Characteristics   4 Electrical properties of HEMT

   4.1 DC characteristics of HEMTs

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HEMT, altrought different to a conventional MOSFET in terms of operation, is still a field effect transistor. Thus as a transistor it was developed in order to control large currents using small driving signals. HEMTs have a high potential especially in the high power and high frequency applications. To characterise the DC properties of a transistor a series of graphs is used, among which the I-V (current-voltage) characteristics called output and  transfer characteristics are most known.(1)

HEMT theory modelHEMT DC ideal

Fig. 4.2 Structure for HEMT simulation (left) and theoretical ideal characteristics Ids-Vds for an AlGaN/GaN HEMT at different gate Voltage (right)(2)

HEMT primarily works in depletion mode, i.e. current flows through the device even with no gate drive voltage. Voltage which has to be applied on the gate electrode in order to  stop the current flow is called the pitch off voltage Vp. The best way to understand the HEMT electrical behavior is to develop a theorethical model and simulate the I-V characteristics. Figure 4.2 (left)  shows the cross-sectional view of an AlGaN/GaN HEMT used for the simulation. The layer sequence is, from top to bottom, metal/n-AlGaN/undoped-AlGaN/undoped-GaN with a 2-DEG formed at the unintentionally doped (UID)-AlGaN/GaN interface. Figure 4.2 (right) shows the output Current-Voltage characteristics for different gate voltage Vgs ranging from −4 V to 0 V where -4 V is pitch off voltage Vp. For better understanding of this characteristics it is good to know the HEMT operation principle described in chapter 3.3. These characteristics correspond to an ideal HEMT structure, i.e. without considering the anomalies that may be present in this type of component. It appears clearly that the saturated drain current increases when increasing gate voltage.(2)


DC characteristics

Fig. 4.3 HEMT DC characteristics (click to run)

A transfer characteristics shown on Fig. 4.3. displays the relation between gate voltage and drain current with a constant drain-source voltage Vds. To run the interactive animation click on the figure.  

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(1) Anders Lundskog, Characterization of advanced AlGaN HEMT structures, Master thesis, 2007, [online]
(2) M. Charfeddine, H. Belmabrouk, M. A. Zaidi, H. Maaref, 2-D Theoretical Model for Current–Voltage Characteristics in AlGaN/GaN HEMT’s, Journal of Modern Physics, Vol.3 No.8, 2012, [online]
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