ELCT564 Spring 2012

1. ELCT564 Spring 2012 Diodes, Transistors and Mixers ELCT564

2. Diodes • Diodes are non-linear devices Passive diodes --- signal detection, phase shifting • Non-linear devices Active FET, MESFET, HBT, BiCMOS -- signal generation and amplification • Rectifying properties of semiconductor materials • Known since 1874 --- created by the establishment of electrostatic barrier in • the path of the current flow • Electrostatic barrier can be created by junctions Conductivity when charge carriers have enough energy to overcome barrier (usually thermionic emission) PN Junctions • When an n-type and a p-type semiconductor are brought into contact a pn junction is formed --- acts as a diode • Carriers: electrons and lack of electrons (holes) ELCT564

3. Diodes • n-type: majority electrons, p-type: majority holes • When in contact electrons + holes diffuse across the interface and they recombine when they reach the opposite layer—until thermal equilibrium • Electric field that stops diffusion is set up – V0 contact potential depends on doping concentrations on each side Depletion region is formed (no carriers) • If external voltage is applied --- junction is biased, net flow of charge and energy • Forward bias: reduces potential difference from |V0| to |V0 | - |Vext| • net current flow from p-side to n-side • Reverse bias: increases potential difference from |V0 | to |V0 | + |Vext| • carriers cannot overcome the additional barrier--- little current due to the charge densities ELCT564

4. Diodes • When an ohmic contact is placed on the surface of the n-doped semiconductor only majority carrier diffusion dominates the junction performance • Free electrons that enter the metal have positively charged region depleted of carriers just underneath the metal contact • This positive charge gives rise to an electrostatic barrier V0 • Fwd-bias diffusion current from electrons in semiconductor dominates ELCT564

5. Practical Diodes • Finite resistivity of the bulk semiconductor----voltage drop--- reduces the fraction of the applied voltage that appears across the junction • Current shows a saturation tendency ELCT564

6. Transistor GaAs MESFET BJTs are planar and made of si in the npn type Low cost and reliable solution below 4 GHz Applications: small signal amplifiers, linear power amplifiers, low noise amplifiers and oscillaors BJT Minimum transit time from emitter to collector Use ion-implantation for fabrication and self-alignment: Multi-finger emitter-base construction ELCT564

7. Transistor Model More advanced hybrid-p model Model for packaged transistors Lp ~ 0.2 to 1nH Cp ~ 0.01 to 0.05 pF Advanced CAD programs allow designers to compare measured s-parameters of transistor with those obtained from model. ELCT564

8. Field Effect Transistors Metal- semiconductor field-effect transistor Gate terminal uses a Schottky barrier gate Microwave FETS with GaAs– greater e- mobility– f>4GHZ Excellent freq. response & noise performance ELCT564

9. Mixers and Detectors Use non-linear characteristic of device to generate an output signal containing many frequency components Detection Diode is receiving only one high frequency ωRF DC term which includes information about the amplitude of the RF signal Diode sensitivity = PDC/ PRF Frequency diagram of a detector diode ELCT564

10. Mixing Required in order to get amplitude +phase information Need two high (RF) frequencies close to each other RF frequency and LO frequency (produced by a local oscillator) Goals for the design of a good mixer Optimum match for the RF and LO signals at RF & LO ports Suppression of the DC component at RF, LO, IF ports Optimum match for the IF at the IF port Optimum termination of higher harmonics at RF, LO & IF ports ELCT564

11. Mixer Diode Let’s assume that the incoming signal to the diode is the superposition of the RF & LO as shown below: ELCT564

12. Single Diode Mixers Fundamental to the design of all mixers Every mixer can be scaled down to a single diode one At very high frequencies exhibit best performance in terms of Lc But not that good in terms of RF/LO isolation Problems : Leakage of IF through RF/LO ports, leakage of RF/LO through IF port, leakage of DC through all ports, coupling between RF and LO, mismatch between RF/LO and IF ports and the diode --- need to modify circuits • Transfer of RF, LO powers to the diode is optimized by using matching networks • Transfer of IF power from diode to connecting circuits is optimized by appropriate design + choice of diode • Image frequency has to be determined & isolated from RF/LO & IF ports ELCT564

13. Microstrip Single Diode Mixer Single diode mixers are not used effectively or frequently at microwave frequencies Found more often in single balanced mixers Single stub match for ZRF /LO fLO =20 GHz, fIF =1 GHz fRF fLO+ fIF IF filter extracts only desired frequencies and matches ZIF ELCT564

14. Mixer Measurements Conversion loss: Lc =10log(PRF /PIF) (if using BJT & FET – conversion gain!!) Calibrate RF, LO powers before measurement Find cable losses Calibrate Spectrum analyzer Noise Figure Use N.F. meter ELCT564

15. Balanced Mixers Single diode mixers are easy to make but have difficulty in isolating the RF from LO ports --- balanced mixers overcome this problem Also give cancellation of AM noise from the local oscillator Single balanced mixers • 2 complete single diode mixers connected to mutually isolated ports of a hybrid • RF & LO connecting to mutually isolated ports • IF outputs are usually connected in parallel or combined through another hybrid ELCT564

16. 180o Hybrid Balanced Mixer Microstrip configuration Vd1=VRF + VLO Vd2=VRF - VLO In to get a non-zero IF --- 2 diodes are connected in an anti-parallel configuration. ELCT564

17. Rat-race Mixer not appropriate for all applications but have simple design, low LO requirements and simple bias circuit Example of a mixer used in an integrated low-noise receiver IF output is filtered directly from the ring and low frequency blocks are used on the RF and LO lines Anti-parallel diode mixer ELCT564

18. Double Balanced Mixer • Suppresses even harmonics of both LO + RF • Four diodes in configuration • Very low conversion loss Image Rejection Mixer Fabricated Monolithic FGC Mixer ELCT564