8.1 Other Field Effect Transistors - Introduction

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8.1 Introduction

While the MOSFET is the most common type of Field Effect Transistor, there are several other types of FETs. One crucial difference, which in part also explains the fact that these FETs are less common, is that the isolation between the gate and the channel is obtained by a reversed biased diode. The operation of the transistor is therefore limited to the voltage range for which that diode is indeed reversed biased. Especially enhancement-mode transistors are affected by this restriction, which limits the useful voltage range.

The Junction Field Effect Transistor (J-FET) uses a p-n diode to isolate the channel from the gate. No inversion layer exists in the device, instead the channel is depleted by reverse biasing the p-n diode between the channel and the gate. The advantage of a j-FET is that the carriers flow within the semiconductor rather than at the surface as in a MOSFET, yielding a higher carrier mobility and better noise performance. j-FETs can therefore be found in low noise amplifiers. One disadvantage of the j-FET is that the high-frequency behavior is adversely affected by the diffusion capacitance, due to the minority carriers which accumulate under forward bias, and the parasitic capacitance of the p-n diode.

The Metal-Semiconductor Field Effect Transistor (MESFET) uses a Shottky barrier diode to isolate the channel from the gate. Again, the channel is depleted by reverse biasing the diode and this structure has the same advantages as the j-FET. In addition it is easier to avoid parasitic capacitances, while the Schottky diode does not store minority carriers under forward bias and therefore does not have the associated diffusion capacitance. The MESFET is therefore a device which tends to be popular if the semiconductor of interest does not lend itself to the fabrication of MOSFETs. A classical example is the gallium arsenide (GaAs) MESFET, a frequently used microwave transistor.

The Modulation-Doped Field Effect Transistor (MODFET) also uses a Shottky barrier diode to isolate the channel from the gate and has similar advantages as the MESFET. The primary difference is that the channel is not a simply doped region, but instead consists of carriers which come from a doped region with a higher bandgap. This device is also refered to as the High Electron Mobility Transistor (HEMT) which better highlights the fact that the electrons are physically separated from their donor atoms. The separation of the carriers from the donors yield a higher mobility and hence the name. The term MODFET is more general since it is not restricted to n-type devices.

The MODFET structure requires a heterojunction material system. One common example is the GaAs/AlGaAs MODFET in which the electrons transfer from the n-type doped wider-bandgap aluminum gallium arsenide (AlGaAs) to the undoped GaAs channel. Submicron gate length MODFETs or HEMTs are known for their superior high-frequency characteristics.

8.0 ® 8.2

Bart J. Van Zeghbroeck, 1998