 to 3 mm W = 0.2 to 100 mm Tox= 2 to 50 nm SJTU Zhou Lingling

Drain current controlled by vDS Drain current controlled by vGS Physical Operation Creating an n channel Drain current controlled by vDS Drain current controlled by vGS SJTU Zhou Lingling

Creating a Channel for Current Flow The enhancement-type NMOS transistor with a positive voltage applied to the gate. An n channel is induced at the top of the substrate beneath the gate. Inversion layer Threshold voltage SJTU Zhou Lingling

Drain Current Controlled by Small Voltage vDS An NMOS transistor with vGS > Vt and with a small vDS applied. The channel depth is uniform. The device acts as a resistance. The channel conductance is proportional to effective voltage. Drain current is proportional to (vGS – Vt) vDS. SJTU Zhou Lingling

vDS Increased Operation of the enhancement NMOS transistor as vDS is increased. The induced channel acquires a tapered shape. Channel resistance increases as vDS is increased. Drain current is controlled by both of the two voltages. SJTU Zhou Lingling

Channel Pinched Off Channel is pinched off Inversion layer disappeared at the drain point Drain current isn’t disappeared Drain current is saturated and only controlled by the vGS Triode region and saturation region Channel length modulation SJTU Zhou Lingling

Drain Current Controlled by vGS vGS creates the channel. Increasing vGS will increase the conductance of the channel. At saturation region only the vGS controls the drain current. At subthreshold region, drain current has the exponential relationship with vGS SJTU Zhou Lingling

p Channel Device Two reasons for readers to be familiar with p channel device Existence in discrete-circuit. More important reason is the utilization of CMOS circuits. Structure of p channel device The substrate is n type and the inversion layer is p type. Carrier is hole. Threshold voltage is negative. All the voltages and currents are opposite to the ones of n channel device. Physical operation is similar to that of n channel device. SJTU Zhou Lingling

Complementary MOS or CMOS The PMOS transistor is formed in n well. Another arrangement is also possible in which an n-type body is used and the n device is formed in a p well. CMOS is the most widely used of all the analog and digital IC circuits. SJTU Zhou Lingling

Current-Voltage Characteristics Circuit symbol Output characteristic curves Channel length modulation Characteristics of p channel device Body effect Temperature effects and Breakdown Region SJTU Zhou Lingling

Circuit Symbol Circuit symbol for the n-channel enhancement-type MOSFET. Modified circuit symbol with an arrowhead on the source terminal to distinguish it from the drain and to indicate device polarity (i.e., n channel). (c) Simplified circuit symbol to be used when the source is connected to the body or when the effect of the body on device operation is unimportant. SJTU Zhou Lingling

Output Characteristic Curves An n-channel enhancement-type MOSFET with vGS and vDS applied and with the normal directions of current flow indicated. The iD–vDS characteristics for a device with k’n (W/L) = 1.0 mA/V2. SJTU Zhou Lingling

Output Characteristic Curves Three distinct region Cutoff region Triode region Saturation region Characteristic equations Circuit model SJTU Zhou Lingling

Cutoff Region Biased voltage The transistor is turned off. Operating in cutoff region as a switch. SJTU Zhou Lingling

Triode Region Biased voltage The channel depth from uniform to tapered shape. Drain current is controlled not only by vDS but also by vGS SJTU Zhou Lingling

Triode Region Assuming that the drain-t-source voltage is sufficiently small. The MOS operates as a linear resistance SJTU Zhou Lingling

Saturation Region Biased voltage The channel is pinched off. Drain current is controlled only by vGS Drain current is independent of vDS and behaves as an ideal current source. SJTU Zhou Lingling

Saturation Region The iD–vGS characteristic for an enhancement-type NMOS transistor in saturation Vt = 1 V, k’n W/L = 1.0 mA/V2 Square law of iD–vGS characteristic curve. SJTU Zhou Lingling

Channel Length Modulation Explanation for channel length modulation Pinched point moves to source terminal with the voltage vDS increased. Effective channel length reduced Channel resistance decreased Drain current increases with the voltage vDS increased. Current drain is modified by the channel length modulation SJTU Zhou Lingling

Channel Length Modulation The MOSFET parameter VA depends on the process technology and, for a given process, is proportional to the channel length L. SJTU Zhou Lingling

Channel Length Modulation MOS transistors don’t behave an ideal current source due to channel length modulation. The output resistance is finite. The output resistance is inversely proportional to the drain current. SJTU Zhou Lingling

Characteristics of p Channel Device Circuit symbol for the p-channel enhancement-type MOSFET. Modified symbol with an arrowhead on the source lead. Simplified circuit symbol for the case where the source is connected to the body. SJTU Zhou Lingling

Characteristics of p Channel Device The MOSFET with voltages applied and the directions of current flow indicated. The relative levels of the terminal voltages of the enhancement-type PMOS transistor for operation in the triode region and in the saturation region. SJTU Zhou Lingling

MOSFET Circuit at DC Assuming device operates in saturation thus iD satisfies with iD~vGS equation. According to biasing method, write voltage loop equation. Combining above two equations and solve these equations. Usually we can get two value of vGS, only the one of two has physical meaning. Checking the value of vDS if vDS≥vGS-Vt, the assuming is correct. if vDS≤vGS-Vt, the assuming is not correct. We shall use triode region equation to solve the problem again. SJTU Zhou Lingling

MOSFET Circuit at DC The NMOS transistor is operating in the saturation region due to SJTU Zhou Lingling

MOSFET Circuit at DC Assuming the MOSFET operate in the saturation region Checking the validity of the assumption If not to be valid, solve the problem again for triode region SJTU Zhou Lingling

The MOSFET As an Amplifier Basic structure of the common-source amplifier. Graphical construction to determine the transfer characteristic of the amplifier in (a). SJTU Zhou Lingling

The MOSFET As an Amplifier Transfer characteristic showing operation as an amplifier biased at point Q. Three segments: XA---the cutoff region segment AQB---the saturation region segment BC---the triode region segment SJTU Zhou Lingling

Biasing in MOS Amplifier Circuits Voltage biasing scheme Current-source biasing scheme SJTU Zhou Lingling

Biasing in MOS Amplifier Circuits (d) coupling of a signal source to the gate using a capacitor CC1; (e) practical implementation using two supplies. SJTU Zhou Lingling

Biasing in MOS Amplifier Circuits Biasing the MOSFET using a constant-current source I. Implementation of the constant-current source I using a current mirror. SJTU Zhou Lingling

Small-Signal Operation and Models The ac characteristic Definition of transconductance Definition of output resistance Definition of voltage gain Small-signal model Hybrid π model T model Modeling the body effect SJTU Zhou Lingling

The ac Characteristics The definition of transconductance The definition of output resistance The definition of voltage gain SJTU Zhou Lingling

The Small-Signal Models neglecting the the channel-length modulation effect including the effect of channel-length modulation, modeled by output resistance ro = |VA| /ID. SJTU Zhou Lingling

The Small-Signal Models The T model of the MOSFET augmented with the drain-to-source resistance ro. An alternative representation of the T model. SJTU Zhou Lingling

Single-Stage MOS Amplifier Characteristic parameters Basic structure Three configurations Common-source configuration Common-drain configuration Common-gate configuration SJTU Zhou Lingling

Characteristic Parameters of Amplifier This is the two-port network of amplifier. Voltage signal source. Output signal is obtained from the load resistor. SJTU Zhou Lingling

Definitions Input resistance with no load Input resistance Open-circuit voltage gain Voltage gain SJTU Zhou Lingling

Definitions(cont’d) Short-circuit current gain Current gain Short-circuit transconductance gain SJTU Zhou Lingling

Definitions(cont’d) Open-circuit overall voltage gain SJTU Zhou Lingling

Definitions(cont’d) Output resistance of amplifier proper SJTU Zhou Lingling

Definitions(cont’d) Voltage amplifier SJTU Zhou Lingling

Definitions(cont’d) Transconductance amplifier SJTU Zhou Lingling

Relationships Voltage divided coefficient SJTU Zhou Lingling

Basic Structure of the Circuit Basic structure of the circuit used to realize single-stage discrete-circuit MOS amplifier configurations. SJTU Zhou Lingling

The Common-Source Amplifier Common-source amplifier based on the circuit of basic structure. Biasing with constant-current source. CC1 And CC2 are coupling capacitors. CS is the bypass capacitor. SJTU Zhou Lingling

Equivalent Circuit of the CS Amplifier SJTU Zhou Lingling

Equivalent Circuit of the CS Amplifier Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized. SJTU Zhou Lingling

Characteristics of CS Amplifier Input resistance Voltage gain Overall voltage gain Output resistance SJTU Zhou Lingling

Summary of CS Amplifier Very high input resistance Moderately high voltage gain Relatively high output resistance SJTU Zhou Lingling

The Common-Source Amplifier with a Source Resistance SJTU Zhou Lingling

Small-signal Equivalent Circuit with ro Neglected SJTU Zhou Lingling

Characteristics of CS Amplifier with a Source Resistance Input resistance Voltage gain Overall voltage gain Output resistance SJTU Zhou Lingling

Summary of CS Amplifier with a Source Resistance Including RS results in a gain reduction by the factor (1+gmRS) RS takes the effect of negative feedback. SJTU Zhou Lingling

The Common-Gate Amplifier Biasing with constant current source I Input signal vsig is applied to the source Output is taken at the drain Gate is signal grounded CC1 and CC2 are coupling capacitors SJTU Zhou Lingling

The Common-Gate Amplifier A small-signal equivalent circuit of the amplifier in fig. (a). T model is used in preference to the π model Neglecting ro SJTU Zhou Lingling

The Common-Gate Amplifier Fed with a Current-Signal Input SJTU Zhou Lingling

Characteristics of CG Amplifier Input resistance Voltage gain Overall voltage gain Output resistance SJTU Zhou Lingling

Summary of CG Amplifier Noninverting amplifier Low input resistance Has nearly identical voltage gain of CS amplifier, but the overall voltage gain is smaller by the factor (1+gmRsig） Relatively high output resistance Current follower Superior high-frequency performance SJTU Zhou Lingling

The Common-Drain or Source-Follower Amplifier Biasing with current source Input signal is applied to gate, output signal is taken at the source. SJTU Zhou Lingling

The Common-Drain or Source-Follower Amplifier Small-signal equivalent-circuit model T model makes analysis simpler Drain is signal grounded SJTU Zhou Lingling

Small-Signal Analysis Performed Directly on the Circuit SJTU Zhou Lingling

Circuit for Determining the Output Resistance of CD Amplifier SJTU Zhou Lingling

Characteristics of CD Amplifier Input resistance Voltage gain Overall voltage gain Output resistance SJTU Zhou Lingling

Summary of CD or Source-Follow Amplifier Very high input resistance Voltage gain is less than but close to unity Relatively low output resistance Voltage buffer amplifier Power amplifier SJTU Zhou Lingling

Summary and Comparisons The CS amplifier is the best suited for obtaining the bulk of gain required in an amplifier. Including resistance RS in the source lead of CS amplifier provides a number of improvements in its performance. The low input resistance of CG amplifier makes it useful only in specific application. It has excellent high-frequency response. Can be used as a current buffer. Source follower finds application as a voltage buffer and as the output stage in a multistage amplifier. SJTU Zhou Lingling

High-Frequency Model (b) The equivalent circuit for the case in which the source is connected to the substrate (body). (c) The equivalent circuit model of (b) with Cdb neglected (to simplify analysis). SJTU Zhou Lingling

The MOSFET Unity-Gain Frequency Current gain Unity-gain frequency SJTU Zhou Lingling

Frequency Response of the CS Amplifier Capacitively coupled common-source amplifier. SJTU Zhou Lingling

Frequency Response of the CS Amplifier A sketch of the frequency response of the amplifier in (a) delineating the three frequency bands of interest. SJTU Zhou Lingling

The High-frequency Response Equivalent Circuit SJTU Zhou Lingling

The High-frequency Response The circuit of (a) simplified at the input and the output SJTU Zhou Lingling

The High-frequency Response The equivalent circuit with Cgd replaced at the input side with the equivalent capacitance Ceq; SJTU Zhou Lingling

The High-frequency Response The frequency response plot, which is that of a low-pass single-time-constant circuit. SJTU Zhou Lingling

The High-frequency Response A large value of will cause to be lowered. Although is a very small capacitance, its effect on the amplifier frequency response can be very significant as a result of its multiplication by the factor . This effect is known as Miller effect. To extend the high-frequency response of a MOSFET amplifier, we have to find configuration in which the Miller effect is absent or at least reduced. SJTU Zhou Lingling

The Depletion-Type MOSFET Circuits symbol Structure Characteristic curves SJTU Zhou Lingling

Circuit Symbol for the n-Channel Depletion-Type MOSFET Simplified circuit symbol applicable for the case the substrate (B) is connected to the source (S). SJTU Zhou Lingling

Physical Structure The structure of depletion-type MOSFET is similar to that of enhancement-type MOSFET with one important difference: the depletion-type MOSFET has a physically implanted channel. There is no need to induce a channel. The depletion MOSFET can be operated at both enhancement mode and depletion mode. SJTU Zhou Lingling

Characteristic Curves Transistor with current and voltage polarities indicated. Typical value for discrete transistor: Vt = –4 V and k¢n(W/L) = 2 mA/V2 SJTU Zhou Lingling

The Output Characteristic Curves SJTU Zhou Lingling

The iD–vGS Characteristic in Saturation Expression of characteristic equation Drain current with SJTU Zhou Lingling

The iD–vGS Characteristic in Saturation Sketches of the iD–vGS characteristics for MOSFETs of enhancement and depletion types The characteristic curves intersect the vGS axis at Vt. SJTU Zhou Lingling