The thesis examines the prospects of using the superconductor NbN as the gatemetal for an InP HEMT. A HEMT or High Electron Mobility Transistor is aheterostructure transistor engineered to reach very high electron mobility. InPHEMTs are used as cryogenic Low Noise Amplifiers (LNAs), which have increasedin demand as quantum computing is scaling up. A superconducting NbN gate isof interest as it has the potential to decrease the amount of noise generated by theHEMT LNAs.A gate width dependence for both the transconductance (gm) and the large-signal HEMT channel resistance (RON ) of the NbN HEMTs at room temperaturehas been observed, and the first goal pf the thesis is to determine the originof the dependence. Moreover, the measured RF characteristics of the NbNdevices tend to deviate from the norm of a standard HEMT, and the secondgoal is to understand why. The third goal is to determine if the NbN gate stayssuperconducting at cryogenic temperatures or if self-heating from the channelduring DC operations will break superconductivity.In the thesis, it was possible to recreate the observed gate width dependence withnew devices, and additionally, a gate width dependence in the threshold voltageis observed. The origin of width dependence is most likely related to the straincreated by the NbN gate. At DC, extremely high peaks in the transconductanceare observed, which is most likely related to impact ionization and a subsequentincrease in hole trapping caused by the introduction of the NbN gate.Using simulations, it was possible to accurately recreate the observed deviantbehaviour, likely associated with the NbN gate’s high capacitance, inductance andresistance at room temperature. The high capacitance is likely partly related tosome NbN gates of the HEMTs being broken. Finally, the HEMT can operatein DC at 2 K with VG = 0.3 V and a maximum VD = 0.1 V before self-heatingfrom the channel will break the NbN superconductivity of the gate. This is oneof the critical conclusions of the work because it shows that a superconductinggate electrode can be implemented and functional in a high-performance HEMTdevice structure and under realistic operating bias conditions. As long as it can bedemonstrated that the superconductivity does not break when operating in RF, aNbN gate is a promising avenue to increase the noise performance of the cryogenicHEMT. / Detta arbete utforskar möjligheterna att använda supraledaren NbN som metallför gaten av en InP HEMT. En HEMT eller High Electron Mobility Transistor är entyp av transistor som är designad för väldigt hög elektron mobilitet. InP HEMTsanvänds som lågbrusförstärkare eller på engelska Low Noise Amplifiers (LNAs),vilket har ökat i efterfrågan när kvantdatorer nu skalar upp. En supraledande gateär av intresse för att den har potentialen att minska mängden brus som skapas ien HEMT LNA.Ett gatebredd beroende för både transkonduktans och RON har observerats och ettmål i arbetet är försöka fastställa dess ursprung. Utöver detta tenderar beteendet,av HEMTs med en NbN gate, att avvika från normen av en standard HEMT närden drivs i RF, och det andra målet är att förstå varför. Det tredje målet äratt fastställa om en NbN gate kommer supraleda i kryogen temperatur eller omsjälvuppvärmning från kanalen i HEMTen kommer bryta supraledningen.I arbetet var det möjligt att återskapa beroendet av gatebredden för nyaHEMTs och ett gatebredd beroende för HEMTens tröskelström observeradesockså. Gatebredd beroendet kommer troligtvis från spänning i HEMTen skapatav introduktionen av NbN gaten. I DC observerades väldigt höga toppar itranskonduktansen, vilket troligt är relaterat till processen ”impact ionization”och en efterföljande ökning i infångningen av hål som skapats på grund av NbNgaten.Genom simulationer är det möjligt att återskapa det observerade avvikandebeteendet i RF, vilket troligtvis är relaterat till NbN gatens höga resistans,kapacitans och induktans i rumstemperatur. Den höga kapacitansen är möjligtvisrelaterad till att vissa gates är trasiga. Slutligen fastställer vi att HEMTen kanvara i funktion under DC med T = 2 K, VG = 0.3 V och en max source-drainpotential av VD = 0.1 V, innan självuppvärmning bryter supraledningen. Dettaär den viktigaste slutsatsen i arbetet, eftersom den visar att en supraledandegateelektrod kan implementeras i en högpresterande HEMT och fungera underrimliga driftförhållanden. Så länge det går att visa att supraledningen inte brytsunder RF, är en HEMT med en supraledande NbN gate en lovande väg framåt föratt förbättra brusprestandan för en kryogenisk HEMT.

The objective of the proposed work is to study the behavior of SiGe HBTs at cryogenic temperatures and its relation to device scaling and optimization. Not only is cryogenic operation of these devices required by space missions, but characterizing their cryogenic behavior also helps to investigate the performance limits of SiGe HBTs and provides essential information for further device scaling. Technology computer aided design (TCAD) and sophisticated on-wafer DC and RF measurements are essential in this research. Drift-diffusion (DD) theory is used to investigate a novel negative differential resistance (NDR) effect and a collector current kink effect in first-generation SiGe HBTs at deep cryogenic temperatures. A theory of positive feedback due to the enhanced heterojunction barrier effect at deep cryogenic temperatures is proposed to explain such effects. Intricate design of the germanium and base doping profiles can greatly suppress both carrier freezeout and the heterojunction barrier effect, leading to a significant improvement in the DC and RF performance for NASA lunar missions. Furthermore, cooling is used as a tuning knob to better understand the performance limits of SiGe HBTs. The consequences of cooling SiGe HBTs are in many ways similar to those of combined vertical and lateral device scaling. A case study of low-temperature DC and RF performance of prototype fourth-generation SiGe HBTs is presented. This study summarizes the performance of all three prototypes of these fourth-generation SiGe HBTs within the temperature range of 4.5 to 300 K. Temperature dependence of a fourth-generation SiGe CML gate delay is also examined, leading to record performance of Si-based IC. This work helps to analyze the key optimization issues associated with device scaling to terahertz speeds at room temperature. As an alternative method, an fT -doubler technique is presented as an attempt to reach half-terahertz speeds. In addition, a roadmap for terahertz device scaling is given, and the potential relevant physics associated with future device scaling are examined. Subsequently, a novel superjunction collector design is proposed for higher breakdown voltages. Hydrodynamic models are used for the TCAD studies that complete this part of the work. Finally, Monte Carlo simulations are explored in the analysis of aggressively-scaled SiGe HBTs.

As Si-manufacturing compatible SiGe HBTs are making rapid in-roads into RF through mm-wave circuit applications, with performance levels steadily marching upward, the use of these devices under extreme environment conditions are being studied extensively. In this work, test structures of SiGe HBTs were designed and put into extremely low temperatures, and a new negative differential resistance effect and a novel collector current kink effect are investigated in the cryogenically-operated SiGe HBTs. Theory based on an enhanced positive feedback mechanism associated with heterojunction barrier effect at deep cryogenic temperatures is proposed. The accumulated charge induced by the barrier effect acts at low temperatures to enhance the total collector current, indirectly producing both phenomena. This theory is confirmed using calibrated 2-D DESSIS simulations over temperature. These unique cryogenic effects also have significant impact on the ac performance of SiGe HBTs operating at high-injection. Technology evolution plays an important role in determining the magnitude of the observed phenomena, and the scaling implications are addressed. Circuit implication is discussed.

Ohmic contacts are crucial components in semiconductor devices such as transistors and diodes, and lowering their contact resistance is an important factor in device performance enhancement. This is especially important for low-noise amplifiers (LNAs) where device noise temperature decreases both directly and indirectly with decreasing contact resistance. This becomes relevant in quantum computers operated at cryogenic temperatures as LNAs constitutes the 4 K quantum bit (qubit) readout signal amplification chain. The goal of this project is to investigated the superconducting element niobium (Nb) as contact material for indium phosphide (InP) high-electron-mobility transistors (HEMTs), being the active component in cryogenic high-frequency LNAs. For contact and barrier resistance determination, test structures were fabricated and utilized according to the transfer length method(TLM) and the recess TLM respectively. Measurements were performed in room temperature as well as in cryogenic temperatures below and above Nb’s bulk transition temperature of 9.25 K. The results show low-resistance Nb-based ohmic contacts for n-In0.65Ga0.35As, with the non-alloyed Nb(50 nm)/Au(100 nm) stack yielding a room temperature contact resistivity of (9.4 ± 0.5) × 10−8 Ωcm2. For all contacts the contact resistivity increased moving to cryogenic temperatures, as expected when electron occupation of high-energy states decreases. At cryogenic temperatures nosuperconducting transition was observed, attributed to the Nb layer thickness being roughly equal to its coherence length. Considering the effective barrier resistance, the Ni/Ge/Au/Nb/Au alloyed contact had the lowest room temperature resistance, reporting 143 Ω µm. In cryogenic temperatures the effective barrier resistance unexpectedly decreased in all contacts. The Nb/Au contact showed the best cryogenic performance, with a barrier resistance of 28 − 37 Ω µm. This indicates great potential for non-alloyed Nb/Au contacts in cryogenic InP HEMTs. / Alla halvledarkomponenter, som dioder och transistorer, har ohmska kontakter. Att sänka kontaktresistansen hos de ohmska kontakterna är ett sätt att höja prestandan hos en komponent. Särskilt gäller detta för lågbrusförstärkare, som har en brustemperatur som minskar både direkt och indirekt med avtagande kontaktresistans. För kvantdatorer som måste kylas till kryogena temperaturer för att fungera är detta relevant eftersom förstärkningen av utläsningssignalen från kvantbitar sker via lågbrusförstärkare vid 4 K. Målet för detta examensprojekt är att undersöka ohmska kontakter baserade på det supraledande materialet niobium (Nb) i indiumfosfidbaserade högelektronmobilitetstransistorer, som är den aktivakomponenten i kryogena högfrekvens-lågbrusförstärkare. För bestämning av kontaktoch barriärresistans producerades teststrukturer enligt Transfer Length-metoden (TLM) respektive etsad TLM. Mätningar genomfördes i rumstemperatur samt vid kryogena temperaturer både över och under niobiumets kritiska temperatur på 9.25 K. Resultatet visar låg kontaktresistans för Nb-baserade ohmska kontakter på n-In0.65Ga0.35As. Den icke-legerade Nb(50 nm)/Au(100 nm)-kontakten hade en kontaktresistivitet på (9.4 ± 0.5) × 10−8 Ωcm2 . Vid kryogena temperaturer ökade kontaktresistansen för samtliga Nb-baserade kontakter, vilket är förväntat då färre elektroner fyller högenergitillstånd. Inget supraledande tillstånd observerades vid kryogena temperaturer, vilket kan förklaras av att tjockleken på niobiumlagret var ungefär lika med dess koherenslängd. Lägst barriärresistans vid rumstemperatur hade den legerade Ni/Ge/Au/Nb/Au-kontakten, med ett värde på 143 Ω µm. Vid kryogena temperaturer skedde en oväntad minskning hos barriärsresistansen hos samtliga kontakter, där den lägst barriärsresistans uppmättes på den icke-legerade Nb/Au-kontakten, 28 − 37 Ω µm. Slutsatsen som dras är att det finns stor potential för användning av icke-legerade Nb/Au-kontakter för kryogena lågbrusförstärkare baserade på indiumfosfid.