3.6 Metal-Semiconductor Field Effect Transistors (MESFETs)

3.6 Metal-Semiconductor Field Effect Transistors (MESFETs)
3.6. Metal-Semiconductor Field Effect Transistor (MESFETs)
The Metal-Semiconductor-Field-Effect-Transistor (MESFET) consists of a conducting channel
positioned between a source and drain contact region as shown in the Figure 3.6.1. The carrier
flow from source to drain is controlled by a Schottky metal gate. The control of the channel is
obtained by varying the depletion layer width underneath the metal contact which modulates the
thickness of the conducting channel and thereby the current between source and drain.
Source
Gate
n+
Drain
d
n+
L
n-channel
depletion region
p-type or semi-insulating substrate
Figure 3.6.1
Structure of a MESFET with gate length, L, and channel thickness, d.
The key advantage of the MESFET is the higher mobility of the carriers in the channel as
compared to the MOSFET. Since the carriers located in the inversion layer of a MOSFET have a
wavefunction, which extends into the oxide, their mobility - also referred to as surface mobility is less than half of the mobility of bulk material. As the depletion region separates the carriers
from the surface their mobility is close to that of bulk material. The higher mobility leads to a
higher current, transconductance and transit frequency of the device.
The disadvantage of the MESFET structure is the presence of the Schottky metal gate. It limits
the forward bias voltage on the gate to the turn-on voltage of the Schottky diode. This turn-on
voltage is typically 0.7 V for GaAs Schottky diodes. The threshold voltage therefore must be
lower than this turn-on voltage. As a result it is more difficult to fabricate circuits containing a
large number of enhancement-mode MESFET.
The higher transit frequency of the MESFET makes it particularly of interest for microwave
circuits. While the advantage of the MESFET provides a superior microwave amplifier or circuit,
the limitation by the diode turn-on is easily tolerated. Typically depletion-mode devices are used
since they provide a larger current and larger transconductance and the circuits contain only a
few transistors, so that threshold control is not a limiting factor. The buried channel also yields a
better noise performance as trapping and release of carriers into and from surface states and
defects is eliminated.
The use of GaAs rather than silicon MESFETs provides two more significant advantages: first,
the electron mobility at room temperature is more than 5 times larger, while the peak electron
velocity is about twice that of silicon. Second, it is possible to fabricate semi-insulating (SI)
GaAs substrates, which eliminates the problem of absorbing microwave power in the substrate
due to free carrier absorption.
The threshold voltage, VT, of a MESFET is the voltage required to fully deplete the doped
channel layer. This threshold voltage equals:
VT = φ i −
(3.6.1)
qN d d 2
2ε s
where φ i is the built-in potential and d is the thickness of the doped region. This threshold
voltage can also be written as a function of the pinch-off voltage VP:
VT = φ i − VP
(3.6.2)
qN d d 2
VP =
2ε s
(3.6.3)
Where the pinch-off voltage equals:
The derivation of the current in a MESFET starts by considering a small section of the device
between y and y + dy. The current density at that point can be written as a function of the
gradient of the channel voltage:
J = qnv = qN d µ n E = −qN d µ n
(3.6.4)
dVC ( y )
dy
The drain current is related to the current density and the part of the MESFET channel that is not
depleted.
I D = − JW ( d − xn ( y ))
(3.6.5)
Where the depletion layer width at position y is related to the channel voltage, VC(y), by:
xn ( y) =
2ε s (φ i − VG + VC ( y ))
qN d
(3.6.6)
The equation for the current can now be integrated from source to drain, yielding:
L
VD
0
0
∫ I D dy = qN d µ n dW ∫ (1 −
φi − VG + VC
) dVC
VP
(3.6.7)
Since the steady-state current in the device is independent of position, the left hand term equals
ID times L so that:
W
I D = qN d µ nd (VC
L
VD
0
3/2
(
φi − VG + VC )
−
VP
VD
(3.6.8)
)
0
Integration results in:
W
I D = qµ n N d d
L

 (φ − V + V ) 3 / 2 (φ − V ) 3 / 2 
G
D
G

VD − 2  i
− i
3


V
V
P
P


(3.6.9)
This result is valid as long as the width of the un-depleted channel (d – x n (y)) is positive, namely
for:
VD ≤ VG − VT
(3.6.10)
This condition also defines the quadratic region of a MESFET. For larger drain voltage, the
current saturates and equals that at
VD = VG − VT = VD , sat
(3.6.11)
The corresponding current is the saturation current, ID,sat:
3 / 2 

W
(φ i − VG )
2


I D, sat = qµ n N d d VG − VT − 3 V P −


L
VP



(3.6.12)
An example of the resulting I-V characteristics is shown in
Figure 3.6.2. The corresponding device parameters are listed in Table 3.6.1.
Drain current (mA)
7
6
5
4
3
2
1
0
0
2
4
6
Drain-Source Voltage (V)
Figure 3.6.2
Drain current versus Drain-Source voltage at a gate-source voltage of 0.2, 0.4, 0.6
0.8 and 1.0 Volt for a silicon MESFET with built-in potential of 1 V. Channel
parameters and device dimensions are listed in Table 3.6.1.
Parameter
Table 3.6.1
Symbol
Value
Channel width
W
1 mm
Channel length
L
1 µm
Channel mobility
µn
100 cm2 /V-s
Channel doping
Nd
1017 cm-3
Channel thickness
D
115 nm
Built-in potential
φI
1V
MESFET parameters used to calculate
Figure 3.6.2.
The transfer characteristic of a MESFET is shown in Figure 3.6.3 and compared to a quadratic
expression of the form:
ε W (VG − VT )
I D, sat = µ n s
w L
2
(3.6.13)
2
(Drain current)
1/2
1/2
(A )
where w is the average depletion layer width in the channel layer. The quadratic expression
yields the same current at VG = φ i for w = 3d/8. The close fit is at times used to justify using the
simpler quadratic equation.
0.03
0.025
0.02
0.015
0.01
0.005
0
-0.5
0
0.5
1
Gate Voltage (V)
1.5
Figure 3.6.3
Transfer characteristic of a MESFET. Shown is the square root of the drain
current of the MESFET (solid line) and a quadratic fit with w =3d/8 (dotted line).
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