Active solid-state devices (e.g. – transistors – solid-state diode – Specified wide band gap semiconductor material other than... – Diamond or silicon carbide
Reexamination Certificate
1999-03-19
2001-06-26
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Specified wide band gap semiconductor material other than...
Diamond or silicon carbide
C257S604000
Reexamination Certificate
active
06252250
ABSTRACT:
FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a high power IMPATT (Impact Avalanche Transit Time) diode for generating high frequency signals having two electrodes, an anode and a cathode, and a semiconductor layer therebetween with a junction for blocking conduction of the diode when a voltage is applied in a reverse direction across the electrodes, said semiconductor layer comprising a low doped n-type drift layer between the junction and a first of said electrodes through which charge carriers are transported upon avalanche breakdown in the semiconductor layer and avalanche multiplication of charge carriers when a voltage high enough is applied across the electrodes in said reverse direction.
Such a diode employs the avalanche and carrier drift processes occurring in a semiconductor to produce a dynamic negative resistance at microwave frequencies. This diode is one of the most powerful sources of microwave power. It may be used to produce high power signals for frequencies above 30 GHz.
This type of diode has been described in the literature by for instance READ, and the general function thereof is that a direct voltage is applied across the diode in the reverse direction thereof, and an alternating voltage source is generated within an external resonance circuit for which the IMPATT diode provides the active power input, said alternating voltage having a lower amplitude than the magnitude of the direct voltage so as to change the reverse voltage across the diode with time, This is done so that the electric field will during at least a part of the positive half period of the alternating voltage with respect to the direct voltage be above the breakdown field for the semiconductor in question, so that avalanche breakdown will occur and an exponential growth of the electron-hole concentration in the avalanche region of the semiconductor layer will result as a consequence of the production of secondary electron-hole pairs by impact ionization. The electrons so formed move through the drift layer to the anode with a drift velocity corresponding to the saturation drift velocity. The transit time of said electrons from the avalanche region to the anode is preferably half the period of the alternating voltage, so that the electrons will reach the anode when the alternating voltage is zero thereby minimising losses.
It is desired to produce such diodes being able to deliver a high power output at high operation frequencies. The power output decreases with operation frequency f as 1/f or 1/f
2
. An ideal semiconductor material for a diode of this type should have a high breakdown field, a high saturation drift velocity and a high thermal conductivity. With a high breakdown field the diodes with a given transit time can have much higher operation voltages and by that deliver higher powers. A high saturation drift velocity further decreases the transit time for the same drift region thickness and by that increases the possible frequency. A high thermal conductivity is required to overcome thermal limitations. Such thermal limitations are highly relevant for this type of devices, since the power dissipation is very high, namely more than 50% of the electric power supplied to an IMPATT diode is dissipated within the device as compared to lower frequency power semiconductor switches for which mostly less than 1% of the electric power supplied is dissipated.
A possible material for a high power IMPATT diode with an output power significantly increased with respect to such a diode of silicon would be silicon carbide, since it has a high breakdown field, a high saturation drift velocity as well as a high thermal conductivity and is stable at high temperatures. However, for different reasons it has until now not been possible to produce any working IMPATT diode of silicon carbide. The main problem seems to be to take care of the high power dissipated by such a diode. The high breakdown field of silicon carbide will namely result in such a high breakdown voltage and accordingly operation voltage of the diode that such a diode could not withstand care of the heat dissipated, since the current density has to have a certain value for obtaining avalanche resonance. This means that thermal limitations have until now prevented the use of this promising material (silicon carbide) for IMPATT diodes.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a high power IMPATT diode of the type defined in the introduction being able to deliver a much higher output power than such diodes already known and working.
This object is according to the invention obtained by making the semiconductor layer of such a diode of crystalline SiC and providing the semiconductor layer with means adapted to locally increase the electric field in the drift layer substantially with respect to the average electric field therein for generating said avalanche breakdown at a considerably lower voltage across said electrodes than it would be if the doping of the active region of the diode were substantially constant across the entire active-region layer. Accordingly, said means will “trigger” the diode at an earlier stage, so that the operation voltage thereof will be considerably decreased for a given drift region length. This means that a higher current density may be achieved with the same power dissipation, or the power dissipation will be considerably reduced for a given current density. Accordingly, the thermal limitation of such diodes of SiC has been overcome by sacrificing a significant part of the theoretically available power density and partly the efficiency. Furthermore, the current density may be given such a value, that so-called near-resonance conditions could be achieved in the avalanche region and by that more optimized conditions of generation under continious-wave operation will be ensured.
According to preferred embodiments of the invention said means is adapted to cause an avalanche breakdown at a voltage across said electrodes being a factor of less than 50% and as an alternative 30% of the breakdown voltage of the diode had the active region doping been substantially constant across the entire active region thickness.
According to another preferred embodiment of the invention said means comprises a thin n-type layer with a substantially higher doping concentration than the drift layer arranged in the drift layer at a distance from said junction for locally increasing the electric field in a region of the drift,layer between said thin layer and the junction with respect to the average electric field in the drift layer. Such a thin layer with a higher doping concentration will result in a high, nearly constant electric field and accordingly a well defined avalanche region between said junction and said thin layer. The breakdown voltage of the diode may in this way be lowered substantially.
According to another preferred embodiment of the invention the distance between said junction and said thin layer is short with respect to the thickness of the drift layer, preferably less than ⅕, and most preferred less than {fraction (1/10)} thereof. It has been found that the current required to achieve the avalanche resonance at a given frequency is rapidly decreased with a decreasing distance between the junction and the thin layer, which makes it easier to arrive to a negative differential resistance operation mode, i.e. operation with a frequency close to the avalanche resonance frequency.
According to another preferred embodiment of the invention the doping concentration of said thin layer is at least an order of magnitude, preferably at least two orders of magnitude higher than the doping concentration of the drift layer. Such a high doping concentration will considerably reduce the breakdown voltage of the diode.
According to another very preferred embodiment of the invention said semiconductor layer is made of crystalline SiC of the 4H polytype.
Another problem encountered in the attempts to produce this type of device in SiC and not addressed sofar, is that a
Harris Christopher
Konstantinov Andrei
Acreo AB
Chaudhuri Olik
Dilworth & Barrese
Wille Douglas A.
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