SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. JFETs, MESFETs, and MODFETs 2013.01.26 SD Lab. SOGANG Univ. Gil Yong Song

SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.

JFETs, MESFETs, and MOD-FETs2013.01.26

SD Lab. SOGANG Univ.Gil Yong Song


Contents

1. JFET and MESFET

I-V characteristics

Microwave performance

Device structures

2. MODFET

Device structures

I-V characteristics

Equivalent circuit and microwave performance


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


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,


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


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


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 )


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


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


JFET and MESFET

• Velocity Saturation

- velocity saturation model : short gates where

- transferred-electron effect

- ballistic effect

(a) constant mobility model (b) velocity saturation


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


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.


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



• 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



• 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(.



• 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.



• 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


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


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,


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,


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 :


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.


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


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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SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. JFETs, MESFETs, and MODFETs 2013.01.26 SD Lab. SOGANG Univ. Gil Yong Song