Lecture 6: III-V FET DC I - MESFETs
Metal-Semiconductor Junction Basic MESFET Operation
2014-01-28 Lecture 6, High Speed Devices 2014 1 Field Effect Transistors
V Lg g W Gate Source Drain V y DS
N+ N+
x
• The gate electrode controls the carrier concentration in the channel • Source/Drain set the potential at the source/drain side
• Electrons flow from source to drain IDS and n(x,y) depend on geometry and transport properties. • 2D problem (in x and y)
2014-01-28 Lecture 6, High Speed Devices 2014 2 Field Effect Transistors
V =1V Bulk MOSFET g SOI, Quantum Well MOSFET Metal Metal VD=1V Oxide Oxide - n+ n+ n+ n n+ p-type Oxide or wide bandgap Semicondudctor p-type, S.I. Insulating
MESFET Vg=-1V Metal HEMT Metal Depletion Wide band region semiconductor n+ n+ n+ n+ - n n Wide band semiconductor p-type, S.I. Insulating p-type, S.I. Insulating
2014-01-28 Lecture 6, High Speed Devices 2014 3 Si vs. III-V Field Effect Transistors (FETs)
• Si: µ ≤ 1300 Vs/cm2. v ≈ 8×106 cm/s III-V MOSFETs are difficult to n sat fabricate 2 7 • Alternative: Semiconductor InGaAs µn ≈ 14000 cm /Vs! vsat ≈ 2×10 cm/s to isolate the gate from the SiO2-Si excellent interface channel • InGaAs- GaO InO AsO – poor interface Simple: MESFETs x x x • Better: HEMTs or III-V MOSFETs
SiO2/HfSiO2 Schottky
Barrier
n+ n+ n+ GaAs n n+ Si p-type GaAs Semi-Insulating
An Si MOSFET uses an oxide (SiO2, A III-V Metal-Semiconductor FET (MESFET) SiHfO2) to isolate the gate
2014-01-28 Lecture 6, High Speed Devices 2014 4 Metal-Semiconductor Junction
Similar to a p+N junction!
qcs qfm
qfn Fb fbi Fb
qfb qfm qcs Schottky barrier height
Fn Too simplistic! f f Build in potential bi b q
However, now we ignored that we terminated the crystal and created a lot of surface states....
2014-01-28 Lecture 6, High Speed Devices 2014 5 Metal-Semiconductor Junction II
Experiments show only a very weak
dependence of fb as a function of metal work function fb
qfb
fm
Surface reconstruction and surface defects create a large Fn number of states in the bandgap, which “pins” the Fermi- fbi fb energy q The energy position of these surface states sets the
Schottky barrier height. For GaAs, fb≈0.7-0.8V fb is a material InP fb ≈ 0.3V, InAs fb ≈ -0.1V, In0.53Ga0.47As fb ≈ 0.1V parameter!
2014-01-28 Lecture 6, High Speed Devices 2014 6 MESFET Structure – simplest FET transistor
Vgs (negative) Source Gate Drain x Depletion Region: n≈0 a Nd b y
Semi-insulating
•Schottky depletion under gate modulates channel thickness b
•VDS causes current to flow from source to drain, which can be
modulated by Vgs
2014-01-28 Lecture 6, High Speed Devices 2014 7 Metal-Semiconductor Junction III
d q Nd dy s q df y N X y s (X ) (X ) 0 r d dep dep dep
s dy q y 2 f y N X y Ref. s d dep fs (0) 0 X s 2 potential
dep
a
y
2 s Depletion thickness, maximum Xdep=a X dep fbi Va qNd
Potential needed to fully qNd 2 f00 a deplete down to a 2 s
2014-01-28 Lecture 6, High Speed Devices 2014 8 2 minute exercise – part 1
Depletion Edge
Vgs=0 Vgs=-1V A B
C D
Black – Vg = 0V
Which green plot corresponds to Vgs=-1 V?
Remember: negative bias increases the potential energy of an electron!! 2014-01-28 Lecture 6, High Speed Devices 2014 9 2 minute exercise – part 2
Depletion Vsub=1V Vgs Vgs=-1V egdes + Vsub -
Black plot – Vg and Vsub = 0V
Which red plot corresponds to Vgs=-1 V and Vsub = 1V?
2014-01-28 Lecture 6, High Speed Devices 2014 10 MESFET Operation
+ VDS - Vgs
I Id d
Vcs=0 Vcs(x) Vcs=Vds Resistive voltage drop along channel
DV DV DVgs gs gs
VDS
The potential under the gate is set by the channel-gate potential Vcs(x)
2014-01-28 Lecture 6, High Speed Devices 2014 11 Calculation of the current
V + DS f - Vgs s(x,0)=0 퐽푛 = 푞휇푛푛훻푉 훻 ∙ 퐽푛 = 0 휕2휙 휕2휙 −푞 2 + 2 = ∆푉 = 푁푑 − 푛 Id 휕푥 휕푦 휀푠
Complicated 2D problem!
x y GCA – Gradual Channel approximation: d2f/dx2< 휕2휙 −푞 2 = 푁푑 휕푦 휀푠 Xdep(x) varies slowly with x 휕휙(푥, 푏) Electric field in x-direction is ‘small’ 퐽 = 푞휇 푛 푛 푛 휕푥 푑퐽푛(푥)/푑푥 = 0 2014-01-28 Lecture 6, High Speed Devices 2014 12 Drift Current ich x qWNd n xbx Drift current Gradual channel approximation b(x) varies slowly with x ich f 0 i x I Continuity b(x) is determined by solving s(y) x ch D f x f x X a s bx a X a1 s Depletion dep dep f f00 00 f x f x f V V (x) x s E-field s bi gs cs x L L f x f x fL f s s s I Ddx qWNd na 1 dx qWNd na 1 dfs x f x x f00 x f00 2014-01-28 Lecture 6, High Speed Devices 2014 13 Saturation Voltage, Pinch-off Vds=0V Vds>0>VDS,sat Vds=VDS,sat Vds>VDS,sat At pinch-off, the depletion region reaches the S.I region Our 1D-decoupled model breaks down: (d2f/dx2>>0) we need to solve 2D possion equation, Dfs(x,y) (numerical solutions needed) Result: Channel of finite thickness forms at channel edge. Increased Vds drops inside this VDS ,sat f00 fbi Vgs region, or between channel-drain. Current remains independent of Vds after Vds,sat 2014-01-28 Lecture 6, High Speed Devices 2014 14 Current-Voltage Characteristics f x f 0 f L u(x) s s s d s f V f00 f00 f00 s bi gs f 00 qWNd naf00 2 3/ 2 2 3/ 2 fbi Vgs VDS I D d s d s d V V L 3 3 DS DS ,sat f 00 fbi Vgs VDS ,sat 0.16 d 1 VDS VDS ,sat f 00 0.14 Vgs=0 0.12 qNd 2 0.1 f00 a 0.08 2 s 0.06 Ids - arb units arb - Ids V <0 gs V f f V Pinch-off voltage 0.04 DS ,sat 00 bi gs 0.02 Threshold Voltage VT fbi f00 0 0 0.5 1 1.5 2 2.5 3 Vds Vgs=-|VT| 2014-01-28 Lecture 6 High Speed Devices 2014 15 MESFET limitations qWNd naf00 1 2 3/ 2 I D,sat s s L 3 3 dI qWN a f V g D,sat d n 1 bi gs m dV L f gs 00 • We want a high gm – but: • Positive gate-voltage – very high gate leakage! • From electrostatics: a • Increase ND – but this lowers µn due to impurity scattering and increases gate leakage! • Increase µn by material choice: Need to have a high Schottky barrier! (InGaAs, InAs can’t be used!) • We can do better using heterostructures or MOSFETs! 2014-01-28 Lecture 6, High Speed Devices 2014 16