An MMIC (Monolithic Microwave Integrated Circuit) that includes an LNA (Low Noise Amplifier) and a detector may be used to detect a weak MMW (Milli-Meter Wave). FIG. 1 illustrates a circuit diagram of an MMIC 300 that includes an LNA 302 and a detector 303. As illustrated in FIG. 1, the MMW received by an antenna 301 is amplified by the LNA 302, and after being converted into a DC (Direct Current) voltage in the detector 303, is output from an output terminal 304 as a voltage Vdet.
The MMIC 300 includes the LNA 302 and the detector 303, and an MMW detection sensitivity is greatly affected by the detector 303. Generally, a Schottky diode is used in many cases for the detector 303, however, it is difficult to obtain a sufficient detection performance in a vicinity of a 0 V bias.
For this reason, Japanese Laid-Open Patent Publication No. 2010-251689 proposes a backward diode which may be used in place of the above Schottky diode and obtain a sufficient detection performance in the vicinity of the 0 V bias.
The backward diode is basically a diode having a heterojunction, and is characterized by its band junction condition. More particularly, the backward diode has the so-called type II heterojunction in a flat band state, and the energy of the conduction band of an n-type semiconductor layer is higher than that of the valence band of a p-type semiconductor layer. The type II heterojunction refers to a heterojunction in which the energy of the conduction band of the n-type semiconductor is lower than that of conduction band of the p-type semiconductor layer, and the energy of the valence band of the n-type semiconductor layer is lower than that of the valence band of the p-type semiconductor layer.
As an example, FIGS. 2A and 2B illustrate energy band diagrams of the backward diode having a structure in which an n+-InGaAs layer 311 provided as the n-type semiconductor layer, a non-doped InAlAs layer 312, and a p+-GaAsSb layer 313 provided as the p-type semiconductor layer are stacked. FIG. 2A is the energy band diagram in a flat band state, and FIG. 2B is the energy band diagram in a balanced state.
The non-doped InAlAs layer 312 functions as a barrier layer, and has a band gap wider than those of the n+-InGaAs layer 311 and the p+-GaAsSb layer 313. In addition, one of the p-type semiconductor layer and the n-type semiconductor layer has a high doping concentration of an impurity element to a degenerating extent. In the case of the backward diode having the energy bands illustrated in FIGS. 2A and 2B, the n+-InGaAs layer 311 provided as the n-type semiconductor layer is doped with an n-type impurity element to a high concentration, and the p+-GaAsSb layer 313 provided as the p-type semiconductor layer is doped with a p-type impurity element to a high concentration. In the balanced state of the backward diode illustrated in FIG. 2B, an upper end energy Evp of the valence band of the p+-GaAsSb layer 313 and a lower end energy Ecn of the valence band of the n+-InGaAs layer 311 are approximately at the same level. In other words, although Evp<Ecn in the flat band illustrated in FIG. 2A, Evp and Ecn are approximately equal (Evp≈Ecn) in the balanced state illustrated in FIG. 2B. In FIG. 2B, a one-dot chain line indicates a Fermi level.
FIG. 3 illustrates a relationship between an applied voltage and current in the backward diode having the energy band structure illustrated in FIGS. 2A and 2B. As illustrated in FIG. 3, when a voltage is applied to the backward diode in a reverse direction as illustrated in FIG. 4A, the voltage is applied in the negative direction, and electrons flow as a tunneling current from the valence band of the p+-GaAsSb layer 313 to the conduction band of the n+-InGaAs layer 311. On the other hand, when a voltage is applied to the backward diode in a forward direction as illustrated in FIG. 4B, the voltage is applied in the positive direction, to form a barrier with respect to the electrons and holes, and virtually no current flows until a predetermined voltage is applied. Hence, the backward diode is highly non-liner in the vicinity of 0 V.
The applicant is aware of Japanese Laid-Open Patent Publications No. 2010-251689, No. 1-37858, No. 2004-39893, and No. 2011-61086.
Various methods to improve the detection characteristic of the backward diode are conceivable. For example, as illustrated in FIG. 5A, a conceivable backward diode may have a structure in which an n+-InGaAs layer 324 doped with an impurity element to a high concentration is provided between the InAlAs layer 312 and an n-InGaAs layer 321. By providing the n+-InGaAs layer 324, a curve in the conduction band of the n+-InGaAs layer 324 may be made sharp, and a depletion layer that is formed may be made narrow. As a result, the width of a forbidden band becomes narrow at the energy level of the Fermi level, and the tunneling current becomes easier to flow. In this state, the concentration of the impurity element that is doped may be 1×1018 cm−3 for the n-InGaAs layer 321 and 8×1018 cm−3 for the n+-InGaAs layer 324. The thickness of the n+-InGaAs layer 324 is preferably on the order of the thickness of the depletion layer. In addition, a p-type semiconductor layer that is doped with a p-type impurity element to a high concentration may be provided between the p+-GaAsSb layer 313 and the InAlAs layer 312. Furthermore, the p-type semiconductor layer and the n-type semiconductor layer may be bonded directly, without forming the InAlAs layer 312.
For example, as illustrated in FIG. 5B, another conceivable backward diode may have a structure in which a band adjusting layer 325 having a band gap narrower than that of the n-InGaAs layer 321 is provided between the InAlAs layer 312 and the n-InGaAs layer 321. The band adjusting layer 325 is made of a material having a lower end of the conduction band lower than that of the n-InGaAs layer 321. For example, when In0.53Ga0.47As is used for the n-InGaAs layer 321, InxGa1-xAs (x>0.53) having a band gap narrower than that of the n-InGaAs layer 321 is used for the band adjusting layer 325. Hence, the tunneling current may more easily flow due to the decrease of the conduction band of the band adjusting layer 325, without having to make the curve in the conduction band of the band adjusting layer 325 sharp. A critical thickness of InxGa1-xAs becomes thin as the value of x increases, and thus, the value of x may preferably be on the order of 0.53<x<0.7. In addition, the thickness of the band adjusting layer 325 may preferably be on the order of 10 nm. Moreover, a p-type semiconductor layer that is doped with a p-type impurity element to a high concentration may be provided between the p+-GaAsSb layer 313 and the InAlAs layer 312. Furthermore, the p-type semiconductor layer and the n-type semiconductor layer may be bonded directly, without forming the InAlAs layer 312.
However, according to the backward diodes having the structures described above, the pn-junction is formed by the n-type semiconductor layer and the p-type semiconductor layer that are doped with the impurity element to a high concentration, and for this reason, a junction capacitance may become high. In addition, in a case in which the energy difference in the conduction band or the valence band of the backward diodes having the structures described above is large, the resistance of the backward diode may become high.