A p-Channel GaN Heterostructure Tunnel FET With High ON/OFF Current Ratio
Summary (2 min read)
I. INTRODUCTION
- Forming a tunnel junction in GaN with a low tunnel resistance is challenging, in part due to the large band gap that increases the tunneling barrier height and electric field required to produce sharp band bending.
- Therefore, a thin layer of either AlN [9] , [10] , InGaN [11] - [13] or InN [14] is sandwiched between pand n-type GaN regions to introduce additional polarization charge at each of its interfaces to raise the electric field between the p-and n-type regions, thereby facilitating tunneling.
- Moreover, conventional TFETs suffer from ambipolarity which results in high offcurrent [19] and limits their applicability in complementary circuits [20] .
- The work was partially funded by ENIAC-JU project E2SG under grant contract number 296131.
II. MODEL AND CALIBRATION
- All results are obtained using Silvaco TCAD [25] , where the inbuilt non-local band-to-band tunneling (BBT) model along with III-nitride specific field-dependent mobility model [26] , [27] , Shockley-Read-Hall, and Augur recombination are selected for the tunneling current and device electrical characteristics.
- Unlike the local tunneling models, where the tunneling rate at each point is calculated from the localised value of electric field, a non-local BBT model that includes a local variation of energy bands is employed in the interests of accuracy [27] , while quantum confinement effects are neglected.
- In conventional tunneling devices, such as Zener diodes in Si, degenerately doped regions located adjacently, achieve high band bending that facilitates carrier tunneling through the energy band gap.
- To overcome this, a thin layer of AlN is sandwiched between p-and n-type regions (inset Fig. 1 (b) ), which introduces polarization charge at each of its interfaces with GaN.
- This is because a thin AlN layer fails to provide sufficient band bending required for the BBT, while a thicker AlN increases the length of the tunnel barrier, resulting in exponential degradation of the tunnelling current.
III. NON-AMBIPOLAR OPERATION OF P-CHANNEL GAN HTFET
- Fig. 3 (a) shows a schematic diagram of the p-channel GaN HTFET with its vertical direction along [0001] .
- As shown in the corresponding band diagram in Fig. 3 (b), the u-GaN acts as channel and maintains the valence band sufficiently lower than both the hole quasi fermi level (ℎ + 𝑄𝐹𝐿) as well as the conduction band in the n-GaN, thus preventing the tunneling of carriers when the gate bias is zero.
- The bias requirement of this device also increases to raise the energy of the valence band for alignment with the conduction band across the AlN layer.
- A lightly doped channel could also be employed to reduce the operating bias of this device.
- As can be observed, the drain current remains orders of magnitude lower at positive gate bias than at negative gate bias, thus confirming non-ambipolar behaviour, as indicated by the band diagrams (Fig. 3 (d) ).
IV. OPTIMISED CYLINDRICAL P-CHANNEL GAN HTFET
- The most common technique to improve the on-current in TFETs is to introduce a highly doped pocket of opposite polarity in the vicinity of the source edge of the channel to enhance the electric field across the tunneling junction [39] , [40] .
- The current is normalised to the diameter of the cylindrical geometry.
- In the absence of the thin AlN layer at the top edge of the gate, a positive gate bias alone is insufficient to produce a large band bending at this interface, hence the device continues to remain non-ambipolar.
- Owing to the higher recombination, the maximum drain current of the device degrades.
- This increase comes at the cost of increase in SS and leakage current, which arises from an inability of the gate to maintain the same potential across a wider channel.
V. ANALYSIS OF TUNNEL DISTANCE
- To contrast the operation of the PITJ HTFET, from a conventional TFET employing group IV semiconductor such as Si, in Figs. 7 (a ) and (b), the band diagrams during the OFF and ON states are compared with a conventional double gated p-i-n TFET in Si.
- The figures indicate the tunnel distance, defined as the minimum horizontal distance between the valence and conductance bands.
- The tunneling region also moves away from the gated channel region therefore resulting in a weaker gate control.
- To further highlight the distinction in operation, the, transfer characteristics and tunneling distances of the two devices are compared in Fig. 8 .
- Due to a large bandgap even though the maximum on-current is smaller, a wider band gap in GaN as well as a better control of the tunneling distance, limited only by the thickness of the tunnel barrier, lead to a higher ON/OFF current ratio and a steeper SS.
VI. CONCLUSION
- In summary, an analysis of a p-channel heterostructure TFET in GaN reveals that owing to a polarization induced tunnel junction, transfer characteristics do not suffer from ambipolarity.
- Unlike contemporary p-channel MOSHFETs in GaN, the transfer characteristics show normally-off operation with a threshold voltage greater than |−4| 𝑉, along with a subthreshold swing of 36 𝑚𝑉/𝑑𝑒𝑐.
- In addition, since the region of tunneling is pinched to the location of the PITJ, a better electrostatic control over the tunneling region via the gate and reduction in the tunnel distance by a factor of 2 are shown in the present device compared to the conventional TFETs.
- Further improvements in the on-current and reduction in the supply voltage are expected for the PITJ based on smaller band gap materials such as InGaN or InN instead of AlN.
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Cites background from "A p-Channel GaN Heterostructure Tun..."
...In this paper, we have reported a vertically grown pchannel heterojunction GaN TFET where a thin layer of InGaN is introduced as a PITJ to enhance the ON current [27]....
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References
90 citations
"A p-Channel GaN Heterostructure Tun..." refers background or methods in this paper
...[15] have previously reported that negligence of such quantum effects in TCAD models leads to an underestimation of the drain current compared to nonequilibrium Green’s function (NEGF) simulations of their HTFET utilizing an InN-based PITJ....
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...study, an inline-gated rectangular TFET with InN-based PITJ demonstrated an ON-current of 73 mA/mm with an SS of 15 mV/dec, and an ON/OFF current ratio of 5 orders of magnitude [15], while the maximum bias was kept at 0....
[...]
86 citations
"A p-Channel GaN Heterostructure Tun..." refers background in this paper
...INCREASING attention is being divested currently in low-resistance tunnel junctions in III-nitrides in order to improve the efficiency of visible and ultraviolet light-emitting diodes (LEDs) [1]–[3] by the elimination of p-type contacts in GaN [4]–[6]....
[...]
...Recent progress in p-type doping in excess of 1020 cm−3 facilitated by low-temperature molecular beam epitaxy growth has led to the demonstration of a direct tunnel junction between degenerately doped p- and n-type GaN [6], [16]....
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84 citations
"A p-Channel GaN Heterostructure Tun..." refers background in this paper
...INCREASING attention is being divested currently in low-resistance tunnel junctions in III-nitrides in order to improve the efficiency of visible and ultraviolet light-emitting diodes (LEDs) [1]–[3] by the elimination of p-type contacts in GaN [4]–[6]....
[...]
81 citations
"A p-Channel GaN Heterostructure Tun..." refers background in this paper
...INCREASING attention is being divested currently in low-resistance tunnel junctions in III-nitrides in order to improve the efficiency of visible and ultraviolet light-emitting diodes (LEDs) [1]–[3] by the elimination of p-type contacts in GaN [4]–[6]....
[...]
79 citations