Wednesday, December 1, 2010

Metal Semi-conductor FET (MESFET)

The gallium arsenide field-effect transistor, a bulk-current-conduction majority-carrier device, is fabricated from bulk gallium arsenide by high-resolution photolithography and ion implantation into a semi-insulating GaAs substrate. Processing is relatively simple, requiring no more than six to eight masking stages. For the purpose of comparison, the figure below shows the evolution of process complexity in terms of mask count as function of time for both silicon and gallium arsenide technologies.

The structure of the basic MESFET as shown in the figure below is very simple. It consists of a thin n-type active region joining two ohmic contacts with a narrow metal Schottky barrier gate that separates the more heavily doped drain and source.

MESFET Structure
MESFET Structure
GaAs MESFETs are similar to silicon MOSFETs. The major difference is the presence of a Schottky diode at the gate region which separates two thin n-type active regions, that is, source and drain, connected by ohmic contacts. It should be noted that both D type and E type MESFETs, that is, ‘ON’ and ‘OFF’ devices, operate by the depletion of an existing doped channel. This can be contrasted with silicon MOS devices where the E [enhancement] mode transistor functions by inverting the region below the gate to produce a channel, while the D [depletion] mode device operates by doping the region under the gate slightly in order shift the threshold to a normally ‘ON’ condition.
This similarity provides us with the basis for extending to gallium arsenic the design methodology used so successfully in silicon to simplify circuit as system design and layout issues.
The D-MESFET is normally ‘ON’ and its threshold voltage, Vtdep, is negative. The E-MES FET is normally ‘OFF’ and its threshold Vtenh is positive. The threshold voltage is determined by the channel thickness, a, and concentration density of the implanted impurity, ND. A highly doped, thick channel exhibits a larger negative threshold voltage. By reducing the channel thickness, and decreasing the concentration density a normally ‘ OFF’ enhancement mode MESFET with a positive threshold voltage can be fabricated. Circuit symbols for the depletion and enhancement mode MESFETs arc set out in the figure below.

D-MESFET and E-MESFET Circuit Symbol
D-MESFET and E-MESFET Circuit Symbol
The MESFET has a maximum gate to source voltage V of about 0.7-0.8 volt owing to the diode action of the Schottky diode gate. Since the principle underlying the operation of MESFETs is based upon the behaviour of metal-semiconductor interface, we will briefly outline some of the features that characterize such an interface.

Characteristics of Schottky Barriers

When a metal is brought into contact with a semiconductor, an electrostatic potential barrier (referred to as Schottky barrier) is created at the interface as the result of the difference in the work function of the two materials. To appreciate the physical nature of the barrier we can model the interface by visualizing a situation whereby the metal is gradually brought toward the semiconductor surface until the separation becomes zero.
As this separation between the metal-semiconductor surface is reduced, the induced charge in the semiconductor increases, while at the same time the space charge layer widens. A greater part of the contact potential difference begins to appear across the space charge layer within the semiconductor. Because the carrier concentration in the metal is several orders of magnitude larger than that in the semiconductor when the separation is brought to zero, the entire potential drop then appears within the semiconductor itself. This is in the form of a depletion layer situated adjacent to the metal and extending into the semiconductor.

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