Transistors & MOSFETs

PNP BJT

NPN BJT

Transistors are semiconductors that amplify signals and operate as a solid state switch. They are constructed as a 3 “layer cake” with PNP or NPN the collector emitter and base can be identified by simply diode testing each connection. The higher reading is the emitter (B-E) - approx 0.7V is the forward bias voltage, the lower value is the collector (B-C) and the common with both terminals is the base. If this is found using the positive lead on it then the base is an NPN and vis-versa for PNP all other test should give OL if not there is a short or an earth in the BJT. 

 AC Amplification BJT Circuit 

An easy way to remember the format of the diodes is NPN: “not pointing in” and PNP: “pointing in please”. This was tested and the results shown next confirm the above method of testing the BJT leads. See the image beside for the pro electron codes.

Basic Switching Transistor Circuit

P is the anode an N is the Cathode.

Diode Test meter reading (volts) of two different working BJT’s;

BJT: Tip32C NPN; Vbe 0.685, Veb OL, Vbc 0.682, Vcb OL, Vce OL and Vec OL.

BC547 PNP; Vbe OL, Veb 0.601, Vbc OL, Vcb 0.600, Vce OL, Vec OL

In a basic switching transistor Experiment a circuit is connected using a small signal NPN transistor as a switch, a 15V power supply and 2 resistors (Rb=10K, RC=1K). A Voltmeter connected in parallel across the base and emitter gives a reading of 0.79V this reading indicates the forward bias voltage. The same test across the Collector and Emitter gave a reading of 0.53V which is close to zero showing that the saturated transistor is acting as a switch in the on position.

BJT’s Operating as a solid state switch but are much faster and more reliable than a relay because they are electronic instead of mechanical.  They switch on by saturating at the end of the load line that Vce is equal to or close to zero and they switch off by being placed in the cut off bias – base current equals zero so no collector current will flow. In both the above regions there is minimum power dissipation.

The transistor working in the regions on the graph (shown next) of collector current(IC) vs. collector to emitter voltage (VCE) and base current (Ib). The area “A” is the saturated region, where VCE is approx equal to zero and collector current is determined by load resistance, and where the BJT is “on”. “B” is the cut off region where no current flows, Ic = 0, full current is used through load resistance or the power supply drops across RL. From this graph we can also see that the power dissipated (PD) by the BJT at 3 volts is 42mW (3*14). Beta (β)-the ratio of collector current to base current, also called gain or hfe (which stands for H parameters, forward current transfer ratio, common emitter configuration) is equal to Ic/Ib in this example at Vce of 2,3 and 4 volts β =25/1, 20/0.8 or 15/0.6 = 25.

MY next BJT Experiment (circuit diagram and graph shown) is changing the resistance of Rb (2K2 – 330K) and measuring to voltage and current, for Vce, Vbe, Ic and Ib and plotting these results on a graph. From this it was noted that as Rb increased so did Vce, Vbe remained constant between 0.68 and 0.79V this is its forward bias voltage. The base current was 1.9mA with the smallest (2K2) base resistor and the Ib decreased to 0.01mA with lager base resistors. The collector current decreased by a small 3 to 4mA drop as Rb increased. The Ib lines on the graph were later changed to the same scale as Vce and a reliable β vale of 3.5 to 4.0 was calculated from using the formula β= Ic/Ib. Lastly the load line is used to describe the saturation region – at one end Vce is large(Ic close to zero cut off region) and at the other extreme Ic is greater this is the saturation region were maximum collector current can flow but Vce is close to zero

Germanium BJT's

An interesting fact is that British and European transistors are issued with a unique combination of letters and numbers.
The first letter identifies the semiconductor type: 
· A = Germanium 
· B = Silicon 
· C = Gallium Arsenide 
· D = other compound semiconductor material 
The second letter indicates the intended use: 
· A = Small signal diode 
· B = Varicap diode 
· C = Small signal LF transistor 
· D = LF power transistor 
· E = Tunnel (Ersaki) diode 
· F = RF small signal transistor 
· K = Hall effect device 
· L = RF power device 
· N = Optocoupler 
· P = Radiation sensitive device (e.g photo transistor)
· Q = Radiation emitting device (e.g LED)
· R = Low power SCR 
· T = High power SCR or triac 
· U = High voltage switching transistor 
· Y = Rectifier diode 
· Z = Zener diode 


Any third letter indicates the device is primarily intended for professional and industrial use. This letter is usually a V, W, X or Y and can be ignored. Many devices have a suffix letter after the number: A = low gain, B = mid gain, C = high gain.
Devices with no suffix can have an hfe anywhere within the part's specification. Military devices are generally marked with a "CV" (Common Valve) number rather than the civilian code. For instance the CV7003 is an OC44 in uniform.


MOSFET

Metal Oxide Silicon Field Effect Transistors (MOSFET)

There are ‘P’ and ‘N’ type MOSFET’s, they have 2 modes enhancement - used for applications such as switching due their low ‘on’ resistance and high off resistance and also the less common depletion mode. Dashed lines show that a MOSFET is “NO” (normally open)

Voltage controlled MOSFET’s have; fast switching time, low voltage gain, high current gain, low noise generation, very high input impedance and high output impedance but can be easily damaged by static so it is recommended using an anti static wrist strap. Some MOSFET’s require an input to turn it “OFF”. Hence this semiconductor is best for use as a switch when compared to a BJT.

In comparison the cheaper, current controlled Bipolar Transistor are robust, have; medium noise generation, high voltage gain, low current gain, low input impedance, low output impedance and medium switching time, these also requires zero input to turn them “OFF”

Some of the Electrical characteristics of these transistors are as follows;

The resistance(R) between the drain (D) and source (S) terminals when the device is fully turned on is known as the RDS (ON)

 The ‘IDSS’ is the drain to source leakage current and is the amount of current that can flow from drain to source when the device is turned off.

The ‘VGS (TH)’ is the gate threshold voltage and is the minimum voltage required to turn the device on but a MOSFET will require more than this to turn it fully on.

Transconductance (gfs) is the ratio of drain current (ID) to gate to source voltage (VGS). The current over voltage (I/V) ratio is commonly referred to as gain or hfe (parameters, forward current transfer ratio, common emitter configuration)

When using MOSFET’s be aware of the current rating and temperature ratings given in datasheet for that specific MOSFET.

A MOSFET is like a BJT in that the Gate is like the base, the drain – collector and  the source – emitter also if you short the gate to source maximum drain current flows.

For an N-channel MOSFET charge carriers contribute to the current flow

As the depletion of charge carriers control the MOSFET depletion. When the Gate is made more negative, it depletes the majority carriers from a larger depletion zone around the gate. This reduces the current flow for a given value of Source-to-Drain voltage.