Test: Basic Electronics- 1 - Electrical Engineering (EE) MCQ

In a unipolar device current is due to only one type of carriers (i.e. electrons or holes), while in bipolar device current is due to both holes and electrons.

Schottky diode or hot-carrier diode is a junction of n-type semiconductor and metal, in it electrons are sole carriers.

A diode in forward-bias conducts current just like normal conductor, while in reverse-bias prohibits current and only very small amount of current flows, called reverse saturation current.

After certain reverse voltage called reverse breakdown voltage the diode breaks down and conducts huge current.

Thermal runaway is a situation in which increase in temperature changes the conditions in a way that further causes temperature to increase.

In BJTs the collector current dissipates lot of heat, this increase in temperature causes increased flow of minority carriers from collector to base, further increasing the collector current. This increase in collector current further raise temperature, thus thermal runaway occurs ending in burnout of BJT.

Transistor switches can be used to switch low DC voltages ON or OFF by operating it in saturation or cut-off mode

The CE terminals act as ON/OFF terminals as shown in figure above.

LEDs are fabricated using direct bandgap semiconductors.In which, when electron transfers from conductor band to valence band and recombines with the holes the energy difference between the both bandsget converted in to light with frequency ‘ν’, it is related with energy as,

E = hν, where h is Planck’s constant.

Uni Junction Transistor is a three terminal semiconductor device formed by single P-N junction. It functions as an ON-OFF switch and has unidirectional conductivity and negative resistance characteristics.

It is used to trigger SCR devices and used in relaxation oscillator.

In JFET Gate and channel are opposite type of semiconductors with depletion layers in between them.

If reverse voltage is applied to Gate terminal the depletion region increases and channel width is reduced, this effect is called pinch-off effect, limiting the drain current

A transistor operation needs very small base width to allow the minority carriers, drifted from emitter to diffuse into collector.

By connecting two PN diodes back to back the transistor will have wide base causing the minority carriers from emitter to recombine in base region before they diffuse into collector. This reduces collector current or cut off entirely.

A common emitter amplifier acts as a bandpass amplifier due to reduced gain at low and high frequencies. This is due to source, emitter and parasitic capacitances.

In transistor relay circuit when relay is in OFF state, high reverse voltage is developed across relay terminals, this can damage the transistor. Hence a diode is connected parallel to the relay, this clamps the voltage across the relay and helps dissipate the energy in diode.

Gunn diode, FET and UJT are unipolar devices in which current is due to only one type of carrier. While tunnel diode is bipolar device in which both electrons and holes contribute to current.

Biasing of a transistor is done to keep the stable DC condition like V_BE, I_C needed for specific action of transistor, like amplification, switch, etc.

In an amplifier the maximum power is transferred to the load when the load resistance is equal to output impedance of amplifier

In push-pull amplifiers each transistor operates only for one half cycle of AC voltage, in it even harmonics are minimised, so distortion is mainly due to odd harmonics.

Hence of the given options third harmonics needs to be eliminated

In semiconductors like GaAs the elements Ga and As are in covalent bond. Arsenic has five valence electrons while gallium has three valence electrons. Therefore, in GaAs crystal Arsenic contribute one electron to Gallium and it is shared between both atoms in a covalent bond

I_CBO is the reverse leakage current between collector and base while emitter is open it is given by I_CBO=(1−α)I_C−αI_B. It is the reverse current in base collector junction, which is dependent on temperature at the junction and is reduced with decreasing current.

Zener diodes in reverse bias starts conduct current at a certain reverse voltage known as Zener voltage. Above the Zener voltage the diode maintains constant voltage across its terminals for varying values of current, hence Zener diodes are used as voltage regulators.

In a JFET transconductance is defined as 

Hence it is computed at constant V_DS by ratio of change in l_d to change of V_gs.

The intersection of DC load line with the parameter graph of a semiconductor device (for example base current vs V_CE in transistor) gives operating point of the said device

In transistor the Gain Bandwidth product is equal to unity, since gain is proportional to DC current gain h_fe the transistor with lower h_fe will have higher bandwidth.