5 Metal Semiconductor Field Effect Transistor (MESFET)

Dr. Monika Tomar and Dr. Ayushi Paliwal

Introduction

Three-terminal devices such as MESFETs are unipolar, i.e. they use only one type of carrier. This makes optimization easier than if two types of carriers have to be considered. Most of the materials used in three-terminal devices, such as GaA8, InP, etc. have high mobility compared with silicon. A large variety of forms and applications are possible. Monolithic integration of devices on a semi-insulating substrate is easier to accomplish than for two-terminal devices. The availability of three terminals means that uni-directional amplification is possible, and circulators are not usually needed A partial list of the type of applications which have been implemented contains amplifiers (low-noise and power-), limiters, harmonic frequency multipliers, mixers, phase-shifters and oscillators. The present-day microwave three-terminal devices of course have as their ancestor the bipolar transistor which was invented in the late 1940’s. It took a long time before transistors were fast enough to be useful at microwave frequencies, however. Not until 1965 did germanium bipolar transistors start to be employed at L-band (1 – 2 GHz). The idea of a field-effect transistor goes back to Shockley (1952), but took even longer to realize for microwave frequencies. Early field-effect transistors had cut-off frequencies which were inferior to those ofbipolars. For example, the first microwave FET using GaAs was reported on in 1968, and had a maximum oscillation frequency (fmax)*= 3 GHz, but already in 1970, Drangeid et al. (1970) had developed a GaAB FET with fmax = 30 GHz, which far surpassed the performance of silicon bipolar devices. Since that time, oscillation has been observed above 100 GH., and noise figures of GaAs. FETs have been shown to be superior at essentially all microwave frequencies. It is now mainly the HFET, a development of the 1980’s, which can compete in terms of noise figure.

Schematic and fabrication of MESFET

Metal Semiconductor Field Effect Transistor (MESFET): Figure 1 (a) shows the schematic diagram of MESFET. MESFET consists of a thin layer of n-type semiconductor on a semi-infinite substrate. Ohmic metal contacts are provided at two ends of n-type semiconductor to serve as ‘source’ and ‘drain’. A Schottky metal contact is also with semiconducting layer to act as ‘Gate’ a shown in figure 1

(a). Carrier flow across the channel (n-type layer) from ‘source’ to ‘drain’. Thickness of the channel is controlled by variation in the depletion region underneath the metal gate this can vary the current between source and drain. Figure 1 (b) shows the process steps used for the fabrication of a MESFET. This figure summarizes the process starting from the initial GaAs wafer and the ending with the final device. An important process of the MESFET fabrication process is the Mask and wafer alignment and the exposure. A special technique used to make sure of the alignment of all of the dies on the wafer is the Split-Field alignment. This technique allows us to view two different sites at a time and align them.

In the case of JFET channel conductance is there with no biasing and ‘Source’ is not isolated from the ‘Drain’. Thus, channel is available for the conduction of electrons and this device is ‘Normally On’ device or ‘Depletion mode’ device. But in the case of MESFET, it is possible to design the device with both ‘Normally On’ and ‘Normally Off’ features.

Operation of MESFET

3.1 ENHANCEMENT MODE: Normally OFF

In enhancement mode, width of depletion region in n-type semiconductor (underneath Schottky contact) is wide is wide enough without applied voltage which can pinch off the channel as shown in Fig. 2. In enhancement mode, the thickness (t) of n-type semiconducting (n-GaAs) layer is less than width (a) of depletion region due to Schottky contact. Thus, MESFET is considered naturally in “OFF” state in the enhancement mode. When some positive voltage bias (VT>0) is applied across the ‘Gate’ with respect to ‘Source’, Schottky diode becomes forward bias, and the width of depletion region starts reducing. This results in the opening of the channel for the flow of charge carriers and makes the channel conducting. Hence, a large drain current (ID>0) can flow for applied positive bias at drain (VD) as shown in Fig. 3.

3.2 DEPLETION MODE: Normally ON

If the width (a) of depletion region in the n-type semiconductor (underneath Schottky Gate) does not extend to the entire thickness (t) of p-type substrate, the MESFET is in depletion-mode as shown in Fig.Here, depletion width a t (thickness of n-type layer). Under thermal equilibrium condition, the channel is always available for the flow of charge carriers. Thus, when the ‘Gate’-to-‘Source’ voltage is not applied, the MESFET becomes conductive or in “ON” state.

To switch-off the MESFET, a negative voltage (VT 0) is required at ‘Gate’ with respect to the source. The negative applied voltage at ‘Gate’ makes Schottky diode in reverse bias configuration, and resulting in an increase in the depletion width, such that it ‘pinches off’ the channel as shown in figure 5. Thus, drain current (ID) flows when VG Vth, and becomes zero for VG = Vth, where VT 0.

4. I-V CHARACTERSTICS

The ‘Source’ is grounded and act as reference voltage (zero) for MESFET device. The Drain is kept at positive voltage with respect to source. The metal ‘Gate’ which is in reverse biased mode with the n-type semiconductor forms a Schottky diode, so there is no gate current, except for some leakage current in small amount. The applied gate voltage can modulate the depletion width, due to which the conductivity of the channel is modulated.

When the drain (VD) is kept small, the depletion width (  depl) in the channel can be calculated from the Schottky diode theory as explained below:

At threshold voltage, VG = Vth, the width of depletion region is =a. Using the equation 1, the threshold voltage could be estimated as:

This threshold voltage can be either negative or positive which depends upon the thickness of the n-type semiconducting layer, the doping concentration (Nd) or the metal which is used to form the Schottky gate. For positive threshold voltage, the MESFET is in enhancement mode and for negative value of Vth, it is in depletion mode.

Fig. 6 shows the variation of Drain current (ID) as a function of Gate voltage (VG) and Drain voltage (VD) in both the Enhancement and Depletion modes. Since the channel provides resistance to the flow of charge carriers from source to drain, it gives rise to a major potential drop along the channel. Consider, VD is the applied Drain voltage with respect to the source, and V(y) is the voltage at any point y (from source) in the channel. The applied voltage is uniformly distributed in the channel and varying from V(y=0)=VS=0 at the source to V(y=L)=VD at the drain.

In every section located vertically at a position ‘y’, the reverse bias across the Schottky junction is equal to VG-V(y), due to which the depletion width varies as a function of ‘y’. the depletion width will be

Figure 6: I-V characteristic curve of MESFET in Ehnacement mode and Depletion mode

Consider a small element dy of the channel which offer a small resistance ‘dR’ to the Drain current ID.The Drain current is estimated by integrating Ohm’s law from source to drain:

where, W is the width of device as shown in Fig.1 and dV is the voltage drop across small element ‘dy’ of the channel.

Replacing xdepl(y) from equation 3 in equation 4:

These equations hold if the VD is small and channel is not pinched-off. It may be noted from equation 7 that ID increases with increasing VD for fixed value of VG (figure 6). The saturation value of Drain current, ID = IDsat; occurs when xdepl (L)=a, at which point VD=VDsat=VP-Vi+VG.Saturation Drain Voltage is VDsat=VP-Vi+VG=VG-VTH. (8)

The drain saturation current could be estimated by replacing VD by the saturation drain voltage VDsat, in equation 7:3 =  0 ( + 2(  −  )2 − + ) (9) 3 3√ The Drain saturation current (IDsat) is independent of applied Drain voltage. The transconductance in saturation region is given by:

ADVANTAGES AND DISADVANTAGES

MESFET has advantage over MOSFET that the carriers have higher mobility in the channel. Their mobility is less than half of the mobility of bulk counter material, as the charge carriers are situated in the inversion layer, which extends into the oxide layer,. However, as the depletion region starts separating the charge carriers from the surface, their mobility starts increasing and approaches to bulk material which leads to increase in current, transconductance and transit frequency of the MESFET. The presence of Schottky metal gate limits the forward bias voltage on the gate to the turn-on voltage of the Schottky diode which is the disadvantage of the MESFET structure. The value of this turn-on voltage for GaAs Schottky diodes is ~0.7 V and the threshold voltage should be lower than the turn-on voltage. It is more difficult to fabricate circuits containing a large number of MESFET in the enhancement-mode. The advantage of higher transit frequency of the MESFET provides a superior microwave amplifier. Moreover, MESFET in depletion mode is used as it provides a larger current and larger transconductance so that threshold control is not a limiting factor. GaAs is most commonly preferred over silicon for fabricating MESFET as the electron mobility at room temperature is more than 5 times larger, while the peak electron velocity is about twice that of silicon. Also GaAs substrate resolves the problem of absorbing microwave power due to free carrier absorption.

5. Small-Signal equivalent circuit model

The small-signal equivalent circuit of a MESFET is shown in Figure 7. This model is quite similar to other well-known models of FETs. Measured values for the elements in a circuit model must be obtained by first measuring the small-signal S-parameters of the device, and then fitting these to agree with the equivalent circuit model. and considerable errors may result. Recently introduced microprobes contact the device directly on its. “chip” and allow determination of the equivalent circuit parameters with greater accuracy. It is important to note that DC-measurements of especially Rds but also gm often give results which are very noticeably different from measurements at microwave frequencies. Frequency-dispersion of these parameters is observed in the frequency-range of Hz-kHz, and has been correlated with traps, either at the surface, or at the buffer-layer/active layer interface. One thus must be quite careful in using DC data, instead of S-parameter data, for some of the parameters.

gm – Since the drain current is delayed by a time = the transit-time, with respect to the gate voltage, we must also use a phase-factor, exp(-j 0), to take this into account (not included in the PHS model). We also must distinguish the external gm,e (measured from the terminals) from the intrinsic value, gm,o. These are related as follows:

Cdg,  and  Cgs are capacitances from the gate to the channel (source end and drain end, respectively). We of course can not exactly pin-point the channel location of the lower capacitance electrode, but the success of the equivalent circuit model depends on the extent to which measured S-parameter data can be fitted to the predictions based on the model. Cde also ends at the source end of the channel. The electric field lines in Cgs, are primarily routed through the semi-insulating substrate.

Other series resistances Rs, Rd connect the “actual channel” with the source and drain contacts, respectively. Also, the gate fingers have a resistance, Rg, Rds, is the actual channel resistance.

Cut-Off Frequencies: Several different cut-off frequencies can be defined on the basis of the equivalent circuit model.

6. Summary

Schematic and fabrication steps of MESFET ENHANCEMENT MODE (Normally OFF) and DEPLETION MODE (Normally ON) Study the IV characteristics along with their advantages and disadvantages SMALL-SIGNAL EQUIVALENT CIRCUIT MODEL and the expression of cut off frequency