4.1.2 Structure of the detector diode
There are three types of detector devices in total: point (crystal detector); mesastructure and planar detector diode. Let's consider the structure of each of these detector devices.
Picture 1 – Point (crystal detector)
The point
diode consists of a polycrystalline silicon sample of p-type
1, in contact with which there is a spring made of a pointed tungsten
wire 2 at the end. The electrodes 3 and 4 are soldered to a ceramic
housing 5. The tip of the spring is welded to the semiconductor by
passing a current pulse. A Schottky barrier with a small area (units
of
)
is formed at the welding site, which causes a small transition
capacity. Such a design is characterized by a high value RS
and
a bad coefficient of imperfection n
(3
4),
thus, such a structure is far from perfect [4].
These disadvantages are absent in epitaxial structures with a sprayed metal film.
Picture 2 – Mesastructure
Such a detector diode is a mesastructure of gallium arsenide (GaAs). Metallization 1, applied by vacuum evaporation, forms a Schottky barrier with an epitaxial film of n-type 2, which is close to ideal in its properties. Highly alloyed n+ substrate 3 contributes to low resistance RS. In view of this, despite the large capacitance compared to a point diode, such a detector diode has higher critical frequency values with a lower noise level [4].
Picture 3 – Planar structure
The barrier in such a planar structure is realized at the point of contact of the nickel electrode with the n-type semiconductor. The gold coating of the contacts ensures the stability of the operation of such devices during operation [4].
4.1.3 Parameters of the detector diode
Here is an equivalent circuit of a detector diode [5]:
Picture 4 – Equivalent detector diode circuit
The main parameters and characteristics of detector diodes are:
1. Volt-ampere characteristic:
Picture 5 – Volt-ampere characteristic of the detector diode
The theoretical formula of the volt-ampere characteristic of a diode with a Schottky barrier has the following form [6]:
(6)
where IS – saturation current at reverse bias (thermal current); q – electron charge; U – diode bias voltage; n – the coefficient of ideality reflecting the effect of tunneling and the presence of a contact dielectric layer.
Consider the negative branch of the volt-ampere characteristic. With small reverse displacements, the current through the diode is practically unchanged and is determined by the current of non-primary charge carriers. The IS current is created by non-basic carriers that have fallen into the region of the pulling electric field from the diffusion transition regions. Such a current is called a thermal current. With large reverse offsets, the current flowing through the diode goes into avalanche breakdown mode. However, if the diode works in this mode for a long time, it may fail [6].
At the same time, a direct branch of the volt-ampere characteristic is of particular interest. If you plot its initial section on a logarithmic scale, it is not difficult to notice that the resulting graph has three distinct sections. The middle section II is characterized by an exponential increase in current and is in good agreement with the known theoretical relations. The deviation from the exponential dependence on the section I of the direct branch of the volt-ampere characteristic of the diode is due to the presence of leakage currents that are not taken into account when deriving the theoretical formula. In the region of large direct displacements (section III), the forward voltage at the junction is such that it is completely open (approximately 0,5 V is enough to open the junction). The deviation of the real volt-ampere characteristic from the exponential growth in this area is determined by the influence of the resistive regions of the diode, which is taken into account by the RS resistor in the equivalent circuit (picture 4). This area is called resistive [6].