sogang university sogang university. semiconductor device lab. jfets, mesfets, and modfets...
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
JFETs, MESFETs, and MOD-FETs2013.01.26
SD Lab. SOGANG Univ.Gil Yong Song
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
Contents
1. JFET and MESFET
I-V characteristics
Microwave performance
Device structures
2. MODFET
Device structures
I-V characteristics
Equivalent circuit and microwave performance
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
JFET and MESFET
I-V characteristics
- two ohmic conatct : source, drain
- positive : electrons flow from source to drain
- gate controls the net opening of the channel by
varying the depletion width.
- JFET : p-n junction, MESFET : Schottky junction
- voltage controlled register
- depletion mode : normally on with =0, is negative.
- channel current increases with the drain voltage → saturate
- assumption :
uniform channel doping
gradual-channel approximation
abrupt depletion layer
negligible gate current
L : channel length
a : channel depth
: depletion depth
b : net channel opening
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
JFET and MESFET
• Channel-charge distribution
- The depletion width varies along the channel(x-direction)
- By using Poisson’s equation,
- one-sided abrupt-junction,
- built-in potential for JFET is,
for MESFET,
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
JFET and MESFET
• Channel-charge distribution
- Potential difference between source and drain in neutral channel
- The depletion width at the source and drain ends :
- When , =0 (flat band).
The maximum value of is equal to a (pinch off potential)
- Current :
- Current saturation mechanism
1. long channel(channel pinch off) : mobility is constant
2. short channel : At high field, mobility is no longer constant
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
JFET and MESFET
• Constant mobility
- is assumed to hold without limit. Then,
where
- In the linear region,
- For more simple equation around
with
- For non-linear condition(when drain bias continues to increase),
pinch-off condition when
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
JFET and MESFET
• Constant mobility
- transconductance is given by
- For drain bias higher than , the pinch-off starts to migrate toward the source.
However, potential remains independent of . Thus field remains constant too.
- Practical devices show that doesn’t saturate with due to the reduction in the effective channel length.
- can be simplified to be(when )
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
JFET and MESFET
• Velocity-Field Relationship
- Long channel device : constant mobility
- Short channel device : At higher fields, the carrier velocity saturates
to a value called saturation velocity .
• Field dependent Mobility : Two-Piece Linear Approximation
- constant mobility (maximum field reaches critical field)
- current saturates as approaches
Long channel
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
JFET and MESFET
• Field-Dependent Mobility : Empirical Formula
- current is reduced by a factor of from that of
constant mobility model.
- In order to obtain , we set the transcendental
equation for as
- saturation current(transcendental equation into
Empirical formula
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
JFET and MESFET
• Velocity Saturation
- velocity saturation model : short gates where
- transferred-electron effect
- ballistic effect
(a) constant mobility model (b) velocity saturation
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
JFET and MESFET
• Dipole-Layer Formation
- Before the saturation drain bias , the potential
along the channel is increases from 0(source) to (drain)
→ depletion width becomes wider and channel width decreases.
- fig 8(a)
- fig 8(b)
channel width decreases as depletion region increases
: negative charge changes to positive space charge
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
JFET and MESFET
• Breakdown
- As the drain voltage increases further, breakdown occurs.
- The fundamental mechanism of breakdown : impact ionization
- one dimension, treating the gate-drain structure as reverse-biased diode,
the drain breakdown voltage is
- fig 9(a) : for higher , the drain breakdown voltage becomes higher.
→ Bur for MESFETS on GaAs, the breakdown mechanisms are changed.
- MESFETs have a gap between the gate and the source/drain contacts.
In gate-drain distance region, the doping level is the same as the channel.
→ surface effect could be occurred and affect the field distribution.
- tunneling current associated with the Schottky-barrier gate contact.
Surface potential created by surface traps.
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
Microwave Performance
• Small-Signal Equivalent circuit
- total gate-channel capacitance :
- channel resistance :
- series resistance(source,drain,gate) :
- parasitic input capacitance :
- output capacitance :
- leakage current in the gate-to-channel junction :
- Input resistance :
- In the linear region, effective are
- In saturation region, measured extrinsic transconductance
is equal to
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
Microwave Performance
• Cutoff Frequency
- For a measure of the high-speed capability, is used.
- is defined as the frequency of unity gain,
- total input capacitance
- for ideal case of zero input capacitance,
L/v : the transit time for a carrier to travel from source to drain.
- more complete equation containing series components,
- Geometry affects the cutoff frequency. Decreasing gate length(L) will decrease gate capacitance
and increase transconductance. increases
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
Microwave Performance
• Maximum Frequency of Oscillation.
- for measure of the high-speed capability, is used.
- definition : maximum frequency at which the device can provide power gain.
- To maximize , must be optimized in the intrinsic FET and must be minimized.
• Power-Frequency Limitations
- For power applications, both high voltage and high current are required.
- For high current, the total channel dose has to be high.
- For high BV, doping level cannot to be high and L cannot be small.
- For a high , L has to be minimized and as a consequence, has to increase.
-
- In high power operation, the device temperature increases reduction of the mobility(
saturation velocity(.
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
Microwave Performance
• Noise Behavior
- MESFET, JFET : low-noise devices (only majority carriers
participate in their operations)
- In practical devices, parasitic resistances are responsible
for the noise behavior.
-
- The noise figure is defined as the ratio of the total noise power to the noise power generated
from the source impedance.
- minimum noise figure :
- For low-noise performance, parasitic gate resistance and source resistance should be minimized.
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
Microwave Performance
• Device Structures
- semiinsulating(SI) substrate : for compound semiconductors
such as GaAs.
- Fig 16(a) : Ion-implanted planar structure
(1) self aligned process : the gate is formed first, and the
source/drain ion implantation is self-alinged to the gate.
(2) ohmic-priority : source/drain implantation and anneal are done before the gate formation
- Fig 16(b) : recessed-channel structure.
buffer layer : to eliminate defects duplicating from the SI substrate
n+ layer : to reduce the source and drain contact resistance
n+ layer is selectively removed for gate formation.
advantage : surface is further away from the n-channel so that surface effects are minimized
- T-gate
shorter dimension of bottom : to optimize and
wider dimension of top : to reduce the gate resistance
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
MODFET
- Modulated-doped field-effect transistor (also known as HEMT (high-electron mobility transistor))
- Hetero structure : wide band gap material is doped and carriers diffuse to the undoped
narrow bandgap layer at which heterointerface the channel is formed.
- channel carreirs in the undoped heterointerface are spatially separated from the doped region and
have high mobilities because there is no impurity scattering.
- The main advantage of modulation doping is the
superior mobility. (no scattering)
- electron gas.
lattice scattering
Impurity scattering
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
MODFET
• Basic device structure
- AlGaAs/GaAs heterointerface.
- barrier layer AlGaAs under the gate is doped
- channel layer GaAs is undoped
- principle of modulation doping :
Carriers from the doped barrier layer are transferred to reside
at the heterointerface and are away from the doped region to avoid
impurity scattering.
• I-V Characteristics
- The impurities within the barrier layer are ionized and carriers
depleted away.
- potential variation within the depletion region :
- For uniform doping profile,
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
MODFET
- Threshold voltage : when the at the GaAs surface coincide
with the conduction-band edge .
- By choosing the doping profile and , can be varied.
- With gate voltage larger than the threshold voltage,
charge sheet in the channel is given by
- The channel has a variable potential with distance,
- Channel current is constant through out the channel,
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
MODFET
→ Constant mobility
- drift velocity :
-
- In the linear region where ,
- At high , pinch off is occurred and current saturates with .
saturation drain bias is ,then
- transconductance :
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
MODFET
→ Field-Dependent mobility
- current becomes saturated with before the pinch-off occurs,
due to the fact that carrier drift velocity no longer is linearly
proportional to the electric field. In high fields, the mobility becomes
field dependent.
→ Velocity Saturation
- In the case of short-channel devices, velocity saturation is approached
and simpler equations can be used.
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
MODFET
• Equivalent circuit and microwave performance
- From the equivalent circuit, in the presence of parasitic source resistance,
the extrinsic transconductance is degraded by
- cutoff frequency , maximum frequency :
- minimum noise figure :
- Since gate-channel capacitance shorter channels have better noise performance.
- mobility : MODFET>MESFET so, speed : MODFET>MESFET
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
MODFET
- Right side : identical(same amount of channel charge)
- Left side :
1. the threshold voltage of the MODFET is lowered
2. the built-in potential within the barrier layer
increases the total barrier for carrier confinement.
The higher barrier enables a higher gate bias
before excessive gate current takes place.