GaN tunnel switch diodes

Tunnel Switch Diodes (TSDs) exhibit a S-shaped IV curve with negative differential resistance. Because of tunneling, they are able to switch between a low-current, high-resistance state (HRS) and a high-current, low-resistance state (LRS) fast, making them promising for high-speed memory [1]. The TSD consists of a thin tunnel barrier on top of a pn junction, [Fig 1(a)]. In the HRS state, the barrier allows only a small tunneling current through. When biased beyond a switching voltage the p-layer of the pn-junction depletes from the surface field-effect, and the surface depletion edge reaches the depletion edge of the buried pn diode. This turns the buried diode on, which floods the p-layer with electrons. Much of these electrons become trapped in the top triangular quantum well barrier, self-biasing the device. Since this state is unstable, the drop in electric field moves from p-layer to barrer, with a sudden onset of tunneling current when the TSD reaches the LRS [Fig. 1(b)]. Reducing the applied bias reverses this process and the TSD switches back to the HRS state. The switching voltage can be found with V
s
= qN
A
(t
p
− x
dep.
)
²
)/2∊
semi
∊
o
+ t
barrier
√2q∊
semi
N
A
φ
s
/∊
barrier
∊
o
, where N
A
is the acceptor doping concentration, x
dep
is the pn junction depletion width in the p-layer, t
p
and t
barrier
are the thicknesses of the p-layer and tunneling barrier, and ∊
semi
and ∊
barrier
are the relative dielectric constants of p-layer and the barrier and φ
s
is the surface potential needed to deplete the p-layer. TSDs were studied in SiO
2
/Si and in AlSb/GaSb heterostructures recently [2,3]. Advances in GaN pn-diodes and polarization physics [4] present an exciting opportunity to realize TSDs with new functionality. This work demonstrates GaN homojunction and heterojunction TSDs for the first time.