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Unformatted text preview: ECE 654 Solid State Devices II Prof. S. Mohammadi - 113 - Chapter 7 High Electron Mobility Transistors (HEMTs) High electron mobility transistors (HEMTs) also known as Modulation Doped Field Effect Transistors (MODFETs) or Heterostructure Field Effect transistors (HFETs) were first introduced in 1980s. These devices take advantage of the formation of 2-Dimensional electron gas at the vicinity of a wide bandgap and small bandgap material junction. The formation of the 2-D channel boost the transport properties inside the channel resulting in excellent high frequency and noise properties for these devices as shall be discussed in details in this chapter. HEMTs can be made of different material systems in lattice-match, pseudomorphic and metamorphic fashions. Basic Structure Fig. 7.1 shows a conventional AlGaAs/GaAs HEMT structure with a recess gate technology. Note that unlike MESFETs where recess gate is used to change the threshold voltage of the transistor, the recess gate technology for HEMTs is necessary for the proper device operation. HEMT layers are grown on top of semi-insulating GaAs substrates with typical resistivity of 10 8 cm. The un-doped or lightly p-doped GaAs buffer layer is used for the formation of the two Source 2-DEG Semi-insulating GaAs Substrate un-doped GaAs N+ AlGaAs N+ GaAs Drain Gate undoped AlGaAs Metal Gate N+ AlGaAs AlGaAs spacer GaAs buffer 80A E F 2-DEG Source 2-DEG Semi-insulating GaAs Substrate un-doped GaAs N+ AlGaAs N+ GaAs Drain Gate undoped AlGaAs Metal Gate N+ AlGaAs AlGaAs spacer GaAs buffer 80A E F 2-DEG Fig.7.1. Cross section of a conventional lattice-matched AlGaAs/GaAs HEMT with the formation of a 2-D Electron Gas (2-DEG) channel in the undoped GaAs region. Quantization helps improving the transport properties of GaAs resulting in excellent speed and noise characteristics for the device. ECE 654 Solid State Devices II Prof. S. Mohammadi - 114 - dimensional electron gas. The fact that the buffer layer is undoped helps suppressing the ionized impurity scattering in the channel, thus improving the mobility. This is evident at reduced temperatures where ionized impurity scattering is the dominant scattering mechanism. The mobility of undoped GaAs improves from 8000 cm 2 /V-sec at room temperature to 200,000 cm 2 /V-sec at 77K and 1.510 6 cm 2 /V-sec at 4K. The undoped AlGaAs spacer layer grown on top of the GaAs buffer layer has a typical thickness of 20A and is used to balance the requirement of high mobility and high current capability. Thick spacer layer separates the carriers in the 2- DEG channel from ionized impurities in the N+ AlGaAs layer resulting in an increase in their mobility. At the same time due to a wide separation of the N+ AlGaAs layer and the 2-DEG due to thick spacer layer, the number of free electrons in the channel drops resulting in lower current capability as well as lower device transconductance. The AlGaAs layer above the spacer layer is called donor layer and is a heavily doped N+ region. If the AlGaAs donor layer is too thick, or is called donor layer and is a heavily doped N+ region....
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This note was uploaded on 02/19/2012 for the course ECE 654 taught by Professor Mohammadi during the Spring '08 term at Purdue University-West Lafayette.
- Spring '08