Electronic Circuits Junction Field-effect Transistors Dr. Manar Mohaisen Office: F208 Email: manar.subhi@kut.ac.kr Department of EECE
Review of the Precedent Lecture Explain the Operation Class A Power Amplifier Explain the Operation of Class B and AB Amplifiers Explain The Operation of the Class C Amplifiers
Class Objectives Introduce the Junction Field-effect Transistor (JFET) Structure, characteristics and parameters, and biasing Introduce the Metal Oxide Semiconductor FET (MOSFET) Types, characteristics, power MOSFET, and biasing
JFET Two Types: n channel and p channel Junction Field-effect Transistor (JFET) The leads are connected to each end of the channel Drain: Connected to the upper end of the channel Source: Connected to the lower end of the channel In case of n channel JFET Two p-type regions are diffused in the n-type material to form a channel. In case of p channel JFET, two n-type regions are used.
Junction Field-effect Transistor (JFET) contd. Basic Operation V DD provides drain-to-source voltage. V GG sets the reverse-bias voltage between the gate and source. JFET is always operated with the gate-source junction reverse-biased.
Junction Field-effect Transistor (JFET) contd.
JFET Characteristics and Parameters Zero V GG Ohmic Region When V DS increases, I D will increase proportionally. Channel resistance is essentially constant. Active Region I D becomes essentially constant. Starting at the Pinch-Off Voltage, V p. V GD increases and creates a depletion region that offsets the increase in V GS.
JFET Characteristics and Parameters contd. Zero V GG contd. Breakdown The point at which I D starts to increase very rapidly with any increase in V DS. Usual Operation of the JFET In the active region.
JFET Characteristics and Parameters contd. V GS Controls I D I D decreases as the value of V GS increases. The depletion region is increased and the channel is narrowed.
JFET Characteristics and Parameters contd. Cutoff Voltage The value of V GS that makes I D approximately zero. Therefore, the JFET must be operated Between V GS = 0 V and V GS(off). I D will, therefore, vary between I DSS (max value) and zero.
JFET Characteristics and Parameters contd. Remarks The pinch-off voltage Vp is measured when V GS = 0. I D starts to be constant. I D is maximum and equals to I DSS. The pinch-off occurs for any value of V GS (less than 0) I D starts to be constant. I D is less than I DSS. V DS is less than Vp. Cutoff Voltage and Pinch-off voltage They are equal in magnitude and opposite in sign.
Example 8-1 V GS(off) = -4 V and I DSS = 12 ma. JFET Characteristics and Parameters contd. Determine the minimum value of V DD to put the device in the constant-current region of operation when V GS = 0V. V DS = V P = - V GS(off) = 4 V. I 12 ma D = IDSS = V = I R = (12mA)(560 Ω ) = 6.72 V R D D D V = 4 V 6.72V 10.72 V DD V + DS V = + = R D
JFET Universal Transfer Characteristic Transconductance curve. JFET Characteristics and Parameters contd. I = 0, when V = V D GS GS GS(off) I I = DSS, when V = 0.5V D 4 GS I I = DSS, when V = 0.3V D 2 GS and I = I, when V = 0 D DSS GS(off) GS(off)
JFET Characteristics and Parameters contd. Characteristic Curve It is expressed approximately by I D I DSS 1 V 2 VGS GS(off) JFETs and MOSFETs are known as square-law devices
The Forward Transconductance JFET Characteristics and Parameters contd. The change in drain current for a given change in gate-to-source voltage. ΔI g D m = Δ V GS g m m0 = g 1 V VGS GS(off) g m0 2I = V DSS GS(off) with g m0 as the minimum transconductance.
JFET Characteristics and Parameters contd. Input Resistance and Capacitance The JFET operates with a reverse-biased gate-source junction. Therefore, the gate resistance is very high. R IN = V I GS GSS With I GSS as the reverse gate-to-source current. AC Drain-to-Source Resistance Above pinch-off, the drain current is almost constant. The ac drain-to-source resistance is given by: r ' ds ΔV = Δ I DS D
JFET Biasing Self Biasing The gate resistor does not affect the bias because its voltage = 0. However, it forces the gate to be at 0 V and block ground ac signal.
JFET Biasing contd. Self Biasing contd. V = V V = 0 I R = I R GS G S D S D S V = V V V DS DD R R S D = V I ( R + R ) DD D S D
JFET Biasing contd. Example 8-6 Self Biasing V = V V = 15V (5mA)(1,0k Ω ) = 10 V D DD R D V = I R = (5mA)(220 Ω ) = 1.1V GS D S V = 15V 5V 1.1V 8.9 V DS V V V = = DD Rs R D
JFET Biasing contd. Self Biasing contd. Setting the Q-point We set Rs to achieve the required Q-point R s = V I GS D Note that: Midpoint Bias I D I DSS 1 V 2 VGS GS(off) 2 V /3.4 GS(off) 1 0.5 D DSS = V DSS GS(off) I I I
JFET Biasing contd. Self Biasing Example 8-9 I DSS = 1.0 ma, V GS(off) = -0.5 V Find Rs for midpoint bias. I 1.0 ma 0.5 ma 2 2 I = DSS = = D V GS VGS(off) = = 0.5 V = 147 mv 3.4 3.4 R V 147 mv 294 = GS = = Ω S ID 0.5 ma R V V = DD D = 12 kω D ID
JFET Biasing contd. Graphical Analysis of a Self-Biased JFET The Q-point can be found from the graph.
JFET Biasing contd. Voltage-Divider Bias V G = R R 2 + R 1 2 V DD V = V V = V I R GS G S G D S I D V = G V R S GS
JFET Biasing contd. Voltage-Divider Bias contd. V GS does not equal zero when I D = 0. (V GS = V G ) When V GS = 0. I D = V G / R S
JFET Biasing contd. Q-Point Stability Transfer characteristic differ considerably from a device to another.
JFET Biasing contd. Stability Self-bias versus voltage-divider bias I D is much more stable in case of voltage-divider bias.
JFET Biasing contd. Current-Source Bias Current-source method for increasing the Q-point stability. The emitter current is almost constant I E V V = EE BE R D E E I I
The MOSFET Metal Oxide Semiconductor FET (MOSFET) The MOSFET does not have a pn junction The gate is insulated from the channel by a silicon dioxide (SiO2) Basic Types of MOSFET Enhancement MOSFET (E-MOSFET) Depletion MOSFET (D-MOSFET)
The MOSFET contd. Enhancement MOSFET (E-MOSFET) It does not have structural channel. The +ve gate voltage must be above a threshold to induce a channel. V DD pulls the electrons and makes the current flow.
The MOSFET contd. Depletion MOSFEST (D-MOSFET) It has a structural channel. It has two modes of operation (n-channel) Depletion mode: When a negative gate-to-source is applied (General operation mode.) Enhancement mode: When a positive gate-to-source is applied. The p-channel is similar with reversed voltage polarities.
The MOSFET contd. Depletion MOSFEST (D-MOSFET) contd. Depletion Mode The gate is a plate and the channel as the other plate of a capacitor. Negative voltage at the gate will deplete electrons from the channel. Therefore, the conductivity of the channel will be reduced. At a sufficiently large gate voltage, V GS(off), I D is zero.
The MOSFET contd. Depletion MOSFEST (D-MOSFET) contd. Enhancement Mode Positive voltage at the gate will attract more electrons from the channel. Therefore, the conductivity of the channel will be increased.
Power MOSFET Structures Conventional Enhancement MOSFET (E-MOSFET) The channel is long, therefore the resistance is high between the drain and the source. Not designed for power applications. Laterally Diffused MOSFET (LDMOSFET) It has a lateral channel structure designed for power applications. Shorter channel bet. drain and source results in lower resistance. +ve gate voltage induces a very short n channel. E-MOSFET LDMOSFET
Power MOSFET Structures contd. The V-Groove MOSFET (VMOSFET) Designed for high power applications Short (Vertical) and wide channel between D and S is created It has 2 source connections. The TMOSFET Short (Vertical) and wide channel between D and S is created
E-MOSFET General Transfer Char. Curves MOSFET Characteristics and Parameters The Drain Current I = K( V V ) D GS GS(th) K is a constant dependant on the particular MOSFET. 2
D-MOSFET General Transfer Char. Curves MOSFET Characteristics and Parameters The Drain Current I D I DSS 1 V 2 VGS GS(off)
MOSFET Biasing E-MOSFET Bias I = K( V V ) D GS GS(th) 2 Voltage-divider bias Drain-feedback bias V GS DS = R 2 R + R 1 2 V V = V I R DD DD D D V GS = V DS
MOSFET Biasing contd. D-MOSFET Bias Zero-biased D-MOSFET No AC source V GS = 0. AC Operation AC input varies V GS to positive and negative values.
Lecture Summary Introduced the Junction Field-effect Transistor (JFET) Structure, characteristics and parameters, and biasing Introduced the Metal Oxide Semiconductor FET (MOSFET) Types, characteristics, power MOSFET, and biasing
Discussion & Notes K K A K A A A A K K K K A K A K K A K A K A