Abstract:
The present invention relates to a field effect transistor having heterostructure with a buffer layer or substrate. A channel is arranged on the buffer layer or on the substrate, and a capping layer is arranged on the channel. The channel consists of a piezopolar material and either the region around the boundary interface between the buffer layer or substrate and channel or the region around the boundary interface between the channel and capping layer is doped in a manner such that the piezocharges occurring at the respective boundary interface are compensated.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 10/297,234, having a filing date of Feb. 12, 2003. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a heterostructure as is for example applied in field effect transistors in power electronics and high temperature electronics. With this, in particular heterostructures which have an In(Ga)N channel, an (Al)GaN capping layer and a GaN buffer layer are used. On account of the material properties of GaN and their compounds such as (Al)GaN and In(Ga)N, these heterostructures have been intensively researched in order to permit applications in power electronics and high temperature electronics. 
     With heterostructures having an In(Ga)N channel, an (Al)GaN capping layer and a GaN buffer layer or GaN substrate, the In(Ga)N (hereinafter InGaN) channel is stressed on account of the differing lattice constants. On account of the material, by way of this in the GaN/InGaN/GaN system there arise piezoelectric fields, which at the boundary interfaces produce polarized charges (piezocharges). A GaN/InGaN/GaN heterostructure at the same time has a positive boundary interface charge at the InGaN/GaN transition and a negative boundary interface charge at the InGaN/GaN transition (interface). With an indium content of 100%, the produced boundary interface charges may reach a charge carrier density of 1.3×10 14  cm −2 . The advantage of this structure lies in the fact that the charges produced by the piezoelectric fields are located in the channel. On account of the material properties of GaN there additionally form spontaneous polarization charges on the surface and on the buffer rear-face, which however only have a secondary influence on the transistor properties. 
     Piezoelectric fields in GaN/InGaN heterostructures play a very important part if these structures are applied as irradiation-emitting semiconductor chips (light diodes). In WO 00/21143, the active layer in such a heterostructure comprises a single- or multi-quantum wave structure. In order to eliminate the influence on the piezoelectric fields in the quantum films, which compromises the irradiation behavior of such semiconductor chips, these fields are eliminated as much as possible by way of doping. 
       FIG. 1  shows in its part picture  1 A such a heterostructure with a buffer  1 , with a channel  2 , with a capping layer  3  and the boundary interfaces  5  and  6  between the layers  1 / 2  and the layers  2 / 3 , respectively. Furthermore, the piezocharges are shown wherein one may recognize that the produced positive piezocharge, or piezoinduced positive charges,  10  is located on the boundary interface  5  and the produced negative piezocharge, or piezoinduced negative charges,  11  is located on the boundary interface  6 . The piezocharges  10 ,  11  then move oppositely according to the arrows A and B under the influence of an applied external field E. 
     From literature there are known modulation-doped (MODFET), channel-doped and undoped (piezo-FET) AlGaN/GaN field effect transistors. In the AlGaN/GaN system, stresses also lead to piezoelectric fields that form polarized charges at the boundary interfaces. In contrast to a GaN/InGaN/GaN system, both boundary interface charges are not located in the channel, but the negative charge is located at the AlGaN/GaN interface as 2DEG (2-dimensional electron gas) and the positive charge is located on the AlGaN surface. 
     The disadvantage of such a structure is the fact that the bonded boundary interface charge at the AlGaN/GaN boundary interface is in interaction with the counter-pole surface charge by way of the polarized fields. The surface charges in DC operation of a GaN-based transistor have little influence on the output characteristics. In RF operation the charge on the surface must follow the channel charge on account of its interaction. Assuming the charges in the channel move at a higher speed than the surface charge, then with a characteristic frequency in RF operation the surface charge no longer follows the channel charge. This now counter-pole, quasi-stationary charge violates the neutrality condition and may now trap the free charges in the channel with the result that the output current reduces and the maximal RF power reduces with respect to the DC power. 
     In contrast to the AlGaN/GaN system, with the GaN/InGaN/GaN system no boundary interface charge produced by the piezoelectric polarization is located on the sample surface. An interaction between the piezoelectric polarization charge and the surface charge is avoided. By way of this, the effect of current compensation and the resulting power reduction with a high-frequency entry power observed with AlGaN/GaN transistors is reduced. 
     In a GaN/InGaN/GaN structure, the positive boundary interface charge on the rear-face InGaN/GaN interface and the induced negative boundary interface charge on the GaN/InGaN front-face interface contribute to the transport of current. In comparison to GaN or AlGaN, the electron movability in InGaN is considerably higher and according to theoretical computations may be 4500 cm 2 /Vsec. The movability of holes in the InGaN is however a multiple smaller than the electron movability. The resulting net current flow given by the positive boundary interface charge and negative boundary interface charge is determined by the condition of the charge neutrality, i.e., the slower hole current determines the resulting cut-off frequency of the component. Furthermore the channel may be blocked by a pn-transition. 
     It is then the object of the present invention to avoid the above mentioned disadvantages with heterostructures with a channel of a piezopolar. 
     SUMMARY OF THE INVENTION 
     This object is achieved by the heterostructure having a substrate with a channel consisting of piezopolar material. The region around the boundary interface between the buffer layer and channel or between the channel and capping layer is doped in a manner such that the piezocharges occurring at the respective boundary interface are compensated. A field effect transistor with the heterostructure has on the surface of the capping structure distant to the channel there are arranged two contact electrodes and a gate electrode. In one embodiment, the doped region is designed as an intermediate layer between the buffer layer or substrate and the channel or between the channel and the capping layer. Advantageous further formations of the present invention are also provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic view of a heterostructure; 
         FIG. 1B  is a schematic view of a heterostructure with a stationary n-doped donor layer formed in the channel layer at the boundary of the buffer and the channel; 
         FIG. 1C  is a schematic view of a heterostructure with a stationary n-doped donor layer formed in the buffer layer at the boundary of the buffer and the channel; 
         FIGS. 2A and 2B  are graphs of a simulated band course and charge carrier distribution, respectively, of the heterostructure according to  FIG. 1A  with undoped layers and indium content of 20%; 
         FIGS. 3A and 3B  are graphs of a simulated band course and charge carrier distribution, respectively, of the heterostructure according to  FIG. 1B  with a 10 nm thick N-compensation doping in the InGaN channel; 
         FIGS. 4A and 4B  are graphs of a simulated band course and charge carrier distribution, respectively, of the heterostructure according to  FIG. 1C  with a 10 nm thick N-compensation doping in the GaN buffer below the channel; 
         FIG. 5  is a schematic of a layer construction of an inverted Ga/InGaN/GaN field effect transistor with a rear-face donor doping; 
         FIG. 6  is a graph of the output characteristics of the FET structure according to  FIG. 5 ; 
         FIG. 7  is a graph of the maximal RF output current normalized to the maximum DC output current of the FET structure according to  FIG. 5 ; 
         FIG. 8  is a schematic of a layer construction of an inverted InGaN-based HEMT with a rear face donor-doping; 
         FIGS. 9 and 10  are graphs showing output characteristics of the construction shown in  FIG. 8  with various gate lengths; 
         FIG. 11  is a schematic of an invertor with an n-channel and a p-channel of InGaN-based HFET; 
         FIGS. 12 and 13  are graphs respectively showing the band course and charge carrier distribution of the p-channel InGaN-based heterostructure with n-compensation doping according to  FIG. 11 ; and 
         FIG. 14  is a graph of the output characteristic line of the p-channel transistor according to  FIG. 11  with 0.5 μm gate length. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     According to the invention for example by way of a thin n-doping in the channel or buffer or substrate the positive polarization charge on the boundary interface between the buffer and channel is compensated so that there results only one electron channel. This is shown with reference to  FIG. 1B  as well as  1 C.  FIG. 1B  shows how, by way of doping a portion  4 . 2  of the channel  2  with a stationary donor  12 , one may bring movable electrons  13  into the channel  2 , which recombine with the piezocharge  10  so that there merely remains the electrons  11  as movable charge carriers in the channel  2 . 
       FIG. 1B  shows the distribution of the charge on an inverted GaN/InGaN/GaN HEMT structure with donor-doping in the InGaN channel  2 .  FIG. 1C  shows a further possibility, specifically the distribution of the charge on an inverted GaN/InGaN/GaN HEMT structure with donor-doping in the GaN buffer  1 . In a similar manner as in  FIG. 1B , one may clearly observe the compensation of the positive piezocharge  10  in the channel  2  by the electron modulation doping from the donor layer  4 . 1 , which is a doped region of the buffer  1 . As a positive counter charge for the electron channel charge, after recombination of the charge  10  and  13  there merely remains a positive stationary donor charge  12  in the doped donor layer  4 . 1  of the buffer  1 .  FIGS. 2A and 2B  show the simulated band course in a 20 nm GaN/20 nm InGaN/GaN buffer system and the charge carrier distribution, respectively. For this simulation all layers are undoped and the indium content is 20%. In contrast,  FIGS. 3A and 3B  show, respectively, the simulated band course and the charge carrier distribution in a GaN/InGaN/GaN system with an n-compensation charge in the InGaN channel  2  as in  FIG. 1B . For this simulation a 10 nm thick n-compensation doping (1.5×10 19  cm −3 ) 4.2 is assumed in the InGaN channel  2 . 
       FIGS. 4A and 4B  show a corresponding simulation of a system as in  FIG. 1C , i.e., with a 10 nm thick n-compensation doping (1×10 19  cm −3 ) as a donor layer  7 . 1  in the GaN buffer  1  below the channel  2 . In both cases of  FIGS. 3 and 4 , the doping is chosen exactly such that the hole channel  2  has been compensated. 
     From  FIG. 3B  and  FIG. 4B , and in comparison to  FIG. 2B , it may be directly recognized that the positive piezocharges on the boundary interface between the buffer  1  and channel  2  have been completely compensated and as a result a hole conduction is no longer present in the channel  2 . 
       FIG. 5  shows the layer construction of an inverted GaN/InGaN/GaN field effect transistor with a rear-face donor doping according to the present invention. Here the doping was effected with silicon as a stationary donor (2×10 18  cm −3 ) in a 10 nm thick InGaN layer  4 . 2  that lies at the boundary interface between the channel  2  and the buffer  1 . The field effect transistor was deposited on a substrate of sapphire and additionally provided with contact electrodes  7 ,  8  and a gate electrode  9 . 
     Indium content in the 10 nm undoped InGaN layer of the channel  2  and in the 10 nm thick doped InGaN:Si layer  4 . 2  is approximately 7%. The 3 μm thick buffer  1  and the 20 nm thick capping layer  3  are in each case undoped. 
       FIG. 6  shows the output characteristic line of this FET structure with 20 nm GaN undoped as a capping layer  3 , 10 nm InGaN undoped as a channel  2 , 10 nm InGaN doped (2×10 18  cm −2 ) as a compensation layer  4 . 2  with an indium content of 7% and a 3 μm thick GaN buffer layer  1 , which is likewise undoped. The gate length was selected at 0.5 μm. With this then a maximal saturation current of 250 mA/mm was achieved. 
     In the pinch condition with such FET, a breakdown voltage of 120 V was achieved. This resulted in an output power of 2.5 W/mm. In RF operation with an entry power of 16 dBm on a 50 Ohm load line up to 10 Hz one could ascertain no current compression. This is represented in  FIG. 7 , which shows the maximal RF output current normalized to the maximum DC output current depending on the frequency with this FET of  FIG. 5 . 
       FIG. 8  shows the layer construction of an inverted InGaN-based HEMT with a rear-face donor-doping, which was likewise realized. 
     The donor doping of 1×10 19  cm −3  is located in a 10 nm thick GaN layer  4  below the InGaN channel  2 , separated from it by a 5 nm thick undoped InGaN spacer layer  21  for separating the doping rumps. The indium content in the 20 nm undoped InGaN layer  2  is 10%. The 3 μm thick GaN buffer  1  and the 20 nm thick GaN capping layer  3  are undoped. In  FIGS. 9 and 10  the output characteristics of this structure are shown. With a transistor with 0.5 μm gate length ( FIG. 9 ) and with a 0.25 μm gate length ( FIG. 10 ) a maximum saturation current of 600 mA/mm and 900 mA/mm, respectively, was achieved. 
     With the previous examples, which essentially correspond to  FIGS. 1B and 1C , it was shown that by way of the donor compensation doping of the free holes in an InGaN-based inverted HEMT structure there arises a free electron channel. It is however basically also possible to carry out an acceptor compensation doping of the free electrons and thus to produce a free hole channel. This permits complementary logic circuits to be constructed (analogous to CMOS circuit). In  FIG. 11  there is shown the construction of such an invertor with an n-channel and a p-channel of InGaN-based HFET. At the same time for the same layers the same reference numerals have been used, wherein in the p-channel system the corresponding reference numerals have been shown as reference numerals with an apostrophe (e.g., 3′ instead of 3 for the capping layer). The layer construction of the n-channel transistor at the same time corresponds to  FIG. 8 . The p-channel transistor consists of a 3 μm thick undoped GaN buffer  1 , which is in common with the n-channel transistor, a 20 nm thick undoped InGaN layer  2 ′ with 10% indium as a channel  2 ′, a 5 nm thick magnesium-doped GaN layer  4 ′ for compensation doping of the electron channel induced with this with an acceptor concentration of 2×10 19  cm −3 , and a 20 nm thick undoped GaN-capping layer  3 ′. In the n-type as well as the p-type transistor between the channel  2 ,  2 ′ and the doped layer  4 ,  4 ′ there is located a spacer layer  21  and  21 ′ respectively. 
     For compensating the two potentials of the n-type transistor or of the p-type transistor below the gate region of the two transistors an isolator  22 ,  22 ′ for example of SiO 2  or SiN is placed between the gates  9 ,  9 ′ and the semiconductor material of the capping layer  3 ,  3 ′. 
       FIGS. 12 and 13  show the band course and the charge carrier distribution respectively of a p-channel InGaN-based heterostructure with n-compensation doping shown constructed as in the right part of the picture of  FIG. 11 . On applying a negative drain source voltage and a positive gate voltage one thus realizes a p-channel transistor. This is shown in  FIG. 14  in which the output characteristics line of such a p-channel HEFT with 0.5 μm gate length is shown.