Three Terminal Devices - field effect transistor (FET)

- bipolar junction transistor (BJT)

- foundation on which modern electronics is built

- active devices

- devices described completely by considering only two ports

i.e.: V3 = V2 - V1

- terminal characteristics of ports usually interdependent. One input and the other output.

Figure 5.1 A threeterminal device canbe described completelyby considering onlytwo of its three ports

Field Effect Transistor

- real FET has three terminal

G = gate

S = source

D = drain

G - S => input port

D - S => output port

VGS controls D - S terminal characteristics and gives family of curves.Only one curve is valid at any given instant.

Field Effect Transistor (con’t)

Figure 5.4 Family of output port v-i characteristics for a typical three-terminal FET

Simple Bias Circuit

- as V1 is changed so is VGS

- V - I characteristics of output port change from one curve to another

Figure 5.5 Output port of three terminal FET connectedto a simple Thevenin circuit;input port connected to voltagesource v1

Simple Biased Circuit (con’t)

Figure 5.6 Load line of circuit of Fig. 5.5 superimosed on the FET output port v-i characteristic

Figure 5.7 Several operating points obtainedgraphically and enteredinto table 5.1

- VTR => turn on voltage

- basic switch function

Figure 5.8Values foundin Fig. 5.7plotted as aninput-output transfercharcteristic

FET

- family of devices

- metal-oxide semiconductor field effect transistor (MOSFET)

- metal semiconductor field effect transistor (MESFET)

- junction field effect transistor (JFET)

- all devices have similar output port V-I characteristics

- all devices operate on fundamental physical mechanism called

“electric field effect”

- two types of devices

n-channel

p-channel

same operation with opposite polarities for voltages and currents

- we will focus on n-channel devices

Physical Structure of MOSFET

Figure 5.9 Schematic cross-section of n-channel MOSFET a) side view; b) top view. The length L and thewidth W of the gate region define the device geometry. The insulating oxide layer between the gate and sub-strate has thickness tOX

Physical Structure of a MOSFET (con’t)

Physical Structure of MOSFET (con’t)

- highly doped n+ implant forms drain and source in p-type substrate

- gate consists of metallic electrode that covers an insulating oxide layerover substrate

- extremely high gate input impedance

- gate electrode width and length determine “active channel”

- active channel along with oxide thickness help determine V-Icharacteristics

- if small voltage applied between drain and source, with no voltageapplied to gate, MOSFET behaves like two PN junctions connectedback to back

- one diode is reversed biased so that no current flows. It does howeverform a depletion region

- if voltage is applied between gate and source, MOSFET can conductcurrent

- voltage applied to gate forms electric field between gate and substrate

- negative acceptor ions are exposed as mobile holes are push away

Figure 5.10 The electric field in the oxide layer originates on the positive gate and terminates in thenegative charge at the substrate surface. When the field becomes strong enough, an inversion layer ofmobile free electrons forms beneath the substrate surface.

- as VGS is increased, reach point where there are a maximum number ofexposed acceptor ion cores near the surface

- if gate voltage is further increased, surface charge must be augmentedby mobile electrons that originate from remote areas or n+ region

- forms thin layer of electrons called inversion layer which behaves liken-type region

- VGS voltage when inversion layer formed is called “threshold voltage”VTR of VT

- inversion layer isolated by bound ion cores

- inversion layer bridges D - S to form a conduction path

- density of electrons in inversion layer is dependent on VGS. Initiallyuniform layer which results in triode region operation

Constant Current

- as VDS becomes comparable to VGS, the channel resistance becomesnon-linear

- voltage at any point along substrate will lie between voltage at drainand source

- magnitude in electric field equal to difference in gate and substratevoltage

- magnitude is maximum at source and decreases as you move towardsdrain

- if VDS > VGS - VTR inversion layer cannot form at drain. Voltage acrossinversion channel remains constant

Constant Current (con’t)

Figure 5.12 MOSFET with vDS > vGS - VTR. The inversion layer stops just to the left of thedrain-to-substrate depletion region at point X

- since channel voltage remains constant for VDS > VGS - VTR the current ID becomes constant

Figure 5.14 Voltage-current characteristics of a typical enhancement-mode MOSFET with K = 0.5mA/V2

and VTR = 2V

Constant Current

ID = k(VGS - VTR)2 VDS > VGS - VTR

VGS > VTR

Triode (Resistive)

ID = k[2(VGS - VTR)VDS - VDS2] 0 < VDS < (VGS - VTR)

VGS > VTR

Cut-off

ID = 0 VGS < VTR

- act as open circuits (i.e.: no currents)

- at high frequency gate capacitance takes effect and no longer opencircuit

k = conductance parameter

k = W

L => typical values: 0.05 to 50 mA / V

= electron mobility = dielectric permittivity

W = width of channel t = oxide thickness

L = length of channel

E 2

E OX

OX

µ ε

µ ε2

OX

OXt

Figure 5.15 Voltage-current characteristic of MOSFET gate-to-source input port

Depletion Mode MOSFET- negative threshold voltage

- modify enhancement mode device by implanting layer of donor ionsinto substrate under gate

- donor ions form conduction channel even when VGS = 0

- require negative VGS to shut devices off

- same equations as enhancements mode device describe behavior

Figure 5.16Circuit symboland v-icharacteristic oftypical depletionmode MOSFETwith K = 0.5mA/V2

VTR = -4V

Junction Field Effect Transistor- V-I characteristics similar to those of MOSFET

- n-channel JFET made by diffusing p-type region into n-type channel

- source and drain made by connecting to either side of channel with ohmic contact- gate connected to p-type region- acts like resistance between drain and source and PN junction between gate and channel- normal operation reverse-bias gate -to- channel. As voltage increases it widens depletion region

- increasing depletion region reduces cross-section of channel

- drain and source function as variable resistor which is controlled bygate-channel voltage

- if VGS large enough channel pinch-off. This pinch-off voltage has samefunction as threshold voltage (VP is defined as VTR)

- for small VDS JFET operates the same as triode region of MOSFET

Figure 5.18 An n-channel JFET with small vDS and vGS = VTR (pinch-off condition)

JFET Constant Current- for large VDS channel resistance non-linear and JFET departs from

resistive behavior

- if VDS large enough (> VGS - VP) channel current becomes constant

- width of depletion region determined gate voltage and local channelvoltage

- if VDS large enough pinch-off region forms at drain. This region stillallows current to flow through small channel

Figure 5.19 An n-channel JFET in the constant-current region with vDS > (vGS - VTR). The channel ispinched off near the drain end

JFET Transfer Function

- JFET has same current equations as MOSFET

- sometimes alternate set used with

V V and I

V k

P TR

DSS

P

2≡ ≡

Figure 5.20 Voltage-current characteristicsof a typical n-channel JFET with parametersK = 0.5mA/V2; VTR = VP = -4V; IDSS = KP

2 =8mA; a) circuit symbol; b) input port; c) outputport

FET Transconductance Curve- transfer function with ID vs. VGS (assume VDS = constant)

Figure 5.21 Transconductance curve of enhancement-mode MOSFET of Fig. 5.14 taken at the valuevDS = 8V (K = 0.5mA/V2; VTR = 2V)

FET Transconductance Curve (con’t)

Figure 5.22 Transconductance curve of depletion-mode MOSFET of Fig. 5.16 taken at the valuevDS = 8V (K = 0.5 mA/V2; VTR = -4V)

FET Transconductance Curve (con’t)- VDS = 8V => constant current region

ID = k(VGS - VTR)2

P-Channel FETs

Figure 5.24 Several p-channel field-effect transistors