The HEMT is a peculiar device, since it can offer optimal characteristics in terms of both high voltage, high-power and high frequency operation. Its operation principle is founded on the presence of the 2DEG at interface of an heterostructure, like for example an AlGaN/GaN system. It is a three terminal device where the current between the two Ohmic contacts of source and drain, flowing through the 2DEG, is controlled by the electrode of gate (typically a Schottky contact). Practically, the bias applied to the gate controls the flow of electrons through the channel. The figure shows a schematic of an HEMT device. To confine the electron flow in the 2DEG and isolate HEMT devices, deep trenches (cutting the 2DEG) or ion implantation are typically used.
The below figure illustrates, in a schematic band diagram of an AlGaN/GaN HEMT structure, how the 2DEG is influenced by the different gate bias conditions. This schematic is reported for the case of a n-type doped AlGaN barrier layer. At Vg = 0V there are allowed levels below the Fermi level in the subbands of the quantum well, resulting in the presence of a high sheet carrier concentration and in the on-state of the device. By increasing the gate bias (Vg is greater than 0 V), the Fermi level rises, increasing the density of allowed states below the Fermi level in the conductive band, and therefore increasing the sheet carrier concentration of the 2DEG. By decreasing the gate bias V towards negative values (V is less than 0 V) the Fermi level drops depleting the 2DEG, until the position of the Fermi level lies below the quantum well Under this condition, the level in the energy subbands are completely empty and the device is in the off-state.
In the following figure we can see the Ids vs Vds characteristics of a HEMT. In the Ids-Vds characteristics by applying a positive potential difference between source and drain (Vds ), the current will start to flow in the 2DEG. By increasing the drain bias, the current flow in the channel will increase linearly up to certain value. After this value the current through the channel starts to saturate. The maximum saturation value Idss depends on the sheet carrier concentration n of the channel. Looking at the trans characteristics, for a fixed Vds the drain current I rises with a parabolic behaviour with increasing gate bias.
The drain current(Ids) can be controlled by the bias applied to the gate electrode. In particular, Ids decreases with increasing the negative value of the gate bias (Vg), since the region of the channel under the gate is depleted. The value of Vg which determines the pinch-off of the channel (where the sheet carrier concentration in the channel becomes zero) is called threshold voltage (Vth) of the device.
In a AlGaN/GaN HEMT at any point x along the channel, neglecting the extrinsic series resistance of source and drain, the sheet carrier concentration depend by the applied Vg
ns(x) = 0AlGaN
qdAlGaN
[V g − V th − V (x)] (45)
where dAlGaN is the distance of the gate to the 2DEG channel, corresponding to the AlGaN thickness. The gate-to-channel capacitance (per unit of area) can be approximately assumed as independent of ns using the expression C 2DEG = 0AlGaN /qdAlGaN .
It is now possible to define the threshold voltage of the device, as the gate bias necessary to turn-off the 2DEG, resulting in a ns =0. Looking at the AlGaN/GaN schematic band diagram showed in above figure, it is clear that the threshold voltage depends on different parameters like the Schottky barrier height φB, the conduction band offset at the AlGaN/GaN interface delta Ec, the concentration of
donor atoms in the AlGaN layer N D, the relative dielectric constant AlGaN , the thickness dAlGaN and the Al concentration of the AlGaN. Besides these parameters, in order to have a complete expression of the threshold voltage the contribution of the polarization induced charge density σ must be taken into account. Thus simply the threshold voltage can be expressed as
V th = ΦB − ∆E C − qN DAlGaN d2AlGaN 2ε0εAlGaN
− σ
εdAlGaN (46)
Assuming a constant mobility and remembering the Ohmic law, for a two-dimensional electron gas the conductivity σ of the channel will be directly proportional to the sheet carrier concentration ns and to the electrons mobility in the channel µ
σ = q.ns.µ (47)
It is possible to write the drain current as:
I D = −µ.W.Q(x)dV (x)
dx (48)
where Q(x) is the charge considered in the channel. Integrating both sides in the all length of the channel and considering the expression of Q(x) we have
I D = µ.W
The drain current of a HEMT in linear region is often expressed in a form similar to that used for a MOSFET, i.e., :
Increasing V DS upto certain value called V DSsat, the drain current I D is constant and so the derivate of I D will be zero.
At bias condition of V DSsat the I D will be expressed as I DSS = 1
2µW
L C 2DEG(V g − V th)2 (53)
The above equation is approximation valid for long channel devices. However, for HEMTs with a short gate length (l is less than 10 um) the electron transport occurs under high electric fields and the expression of the saturation current is different. If the electric fields exceeds a certain critical value, the speed of the electrons in the 2DEG begins to saturate. Taking into account the effects of the saturation velocity model the saturation current is expressed as
I DSS = q.ns.vsat (54)
Considering the expression of the drain current, it is also possible to define the transconductance of the device as the change in drain current I D resulting from a variation of gate voltage V g for a fixed V DS :
gm = ∂I D
∂V (55)
at constant V DS .
Similarly the output conductance of the device is defined as the I D response to a V DS variation for a fixer gate bias V g
gd = ∂I D
∂V DS
(56) at constant V g. [26]
9 Conclusion
As the need for power is ever growing, present technology based on Si is not able to provide sufficient power handling and high frequency operation capabilities. Therefore better alternative to the domi-nant Si based technology has to be looked forward to. For this purpose we explored some alternatives and came to the conclusion that WBG semiconductor like GaN can help us in this deed.
We saw that 2DEG which is formed at the interface of AlGaN/GaN heterostructure can be used to make high power, high frequency transistors, based on HEMT principle. In order to understand about the working principle of HEMTs we went to the basics and started with BJT then proceeding to MOSFETs, MESFETs and finally came to HEMT. On understanding the working principles of all the devices we came to know that in both MOSFET and HEMT 2DEG are formed, but because in MOSFET the 2DEG is formed in the doped region, the scattering there is high, which results in lower saturation velocity. Where as in HEMTs the 2DEG is formed in the undoped region which leads to lack of ion scattering and thus the mobility can be raised highly. Although the low field mobility of GaN is lower as compared to other III-IV materials, but the high saturation velocity and higher bandgap makes it ideal candidate for high power and high frequency device.
We compared the various figure of merits for different capable contenders and again came to the conclusion that our choice for GaN was the best. Further the possibility increasing the polariza-tion (piezoelectric polarizapolariza-tion) by inducing stress or strain on the material makes route for further possibilities which can help in tweaking or enhancing the properties of AlGaN/GaN HEMTs.
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