EDC Lesson 12: Transistor and FET Characteristics Lesson-12: MOSFET (enhancement and depletion mode) Characteristics and Symbols 2008 EDCLesson12- ", Raj Kamal, 1
1. Metal Oxide Semiconductor Field Effect Transistor (MOSFET) 2008 EDCLesson12- ", Raj Kamal, 2
Types of FETs The family of FETs may be divided into : Junction FET Depletion Mode MOSFET Enhancement Mode MOSFET 2008 EDCLesson12- ", Raj Kamal, 3
Remember JFET Definition JFET is a unipolar-transistor, which acts as a voltage controlled current device and is a device in which current at two electrodes is controlled by the action of an electric field at a reversed biased p-n junction. 2008 EDCLesson12- ", Raj Kamal, 4
MOSFET Definition MOSFET Field effect transistor is a unipolartransistor, which acts as a voltage-controlled current device and is a device in which current at two electrodes drain and source is controlled by the action of an electric field at another electrode gate having in-between semiconductor and metal very a thin metal oxide layer. 2008 EDCLesson12- ", Raj Kamal, 5
n-channel depletion Metal-Oxide-Semiconductor FET (MOSFET) n channel p 2008 EDCLesson12- ", Raj Kamal, 6
p-channel depletion Metal-Oxide-Semiconductor FET (MOSFET) p channel n 2008 EDCLesson12- ", Raj Kamal, 7
Effect of Insulating SiO 2 (metal-oxide) layer MOSFET Very high input impedance due to SiO 2 layer compared to even reverse biased p-n junction depletion region input impedance in JFET 2008 EDCLesson12- ", Raj Kamal, 8
n-channel depletion region 2008 EDCLesson12- ", Raj Kamal, 9
n-channel depletion region (normally ON) 2008 EDCLesson12- ", Raj Kamal, 10
Transfer Characteristics of n-channel depletion region MOSFET 2008 EDCLesson12- ", Raj Kamal, 11
There is a convenient relationship between I DS and V GS. Beyond pinch-off I DS 1 V V GS ( off Where I DSS is drain current when V GS = 0 and V GS(off) is defined as V P, that is gate-source voltage that just pinches off the channel. The pinch off voltage V P here is a +ve quantity because it was introduced through V DS(sat). V GS(off) however is negative, -V P. DSS 2008 EDCLesson12- ", Raj Kamal, 12 I GS ) 2
p-channel depletion MOSFET 2008 EDCLesson12- ", Raj Kamal, 13
p-channel depletion MOSFET (Normally ON at VGS = 0) 2008 EDCLesson12- ", Raj Kamal, 14
2. Enhancement Mode MOSFETS 2008 EDCLesson12- ", Raj Kamal, 15
n-channel enhancement mode MOSFET 2008 EDCLesson12- ", Raj Kamal, 16
n-channel enhancement mode Normally Off 2008 EDCLesson12- ", Raj Kamal, 17
n-channel enhancement mode p-channel enhancement mode Note that with a n-channel device we apply a +ve gate voltage to allow source-drain current, with a p-channel device we apply a -ve gate voltage. 2008 EDCLesson12- ", Raj Kamal, 18
p-channel enhancement MOSFET 2008 EDCLesson12- ", Raj Kamal, 19
p-channel enhancement MOSFET (normally OFF) 2008 EDCLesson12- ", Raj Kamal, 20
3. Enhancement Mode MOSFETS in detail 2008 EDCLesson12- ", Raj Kamal, 21
Basic MOSFET (n-channel) Enhancement mode When VGS = 0, the n- channel is very thin and channel width enhances with + V GS 2008 EDCLesson12- ", Raj Kamal, 22
Channel width enhancement with +V GS The gate electrode is placed on top of a very thin insulating layer. There are a pair of small n-type regions just under the drain & source electrodes. If apply a +ve voltage to gate, will push away the holes inside the p-type substrate and attracts the moveable electrons in the n-type regions under the source & drain electrodes. 2008 EDCLesson12- ", Raj Kamal, 23
Enhancement mode MOSFET Increasing the +ve gate voltage pushes the p-type holes further away and enlarges the thickness of the created channel. As a result increases the amount of current which can go from source to drain this is why this kind of transistor is called an enhancement mode MOSFET. 2008 EDCLesson12- ", Raj Kamal, 24
Subthreshold region in n-chennel enhancement mode I D = k exp (qv GS /k B T) where T temperature iu in Kelvin, is Boltzman constant 2008 EDCLesson12- ", Raj Kamal, 25
Subthreshold region in n-chennel enhancement mode I D = k (VGS V T ) 2 2008 EDCLesson12- ", Raj Kamal, 26
Above Threshold ON state I D = k (V GS V T ) 2 = 0.278 (V GS V T ) 2 ma I D V T = 2 V If I D (on) = 10 ma and V GS = 8 V, then k =10 ma/(8v 2V) 2 = 0.278 ma/v 2 0 1 2 3 4 5 6 7 8 V GS 2008 EDCLesson12- ", Raj Kamal, 27
V T = 2 V Above Threshold ON state If I D (on) = 10 ma and I D = k (V GS V T ) 2 V GS = 8 V, then k =10 ma/(8v 2V) 2 = 0.278 ma/v 2 I D = k (V GS V T ) 2 = 0.278 (V GS V T ) 2 ma 2008 EDCLesson12- ", Raj Kamal, 28
Ideal Output Characteristics of MOSFET 2008 EDCLesson12- ", Raj Kamal, 29
Ideal Output linear region before saturation Characteristics of MOSFET 2008 EDCLesson12- ", Raj Kamal, 30
Ideal Output Saturation formula of enhancement mode MOSFET chang 2008 EDCLesson12- ", Raj Kamal, 31
Transfer Characteristics in above threshold region and saturation region in n-channel enhancement mode 7 V 6 V 5 V 4 V 3V V GS = V T = 2V 2008 EDCLesson12- ", Raj Kamal, 32
Transfer Characteristics in Subthreshold region and saturation region in n-channel enhancement mode 2008 EDCLesson12- ", Raj Kamal, 33
4. Symbols of depletion and Enhancement Mode MOSFETS 2008 EDCLesson12- ", Raj Kamal, 34
Symbol of depletion mode MOSFET n-channel Depletion Mode MOSFET p-channel Depletion Mode MOSFET 2008 EDCLesson12- ", Raj Kamal, 35
Symbol of enhancement mode of MOSFET n-channel Enhancement Mode MOSFET p=channel Enhancement Mode MOSFET 2008 EDCLesson12- ", Raj Kamal, 36
Summary 2008 EDCLesson12- ", Raj Kamal, 37
We learnt Definitions of MOSFET n-channel and p-channel Depletion MOSFET normally ON, Pinch off on VGS in n-mosfet and + VGS in p- MOSFET n-channel and p-channel enhancement MOSFET normally OFF, below Threshold and above VT on + VGS in n-mosfet and - VGS in p-mosfet 2008 EDCLesson12- ", Raj Kamal, 38
End of Lesson 12 2008 EDCLesson12- ", Raj Kamal, 39