Heterostructure barrier varactor

Heterostructure barrier varactor diode

The Heterostructure barrier varactor (HBV) diode was invented by Erik Kollberg together with Anders Rydberg in 1989 at Chalmers University of Technology. This semiconductor diode has an anti-symmetric current-voltage relationship and a symmetric capacitance-voltage relationship as shown in the graph below. Also, in this figure the inset shows the circuit schematic symbol of the HBV. We can see from this symbol that, in practice, the HBV consists of two, back to back, anti-serially connected rectifying diodes (such as Schottky diodes for instance). The gap in the middle of the diode symbol represents the inherent capacitance of the device. The main use of the HBV is as a signal source in the mm-wave and THz frequency spectra.The electrical characteristics of the HBV are realized by separating two layers of a semiconductor material (A) with a layer of another semiconductor material (B). The band-gap of material (B) should be larger than for material (A). This results in a barrier for the carriers trying to travel trough the layers (A)-(B)-(A). The (A) layers are usually n-doped which means that electrons are the majority carriers of this device. At different bias voltages the carriers are redistributed and the distance between the carriers on each side of the barrier (B) is different. As a consequence the HBV has electrical properties resembling the parallel plate capacitor with a voltage dependent plate distance d.

The main application for the HBV diode is to generate signals at extremely high frequencies (> 100 GHz) by multiplying lower frequency signals. Since it is considerably easier to generate AC power at lower frequencies. This lower frequency power can then be multiplied to higher frequencies with the HBV. This is made possible by the characteristic voltage dependence of the capacitance C(V), making the HBV diode highly nonlinear. This property is used to generate higher harmonics f₃, f₅, f₇… from an initial input signal f₁ . In other words f₃ =3x f₁ and f₅ =5x f₁ etc. So, in effect, the HBV is able to multiply an incoming signal to higher, odd frequency multiples 3,5,7… The reason why the even multiplications, 2,4,6…, are cancelled is because of the symmetric/anti-symmetric nature of the electrical properties. Also, using this inherent symmetry of the device we can operate it without any DC-biasing. This is an advantage compared to the Schottky diode which has to be biased.

Signals at these frequencies (100 GHz - 3 THz) have applications in diverse areas such as radioastronomy, security imaging, biological and medical imaging and high-speed wireless comunications.