sogang university sogang university. semiconductor device lab.

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Transcript sogang university sogang university. semiconductor device lab. 

JFETs, MESFETs, and
MODFETs
2013.01.26
SD Lab. SOGANG Univ.
Gil Yong Song
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Contents
1. JFET and MESFET
I-V characteristics
Microwave performance
Device structures
2. MODFET
Device structures
I-V characteristics
Equivalent circuit and microwave performance
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JFET and MESFET
I-V characteristics
- two ohmic conatct : source, drain
- positive 𝑉𝐷 : 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
L : channel length
- voltage controlled register
a : channel depth
- depletion mode : normally on with 𝑉𝐺 =0, 𝑉𝑇 is negative.
π‘Šπ· : depletion depth
- channel current increases with the drain voltage β†’ saturate
b : net channel opening
- assumption :
uniform channel doping
gradual-channel approximation
abrupt depletion layer
negligible gate current
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JFET and MESFET
β€’
Channel-charge distribution
- The depletion width π‘Šπ· varies along the channel(x-direction)
- By using Poisson’s equation,
- one-sided abrupt-junction,
- built-in potential for JFET is,
for MESFET,
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JFET and MESFET
β€’
Channel-charge distribution
- Potential difference between source and drain in neutral channel
- The depletion width at the source and drain ends :
- When
, π‘Šπ·π‘  =0 (flat band).
The maximum value of π‘Šπ·π‘‘ 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
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JFET and MESFET
Constant mobility
β€’
-
is assumed to hold without limit. Then,
where
- In the linear region, 𝑉𝐷 β‰ͺ 𝑉𝐺 π‘Žπ‘›π‘‘ 𝑉𝐷 β‰ͺ
- For more simple equation around 𝑉𝐺 = 𝑉𝑇
with
- For non-linear condition(when drain bias continues to increase), π‘‰π·π‘ π‘Žπ‘‘ = 𝑉𝐺 βˆ’ 𝑉𝑇
pinch-off condition when π‘Šπ·π‘‘ = π‘Ž
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JFET and MESFET
β€’
Constant mobility
- transconductance is given by
- For drain bias higher than π‘‰π·π‘ π‘Žπ‘‘ , the pinch-off starts to migrate toward the source.
However, potential remains π‘‰π·π‘ π‘Žπ‘‘ independent of 𝑉𝐷 . Thus field remains constant too.
- Practical devices show that πΌπ·π‘ π‘Žπ‘‘ doesn’t saturate with 𝑉𝐷 due to the reduction in the effective channel length.
- πΌπ·π‘ π‘Žπ‘‘ , π‘”π‘š can be simplified to be(when 𝑉𝐺 = 𝑉𝑇 )
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JFET and MESFET
β€’ Velocity-Field Relationship
- Long channel device : constant mobility
- Short channel device : At higher fields, the carrier velocity saturates
to a value called saturation velocity 𝑣𝑠 .
β€’ Field dependent Mobility : Two-Piece Linear Approximation
- constant mobility (maximum field reaches critical field)
- current saturates as 𝑉𝐷 approaches
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Long channel
JFET and MESFET
β€’ Field-Dependent Mobility : Empirical Formula
- current is reduced by a factor of
from that of
constant mobility model.
- In order to obtain π‘‰π·π‘ π‘Žπ‘‘ , we set
Empirical formula
𝑑𝐼𝐷
𝑑𝑉𝐷
= 0, the transcendental
equation for π‘‰π·π‘ π‘Žπ‘‘ as
- saturation current(transcendental equation into 𝐼𝐷 )
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JFET and MESFET
β€’ Velocity Saturation
- velocity saturation model : short gates where
- transferred-electron effect
- ballistic effect
(a) constant mobility model
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(b) velocity saturation
JFET and MESFET
β€’ Dipole-Layer Formation
- Before the saturation drain bias π‘‰π·π‘ π‘Žπ‘‘ , the potential
along the channel is increases from 0(source) to 𝑉𝐷 (drain)
β†’ depletion width becomes wider and channel width decreases.
- fig 8(a) 𝑉𝐷 = π‘‰π·π‘ π‘Žπ‘‘
- fig 8(b) 𝑉𝐷 > π‘‰π·π‘ π‘Žπ‘‘
π‘₯1 ~π‘₯2 ∢ channel width decreases as depletion region increases
𝑛 > 𝑁𝐷
π‘₯2 ~ : negative charge changes to positive space charge
𝑛 < 𝑁𝐷
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JFET and MESFET
β€’ 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 𝑉𝐷𝐡 is
𝑉𝐷𝐡 = 𝑉𝐡 βˆ’ 𝑉𝐺
- fig 9(a) : for higher 𝑉𝐺 , the drain breakdown voltage becomes higher.
β†’ 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 𝐿𝐺𝐷 region, the doping level is the same as the channel.
β†’ surface effect could be occurred and affect the field distribution.
- tunneling current associated with the Schottky-barrier gate contact.
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Surface potential created by surface traps.
Microwave Performance
β€’ Small-Signal Equivalent circuit
β€²
β€²
- total gate-channel capacitance : 𝐢𝐺𝑆
+ 𝐢𝐺𝐷
- channel resistance : π‘…π‘β„Ž
- series resistance(source,drain,gate) : 𝑅𝑆 , 𝑅𝐷 , 𝑅𝐺
β€²
- parasitic input capacitance : πΆπ‘π‘Žπ‘Ÿ
β€²
- output capacitance : 𝐢𝐷𝑆
- leakage current in the gate-to-channel junction :
- Input resistance :
- In the linear region, effective 𝑉𝐺 , 𝑉𝐷 are 𝑉𝐺 βˆ’ 𝐼𝐷 𝑅𝑆 + 𝑅𝐷 , 𝑉𝐷 βˆ’ 𝐼𝐷 𝑅𝑆
- In saturation region, measured extrinsic transconductance
is equal to
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Microwave Performance
β€’ Cutoff Frequency
- For a measure of the high-speed capability, 𝑓𝑇 is used.
- 𝑓𝑇 is defined as the frequency of unity gain,
β€²
β€²
- total input capacitance 𝐢𝑖𝑛
= CGβ€² + Cpar
- for ideal case of zero input capacitance,
β†’ 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. consequently, fT increases
𝐢𝐺′ ∝ 𝑍 × πΏ;
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Microwave Performance
β€’ Maximum Frequency of Oscillation.
- for measure of the high-speed capability, π‘“π‘šπ‘Žπ‘₯ is used.
- definition : maximum frequency at which the device can provide power gain.
β€²
- To maximize π‘“π‘šπ‘Žπ‘₯ , 𝑓𝑇 must be optimized in the intrinsic FET and 𝑅𝐺 , 𝑅𝑆 π‘Žπ‘›π‘‘ 𝐢𝐺𝐷
must be minimized.
β€’ 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 𝑓𝑇 , L has to be minimized and as a consequence, 𝑁𝐷 has to increase.
- In high power operation, the device temperature increases β†’ reduction of the mobility(∝ 1/𝑇 2 ),
saturation velocity(∝ 1/𝑇).
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Microwave Performance
β€’ 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.
- 𝑖𝑛𝑔 𝑖𝑛𝑑𝑒𝑐𝑒𝑑 π‘”π‘Žπ‘‘π‘’ π‘›π‘œπ‘–π‘ π‘’ , 𝑖𝑛𝑑 𝑖𝑛𝑑𝑒𝑐𝑒𝑑 π‘‘π‘Ÿπ‘Žπ‘–π‘› π‘›π‘œπ‘–π‘ π‘’ ,
𝑒𝑛𝑔 π‘‘β„Žπ‘’π‘Ÿπ‘šπ‘Žπ‘™ π‘›π‘œπ‘–π‘ π‘’π‘  π‘œπ‘“ π‘‘β„Žπ‘’ π‘”π‘Žπ‘‘π‘’ π‘Ÿπ‘’π‘ π‘–π‘ π‘‘π‘Žπ‘›π‘π‘’ ,
𝑒𝑛𝑠 (π‘‘β„Žπ‘’π‘Ÿπ‘šπ‘Žπ‘™ π‘›π‘œπ‘–π‘ π‘’ π‘œπ‘“ π‘ π‘œπ‘’π‘Ÿπ‘π‘’ π‘Ÿπ‘’π‘ π‘–π‘ π‘‘π‘Žπ‘›π‘π‘’)
- 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.
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Microwave Performance
β€’ 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 𝑓𝑇 and π‘”π‘š
wider dimension of top : to reduce the gate resistance
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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.
lattice scattering
- The main advantage of modulation doping is the
superior mobility. (no scattering)
- electron gas.
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Impurity scattering
MODFET
β€’
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.
β€’ 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,
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MODFET
- Threshold voltage : when the 𝐸𝐹 at the GaAs surface coincide
with the conduction-band edge 𝐸𝐢 .
- By choosing the doping profile and
, 𝑉𝑇 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,
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MODFET
β†’ Constant mobility
- drift velocity :
- In the linear region where 𝑉𝐷 β‰ͺ 𝑉𝐺 βˆ’ 𝑉𝑇 ,
- At high 𝑉𝐷 , pinch off is occurred and current saturates with 𝑉𝐷 .
saturation drain bias is
,then
- transconductance :
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MODFET
β†’ Field-Dependent mobility
- current becomes saturated with 𝑉𝐷 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.
β†’ Velocity Saturation
- In the case of short-channel devices, velocity saturation is approached
and simpler equations can be used.
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MODFET
β€’ Equivalent circuit and microwave performance
- From the equivalent circuit, in the presence of parasitic source resistance,
the extrinsic transconductance is degraded by
- cutoff frequency 𝑓𝑇 , maximum frequency π‘“π‘€π‘Žπ‘₯ :
- minimum noise figure :
- Since gate-channel capacitance 𝐢𝐺𝑆 ∝ 𝐿, shorter channels have better noise performance.
- mobility : MODFET>MESFET so, speed : MODFET>MESFET
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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.
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